diff --git a/report.lyx b/report.lyx index 991ecd5..d7c23b7 100644 --- a/report.lyx +++ b/report.lyx @@ -3219,8 +3219,7 @@ Erlang is not a very popular language for game development, therefore the interface to different virtual machines which would run the game code. This way a game developer can write the game in his favorite language while the server part still is written in Erlang and can benefit from all the - advantages of the Erlang language. - In this section, a few potential languages are given. + advantages the Erlang language provides. \end_layout \begin_layout Subsection @@ -3229,20 +3228,21 @@ JavaScript \begin_layout Standard JavaScript is a prime GDL candidate for the GGS. - The language is very flexible, a general knowledge of the language is present - in the computer science community, furthermore there are virtual machines - readily available for JavaScript. + The language is very flexible, and a large base of developers within the + web community is using this language on the client side within the browser + and therefore are used to it. \end_layout \begin_layout Standard -JavaScript has gained a lot of popularity lately, it is used in large projects - such as +JavaScript, as a interpreted script language, has gained a lot of popularity + in other fields of computer science lately. + It is used as a server side language in large projects such as \emph on Riak \emph default \begin_inset Foot -status open +status collapsed \begin_layout Plain Layout \begin_inset Flex URL @@ -3260,7 +3260,7 @@ http://wiki.basho.com/An-Introduction-to-Riak.html \end_inset -, + or \emph on CouchDB \emph default @@ -3315,22 +3315,17 @@ https://github.com/languages/ \end_layout \begin_layout Standard -Since the GGS is intended to be connected to several different GDL VMs the - choice for the first language implemented for the GGS prototype seems not - only to depend on the technical features of the GDL chosen, in this case - JavaScript. - A different, albeit still important non technical feature of JavaScript - is the familiarity with the language of the members of the GGS development - team. +Apart from that there are virtual machines with bindings to Erlang readily + available for JavaScript which are provided by organisations like Mozilla + and companies like Google. + In the end this choice was more or less arbitrary since the GGS is intended + to be able to run several different GDL VMs, and one had to be the first. + \end_layout \begin_layout Standard -The popularity of JavaScript in the programming community, in combination - with the availability of several different JavaScript virtual machines - was an important influence in choosing JavaScript as the main control language - for our GGS prototype. \begin_inset ERT -status collapsed +status open \begin_layout Plain Layout @@ -5019,7 +5014,7 @@ The VM itself makes it possible for the game developer to program in the Since the game VM keeps track of the correct table, the game developer does not need to take this into consideration when programming a game. If a method within the game sends data to a player, the data is delivered - to the player in the correct running game. + to the player in the correct game. The same game token is used to store the game state in the database. Therefore, no game states can be mixed up. \end_layout @@ -5038,7 +5033,7 @@ reference "sec:Communication-with-the-GDL-VM" \end_layout \begin_layout Standard -The code which is run in the VM is uploaded to the GGS prior to each game. +The code which is run in the VM is uploaded to the GGS before each game. Allowing the clients to upload code allows clients to run any game. \end_layout @@ -5062,7 +5057,7 @@ Game data from all games on the GGS is stored in the database backend of In the GGS prototype the database module is using a database management system called Mnesia. Mnesia ships with the standard Erlang distribution and is a key-value store - type database. + type of database. Mnesia is designed to handle the stress of telecoms systems, therefore it has some features specifically tailored for telecoms which are not commonly found in other databases. @@ -5093,9 +5088,9 @@ key "667766" \begin_layout Standard The features of Mnesia originally intended for telecoms prove very useful - for the purposes of the GGS as well. - The fault tolerance and speed of Mnesia are very valuable tools, the fast - key/value lookups permit many lookups per second from the database. + for the GGS as well. + The fault tolerance and speed of Mnesia are valuable tools, the fast key/value + lookups permit many lookups per second from the database. \end_layout \begin_layout Standard @@ -5156,9 +5151,9 @@ Localstorage \noun default . To store a value within the database, not only is the table token and the - name of the namespace required, but a unique key so that the value can + name of the namespace required, but an unique key so that the value can be successfully retrieved or modified later. - The key is fully decidable by the game developer. + The key is decidable by the game developer. \end_layout @@ -5167,8 +5162,8 @@ The interface of the database module is an implementation of the upcoming W3C Web Storage specification. Web Storage is intended for use in web browsers, providing a persistent storage on the local machine for web applications. - The storage can be used to communicate in between browser windows (which - is difficult when using cookies), and to store larger chunks of data + The storage can be used to communicate among browser windows (which is + difficult when using cookies), and to store larger chunks of data \begin_inset CommandInset citation LatexCommand citet key "webstorage:website" diff --git a/report.lyx.orig b/report.lyx.orig deleted file mode 100644 index 9f7f607..0000000 --- a/report.lyx.orig +++ /dev/null @@ -1,7971 +0,0 @@ -#LyX 2.0 created this file. 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-\begin_body - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{textblock*}{ -\backslash -paperwidth}(0mm,40mm) -\end_layout - -\begin_layout Plain Layout - - -\backslash -begin{center} -\end_layout - -\begin_layout Plain Layout - - -\backslash -includegraphics[width= -\backslash -paperwidth/2]{graphics/logo} -\end_layout - -\begin_layout Plain Layout - - -\backslash -end{center} -\end_layout - -\begin_layout Plain Layout - - -\backslash -end{textblock*} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Title -A Reliable Generic Game Server -\end_layout - -\begin_layout Author -Niklas Landin -\begin_inset Newline newline -\end_inset - -Richard Pannek -\begin_inset Newline newline -\end_inset - -Mattias Pettersson -\begin_inset Newline newline -\end_inset - -Jonatan Pålsson -\end_layout - -\begin_layout Abstract -This is the abstract! -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -pagenumbering{roman} -\end_layout - -\begin_layout Plain Layout - - -\backslash -setcounter{page}{3} -\end_layout - -\begin_layout Plain Layout - - -\backslash -renewcommand -\backslash -contentsname{Table of Contents} -\end_layout - -\begin_layout Plain Layout - - -\backslash -renewcommand{ -\backslash -cfttoctitlefont}{ -\backslash -hfill -\backslash -Large} -\backslash -renewcommand{ -\backslash -cftaftertoctitle}{ -\backslash -hfill} -\end_layout - -\begin_layout Plain Layout - - -\backslash -renewcommand -\backslash -cftpartdotsep{6.6} -\end_layout - -\begin_layout Plain Layout - - -\backslash -renewcommand -\backslash -cftchapdotsep{6.6} -\end_layout - -\end_inset - - -\begin_inset CommandInset toc -LatexCommand tableofcontents - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Newpage newpage -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -pagenumbering{arabic} -\end_layout - -\begin_layout Plain Layout - - -\backslash -setcounter{page}{1} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Chapter -Introduction -\end_layout - -\begin_layout Standard -Online gaming, and computer gaming in general has become an important part - in many peoples day-to day lives. - A few years ago, computer games were not at all as popular as they are - today. - With the advances in computer graphics and computer hardware today's games - are much more sophisticated then they were in the days of -\emph on -NetHack -\emph default -, -\emph on -Zork, -\emph default -or -\emph on -Pacman. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Zork}}{A textual computer game developed by students at MIT} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Pacman}}{An early graphical computer game developed by Namco} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{NetHack}}{An early computer game developed by the NetHack team, arguably - the oldest computer game still in development} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -The early computer games featured simple, or no graphics at all -\begin_inset CommandInset citation -LatexCommand citet -key "nethack:website" - -\end_inset - -. - The games often took place in a textual world, leaving the task of picturing - the world up to the player. - Multiplayer games were not as common as they are today, whereas most games - today are expected to have a multiplayer mode, most early games did not. -\end_layout - -\begin_layout Standard -Since these early games, the gaming industry have become much more influential - in many ways. - Many advanced in computer hardware are thought to come from pressure from - the computer game industry. - More powerful games require more powerful, and more easily available hardware -\begin_inset Note Note -status collapsed - -\begin_layout Plain Layout -Drop a reference to the gaming industry pressuring more advanced hardware -\end_layout - -\end_inset - -. - Due to the high entertainment value of modern computer games, gaming has - become a huge industry, where large amounts of money are invested. - The gaming industry is today, in some places even larger than the motion - picture industry. - -\begin_inset CommandInset citation -LatexCommand citet -key "esa:website,thenumbers:website" - -\end_inset - - -\end_layout - -\begin_layout Standard -Due to the increasing importance of computer gaming, more focus should be - spent on improving the quality of the gaming service. - As more and more computer games are gaining multiplayer capabilities, the - demands for multiplayer networking software rises. - The topic of this thesis is techniques for improving the quality of this - networking software. -\end_layout - -\begin_layout Standard -The Reliable Generic Game Server, hereafter known as the GGS -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{GGS}}{Generic Game Server, a software for reliably hosting network - games. - The subject of this thesis.} -\end_layout - -\end_inset - -, is a computer program designed to -\emph on -host -\emph default - network games on one or more server computers. - Hosting, in a network software setting, means allowing client software - connect to the server software, for the purpose of utilizing services provided - by the server. - The GGS software provides games as a service, and the clients connecting - to the GGS can play these games on the GGS. -\end_layout - -\begin_layout Standard -The idea of game servers is not new, network games have been played for - decades. - Early, popular examples of network games include the -\emph on -Quake -\emph default - series, or the -\emph on -Doom -\begin_inset Note Note -status collapsed - -\begin_layout Plain Layout - -\emph on -Come up w/ better game -\end_layout - -\end_inset - - -\emph default - games. - Newer examples of network games include -\emph on -World of Warcraft -\emph default -, and -\emph on -Counter-Strike -\emph default -. - The difference between the GGS and the servers for these games is that - the servers for Doom, Quake, and the others listed, were designed with - these specific games in mind. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Quake}}{A first person shooter series developed by ID software. - The series consists of four games.} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Doom}}{A first person shooter series developed by ID software. - The series consists of three games.} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Counter-Strike}}{A multiplayer first person shooter game, popular - in E-Sports.} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{World of Warcraft}}{A MMORPG game developed by Blizzard. - The world's most popular MMORPG by subscriber count.} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Framework}}{A supporting structure, the GGS is a framework for developing - network games} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{First-person shooter}}{A game in which centers around gun combat - from the first person perspective.} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{MMORPG}}{Massively multiplayer online role playing game. - An online game with several thousand participants.} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -What GGS does is to provide a -\emph on -generic -\emph default - framework for developing network games. - The framework is generic in the sense that it is not bound to a specific - game. - There are many different types of games, some are inherently more time - sensitive than others Strategy games are examples of games which are not - very sensitive to time delays, first-person shooters however, can be very - sensitive. -\end_layout - -\begin_layout Standard -The generic nature of the GGS allows the creation of many different types - of games, the motivation behind this is to remove the necessity of writing - new game servers when developing new games. -\end_layout - -\begin_layout Standard -The GGS is in addition to being generic, also -\emph on -reliable -\emph default - in the sense that the gaming service provided is consistent and available. - A consistent and available server is a server that handles hardware failures - and software failures gracefully. - In the event of a component breaking within the GGS, the error is handled - by fault recovery processes, thereby creating a more reliable system. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Hardware failiure}}{A failiure in hardware (hard drive, memory, processor -, etc) which causes a system to stop functioning} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Software failiure}}{A failiure in software (the GGS, the operating - system, etc) which causes a system to stop functioning} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Background -\begin_inset CommandInset label -LatexCommand label -name "sec:Background" - -\end_inset - - -\end_layout - -\begin_layout Standard -The game industry is a quickly growing industry with high revenues and many - clever computer scientists. - Strangely enough gamers often experience long downtimes due to maintaining - or because of problems with the servers -\begin_inset CommandInset citation -LatexCommand citet -key "news/cnet/com/WoWProblems" - -\end_inset - -. - This is a problem that has existed and been resolved in other industries. - The telecom industry, for instance, has already found solutions to similar - problems. -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{The nine nines}}{A common goal for availability in the telecom business. - A system with nine nines of availability is available 99.999999999% of the - time} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Downtime}}{The amount of time a system is unavailable and does not - function} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Uptime}}{The amount of time a system is available and functions} -\end_layout - -\end_inset - -A common figure often used in telecoms is that of -\emph on -the nine nines -\emph default -, referring to -\begin_inset Formula $99.999999999\%$ -\end_inset - - of availability -\begin_inset CommandInset citation -LatexCommand citet -key "Armstrong03" - -\end_inset - -, or roughly -\begin_inset Formula $15ms$ -\end_inset - - downtime in a year. - The level of instability and bad fault tolerance seen in the game server - industry would not have been accepted in the telecom industry. - This level of instability should not be accepted in the game server industry - either. - An unavailable phone system could potentially have life threatening consequence -s, leaving the public unable to contact emergency services. - The same cannot be said about an unavailable game server. - The statement that game servers are less important than phone systems is - not a reason not to draw wisdom from what the telecoms have already learned. -\end_layout - -\begin_layout Standard -Moving back to the gaming industry. - The main reason to develop reliable servers is a higher revenue, to achive - this it is important for game companies to expand their customer base. - Reliable game servers will create a good image of the company. - In general the downtime of game servers is much higher than the downtime - of telecom systems even so the overall structure of the systems is similar - in many ways. - It should be possible to learn and reuse solutions from the telecom systems - to improve game servers. - -\end_layout - -\begin_layout Standard -In the current state game servers are developed on a per-game basis, often - this seems like bad practice. - Developers of multiplayer games need to understand network programming, - which can be a problem for small companies and independent game developers - who often lack expertise in that field. - A way to help game developers in developign servers would be to offer a - generic game server which gives developers an environment in which they - can implement their game. - This approach would not only make it easier to develop network games, it - would also allow games in different programming languages to be implemented - using the same server. -\end_layout - -\begin_layout Standard -Some key factors to the development of the GGS have been isolated. - Many of these are also found in the telecom sector. - The factors are -\emph on -scalability, fault tolerance -\emph default -and a -\emph on -generic -\emph default - nature. - These terms are defined below. -\end_layout - -\begin_layout Standard -Scalability (see -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Scalability" - -\end_inset - -) in computer science is a large topic and is commonly divided into sub-fields, - two of which are -\emph on -structural scalability -\emph default - and -\emph on -load scalability -\emph default - -\begin_inset CommandInset citation -LatexCommand citet -key "Bondi:2000:CSI:350391.350432" - -\end_inset - -. - These two issues are addressed in this thesis. - Structural scalability means expanding an architecture, e.g. - adding nodes to a system without requiring modification of the system. - Load scalability means using the available resources in a way which allows - handling increasing load, e.g more users, gracefully. -\end_layout - -\begin_layout Standard -Fault tolerance (see -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Fault-Tolerance" - -\end_inset - -) is used to raise the level of -\emph on -dependability -\emph default - in a system, so that the dependability is high even in presence of errors. - Dependability is the statistical probability of the system functioning - as intended at a given point in time. - Fault tolerance is the property of a system always to follow a specification, - even in the presence of errors. - The specification could define error handling procedures which activate - when an error occurs. - This means that a fault tolerant, dependable system will have a very high - probability of functioning at a given point in time, and is exactly what - is desired. - -\begin_inset CommandInset citation -LatexCommand citet -key "Gartner:1999:FFD:311531.311532" - -\end_inset - - -\end_layout - -\begin_layout Standard -A generic (see -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Generic" - -\end_inset - -) game server has to be able to run different client-server network games - regardless of the platform the clients are running on. - It runs network games of different type. - A very rough separation of games is real time games and turn based games. -\end_layout - -\begin_layout Standard -The server behaves in a way similar to an application server, but is designed - to help running games instead pf typical applications. - An application server provides processing ability and time, therefore it - is different from a file- or print-server, which only serves resources - to the clients. - In order to more easily understand the purpose of the GGS, it can be of - use to briefly think of application servers, thereafter viewing the differences - between the GGS and application servers. -\end_layout - -\begin_layout Standard -The most common type of application servers are web servers, where you run - a web application within the server. - The application server provides an environment and interfaces to the outer - world, in which applications run. - Hooks and helpers are provided to use the resources of the server. - Some examples for web application servers are the -\emph on -Glassfish -\emph default - server which allows running applications written in Java or the -\emph on -Google App Engine -\emph default - where you can run applications written in Python or some language which - runs in the -\emph on -Java Virtual Machine -\emph default -. - An example of an application server not powering web applications, but - instead regular business logic, is Oracle’s -\emph on -TUXEDO -\emph default - application server, which can be used to run applications written in COBOL, - C++ and other languages. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{SQL}}{Structured Query Language, a computer language common in querying - certain databases} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{JavaScript}}{A programming language originally developed by Netscape, - common in web programming} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{COBOL}}{Programming language} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{C++}}{Programming language} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Java}}{Programming language} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{AXD301}}{Telephone switch developed by Ericsson} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Erlang}}{A concurrent programming language, often used for telecom - applications. - The main language of the GGS} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -A database server can also be seen as an application server. - Scripts, for example SQL queries or JavaScript, are sent to the server, - which runs them and returns the evaluated data to the clients. -\end_layout - -\begin_layout Standard -The difference between the application servers and database servers described - and the GGS is the purpose of the servers. - Application servers were developed to run applications, often web applications. - The application servers offer appealing features for application developers, - which aid these developers in writing applications. - Database servers were developed in order to provide access to and allow - programming of databases, thus having features specifically tailored for - database development. -\end_layout - -\begin_layout Standard -The GGS on the other hand offers features appealing to game developers. - While it would be technically possible to write both regular applications - and database software using the GGS, this is not the intended usage of - the server, and this is how the GGS differs from other kinds of application - servers. -\end_layout - -\begin_layout Standard -To allow the development of different games, the game server developed need - to be generic, therefore one purpose of this thesis is to investigate how - one could make a game server as generic as possible. - Some important helpers for game developers are discussed, such as abstraction - of the network layer, data store and game specific features. - -\end_layout - -\begin_layout Standard -A prototype has been developed in order to aid the discussion of the theoretical - parts of the GGS. - The prototype does not feature all the characteristics described in this - thesis. - A selection has been made among the features and the most important ones - have been implemented either full or in part in the prototype. -\end_layout - -\begin_layout Standard -The choice of the implementation language for the prototype of the GGS was - made with inspiration from the telecom industry. - The Erlang language was developed by the swedish telecom company Ericsson - to develop highly available and dependable telecom switches. - One of the most reliable systems ever developed by Ericsson, the AXD301 - was developed using Erlang. - The AXD301 has possibly the largest code base even written in a functional - language -\begin_inset CommandInset citation -LatexCommand citep -key "Armstrong03" - -\end_inset - -. - The same language is used to develop the prototype of the GGS. - The usage of Erlang in the GGS is discussed in further detail in section - -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:The-usage-of-erlang" - -\end_inset - -. -\end_layout - -\begin_layout Section -Purpose -\end_layout - -\begin_layout Standard -The purpose of creating a generic and fault tolerant game server is to provide - a good framework for the development of many different types of games. - Allowing the system to scale up and down is a powerful way to maximize - the usage of physical resources. - By scaling up to new machines when load increases, and scaling down from - machines when load decreases costs and energy consumption can be optimized. -\end_layout - -\begin_layout Standard -Fault tolerance is important for the GGS to create a reliable service. - The purpose of a reliable game server is to provide a consistent service - to people using the server. - Going back to the telecom example, the purpose of creating a reliable telecom - system is to allow calls, possibly emergency calls, at any time. - Should the telecom network be unavailable at any time, emergency services - may become unavailable, furthermore the consumer's image of the telecom - system degrades. -\end_layout - -\begin_layout Standard -Returning to the game industry, emergency services will not be contacted - using a game server, however an unavailable server will degrade the consumer's - image of the system. - Consider an online casino company. - The online casino company's servers must be available at all times to allow - customers to play. - If the servers are unavailable customers cannot play and the company loses - money. - In this scenario an unavailable server can be compared to a closed real-world - casino. -\end_layout - -\begin_layout Section -Challenges in developing the prototype -\end_layout - -\begin_layout Standard -The word -\emph on -generic -\emph default - in the name of the GGS implies that the system is able to run a very broad - range of different code, for instance code written in different programming - languages or code written for a broad range of different game types. - To support this, a virtual machine (VM) for each -\emph on -game development language -\emph default - (hereafter GDL for brevity) is used. - -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{GDL}}{Game Development Language, the language used to program games - in the GGS} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{VM}}{Virtual Machine} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -No hard limit has been set on which languages can be used for game development - on the GGS, but there are several factors which should be taken into considerat -ion when deciding the feasibility of a language: -\end_layout - -\begin_layout Itemize -How well it integrates with Erlang, which is used in the core the GGS system? -\end_layout - -\begin_layout Itemize -How easy it is to send messages from the GGS to the GDL VM? -\end_layout - -\begin_layout Itemize -How easy it is to send messages from the GDL VM to the GGS? -\end_layout - -\begin_layout Itemize -Is it possible to sandbox every game with a context or something comparable? -\end_layout - -\begin_layout Standard -Internally the GDL VM needs to interface with the GGS to make use of the - helpers and tools that the GGS provides. - Thus an internal API has to be designed to make the GDL VM be able to interact - with the GGS. - This API is ideally completely independent of the GDL, and reusable for - any GDL. -\end_layout - -\begin_layout Standard -The communication with the gaming clients has to take place with help a - protocol. - Ideally a standard protocol should be used in order to shorten the learning - curve for developers and also make the system as a whole less obscure. - A major challenge during this project is to decide whether an existing - protocol can be used, and if not, how a new protocol can be designed which - performs technically as desired, while still being familiar enough to existing - developers. -\end_layout - -\begin_layout Standard -A great deal of work is devoted to make the GGS -\emph on -reliable -\emph default -. - This includes ensuring that the system scales well and to make sure it - is fault tolerant. - In order to facilitate scalability the GGS needs a storage platform which - is accessible and consistent. - The sclability aspects of the GGS are discussed from a theoretical point - of view, however no practical implementation of the scalability aspects - are found in the prototype. -\end_layout - -\begin_layout Section -Limitations of the prototype -\end_layout - -\begin_layout Standard -The implementation of the GGS protocol together with storage possibilities, - server capacity, and game language support imposes some limitations on - the project. - To get a functional prototype, some limits must be set on the types games - that can be played on the prototype. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{UDP}}{User Datagram Protocol, a connectionless networking protocol} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{TCP}}{Transmission Control Protocol, a streaming network protocol} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -The UDP protocol is not supported for communication between client and server. - The TCP protocol was chosen in favor of UDP, due to the fact that the implement -ation process using TCP was faster and easier than if UDP would have been - used. - UDP is generally considered to be faster than TCP for the transfer of game - (and other) related data, this is discussed in more depth in -\begin_inset CommandInset ref -LatexCommand vref -reference "sec:Choice-of-network" - -\end_inset - -. - In short, the decision of using TCP means that games that requires a high - speed protocol will not be supported by the GGS prototype. - Another limitation necessary to set on the system is the possibility to - have huge game worlds due to the implementation of the scaling mechanism - in the prototype. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Latency}}{A measure of delay, often measured in milliseconds} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -In real time games all players are playing together at the same time. - Latency is a huge problem in real time games, a typical round trip time - for such games are one of -\begin_inset Formula $50$ -\end_inset - - to -\begin_inset Formula $150ms$ -\end_inset - - and everything above -\begin_inset Formula $200ms$ -\end_inset - - is reported to be intolerable (see -\begin_inset CommandInset citation -LatexCommand citet -key "Farber:2002:NGT:566500.566508" - -\end_inset - -). - Latency sensitive games include most of the first person shooters with - multiplayer ability, for example -\emph on -Counter Strike -\emph default - or massively multiplayer online role playing games (MMORPGs), for example - -\emph on -World of Warcraft -\emph default -. -\end_layout - -\begin_layout Standard -In turn based games each player has to wait for their turn. - Latency is not a problem since the gameplay does not require fast interactions - among the players, long round trip times will not be noticed. - Examples of turn based games include board and card games, as well as multiplay -er games like -\emph on -Jeopardy -\emph default -. - Both game types have varying difficulties and needs when it comes to implementi -ng them, a Generic Game Server should address all of these difficulties - in order to provide the tools neccessary for the implementation of both - game types. -\end_layout - -\begin_layout Standard -Due to the limited capability of threading in many GDL VMs, the GGS prototype - will not support MMORPGs. -\end_layout - -\begin_layout Standard -The implementation of the GGS described in this thesis is only a small prototype - and tests will be performed on simple games like pong or chess, thus there - is no need to implement more advanced features in the system. - Note that these limitations only apply for the prototype of the project, - and that further developments to the GGS could be to implement these features. -\end_layout - -\begin_layout Section -Method -\end_layout - -\begin_layout Standard -A prototype was developed early on in the project to carry out experiments. - Using this prototype, the system was divided into modules. - A demand specification was created, using this specification, the modules - were easily identifiable. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Module}}{A part of a larger system} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -The first prototype of the GGS consisted of simple modules, however, due - to the separation of concerns among the modules, they were easily independently - modified and improved. - Once the basic structure of the GGS had been established, the first prototype - was removed, remaining was the structure of the modules and the internal - flow of the application. - This could be seen as an iterative workflow, with the first prototype being - the first iteration. - The second iteration later became the final result of the GGS. -\end_layout - -\begin_layout Standard -The layout of the GGS is both layered and modular. - The first layer handles the most primitive data and produces a higher level - representation of the data, passing it along to different modules of the - GGS. - The modular structure of the GGS plays an important role in making the - system fault tolerant. - The approach to fault tolerance is by replication, and restarting the faulting - modules with the last known good data. -\end_layout - -\begin_layout Standard -An informal specification and list of requirements of the system was outlined - early on in the project. - Usability goals for developers were set. - During the project several demo applications were constructed, by constructing - these applications, the usability goals were enforced. -\end_layout - -\begin_layout Chapter -Theory behind the GGS -\begin_inset CommandInset label -LatexCommand label -name "cha:Theory" - -\end_inset - - -\end_layout - -\begin_layout Standard -In this chapter, the theory behind the techniques used in the GGS are discussed. - Performance issues and the measuring of performance is discussed. - Benchmarking techniques are discussed. - The options when choosing network protocols are given, along with a discussion - of each alternative. - Finally, an overview of scalability, fault tolerance and availability is - presented. -\end_layout - -\begin_layout Section -Design of the GGS system -\begin_inset CommandInset label -LatexCommand label -name "sec:Design-of-the" - -\end_inset - - -\end_layout - -\begin_layout Standard -The GGS is modeled after a real world system performing much of the same - duties as the GGS. - This is common practice -\begin_inset CommandInset citation -LatexCommand citep -key "armstrong2011" - -\end_inset - - in the computer software world to understand complex problems more easily. - While there may not always be a real world example of a system performing - the exact duties of the system being modeled in the computer, it is often - easier to create and analyze requirements for real world systems and processes - than systems existing solely in virtual form in a computer. - The requirements and limitations imposed on the real-world system can, - using the proper tools, be transferred in to the software. -\end_layout - -\begin_layout Standard -The real world system chosen to represent the GGS is a -\begin_inset Quotes eld -\end_inset - -Chess club -\begin_inset Quotes erd -\end_inset - - - a building where chess players can meet and play chess. - In the following text the choice of using a chess club for modelling the - GGS is discussed. - The chess club is described in greater detail, furthermore the corresponding - parts of the chess club in the GGS are described. - Since a real-world scenario is readily available, and to such a large extent - resembles the computer software required for the GGS, the next step in - developing the GGS system is to duplicate this real world scenario in a - software setting. -\end_layout - -\begin_layout Standard -Some requirements, limitations and additions were made to the chess club - system, so that the system would more easily and efficiently be replicated - in a software setting. -\end_layout - -\begin_layout Standard -In the text below, two examples will be presented. - On example is that of a real-world chess club, in which players meet to - play chess against each other, the other example is the GGS, and how it - corresponds to this chess club. - -\begin_inset Float figure -wide false -sideways false -status collapsed - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Graphics - filename graphics/theory_layout.eps - scale 40 - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "fig:theory-layout" - -\end_inset - -The layout of a physical -\begin_inset Quotes eld -\end_inset - -Chess club -\begin_inset Quotes erd -\end_inset - - with two players (P) sitting by each chess table (Table), a coordinator - keeps track of all movements of players in the building. - A player has to pass by the entrance to enter or exit the building. - The building is represented by the outermost box. -\end_layout - -\end_inset - - -\end_layout - -\end_inset - -In figure -\begin_inset CommandInset ref -LatexCommand vref -reference "fig:theory-layout" - -\end_inset - - a graphical representation for the chess club is presented. - The club is seen from above. - The outermost box represents the building. - In the GGS setting, the building would represent one instance of the GGS. - Several buildings linked together would represent a cluster of GGS instances. - In order for a player (the P symbol in the graphic) to enter the theoretical - chess club, the player must pass by the entrance. - By having each player pass by the entrance, a tally -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Does this mean what I think it does? -\begin_inset Quotes eld -\end_inset - -Räkning -\begin_inset Quotes erd -\end_inset - -? -\end_layout - -\begin_layout Plain Layout -Richard: it means a list where you count how many people entered or left - for example -\end_layout - -\end_inset - - can be kept, ensuring that there are not too many players within the building. - In the GGS setting, too many players entering would mean too many connections - have been accepted by the GGS system, and that the structure of the system - thus must be modified, adding additional servers. -\end_layout - -\begin_layout Standard -Once a player has been allowed in to the chess club the player is greeted - by the host of the chess club, in the GGS setting represented by the -\emph on -Coordinator -\emph default -, and is seated by a table. - The coordinator keeps track of all the players in the building, and all - moves made by the players. - The information available to the coordinator means that cheating can be - monitored and book keeping can be performed by this entity. -\end_layout - -\begin_layout Standard -A player can only move the figures on her table in the chess club thus every - game is isolated to a table, just as expected. - This means that communication during a game only has to pass by the players - of that particular game and the coordinator, making sure that no cheating - takes place. -\end_layout - -\begin_layout Standard -This isolation of the games play an important part in many properties of - the GGS, the isolation means that games can for example be transferred - among different chess clubs. - Furthermore, if cheating takes place, corruption can only occur in the - particular table where it was found and cannot spread. -\end_layout - -\begin_layout Standard -Moving chess players from one location to another is one alteration made - to the real world chess club system to make the system more appropriate - for a software setting. - Allowing games to be transferred is not an attribute usually desired in - a real world chess club, where transferring players would mean moving the - players from one building to another. - In the software setting, moving players means moving the game processes - from one system to another, perhaps to balance the system load. - This transfer of players can occur transparently, without notifying the - players. -\end_layout - -\begin_layout Standard -The simplified life cycle of a game in the GGS can be viewed using algorithm - -\begin_inset CommandInset ref -LatexCommand vref -reference "alg:game-lifecycle" - -\end_inset - -. - To make this life cycle as efficient and useful as possible the scalability, - fault tolerance and generic traits are being added to the GGS. - These are not shown in the algorithm since these traits are tools in making - the algorithm behave as efficient as possible and are not the main focus - when studying the life cycle of a game. -\end_layout - -\begin_layout Standard -The limits imposed in -\begin_inset CommandInset ref -LatexCommand vref -reference "alg:game-lifecycle" - -\end_inset - - are arbitrary for this example, there are for example no limits in the - GGS on the number of players connecting. -\end_layout - -\begin_layout Standard -\begin_inset Float algorithm -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{algorithmic}[1] -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -newcommand{ -\backslash -INDSTATE}[1][1]{ -\backslash -STATE -\backslash -hspace{#1 -\backslash -algorithmicindent}} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -STATE -\end_layout - -\end_inset - - -\series bold -while -\series default - -\begin_inset Formula $players<2$ -\end_inset - -: -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - - -\series bold -if -\series default - a player connects, call -\emph on -connected -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -STATE -\end_layout - -\end_inset - - -\series bold -while -\series default -the game commences: -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - -call the function -\emph on -game -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -STATE -\end_layout - -\end_inset - - -\series bold -when -\series default - the game has stopped -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - -call the function -\emph on -endGame -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -STATE -\end_layout - -\end_inset - - -\series bold -function -\series default -connected: -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - -assign the new player an id -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - -alert the coordinator of the new player -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - - -\series bold -if -\series default - a free table does not exist: -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE[2] -\end_layout - -\end_inset - -the coordinator creates a new table -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - -the coordinator places the player by the table, and begins watching the - player -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -STATE -\end_layout - -\end_inset - - -\series bold -function -\series default - game -\series bold -: -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - - perform game-specific functions. - In chess, the rules of chess are placed here -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -STATE -\end_layout - -\end_inset - - -\series bold -function -\series default -endGame: -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - -alert the coordinator, unregistering the players -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - -disconnect the players from the system, freeing system resources -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{algorithmic} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "alg:game-lifecycle" - -\end_inset - -A very simple example of the flow through the GGS system when a game is - played. -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Performance -\end_layout - -\begin_layout Standard -There are many ways in which performance could be measured. - For the clients, time and response times are useful measurements in time - critical settings. - In non-time critical settings, the reliability of message delivery may - be an even more important factor than speed. -\end_layout - -\begin_layout Standard -In a first person shooter game, the speed of delivery of messages with informati -on about the current positions of all players is essential. - The failure to deliver messages in time results in choppy gameplay for - the players. - In strategy games, the reliability of delivery may be more important than - the speed, since the game is not perceived as choppy even if the messages - are delayed. -\end_layout - -\begin_layout Standard -For someone operating a GGS, it is perhaps more interesting to measure the - system load, memory consumption, energy consumption and network saturation. - These topics are discussed in theory in this section. - The implementation for the prototype is discussed in chapter -\begin_inset CommandInset ref -LatexCommand vref -reference "cha:Implementation-of-a" - -\end_inset - -. - For test results, refer to chapter -\begin_inset CommandInset ref -LatexCommand ref -reference "chap:Results-and-discussion" - -\end_inset - -, which contains graphs and a discussion concerning the performance of the - GGS prototype. -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Performance measurements -\end_layout - -\begin_layout Plain Layout - -\end_layout - -\begin_layout Plain Layout -Tue apr 26, 9:15. - Continue from here on. - Discuss which results we may expect in a fully fledged GGS system. - What impedes the speeds, what raises the CPU load (and therefore the temperatur -es & power consumption). - What factors are there in the network saturation problem? -\end_layout - -\begin_layout Plain Layout -Which games are affected by what, and what does this mean for the number - of players a GGS can handle? -\end_layout - -\end_inset - - -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Create a game with several thousand players, see how our server scales, - how can we improve the performance? Sharding isn’t very nice.. - alternatives? Improve the speed of sharding? -\end_layout - -\begin_layout Itemize -See how the server scales -\end_layout - -\begin_deeper -\begin_layout Itemize -When adding many clients -\end_layout - -\begin_deeper -\begin_layout Itemize -Measure in -\begin_inset Formula $ms$ -\end_inset - - (ping to clients) -\end_layout - -\begin_layout Itemize -measure in system load -\end_layout - -\begin_layout Itemize -Measure in loss of messages -\end_layout - -\begin_layout Itemize -Measure in # of timeouts? (if any??) -\end_layout - -\begin_layout Itemize -Measure in time-to-crash -\end_layout - -\end_deeper -\begin_layout Itemize -When adding new machines to the pool -\end_layout - -\begin_deeper -\begin_layout Itemize -% increase of performance per machine -\end_layout - -\end_deeper -\begin_layout Itemize -Single-core CPU vs multi-core CPU -\end_layout - -\begin_deeper -\begin_layout Itemize -It's very important to scale well on multi-core systems, since this is where - the industry is going. - Multicore is the future. -\end_layout - -\end_deeper -\end_deeper -\begin_layout Plain Layout -Find reference on how to benchmark distributed, multiprocess systems -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Choosing a network protocol -\begin_inset CommandInset label -LatexCommand label -name "sec:Choice-of-network" - -\end_inset - - -\end_layout - -\begin_layout Standard -There are two main types of protocols with help of which computer communication - over the Internet usually takes place; TCP and UDP which are known as the - network layer protocols and HTTP which is the most prominent application - layer protocol. - The transport layer protocols are commonly used to transport application - layer protocols such as HTTP, FTP and IRC. - TCP and UDP cannot be used on their own without an application layer protocol - on top of them. - Application layer protocols such as HTTP on the other hand need a transport - layer protocol in order to work. -\end_layout - -\begin_layout Standard -In order for the GGS to communicate with clients over a network, both an - application layer protocol and a network layer protocol must be chosen. - This section outlines some candidates for application and network layer - protocols for the GGS, along with a motivation as to why the described - protocol was or was not chosen. -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{HTTP}}{Hyper Text Transport Protocol, a network protocol commonly - used to deliver web pages} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{FTP}}{File Transfer Protocol} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{IRC}}{Internet Relay Chat} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -UDP -\end_layout - -\begin_layout Standard -Many online games use UDP as the carrier for their application layer protocol. - UDP moves data across a network very quickly, however it does not ensure - that the data transferred arrives in consistent manner. - Data sent via UDP may be repeated, lost or out of order. - To ensure that the data is transferred is in good shape, some sort of error - checking mechanisms must be implemented. - UDP is a good choice for applications where it is more important that data - arrives in a timely manner than that all data arrives undamaged, it is - thus very suitable for media streaming for example. -\end_layout - -\begin_layout Standard -The need to implement custom error checking, and possibly correction makes - UDP a bad candidate for the GGS. - If error checking and correction were to be implemented in the GGS project, - UDP would be a good candidate, however the time neccessary to implement - these features makes this option unfeasable. -\end_layout - -\begin_layout Subsection -TCP -\end_layout - -\begin_layout Standard -For reliable transfers TCP is often used on the Internet. - Built in to the protocol are the error checking and correction mechanisms - missing in UDP. - This ensures the consistency of data, but also makes the transfer slower - than if UDP had been used. - TCP was chosen for the GGS as the network layer protocol even though TCP - can be considerably slower than UDP. - The error checking mechanisms in TCP are reason enough to use TCP instead - of UDP in the GGS prototype. - The implementation of UDP is still possible, it will however not appear - in the prototype. -\end_layout - -\begin_layout Subsection -HTTP -\end_layout - -\begin_layout Standard -Since HTTP is so widely used in web servers on the Internet today, it is - available on most Internet connected devices. - This means that if HTTP is used in the GGS, firewalls will not be a problem, - which is a great benefit. - However, due to the intended usage of HTTP in web servers, the protocol - was designed to be stateless and client-initiated. - In order to maintain a state during a game session using HTTP, some sort - of token would have to be passed between client and server at all times, - much like how a web server works. - These facts combined make HTTP inappropriate for use in the GGS, since - the GGS requires a state to be maintained throughout a session and also - needs to push data from the server to clients without the clients requesting - data. - It should also be mentioned that HTTP uses the TCP protocol for transport. -\end_layout - -\begin_layout Subsection -The GGS Protocol -\end_layout - -\begin_layout Standard -HTTP was designed to be a stateless protocol, which by adding some overhead - is able to remove the need of a permanent connection and a state for each - client. - The GGS however already has a permanent connection to each client because - it needs to push information to the clients. - Therefore it is able to use the state to minimize the overhead in the communica -tion between server and client. - Therefore it was decided to invent a new protocol which was human readable - like HTTP but customized for this special use. - The GGS protocol is described in more detail in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:The-protocol-parser" - -\end_inset - -. -\end_layout - -\begin_layout Section -Generic structure of the GGS -\begin_inset CommandInset label -LatexCommand label -name "sec:Generic" - -\end_inset - - -\end_layout - -\begin_layout Standard -The GGS is a game server. - It was made with a desire to be suitable for many kinds of games. - A game should not only be able to vary in terms of genre, graphics, gameplay - etc, but also in the way the game is implemented for example in different - programming languages. - The GGS should be OS independent and run on Windows, OS X and Linux. - The GGS can be run as a listen server on the players computer and host - games locally. - It could also be a dedicated server running on dedicated independent hardware. - It is meant to run any game in any environment in any way desired, therefore - being as generic as possible. -\end_layout - -\begin_layout Standard -Clients upload the source code of the game it would like to play on the - GGS, this way any client can connect to the server and install the game - through a API without the need of installation through the server provider - or maintainer. -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{API}}{Application programming interface} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Fault tolerance -\begin_inset CommandInset label -LatexCommand label -name "sec:Fault-Tolerance" - -\end_inset - - -\end_layout - -\begin_layout Standard -Merriam-Webster’s dictionary -\begin_inset CommandInset citation -LatexCommand citeyearpar -key "Dictionary.com2011" - -\end_inset - - defines fault tolerance as: -\end_layout - -\begin_layout Quotation -1. - The ability of a system or component to continue normal operation despite - the presence of hardware or software faults. - This often involves some degree of redundancy. -\end_layout - -\begin_layout Quotation -2. - The number of faults a system or component can withstand before normal - operation is impaired. - -\end_layout - -\begin_layout Standard -Fault tolerance is an important factor in servers, a server that is fault - tolerant should be able to follow a given specification when parts of the - system fail. - This means that fault tolerance is different in each system depending on - what specification it has. - It should be noted that it is not possible to achieve complete fault tolerance, - a system will always have a certain risk of failure. - With this in mind the goal is to make the GGS prototype as fault tolerant - as possible. -\end_layout - -\begin_layout Standard -In order to make the GGS prototype fault tolerant the programming language - Erlang has been used. - Erlang will not guarantee a fault tolerant system, however it has features - that support and encourage the development of fault tolerant systems. - In the GGS it is important that the complete system is fault tolerant, - not only small parts. - Crashes of the whole system should be avoided as this would make the system - unusable for a time. - By using supervisor structures it is possible to crash and restart small - parts of the system, this is convenient as faults can be handled within - small modules thus never forcing a crash of the system. -\end_layout - -\begin_layout Standard -The need for fault tolerance in game servers is not as obvious as it may - be for other type of servers. - In general all servers strive to be fault tolerant as fault tolerance means - more uptime and a safer system. - This applies to game servers as well, good fault tolerance is a way of - satisfying customers. - In general, game servers differ from many other fault tolerant systems - in that high-availability is more important than the safety of the system. - For example a simple calculation error will not be critical for a game - server but it may be in a life-critical system and then it is better that - the system crashes than works with the faulty data. - There are cases where safety may be critical in game servers, one example - is in games where in-game money exist. -\begin_inset CommandInset citation -LatexCommand citet -key "Gartner:1999:FFD:311531.311532" - -\end_inset - - -\end_layout - -\begin_layout Section -Availability -\begin_inset CommandInset label -LatexCommand label -name "sec:Availability" - -\end_inset - - -\end_layout - -\begin_layout Standard -One important factor of any server is the availability. - A server which is unreachable is an useless server. - -\end_layout - -\begin_layout Standard -Within the telecom sector high availability has been achieved -\begin_inset CommandInset citation -LatexCommand citet -key "armstrong2011" - -\end_inset - -. - In the game development sector however the lack of high availability problem - has not yet been solved. -\end_layout - -\begin_layout Standard -There are several good papers (e.g. - -\begin_inset CommandInset citation -LatexCommand citet -key "VM:Jin2010,VM:Polze" - -\end_inset - -) on how to migrate whole virtual machines among nodes to replicate them - but for the GGS a different approach has been chosen. - Instead of duplicating a virtual machine, an attempt to lift the state - of the VM to a storage external to the VM is made. - The state is stored in a fast, fault tolerant data store instead of inside - the VM. - In addition to migrating the state of the game VM, the GGS uses tools from - the OTP, some of them are -\emph on -hot code replacement -\emph default -, where code can be updated while the application is running and without - the need to restart it, the -\emph on -supervisor structure -\emph default - provided by -\emph on -OTP -\emph default - and the inter node and process communication via -\emph on -messages -\emph default - instead of shared memory. - We will discuss each of them later on. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Supervisor}}{A process monitoring and hadning crashes in other processes} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Scalability -\begin_inset CommandInset label -LatexCommand label -name "sec:Scalability" - -\end_inset - - -\end_layout - -\begin_layout Standard -Each instance of the GGS contains several so called tables. - Each table is an isolated instance of a game, for instance a chess game - or a poker game. - A possible way for the GGS to scale up is to distribute these tables on - different servers. - In many games it is not necessary for a player to move among tables during - games. - This is for example not a common occurrence in chess, where it would be - represented as a player standing up from her current table and sitting - down at a new table, all within the same game session. - Therefore the main focus of the GGS is not to move players among tables, - but to keep a player seated by a table and to start new tables if needed - instead. - When a server reaches a certain number of players the performance will - start to decrease, or worse, the server may even crash. - To avoid this the GGS will start new tables on another server, using this - technique the players will be close to evenly distributed among the servers. - It is important to investigate the amount of players which is optimal for - each server. - This approach makes it possible to use all resources with a moderate load, - instead of having some resources with heavy load and others with almost - no load. -\end_layout - -\begin_layout Standard -As mentioned in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Background" - -\end_inset - - there are two different types of scalability, structural scalability and - load scalability. - To make the GGS scalable both types of scalability have to be considered. - Structural scalability means - in this case - that it should be possible - to add more servers to an existing cluster of servers. - By adding more servers the limits of with how many users a system can be - burdened with is increased. - Load scalability, in contrast to structural scalability, is not about how - to increase the actual limits of the system, rather it means how good the - system handles increased load. - The GGS should be able to scale well in both categories. -\end_layout - -\begin_layout Subsection -Load balancing -\end_layout - -\begin_layout Standard -The need for load balancing varies among different kind of systems. - Small systems that only use one or a couple of servers can cope with a - simple implementation of a load balancer, while in large systems it is - useful to have extensive and well working load balancing implementations. - The need also depends on what kind of server structure the system is working - on; a static structure where the number of servers is predefined or a dynamic - structure where this number varies. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Amazon EC2}}{A cloud computation service} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -Load balancing and scaling are difficult in different scenarios. - When running in a server park, there is a set number of servers available, - this means that an even distribution on all servers is preferable. - When running the GGS in a cloud, such as Amazon EC2, it is possible to - add an almost infinite number of servers as execution goes on and the load - increases. - In this cloud setting it may be more important to evenly distribute load - on newly added servers. -\end_layout - -\begin_layout Standard -Two methods of balancing load (increasing structure): -\end_layout - -\begin_layout Itemize -Fill up the capacity of one server completely, and then move over to the - next server -\end_layout - -\begin_layout Itemize -Evenly distribute all clients to all servers from the beginning. - When the load becomes too high on all of them a new problem arises: How - do we distribute the load on these new servers? -\end_layout - -\begin_layout Standard -Load balancing is a key component to achieve scalability in network systems. - The GGS is a good example of a system that needs to be scalable, to attain - this, load balancing is necessary. - Optimization of the load balancing for a system is an important task to - provide a stable and fast load balancer. - There are certain persistence problems that can occur with load balancing, - if a player moves from a server to another data loss may occur. - This is an important aspect to consider when a load balancer is designed - and implemented. -\end_layout - -\begin_layout Standard -Load balancing can often be implemented using dedicated software, this means - that in many applications load balancing may not be implemented internally - because better external solutions exist already. - This depends on what specific needs the system has. - A minor goal of this thesis is to analyze whether the GGS can use existing - load balancing tools or if it is necessary, how to implement load balancing - in the project. -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Because P2P game architectures are a constant goal for cheaters and because - “Cheating is a major concern in network games as it degrades the experience - of the majority of players who are honest” and preventing cheating in P2P - game architectures is very difficult game developers try to use Client - - Server architectures which have a natural problem to scale. - In this paper we want to show some strategies to achieve scalability. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -UUID -\begin_inset CommandInset label -LatexCommand label -name "sub:UUID" - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Float algorithm -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{algorithmic}[1] -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -newcommand{ -\backslash -INDSTATE}[1][1]{ -\backslash -STATE -\backslash -hspace{#1 -\backslash -algorithmicindent}} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -STATE -\end_layout - -\end_inset - -global variable -\begin_inset Formula $state:=0$ -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -STATE -\end_layout - -\end_inset - - -\series bold -function -\series default -unique -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - - -\begin_inset Formula $state:=state+1$ -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -INDSTATE -\end_layout - -\end_inset - - -\series bold -return -\begin_inset Formula $state$ -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{algorithmic} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "alg:A-simple-generator" - -\end_inset - -A simple (insufficient) generator for identifiers -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{UUID}}{Universally Unique Identifier} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -Inside the GGS everything needs a unique identifier. - There are identifiers for players, tables and other resources. - When players communicate amongst each other or with tables, they need to - be able to uniquely identify all of these resources. - Within one machine, this is mostly not a problem. - A simple systems a counter can be imagined, where each request for a new - ID increments the previous identifier and returns the new identifier based - on the old one; see algorithm -\begin_inset CommandInset ref -LatexCommand ref -reference "alg:A-simple-generator" - -\end_inset - -. - This solution poses problems when dealing with concurrent and distributed - systems. - In concurrent systems, the simple solution in algorithm -\begin_inset CommandInset ref -LatexCommand ref -reference "alg:A-simple-generator" - -\end_inset - - may yield non-unique identifiers due to the lack of mutual exclusion. -\end_layout - -\begin_layout Standard -The obvious solution to this problem is to ensure mutual exclusion by using - some sort of a lock, which may work well in many concurrent systems. - In a distributed system like the GGS however, this lock, along with the - state, would have to be distributed. - If the lock is not distributed, no guaranties can be made that two nodes - in the distributed system do not generate the same identifier. -\end_layout - -\begin_layout Standard -A different approach is to give each node the ability to generate Universally - Unique Identifiers (UUID), where the state of one machine does not interfere - with the state of another. - According to -\begin_inset CommandInset citation -LatexCommand citet -key "Leach98uuidsand" - -\end_inset - -: -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{MAC Address}}{Media Access Control address, used to identify network - cards} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{SHA-1}}{Cryptigraphic hash function, designed by the National Security - Agency (NSA)} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Quote -A UUID is 128 bits long, and if generated according to the one of the mechanisms - in this document, is either guaranteed to be different from all other UUIDs/GUI -Ds generated until 3400 A.D. - or extremely likely to be different. -\end_layout - -\begin_layout Standard -The generation of a UUID is accomplished by gathering several different - sources of information, such as: time, MAC addresses of network cards; - and operating system data such as; percentage of memory in use, mouse cursor - position and process IDs. - The gathered data is then -\emph on -hashed -\emph default - using an algorithm such as SHA-1. -\end_layout - -\begin_layout Standard -When using system wide unique identifiers it is extremly unlikely to have - identifier collisions when recovering from network splits between GGS clusters. - Consider figure -\begin_inset CommandInset ref -LatexCommand ref -reference "fig:network-split" - -\end_inset - -, where an example of a network split is presented. - When -\emph on - -\emph default -the decoupled node -\emph on - -\emph default -and -\emph on - -\emph default -the rest of the network later re-establish communication, they may have - generated the same IDs if using algorithm -\begin_inset CommandInset ref -LatexCommand ref -reference "alg:A-simple-generator" - -\end_inset - -, even when mutual system-wide exclusion was implemented. - This is exactly the problem UUIDs solve. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Network split}}{Separation of two networks, occurs when two networks - cannot communicate, commonly because of a hardware or software failiure} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Float figure -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Graphics - filename graphics/netsplit2.eps - scale 40 - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "fig:network-split" - -\end_inset - -An example of a network split -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Security -\end_layout - -\begin_layout Standard -The GGS only supports languages running in a sandboxed environment. - Each game session is started in its own sandbox. - The sandboxing isolates the games in such a way that they cannot interfere - with each other. - If sandboxing would not have been in place, one game could potentially - modify the contents of a different game. - A similar approach is taken with the persistent storage provided by the - GGS. - In the storage each game has its own namespace, much like a table in a - relational database. - A game is not allowed to venture outside this namespace and can, because - of that, not modify the persistent data of other games. - -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Sandbox}}{A protected environment in which computer software can - be run safely} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Game Development Language in a Virtual Machine -\begin_inset CommandInset label -LatexCommand label -name "sec:Game-Development-Language" - -\end_inset - - -\end_layout - -\begin_layout Standard -Erlang is not a very popular language for game development, therefore the - GGS needs to be able to run games written in different languages. - The main idea is to offer a replaceable module which would introduce an - interface to different virtual machines which would run the game code. - This way a game developer can write the game in his favorite language while - the server part still is written in Erlang and can benefit from all the - advantages of the Erlang language. - In this section, a few potential languages are given. -\end_layout - -\begin_layout Subsection -JavaScript -\end_layout - -\begin_layout Standard -JavaScript is a prime GDL candidate for the GGS. - The language is very flexible, a general knowledge of the language is present - in the computer science community, furthermore there are virtual machines - readily available for JavaScript. -\end_layout - -\begin_layout Standard -JavaScript has gained a lot of popularity lately, it is used in large projects - such as -\emph on -Riak -\emph default - -\begin_inset Foot -status open - -\begin_layout Plain Layout -\begin_inset Flex URL -status collapsed - -\begin_layout Plain Layout - -http://wiki.basho.com/An-Introduction-to-Riak.html -\end_layout - -\end_inset - - -\end_layout - -\end_inset - -, -\emph on -CouchDB -\emph default - -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -\begin_inset Flex URL -status collapsed - -\begin_layout Plain Layout - -http://couchdb.apache.org -\end_layout - -\end_inset - - -\end_layout - -\end_inset - -. - On the popular social coding site -\emph on -GitHub.com -\emph default -, 18% -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -during the writing of the thesis the percentage went up to 19% -\begin_inset Flex URL -status collapsed - -\begin_layout Plain Layout - -https://github.com/languages/ -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - of all code is written in JavaScript. - -\end_layout - -\begin_layout Standard -Since the GGS is intended to be connected to several different GDL VMs the - choice for the first language implemented for the GGS prototype seems not - only to depend on the technical features of the GDL chosen, in this case - JavaScript. - A different, albeit still important non technical feature of JavaScript - is the familiarity with the language of the members of the GGS development - team. -\end_layout - -\begin_layout Standard -The popularity of JavaScript in the programming community, in combination - with the availability of several different JavaScript virtual machines - was an important influence in choosing JavaScript as the main control language - for our GGS prototype. -\begin_inset ERT -status collapsed - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{CouchDB}}{Database server} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Riak}}{Database server} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{GitHub.com}}{Social coding website} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{ActionScript}}{Programming language} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Lua}}{Programming language} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{.NET}}{Software platform} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -Other languages -\end_layout - -\begin_layout Standard -Other languages like -\emph on -Lua -\emph default - or -\emph on -ActionScript -\emph default - are suitable as well since there is a virtual machine for each of them - which can be -\begin_inset Quotes eld -\end_inset - -plugged in -\begin_inset Quotes erd -\end_inset - - into the GDL VM interface. - With help of the -\emph on -Java Virtual Machine -\emph default - or the -\emph on -.NET -\emph default -environment it is even possible to run nearly every available programming - language in a sandbox as a GDL, however only a VM for JavaScript will be - implemented in the GGS prototype. -\end_layout - -\begin_layout Section -Testing -\end_layout - -\begin_layout Standard -There are several ways in which the GGS can be tested. - The most important aspect has been deemed to be the experience players - have when using the GGS. - To test the user experience of the GGS, a realistic usage scenario has - to be set up. -\end_layout - -\begin_layout Standard -The GGS is intended to be used for powering games which have many concurrent - players. - The players does not need to participate in the same instance of the game, - games such as chess are prime candidates for the GGS. -\end_layout - -\begin_layout Standard -When developing the GGS, two main categories of games exhibiting different - performance requirements were identified; real-time games and turn-based - games. - The real-time games were deemed more demanding than the turn based games. - Tests were carried out using a real time game, since this is the more demanding - type of games. -\end_layout - -\begin_layout Standard -The real time game chosen for testing the GGS is -\emph on -Pong -\emph default -, a game in which two players play a game involving a ball and two paddles. - The goal for each player is to shoot beside the other players paddle while - not allowing the ball to pass by her own paddle. - The game requires real time updates and is demanding when played in several - instances concurrently. -\end_layout - -\begin_layout Standard -There has been some work on the area of testing game servers, see -\begin_inset CommandInset citation -LatexCommand citet -key "Lidholt02designand" - -\end_inset - -, who describes a test bench using -\emph on -bots -\emph default - for testing his generic hazard-gaming server. - Lidholt describes how his server, capable of running several different - casino games are tested using artificial players, so called bots. - Performance is measured in -\begin_inset Quotes eld -\end_inset - -number of clients -\begin_inset Quotes erd -\end_inset - - able to connect to the server, and the system load. -\end_layout - -\begin_layout Standard -Similar tests were performed on the GGS, and the results of these tests - are visible in chapter -\begin_inset CommandInset ref -LatexCommand ref -reference "chap:Results-and-discussion" - -\end_inset - -. - The tests were initially performed by starting an operating system process - for each player. - Due to lack of hardware, not enough player processes could be started in - this way. - The bots were rewritten in Erlang, and due to Erlang's light weigh threads, - enough processes could be created to test successfully the server. -\end_layout - -\begin_layout Chapter -Implementation of a prototype -\begin_inset CommandInset label -LatexCommand label -name "cha:Implementation-of-a" - -\end_inset - - -\end_layout - -\begin_layout Standard -As mentioned earlier in the thesis, a prototype of the GGS has been developed - in order to test hypotheses and provide concrete examples. -\end_layout - -\begin_layout Standard -This chapter contains the realization of much of the principles and techniques - described in chapter -\begin_inset CommandInset ref -LatexCommand vref -reference "cha:Theory" - -\end_inset - -. - Here the problems and their solutions are discussed in greater detail, - and at times the text becomes more specific to the GGS. -\end_layout - -\begin_layout Standard -Much of what is discussed in this chapter has been implemented in the GGS - prototype. - Specific solutions such as -\emph on -supervisor structures -\emph default -and distribution of erlang nodes on physical nodes. - The different means of communications within the GGS and outside the GGS - with third parties are also discussed here. -\end_layout - -\begin_layout Standard -The chapter ends with case studies and code examples of particular features - of the GGS. - The case studies and the code serve as concrete examples of the implementations - outlined in the rest of this chapter. -\end_layout - -\begin_layout Section -Overview of the prototype -\end_layout - -\begin_layout Standard -The prototype of the GGS was developed using the Erlang language. - In Erlang, most things are processes. - The software running the Erlang code is known as the Erlang machine, or - a Erlang node. - Each Erlang node is capable of running several -\emph on -threads -\emph default -(also known as -\emph on -Light Weight Processes; LWP -\emph default -) -\emph on -, -\emph default -much like the threads in an operating system. - There are however differences between operating system threads and the - LWPs of erlang. - Threads in a Linux system, for example, are treated much like operating - system processes in different systems. - Due to the size of data structures related to each process, swapping one - process for another (known as -\emph on -context switching -\emph default -) is an expensive task in many systems -\begin_inset CommandInset citation -LatexCommand citep -after "pg 80" -key "McKusick:2004:DIF:1014910" - -\end_inset - -. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{LWP}}{Light Weight Process} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Context switch}}{The act of switching from one context, commonly - a process, to another. - Used by operating systems to achieve multi tasking} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -The cost of swapping operating system processes becomes a problem when many - processes are involved. - If the GGS system had been developed using regular operating system processes, - it would have had to be designed in a way to minimize the number of processes. - Using Erlang, which is capable of running very many processes, several - times more than an operating system, the relation between the real world - and the GGS (described in -\begin_inset CommandInset ref -LatexCommand vref -reference "sec:Design-of-the" - -\end_inset - -) becomes clearer. -\end_layout - -\begin_layout Standard -Erlang allows the GGS to create several processes for each player connecting, - these processes can handle a multitude of different tasks, parsing data - for example. - Since each task is handled by a different process, the tasks are clearly - separated and the failure of one is easily recovered without affecting - the others. -\end_layout - -\begin_layout Standard -In addition to creating (or -\emph on -spawning -\emph default -) processes specifically to handle new players connecting, the GGS has more - permanent processes running at all times. - The constantly running processes in the GGS system are called -\emph on -modules -\emph default -. - An example of a module in the GGS is the -\emph on -dispatcher module -\emph default -, which handles the initial connection made by a client, passing the connection - along further in to the system. -\end_layout - -\begin_layout Standard -\begin_inset Float figure -wide false -sideways false -status collapsed - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Graphics - filename graphics/Chess_no_text.eps - width 100text% - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "fig:The-layout-of" - -\end_inset - -The layout of the GGS. - The circles marked with 'C' topmost in the picture represent clients. - The cloud marked 'network' pictured directly below the clients can be any - network, for example the Internet. - The barell figure marked 'backup' is a process being fed backup data from - the coordinator. - The barrel marked 'State' contains the state of a table, and this is fed - into the box marked 'Mnesia' which is database. - Finally the figure shaped as a shield marked 'GameVM' contains the actual - game process. -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -In figure -\begin_inset CommandInset ref -LatexCommand vref -reference "fig:The-layout-of" - -\end_inset - - the entire GGS system is represented graphically. - The circles marked with 'C' topmost in the picture represents game clients. - These circles represent processes running on gamers computers, and not - on the GGS machine. - If a game of chess is to be played on the server, the clients on the machines - of the gamers will be chess game clients. - Clients connect through a network, pictured as a cloud, to the dispatcher - process in the GGS. - The dispatcher process and all other modules are discussed in -\begin_inset CommandInset ref -LatexCommand vref -reference "sec:The-modular-structure" - -\end_inset - -. - For each connection, a new player process is spawned, which immediately - after spawning is integrated in to the GGS by the coordinator process. -\end_layout - -\begin_layout Section -The usage of Erlang in the GGS -\begin_inset CommandInset label -LatexCommand label -name "sec:The-usage-of-erlang" - -\end_inset - - -\end_layout - -\begin_layout Standard -As mentioned earlier, the GGS prototype is implemented in Erlang. - The current section and the subsequent section function as a short introduction - to Erlang, focusing on the parts of the language neccessary to understand - the material regarding Erlang presented in this thesis. -\end_layout - -\begin_layout Standard -Erlang was designed by Ericsson, beginning in 1986, for the purpose of creating - concurrent applications and improving telecom software. - Features essential for the telecom industry to achieve high availability - in telecom switches were added to the language. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Mutex}}{A construct for achieving mutual exclusion, used to avoid - simultaneous access to shared resources in computer systems} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -Erlang uses message passing in favor of shared memory, mutexes and locks, - something which at the time was controversial among fellow developers -\begin_inset CommandInset citation -LatexCommand citet -key "Armstrong:2010:ERL:1810891.1810910" - -\end_inset - -. - The reason for using message passing, according to Armstrong, was that - applications should operate correctly before optimizations are done, where - efficient internal communication within the Erlang machine was considered - a later optimization. -\end_layout - -\begin_layout Standard -In using message passing in favor of the methods commonly used at the time, - the issues commonly associated with shared memory and locking were avoided. -\end_layout - -\begin_layout Standard -In Erlang, everything is a process, and everything operates in its own memory - space. - Memory cannot be shared among processes, which prohibits a process from - corrupting the memory of a different process. -\end_layout - -\begin_layout Standard -Messages are sent between the processes in an asynchronous manner, and each - process has a mailbox in which these messages can be retrieved. -\end_layout - -\begin_layout Standard -Processes in Erlang are also called -\emph on -Light Weight Processes. - -\emph default -The Erlang processes are inexpensive to create. - Processes exist within an Erlang machine, or Erlang node. - The Erlang machine has its own scheduler and does not rely on the scheduler - of the operating system, this is a main reason of Erlang's capability of - running many concurrent processes -\begin_inset CommandInset citation -LatexCommand citet -key "Armstrong03" - -\end_inset - -. -\end_layout - -\begin_layout Standard -The strong isolation of Erlang processes make them ideal for multi-core - and distributed systems. - Distribution of software is included as a fundamental part in the Erlang - language. - The 'physical' location of a process, e.g. - which computer the process runs on, is not important when communicating - with the process. - Processes can communicate regardless of whether they run on the same system - or not, transparently. -\end_layout - -\begin_layout Standard -The distributed nature of Erlang is something the GGS can make use of, when - scaling across several computers in order to achieve higher performance. - The distribution is also important in creating redundancy. - The GGS prototype does not make use of the distributed nature of Erlang, - however the GGS prototype is constructed in such a way that making use - of the distributed nature should now pose a big ptoblem. -\end_layout - -\begin_layout Standard -Erlang promotes a non-defensive programming style in which processes are - allowed to crash and be restarted in favor of having the processes recover - from errors. - The distributed nature of Erlang means supervisor processes (discussed - in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:Supervisor-structure" - -\end_inset - -) can reside on remote systems, thereby increasing the reliability of the - system as a whole. -\end_layout - -\begin_layout Standard -A very important feature of Erlang, used in the GGS, is the ability to interface - with external hardware and software. - Erlang allows communication with external resources through -\emph on -ports -\emph default - and -\emph on -NIF -\emph default -s (Native implemented functions) -\emph on -. - -\emph default - Through ports communication can take place in a similar way communication - is performed over sockets. - NIFs are called like any other functions without any difference to the - caller but are implemented in C, this implementation makes NIFs a very - efficient way to interface with the outside world -\begin_inset CommandInset citation -LatexCommand citet -key "NIF:website" - -\end_inset - -. -\end_layout - -\begin_layout Standard -The GGS uses Erlang ports for generating UUIDs -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -UUIDs are discussed in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:UUID" - -\end_inset - - -\end_layout - -\end_inset - - and NIFs for interfacing with the virtual machines of games -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -Virtual machines of games are discussed in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Game-Development-Language" - -\end_inset - - -\end_layout - -\end_inset - -. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{OTP}}{Open Telecom Platform, a software suite for Erlang} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Behaviour}}{A design pattern in OTP} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -Development of the GGS would have been hard if not impossible had it not - been for the -\emph on -OTP -\emph default - supplied with the standard Erlang distribution. - The OTP (Open Telecom Platform) is a set of standard libraries and design - patterns, called -\emph on -behaviors -\emph default -, which are used when developing Erlang systems. -\end_layout - -\begin_layout Standard -The GGS makes heavy use of the behaviors supplied in the OTP. - The behaviors impose a programming style suitable for distributed and concurren -t applications. - In particular, the GGS uses the following behaviors: -\end_layout - -\begin_layout Itemize -The -\emph on -supervisor -\emph default - behavior, which is used when creating a supervisor. - Supervisors are used when monitoring processes in the Erlang system. - When a process exits wrongfully, the supervisor monitoring the process - in question decides which action to take. - In the GGS, the most common action is to restart the faulting process. - A more thorough discussion on supervisors can be found in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:Supervisor-structure" - -\end_inset - -. -\end_layout - -\begin_layout Itemize -The -\emph on -gen_tcp -\emph default -behavior, which is used to work with TCP sockets for network communication. - Using the gen_tcp behavior, network messages are converted to internal - Erlang messages and passed to a protocol parser, where the messages are - processed further. -\end_layout - -\begin_layout Itemize -The -\emph on -gen_server -\emph default - behavior, which is used when constructing OTP servers in Erlang. - Using this behavior, a state can easily be kept in a server process, greatly - increasing the usefulness of the server process. - There are many gen_servers in the GGS, it is the most widely used behavior - in the project. - In addition to introducing a state to the server, the gen_server behavior - also imposes patterns for synchronous and asynchronous communication between - other gen_servers and other OTP behaviors. -\end_layout - -\begin_layout Itemize -The -\emph on -gen_fsm -\emph default - behavior is used in the protocol parser module in the GGS. - Using the gen_fsm behavior, finite state machines are easily developed. - Protocol parsers are an ideal example of where to use finite state machines, - which are widely used for parsing strings of text. -\end_layout - -\begin_layout Standard -In addition to supplying behaviors, the OTP also has a style for packaging - and running Erlang applications. - By packaging the GGS as an -\emph on -application -\emph default -the GGS can be started in a way uniform to most erlang software, providing - familiarity for other Erlang users, and eases the incorporation of the - GGS in other applications. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Application}}{A way of packaging Erlang software in a uniform way} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -Short introduction to the Erlang syntax -\end_layout - -\begin_layout Standard -To understand the examples in this thesis, a small subset of Erlang must - be understood. - In this section, the very syntactic basics of Erlang are given. -\end_layout - -\begin_layout Itemize - -\series bold -Variables -\series default - start with an uppercase letter, examples include -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt X, Var}, and { -\backslash -tt Global} -\end_layout - -\end_inset - -. - A variable can only be assigned once. -\end_layout - -\begin_layout Itemize - -\series bold -Atoms -\emph on - -\series default -\emph default -start with lower case letters, for example: -\begin_inset ERT -status open - -\begin_layout Plain Layout - - { -\backslash -tt atom, a} -\end_layout - -\end_inset - -. -\end_layout - -\begin_layout Itemize - -\series bold -Functions -\series default - are defined starting with an atom for the name, parenthesis containing - parameters, an arrow, a function body and finally a dot marking the end - of a function. - -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt square(X) -> X*X.} -\end_layout - -\end_inset - - is an example of a function producing the square of X. -\end_layout - -\begin_layout Itemize -Functions are -\series bold -called -\series default - by suffixing an atom with the function name with parenthesis, for example - -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt square(10)} -\end_layout - -\end_inset - -. - Qualified names can be specified using ':', for example: -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt math:square(10)} -\end_layout - -\end_inset - -. -\end_layout - -\begin_layout Itemize - -\series bold -Tuples -\series default - are containers of fixed type for Erlang data types. - They are constructed using curly brackets, for example: -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt -\backslash -{atom1, atom2, atom3 -\backslash -}}. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Itemize - -\series bold -Lists -\series default - are constructed using [ and ], for example: -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt [1,2,3]} -\end_layout - -\end_inset - -. -\end_layout - -\begin_layout Itemize - -\series bold -Strings -\series default - doubly quoted lists of characters, for example -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt "Hello world"} -\end_layout - -\end_inset - -. -\end_layout - -\begin_layout Itemize - -\series bold -Records -\series default - are erlang tuples coupled with a tag for each tuple element. - This allows referring to elements by name instead of by position. - An example of a record looks like this: -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt -\backslash -#myRecord{}} -\end_layout - -\end_inset - -. -\end_layout - -\begin_layout Section -The modular structure of the GGS prototype -\begin_inset CommandInset label -LatexCommand label -name "sec:The-modular-structure" - -\end_inset - - -\end_layout - -\begin_layout Standard -The separation of concerns and principle of single responsibility -\begin_inset Foot -status open - -\begin_layout Plain Layout -More information on the SRP is available at: -\begin_inset CommandInset href -LatexCommand href -target "http://www.objectmentor.com/resources/articles/srp.pdf" - -\end_inset - - -\end_layout - -\end_inset - - are widely respected as good practices in the world of software engineering - and development. - By dividing the GGS into modules each part of the GGS can be modified - without damaging, or requiring changes in the rest of the system. - Due to the hot code updates featured in Erlang, it is theoretically possible - to update parts of the GGS while the system is running, this has however - not been implemented in the prototype. - The modular composition of the GGS prototype should make a transition to - a folly hot code swappable system relatively easy. - Hot code replacements are discussed in more detail in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:Hot-code-replacement" - -\end_inset - -. -\end_layout - -\begin_layout Standard -The responsibility and concern of each module comes from the responsibility - and concern of the real-world entity the model represents. - The modeling of the GGS after a real world system was discussed in chapter - -\begin_inset CommandInset ref -LatexCommand vref -reference "cha:Theory" - -\end_inset - -. -\end_layout - -\begin_layout Standard -In the text below the different modules of the GGS are presented. - In the text the word module refers to the actual code of the discussed - feature, while the word process is used when referring to a running instance - of the code. -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{SRP}}{Single Responsibility Principle} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Object Oriented Programming}}{A programming paradigm focusing on - objects} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -The dispatcher module -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status collapsed - -\begin_layout Plain Layout -The discussion of the modules is divided into the following parts: -\end_layout - -\begin_layout Itemize -What does the module do? -\end_layout - -\begin_layout Itemize -What happens when the module fails? -\end_layout - -\begin_layout Itemize -How does the module correspond to the real-world scenario of the chess club? -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -The dispatcher module is the first module to have contact with a player. - When a player connects to the GGS, the player is first greeted by the dispatche -r module, which sets up an accepting socket for each player. - The dispatcher is the module which handles the interfacing to the operating - system when working with sockets. - Operating system limits concerning the number of open files, or number - of open sockets are handled here. - The operating system limits can impose problems in the GGS, this is discussed - more in detail in chapter -\begin_inset CommandInset ref -LatexCommand vref -reference "sec:Operating-system-limitations" - -\end_inset - -. -\end_layout - -\begin_layout Standard -Should the dispatcher module fails to function, no new connections to the - GGS can be made. - In the event of a crash in the dispatcher module, a supervisor process - immediately restarts the dispatcher. - There exists a window of time between the crashing of the dispatcher and - the restarting of the dispatcher, this window is very short, and only during - this window is the GGS unable to process new connection requests. - Due to the modular structure of the GGS, the rest of the system is not - harmed by the dispatcher process not functioning. - The dispatcher process does not contain a state, therefore a simple restart - of the process is sufficient in restoring the GGS to a pristine state after - a dispatcher crash -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Well.. - In theory.. -\end_layout - -\end_inset - -. -\end_layout - -\begin_layout Standard -Returning to the scenario of the chess club, the dispatcher module is the - doorman of the club. - When a player enters the chess club, the player is greeted by the doorman, - letting the player in to the club. - The actual letting in to the club is in the GGS represented by the creation - of a player process discussed in section -\begin_inset CommandInset ref -LatexCommand vref -reference "sub:The-player-module" - -\end_inset - -. - The newly created player process is handed and granted rights to, the socket - of the newly connected player. -\end_layout - -\begin_layout Subsection -The player module -\begin_inset CommandInset label -LatexCommand label -name "sub:The-player-module" - -\end_inset - - -\end_layout - -\begin_layout Standard -The player module is responsible for representing a player in the system. - Each connected player has its own player process. - The player process has access to the connection of the player it represents, - and can communicate with this player. - To communicate with a player, the data to and from the player object must - pass through a protocol parser module, discussed in -\begin_inset CommandInset ref -LatexCommand vref -reference "sub:The-protocol-parser" - -\end_inset - -. - Raw communication, without passing the data through a protocol parser is - in theory possible but it is not useful. -\end_layout - -\begin_layout Standard -In the creation of a player process, the coordinator process, discussed - in -\begin_inset CommandInset ref -LatexCommand vref -reference "sub:The-coordinator-module" - -\end_inset - -, is notified by the newly connected process. -\end_layout - -\begin_layout Standard -In the event of a crash in a player process, several things happen. - -\end_layout - -\begin_layout Enumerate -The player process, which is the only process with a reference to the socket - leading to the remote client software, passes this reference of the socket - to the coordinator process temporarily. -\end_layout - -\begin_layout Enumerate -The player process exits. -\end_layout - -\begin_layout Enumerate -The coordinator spawns a new player process, with the same socket reference - as the old player process had. -\end_layout - -\begin_layout Enumerate -The player process resumes operation, immediately starting a new protocol - parser process, and begins to receive and send network messages again. -\end_layout - -\begin_layout Standard -The window of time between the crash of the player process and the starting - of a new player process is, as with the dispatcher, very short. -<<<<<<< HEAD - Since the connection changes owners for a short period of time, but is - never dropped, the implications of a crash are only noticed, at worst, - as choppy gameplay for the client. - Note however that this crash recovery scheme is only partly implemented - in the GGS prototype. -======= - Since the connection changes owners for a short period of time but is never - dropped, the implications of a crash is only noticed, at worst, as choppy - gameplay for the client. - Note however that this crash recovery scheme is not implemented in the - GGS prototype. ->>>>>>> 20eb91bc412469b90510d7c33d68d8e67ac338a8 - -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Can we do this..? Seems a bit sneaky. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -Moving back to the real world example, the player process represents an - actual person in the chess club. - When a person sits down at a table in the chess club, the person does so - by requesting a seat from some coordinating person, and is then seated - by the same coordinator. - Once seated, the player may make moves on the table he or she is seated - by, this corresponds clearly to how the GGS is structured, as can be seen - in the following sections. -\end_layout - -\begin_layout Subsection -The protocol parser module -\begin_inset CommandInset label -LatexCommand label -name "sub:The-protocol-parser" - -\end_inset - - -\end_layout - -\begin_layout Standard -The protocol parser is an easily interchangeable module in the GGS, handling - the client-to-server, and server-to-client protocol parsing. - In the GGS prototype, there is only one protocol supported, the -\emph on -GGS Protocol -\emph default -. - The role of the protocol parser is to translate the meaning of packets - sent, using the protocol in use, to internal messages of the GGS system. - The GGS protocol, discussed below is used as a sample protocol to explain - how protocol parsers can be built for the GGS. -\end_layout - -\begin_layout Subsubsection -The structure of the GGS Protocol -\begin_inset CommandInset label -LatexCommand label -name "sub:The-structure-of" - -\end_inset - - -\end_layout - -\begin_layout Standard -The GGS protocol is modeled after the HTTP protocol. - The main reason for this is the familiarity many developers already have - with HTTP due to its presence in internet software. - Each GGS protocol packet contains a header section. - The header section is followed by a data section. - In the header section, parameters concerning the packet is placed. - In the data section, the actual data payload of the packet is placed. -\end_layout - -\begin_layout Standard -There is no requirement of any specific order of the parameters in the header - section, however the data section must always follow directly after the - header section. -\end_layout - -\begin_layout Standard -In the example below, line 1 contains a Game-Command parameter. - This parameter is used to determine which game-specific command the client - is trying to perform. - The handling of this parameter is specific to each game, and can be anything. -\end_layout - -\begin_layout Standard -Line 2 specifies a game token. - This is a UUID which is generated for each client upon authentication with - the GGS. - The GGS uses this token if a client is disconnected and the new connection - created when the client reconnects must be re-paired with the player object - inside the GGS. - The UUID is also used as an unique ID within GDL VMs. -\end_layout - -\begin_layout Standard -Line 3 specifies the content type of the payload of this particular packet. - This parameter allows the GGS to invoke special parsers, should the data - be encoded or encrypted. - When encryption is employed, only the payload is encrypted, not the header - section. - This is a scheme which does not allow for strong encryption, but is deemed - feasible for gaming purposes. -\end_layout - -\begin_layout Standard -Line 4 specifies the content length of the payload following immediately - after the header section. -\end_layout - -\begin_layout Standard -The parser of the GGS protocol implemented in the GGS prototype is designed - as a finite state machine using the gen_fsm behavior. - When a full message has been parsed by the parser, the message is converted - into the internal structure of the GGS messages and sent in to the system - from the protocol parser using message passing. -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Packet below is not an algorithm, but I don't know how to change that label.. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Float algorithm -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -lstset{ -\end_layout - -\begin_layout Plain Layout - -backgroundcolor= -\backslash -color{white}, -\end_layout - -\begin_layout Plain Layout - -extendedchars=true, -\end_layout - -\begin_layout Plain Layout - -basicstyle= -\backslash -footnotesize -\backslash -ttfamily, -\end_layout - -\begin_layout Plain Layout - -showstringspaces=false, -\end_layout - -\begin_layout Plain Layout - -showspaces=false, -\end_layout - -\begin_layout Plain Layout - -numbers=left, -\end_layout - -\begin_layout Plain Layout - -numberstyle= -\backslash -footnotesize, -\end_layout - -\begin_layout Plain Layout - -numbersep=9pt, -\end_layout - -\begin_layout Plain Layout - -tabsize=2, -\end_layout - -\begin_layout Plain Layout - -breaklines=true, -\end_layout - -\begin_layout Plain Layout - -showtabs=false, -\end_layout - -\begin_layout Plain Layout - -captionpos=b -\end_layout - -\begin_layout Plain Layout - -} -\end_layout - -\begin_layout Plain Layout - - -\backslash -begin{lstlisting} -\end_layout - -\begin_layout Plain Layout - -Game-Command: chat -\end_layout - -\begin_layout Plain Layout - -Token: e30174d4-185e-493b-a21a-832e2d9d7a1a -\end_layout - -\begin_layout Plain Layout - -Content-Type: text -\end_layout - -\begin_layout Plain Layout - -Content-Length: 18 -\end_layout - -\begin_layout Plain Layout - -\end_layout - -\begin_layout Plain Layout - -Hello world, guys! -\end_layout - -\begin_layout Plain Layout - - -\backslash -end{lstlisting} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "alg:A-sample-packet" - -\end_inset - -A sample packet sent from a client to the GGS during a chat session -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Mention that the protocol is heavily influenced bye HTTP, is parsed using - a FSM, perhaps give a sample packet. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -The coordinator module -\begin_inset CommandInset label -LatexCommand label -name "sub:The-coordinator-module" - -\end_inset - - -\end_layout - -\begin_layout Standard -The coordinator module is responsible for keeping track of all players, - their seats and tables. - Players register with the coordinator process when first connecting to - the server, and the coordinator places each player by their respective - table. -\end_layout - -\begin_layout Standard -The coordinator keeps mappings between each player and table, therefore - it is used to perform lookups on tables and players to find out which are - connected. - The connectivity of players and tables is important when sending messages - to all participants in a game. - A lookup in the coordinator process is performed prior to notifying all - players in a game to ensure the message reaches all players. - The lookup can be performed either using internal identification codes - or using the UUID associated with each client and table. -\end_layout - -\begin_layout Standard -The coordinator process contains important state, therefore a backup process - is kept at all times. - All good data processed by the coordinator is stored for safekeeping in - the backup process as well. - Data which is potentially harmful is not stored in the backup process. -\end_layout - -\begin_layout Standard -Upon a crash, the coordinator process recovers the prior good state from - the backup process and continues where it left off. - A supervisor process monitors the coordinator process and restarts the - process when it malfunctions. - There is a window of time between the crash of the coordinator and the - restarting of the coordinator, during this time, players cannot be seated - by new tables, and cannot disconnect from the server. - This window of time is very small, and the unavailability of the coordinator - process should not be noticed by more than a short time lag for the clients. -\end_layout - -\begin_layout Standard -Moving back to the example of the chess club, the coordinator process can - be seen as a judge, monitoring all moves of the players. - At the same time as acting as a judge, the coordinator process is also - a host in the chess club, seating players by their tables and offering - services to the players. -\end_layout - -\begin_layout Subsection -The table module -\end_layout - -\begin_layout Standard -The table module is mostly a hub used for communication. - New table processes are created by the coordinator on demand. - The table module does not contain any business logic, however each process - contains information concerning which players are seated by that particular - table. -\end_layout - -\begin_layout Standard -The information about which players are seated by each table is used when - notifying all players by a table of an action. - Consider a game of chess, each player notifies the table of its actions, - the table then notifies the rest of the participants of these actions after - having had the actions processed by the game VM, where an action could - be moving a playing piece. -\end_layout - -\begin_layout Standard -Each table is associated with a game VM. - The actions sent to a table are processed by the game VM, this is where - the game logic is implemented. -\end_layout - -\begin_layout Standard -After a crash in a table process, the entire table must be rebuilt and the - players must be re-associated with the table. - Data concerning players is kept in the coordinator process, and is restored - from there. - Data kept in the actual game is not automatically corrupted by the crash - in a table, however the table must be re-associated with the game VM is - was associated with prior to the crash of the table. - The table process maps well into the setting of the real-world chess club - scenario previously discussed. - A table works in the same way in a real world setting as in the GGS setting. -\end_layout - -\begin_layout Subsection -The game virtual machine module -\end_layout - -\begin_layout Standard -This module holds the game logic of a game and is responsible for the VM - associated with each game. - -\end_layout - -\begin_layout Standard -The game VM contains the state of the VM and a table token associated with - a running game. - GameVM is started by the table module. - The table module hands over a token to the game VM during initialization. - During initialization a new VM instance and various objects associated - to the VM instance will be created. - Callbacks to Erlang are registered into the VM and then the source code - of a game is loaded into the VM and the game is ready for startup. - The only means for a game to communicate with the VM is through usage of - a provided interface. - -\end_layout - -\begin_layout Standard -The VM itself makes it possible for the game developer to program in the - programming language covered by the VM. - In future releases, more game VMs will be added to support more programming - languages. - Because the game VM keeps track of the correct table, the game developer - does not need to take this into consideration when programming a game. - If a method within the game sends data to a player, it will be delivered - to the player in the correct running game. - The same game token is used to store the game state in the database. - Therefore, no game states will be mixed up either. -\end_layout - -\begin_layout Standard -This module does not affect game runtime but evaluates a new game state - and handles communication between the game and the players. - A closer look at the structure of this model is given in -\begin_inset CommandInset ref -LatexCommand vref -reference "sec:Communication-with-the-GDL-VM" - -\end_inset - -. -\end_layout - -\begin_layout Standard -The code which is run in the VM is uploaded to the GGS prior to each game. - Allowing the clients to upload code allows clients to run any game. -\end_layout - -\begin_layout Subsection -The database module -\begin_inset CommandInset label -LatexCommand label -name "sub:The-database-module" - -\end_inset - - -\end_layout - -\begin_layout Standard -Game data from all games on the GGS are stored in the database backend of - the database module. -\end_layout - -\begin_layout Standard -In the GGS prototype the database module is using a database management - system called Mnesia. - Mnesia ships with the standard Erlang distribution and is a key-value store - type database. - Mnesia is designed to handle the stress of telecoms systems, and has some - features specifically tailored for telecoms which are not commonly found - in other databases. - Key features of the Mnesia database are: -\end_layout - -\begin_layout Itemize -Fast key/value lookups -\end_layout - -\begin_layout Itemize -Distribution of the database system -\end_layout - -\begin_layout Itemize -Fault tolerance -\end_layout - -\begin_layout Standard -\begin_inset CommandInset citation -LatexCommand citet -key "667766" - -\end_inset - - -\end_layout - -\begin_layout Standard -The features of Mnesia originally intended for telecoms prove very useful - for the purposes of the GGS as well. - The fault tolerance and speed of Mnesia are very valuable tools, the fast - key/value lookups permit many lookups per second to the database. -\end_layout - -\begin_layout Standard -Game data will not be lost when a game is stopped or has gone down for unknown - reasons. - This makes it possible to continue a game just before the failure without - having to start the game from the beginning. - -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Mnesia}}{Database server used in the GGS} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -The GGS stores the game state in the distributed Mnesia database, from which - the state can be restored in the event of a crash. -\end_layout - -\begin_layout Standard -Each game is uniquely identified by a table token and the data of each game - is stored within two different namespaces. - The namespaces are named World and Localstorage. - The World is used contain all game data related to the game state. - This sort of game data may change during the runtime of the game. - The Localstorage should contain data independent of the game state. - Game resources, constants and global variables are all examples of data - that could reside within the Localstorage. - To store a value within the database, not only is the table token and the - name of the namespace required, but a unique key so that the value can - be successfully retrieved or modified later. - The key is fully decidable by the game developer. - -\end_layout - -\begin_layout Standard -The interface of the database module is an implementation of the upcoming - W3C Web Storage specification. - Web Storage is intended for use in web browsers, providing a persistent - storage on the local machine for web applications. - The storage can be used to communicate in between browser windows (which - is difficult when using cookies), and to store larger chunks of data -\begin_inset CommandInset citation -LatexCommand citet -key "webstorage:website" - -\end_inset - -. - Usage of the web storage standard in the GGS provides a well documented - interface to the database backend. -\end_layout - -\begin_layout Section -Communication with the GDL VM -\begin_inset CommandInset label -LatexCommand label -name "sec:Communication-with-the-GDL-VM" - -\end_inset - - -\end_layout - -\begin_layout Standard -A game launched on the GGS is run within a virtual machine. - For each programming language supported, there is a virtual machine that - interprets the game. - Furthermore an interface for communication between the GGS, the game and - the players playing the game is present. -\end_layout - -\begin_layout Standard -Callbacks written in Erlang are registered to the VM for the interface to - work. - It is only with the help of the interface that the game developer can access - the game state and send messages to the clients. - The interface provides access to three objects called -\emph on -world, players -\emph default - and -\emph on -localStorage -\emph default -. - The game state is safely stored in a database and retrieved for manipulation - by a call for the world object. - Interaction with the players is done by using the -\emph on -GGS.sendCommand(player_id, command, args) -\emph default - and -\emph on -GGS. -\emph default -sendCommandToAll(command, args). - The localstorage is a convenient way to store global data and other variables - separated from the game state. - Unique ids called gametokens are generated for hosted games so that they - are not mixed up. -\end_layout - -\begin_layout Standard -A game launched on the GGS is run within a virtual machine. - For each programming language supported, there is a virtual machine that - interprets the game. - Furthermore an interface for communication between the GGS, the game and - the players playing the game must be present. -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout - Unique id:s called gametokens are generated for hosted games so that they - are not mixed up. - -- good text, integrate more. -\end_layout - -\end_inset - - -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{WebStorage}}{A new standard for letting websites store data on visitors' - computers} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -Exposing Erlang functionality to the GDL VM -\begin_inset CommandInset label -LatexCommand label -name "sub:Exposing-Erlang-functionality" - -\end_inset - - -\end_layout - -\begin_layout Standard -This section contains a concrete example of how the localstorage and world - objects are exposed to a GDL VM. - The example comes from the GGS prototype, which uses JavaScript powered - by Google V8 as its GDL VM. -\end_layout - -\begin_layout Standard -The code given in -\begin_inset CommandInset ref -LatexCommand ref -reference "alg:exposing-erlang" - -\end_inset - - is specific to V8 and JavaScript, however implementations for different - GDLs, or different VMs should be similar. -\end_layout - -\begin_layout Standard -In JavaScript is is common to use a top level object, called a global object, - to establish a global scope. - This allows the declaration of global variables and functions. - To gain access to the global object in the GGS, the -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt erlv8 -\backslash -_vm:global(..)} -\end_layout - -\end_inset - - function on line 2 of the example is used. - Using the global object, declarations of the world and GGS object can be - placed in the global scope. -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt Global:set -\backslash -_value(..)} -\end_layout - -\end_inset - - is a call to the global object, declaring new objects in the global scope. - On line 4 the GGS object is declared. - By accessing -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt GGS.localStorage } -\end_layout - -\end_inset - - from within the GDL, access to the localstorage is provided, thus the localstor -age must be connected to the GGS object, this can be seen in line 5. -\end_layout - -\begin_layout Standard -Both the GGS and localstorage objects are dummy objects, which provide no - functionality, these two objects are simply placed in the GDL for the purpose - clearing up the code. - In order to perform an action using the GGS and localstorage objects, the - -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt getItem} and { -\backslash -tt setItem} -\end_layout - -\end_inset - - functions must be used. - These items are directly connected to the database module of the GGS, which - is discussed in more detail in -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:The-database-module" - -\end_inset - -. -\end_layout - -\begin_layout Standard -Similarly the functions -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt sendCommand, sendCommandToAll} and { -\backslash -tt setTimeout} -\end_layout - -\end_inset - - are directly connected to a piece of code in the GGS which performs the - desired action. - The -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt sendCommand} -\end_layout - -\end_inset - - functions are used to send commands or text to participants of the table. - The -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt setTimeout} -\end_layout - -\end_inset - - function introduces timeouts to the V8 engine, which are not available - per default. -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Prior to this section, the Erlang syntax has to be briefly explained. - I think the 'usage of erlang' section is a good place to do this in. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Float algorithm -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -lstset{ -\end_layout - -\begin_layout Plain Layout - -language=Erlang, -\end_layout - -\begin_layout Plain Layout - -backgroundcolor= -\backslash -color{white}, -\end_layout - -\begin_layout Plain Layout - -extendedchars=true, -\end_layout - -\begin_layout Plain Layout - -basicstyle= -\backslash -footnotesize -\backslash -ttfamily, -\end_layout - -\begin_layout Plain Layout - -showstringspaces=false, -\end_layout - -\begin_layout Plain Layout - -showspaces=false, -\end_layout - -\begin_layout Plain Layout - -numbers=left, -\end_layout - -\begin_layout Plain Layout - -numberstyle= -\backslash -footnotesize, -\end_layout - -\begin_layout Plain Layout - -numbersep=9pt, -\end_layout - -\begin_layout Plain Layout - -tabsize=2, -\end_layout - -\begin_layout Plain Layout - -breaklines=true, -\end_layout - -\begin_layout Plain Layout - -showtabs=false, -\end_layout - -\begin_layout Plain Layout - -captionpos=b -\end_layout - -\begin_layout Plain Layout - -} -\end_layout - -\begin_layout Plain Layout - - -\backslash -begin{lstlisting}[language=Erlang] -\end_layout - -\begin_layout Plain Layout - -% @doc Exposes some GGS functions to JavaScript -\end_layout - -\begin_layout Plain Layout - -expose(GameVM, Table) -> -\end_layout - -\begin_layout Plain Layout - - Global = erlv8_vm:global(GameVM), -\end_layout - -\begin_layout Plain Layout - - Global:set_value("GGS", erlv8_object:new([ -\end_layout - -\begin_layout Plain Layout - - {"localStorage", erlv8_object:new([ -\end_layout - -\begin_layout Plain Layout - - {"setItem", fun(#erlv8_fun_invocation{}, [Key, Val])-> -\end_layout - -\begin_layout Plain Layout - - ggs_db:setItem(Table, local_storage, Key, Val) -\end_layout - -\begin_layout Plain Layout - - end}, -\end_layout - -\begin_layout Plain Layout - - {"getItem", fun(#erlv8_fun_invocation{}, [Key])-> -\end_layout - -\begin_layout Plain Layout - - ggs_db:getItem(Table, local_storage, Key) -\end_layout - -\begin_layout Plain Layout - - end} -\end_layout - -\begin_layout Plain Layout - - % more functions ... -\end_layout - -\begin_layout Plain Layout - - ])}, -\end_layout - -\begin_layout Plain Layout - - {"world", erlv8_object:new([ -\end_layout - -\begin_layout Plain Layout - - {"setItem", fun(#erlv8_fun_invocation{}, [Key, Val])-> -\end_layout - -\begin_layout Plain Layout - - ggs_db:setItem(Table, world, Key, Val), -\end_layout - -\begin_layout Plain Layout - - ggs_table:send_command_to_all( -\end_layout - -\begin_layout Plain Layout - - Table, {"world_set", Key ++ "=" ++ Val} -\end_layout - -\begin_layout Plain Layout - - ) -\end_layout - -\begin_layout Plain Layout - - end}, -\end_layout - -\begin_layout Plain Layout - - {"getItem", fun(#erlv8_fun_invocation{}, [Key])-> -\end_layout - -\begin_layout Plain Layout - - ggs_db:getItem(Table, world, Key), -\end_layout - -\begin_layout Plain Layout - - end} -\end_layout - -\begin_layout Plain Layout - - % more functions ... -\end_layout - -\begin_layout Plain Layout - - ])}, -\end_layout - -\begin_layout Plain Layout - - {"sendCommand", fun(#erlv8_fun_invocation{}, [Player, Command, Args])-> -\end_layout - -\begin_layout Plain Layout - - ggs_table:send_command(Table, Player, {Command, Args}) -\end_layout - -\begin_layout Plain Layout - - end}, -\end_layout - -\begin_layout Plain Layout - - {"sendCommandToAll", fun(#erlv8_fun_invocation{}, [Command, Args])-> -\end_layout - -\begin_layout Plain Layout - - ggs_table:send_command_to_all(Table, {Command, Args}) -\end_layout - -\begin_layout Plain Layout - - end} -\end_layout - -\begin_layout Plain Layout - - {"setTimeout", fun(#erlv8_fun_invocation{}, [Time, Function])-> -\end_layout - -\begin_layout Plain Layout - - timer:apply_after(Time, ?MODULE, call_js, [GameVM, Function]) -\end_layout - -\begin_layout Plain Layout - - end} -\end_layout - -\begin_layout Plain Layout - - % more functions ... -\end_layout - -\begin_layout Plain Layout - - ])). -\end_layout - -\begin_layout Plain Layout - - -\backslash -end{lstlisting} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "alg:exposing-erlang" - -\end_inset - -An example of how Erlang functionality is exposed to a JavaScript GDL -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -TODO: Go in to more detail about how the world, player and localstorage - objects are implemented. - Also discuss localstorage and how it derives from the webstorage standard - in detail. - This is a great point on how we try to follow standards. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -My idea here is that we describe the erlang-js (which failed, but nontheless), - v8, UUID and other external communication. - We shouldn't describe sockets here though.. - or.. - maybe? -\end_layout - -\begin_layout Plain Layout -Also discuss how we allow GDLs to communicate with Erlang, this is -\begin_inset Quotes eld -\end_inset - -external -\begin_inset Quotes erd -\end_inset - - to thre GDL. - Discuss the GGS world object (there is a reference to this secxtion for - that purpose) -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Techniques for ensuring reliability -\end_layout - -\begin_layout Standard -One of the main goals of the project is to achieve high reliability. - The term -\begin_inset Quotes eld -\end_inset - -reliable system -\begin_inset Quotes erd -\end_inset - - is defined by the IEEE as a system with -\begin_inset Quotes eld -\end_inset - -the ability of a system or component to perform its required functions under - stated conditions for a specified period of time -\begin_inset Quotes erd -\end_inset - - -\begin_inset CommandInset citation -LatexCommand citet -key "ieee_90" - -\end_inset - -. - There are some tools for creating reliable applications built in to Erlang. - -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{Reliability}}{The ability of a system or component to perform its - required functions under stated conditions for a specified period of time} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{IEEE}}{Institute of Electrical and Electronics Engineers, read "I-triple- -E"} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Itemize -Links between processes. - When a process spawns a new child process, and the child process later - exits, the parent process is notified of the exit. - -\end_layout - -\begin_layout Itemize -Transparent distribution over a network of processors. - When several nodes participate in a network, it does not matter on which - of these machines a process is run. - Communication between processes does not depend on the node in which each - process is run. - -\end_layout - -\begin_layout Itemize -Hot code replacements. - Two versions of the same module can reside in the memory of Erlang at any - time. - This means that a simple swap between these versions can take place very - quickly, and without stopping the machine. -\end_layout - -\begin_layout Standard -These three features are some of the basic building blocks for more sophisticate -d reliability systems in Erlang. - Many times it is not necessary to use these features directly, but rather - through the design patterns described below. -\end_layout - -\begin_layout Subsection -Supervisor structure -\begin_inset CommandInset label -LatexCommand label -name "sub:Supervisor-structure" - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Float figure -wide false -sideways false -status collapsed - -\begin_layout Plain Layout -\begin_inset Note Note -status open - -\begin_layout Plain Layout -We should really do this graphic in EPS instead of PNG -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Graphics - filename graphics/Supervisor_tree_GGS.eps - scale 40 - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -The supervisor structure of GGS -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -By linking processes together and notifying parents when children exit, - supervisors are created. - A supervisor is a common approach in ensuring that an application functions - in the way it was intended -\begin_inset CommandInset citation -LatexCommand citet -key "Savor:1997:HSA:851010.856089" - -\end_inset - -. - When a process misbehaves, the supervisor takes some action to restore - the process to a functional state. - -\end_layout - -\begin_layout Standard -There are several approaches to supervisor design in general (when not just - considering how they work in Erlang). - One common approach is to have the supervisor look in to the state of the - process(es) it supervises, and let the supervisor make decisions based - on this state. - The supervisor has a specification of how the process it supervises should - function, and this is how it makes decisions. - -\end_layout - -\begin_layout Standard -In Erlang, we have a simple version of supervisors. - We do not inspect the state of the processes being supervised. - We do have a specification of how the supervised processes should behave, - but on a higher level. - The specification describes things such as how many times in a given time - interval a child process may crash, which processes need restarting when - crashes occur, and so forth. - -\end_layout - -\begin_layout Standard -When the linking of processes in order to monitor exit behavior is coupled - with the transparent distribution of Erlang, a very powerful supervision - system is created. - For instance, we can restart a failing process on a different, new node, - with minimal impact on the system as a whole. - -\end_layout - -\begin_layout Standard -In the GGS, we have separated the system in to two large supervised parts. - We try to restart a crashing child separately, if this fails too many -\begin_inset Foot -status collapsed - -\begin_layout Plain Layout -Exactly how many -\begin_inset Quotes eld -\end_inset - -too many -\begin_inset Quotes erd -\end_inset - - is depends on a setting in the supervisor, ten crashes per second is a - reasonable upper limit. -\end_layout - -\end_inset - - times, we restart the nearest supervisor of this child. - This ensures separation of the subsystems so that a crash is as isolated - as possible. -\end_layout - -\begin_layout Standard -The graphic above shows our two subsystems, the coordinator subsystem and - the dispatcher subsystem. - Since these two systems perform very different tasks they have been separated. - Each subsystem has one worker process, the coordinator or the dispatcher. - The worker process keeps a state which should not be lost upon a crash. -\end_layout - -\begin_layout Standard -We have chosen to let faulty processes crash very easily when they receive - bad data, or something unexpected happens. - The alternative to crashing would have been to try and fix this faulty - data, or to foresee the unexpected events. - We chose not to do this because it is so simple to monitor and restart - processes, and so difficult to try and mend broken states. - This approach is something widely deployed in the Erlang world, and developers - are often encouraged to “Let it crash”. -\end_layout - -\begin_layout Standard -To prevent any data loss, the good state of the worker processes is stored - in their respective backup processes. - When a worker process (re)starts, it asks the backup process for any previous - state, if there is any that state is loaded in to the worker and it proceeds - where it left off. - If on the other hand no state is available, a special message is delivered - instead, making the worker create a new state, this is what happens when - the workers are first created. -\end_layout - -\begin_layout Subsection -Redundancy -\end_layout - -\begin_layout Standard -The modules in the GGS are built to be capable of redundant operation. - By adding a backup process to sensitive processes, the state can be kept - in the event of a crash. - The coordinator of the GGS prototype has this backup feature built in. - The coordinator passes state along to the backup process which keeps the - data safe. - In the event of a crash, the coordinator recovers the state from the backup - process. - Figure -\begin_inset CommandInset ref -LatexCommand ref -reference "fig:redundancy" - -\end_inset - - depicts the redundancy built in to the coordinator process. -\end_layout - -\begin_layout Standard -\begin_inset Float figure -wide false -sideways false -status collapsed - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Graphics - filename graphics/redundancy.eps - scale 40 - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "fig:redundancy" - -\end_inset - -To the left normal execution is pictured; the server state is backed up. - To the right; the exceptional execution, where the state is retrieved from - the backup to repopulate the server. -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -Hot code replacement -\begin_inset CommandInset label -LatexCommand label -name "sub:Hot-code-replacement" - -\end_inset - - -\end_layout - -\begin_layout Standard -Hot code replacement is a technique used to update systems while they are - running. - The main use of hot code replacement are in critical systems that require - low downtime, such as telecom systems. - By using hot code replacement, systems can be able to achieve as high uptime - as possible and thus improving the reliability of the system. - Code replacement is a feature that exist in Erlang which means that with - some work it could be implemented into the GGS. -\end_layout - -\begin_layout Section -Software testing -\end_layout - -\begin_layout Standard -In order to make sure the GGS prototype adheres to the specification set - two different approaches to software testing are used. - For simpler testing the GGS prototype uses unit tests. - Modules are tested on a high level, making sure each function in the module - tested functiions according to specification. -\end_layout - -\begin_layout Standard -Unit testing is not employed to test the system from the client side. - In order to more accurately simulate real users some randomization is needed -\begin_inset Note Note -status open - -\begin_layout Plain Layout -citation needed -\end_layout - -\end_inset - -, as users do not always act rationally. - In order to introduce random data, the client side of the GGS is simulated - by QuickCheck tests. -\end_layout - -\begin_layout Subsection -Unit testing -\end_layout - -\begin_layout Standard -Unit testing is a way to check if the functionality adheres to the specification - of the system by manually creating test cases for sections of code. - In most cases whole functions. - Unit testing is good, not only for revealing software bugs, but also to - state that a feature is working according to the specification. - Unit testing is a common way to test software and has proven useful within - the GGS when functions take complicated arguments. - In these cases it is easy to set up a scenario that should work. - -\end_layout - -\begin_layout Standard -Unit testing is a useful way to create regression tests. - Regression tests are used to make sure changes made to the GGS do not introduce - new bugs or break the specification. - The regression tests are optimally run very often, such as after each change - to the code. -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Erlang provides a module for unit testing called eunit. - Eunit, being a part of OTP, is rich in functionality and well documented - yet it doesn't allow any means of testing asynchronous behaviours as opposed - to other means of software testing. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -Automated test case generation -\end_layout - -\begin_layout Standard -The problem of writing software tests manually, is that it takes a lot of - time. - There exists other ways to test software that address this problem by generatin -g test cases with certain properties. - This allows for testing functions with a lot of different input parameters - without having to implement each specific test itself. - -\end_layout - -\begin_layout Standard -By having each test automatically generated, each test can be very complex - and long. - In order to generate random, complex tests the GGS uses QuickCheck. - By using QuickCheck the GGS can be tested with input which would be extremely - difficult to construct using manual testing methods. - Regression tests, such as the unit tests used by the GGS are more useful - for ensuring the system does not diverge from a working scenario than for - finding new cases where the specification does not hold -\begin_inset CommandInset citation -LatexCommand citet -key "Arts:2006:TTS:1159789.1159792" - -\end_inset - -. -\end_layout - -\begin_layout Standard -The entire GGS was not tested using QuickCheck, nor was the entire client - protocol for a game tested using QuickCheck, however the tests performed - using QuickCheck show that an automated testing system such as QuickCheck - is a very viable testing method for the GGS. -\end_layout - -\begin_layout Standard -QuickCheck has features to generate very large and complex tests, the results - of which can be hard to analyze. - The solution to reading these complex test is to extract a -\emph on -minimal failing test case -\emph default - which contains the smalles failing test sequence. - By applying a very large test and gradually simplifying the test to find - the smallest failing sequence, many bugs which would other wise have been - hard to catch can be caught -\begin_inset CommandInset citation -LatexCommand citet -key "Arts:2006:TTS:1159789.1159792" - -\end_inset - -. -\end_layout - -\begin_layout Standard -QuickCheck was originally made for the programming language Haskell. - There are a lot of reimplementations of QuickCheck in various programming - languages. - Erlang QuickCheck (EQC) and Triq are two variants of QuickCheck for Erlang. - EQC was chosen for testing the GGS. - Besides the standard functionality that QuickCheck provides, EQC is capable - of testing concurrency within a program. -\end_layout - -\begin_layout Section -Case studies -\end_layout - -\begin_layout Standard -This section contains three case studies. - These case studies have been written to provide examples of how the flow - through the GGS can look when performing different tasks. - The first case study outlines the flow of sending a common message to the - GDL VM and receiving a response. - The second case study provides an example of the process of connecting - to the GGS to set up a game. - The third and final case study is a section of code from a part of a game - for the GGS. - The code in the third study shows how a simple chat server can be implemented - in the GGS using JavaScript as GDL. -\end_layout - -\begin_layout Subsection -Typical communication -\end_layout - -\begin_layout Standard -This case study describes the flow through the GGS when a typical command - is encountered. - Below is a case study where a chat client sends a message to change the - nick of a user. - The actual code performing the change of a nick in JavaScript is discussed - in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Example-of-a-GGS-app" - -\end_inset - -. - All communication between modules is asynchronous, nothing is blocking, - which is very important in concurrent systems. - To follow the example more easily, looking at the graphic in section -\begin_inset CommandInset ref -LatexCommand vref -reference "fig:The-layout-of" - -\end_inset - - is recommended. -\end_layout - -\begin_layout Enumerate -The client packages a Game-Command into a -\emph on -GGS protocol packet -\emph default - which conforms to the protocol structure the GGS is using and sends it - over the network. -\end_layout - -\begin_layout Enumerate -The player process, which is coupled to the TCP-process which reacts on - incoming messages, accepts the message and forwards the raw data to the - protocol parser process. -\end_layout - -\begin_layout Enumerate -The protocol parser process parses the message and brings it into the format - of the internal GGS presentation of such a message, which is just a specialized - Erlang tuple. -\end_layout - -\begin_layout Enumerate -The protocol parser sends this Erlang touple back to the player process. -\end_layout - -\begin_layout Enumerate -The player process checks if it is a Server-Command or a Game-Command. - In our example it is a Game-Command and it sends the message to the table - process. -\end_layout - -\begin_layout Enumerate -The table process sends it to its own Game VM process. -\end_layout - -\begin_layout Enumerate -The game VM process calls the function -\emph on -playerCommand( -\begin_inset Quotes eld -\end_inset - -278d5002-77d6-11e0-b772-af884def5349 -\begin_inset Quotes erd -\end_inset - -, -\begin_inset Quotes eld -\end_inset - -nick -\begin_inset Quotes erd -\end_inset - -, -\begin_inset Quotes eld -\end_inset - -Peter -\begin_inset Quotes erd -\end_inset - -) -\emph default - within the JavaScript VM. -\end_layout - -\begin_layout Enumerate -The JavaScript VM (JSVM) - at this stage Googles V8 JavaScript Engine - - evaluates the function within the sandboxed game context which has been - established earlier during the setup of the game. -\end_layout - -\begin_layout Enumerate -In the example in section -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Example-of-a-GGS-app" - -\end_inset - - we see that the GGS-functions -\emph on -GGS.localStorage.setItem(key, value) -\emph default - and -\emph on -GGS.localStorage(key) -\emph default - are used. - Both are callbacks coupled to the database module functions. -\end_layout - -\begin_layout Enumerate -Data is being read from and written to the database and handed over to the - JSVM via the database process. -\end_layout - -\begin_layout Enumerate -In the example the -\emph on -GGS.sendCommandToAll() -\emph default - is being called then which is a callback to a function of the table module - which iterates through its player list and sends the command to every player. -\end_layout - -\begin_layout Enumerate -The table process sends every player process the message to send the message - with the change of a nickname of a particular user to its own client. -\end_layout - -\begin_layout Enumerate -The player process asks the protocol process to create a message conforming - to the protocol which is being used. -\end_layout - -\begin_layout Enumerate -The protocol process creates a string according to the protocol and returns - it to the player process. -\end_layout - -\begin_layout Enumerate -The player process sends the message with help of the gen_tcp module to - the client. -\end_layout - -\begin_layout Subsection -Initialization and life cycle of a game -\end_layout - -\begin_layout Standard -This case study describes the initialization and definition of a game and - in roughly its life cycle untill it is removed from the GGS. -\end_layout - -\begin_layout Subsubsection -Initialization -\end_layout - -\begin_layout Enumerate -A client connects via TCP to the GGS. -\end_layout - -\begin_layout Enumerate -The dispatcher process reacts on the incomming connecction and creates a - new player process. -\end_layout - -\begin_layout Enumerate -The dispatcher process couples the TCP connection to the newly created player - process, this way the new player process is responsible to react on incoming - messages. -\end_layout - -\begin_layout Enumerate -The client sends a message with a -\noun on -hello -\noun default - Server-Command to initiate a handshake. -\end_layout - -\begin_layout Enumerate -The player module parses the message with help of the protocol module. -\end_layout - -\begin_layout Enumerate -If the message was just a plain -\noun on -hello -\noun default -, without a table token, then the player process asks the coordinator process - to create a new table process and add this player process to this newly - created table. - If the client did send a table token then the player process asks the coordinat -or to att the player process to this table. -\end_layout - -\begin_layout Enumerate -During the creation of a new table the table process creates a new game - VM process which creates its own game context within the JavaScript VM. -\end_layout - -\begin_layout Enumerate -The player process answers to the client with a -\noun on -hello -\noun default - Client-Command and passes on the clients player token along with the informatio -n about if it should define a game - because it is the first client to connect - to this table - or not and the table token it was assigned to. -\end_layout - -\begin_layout Subsubsection -Defining a game -\end_layout - -\begin_layout Standard -The generic nature of the GGS leaves it up to the client to define which - game should be run. - The definition is done in the GDL, in this example, the GDL is JavaScript. - It is possible to alter the GGS prototype so that only the server maintainer - is able to install new games on the server however the current implementation - of the GGS is much more generic. -\end_layout - -\begin_layout Standard -The first client which connects to a table is responsible to provide the - JavaScript server source code. - To do so there is a -\noun on -define -\noun default - Server-Command. -\end_layout - -\begin_layout Enumerate -If during the handshake with the -\noun on -hello -\noun default - command the client is assigned the task of providing the server source - code then the client must send a -\noun on -define -\noun default - Server-Command message with the source code as its parameter. - Only the first client will get the information about the need of defining - a game during the handshake. -\end_layout - -\begin_layout Enumerate -The player process parses the message, with help of the protocol module. -\end_layout - -\begin_layout Enumerate -The player process sends the source code to the table process assigned to - the player as a -\noun on -define -\noun default - message. -\end_layout - -\begin_layout Enumerate -The table process forwards the source code to the game VM process. -\end_layout - -\begin_layout Enumerate -The game VM process executes the source code within the JavaScript VM. -\end_layout - -\begin_layout Enumerate -The JavaScript VM evaluates the source code - which has to implement the - playerCommand() function - within the context of the game. -\end_layout - -\begin_layout Enumerate -The game is at this point fully initialized and can be used by all clients - with help of the playerCommand() function. -\end_layout - -\begin_layout Enumerate -The table process saves the source code in the database for backup reasons - (this is not yet implemented). -\end_layout - -\begin_layout Enumerate -The player process sends a -\noun on -defined -\noun default - Client-Command to the client. - This way the client is notified that everything went well and it can start - the game. -\end_layout - -\begin_layout Subsubsection -Life cycle -\end_layout - -\begin_layout Enumerate -Initialization -\end_layout - -\begin_layout Enumerate -Defining a game -\end_layout - -\begin_layout Enumerate -Other clients connect and initialize but do not define anything. -\end_layout - -\begin_layout Enumerate -Typical communication -\end_layout - -\begin_layout Enumerate -Clients disconnect -\end_layout - -\begin_layout Enumerate -When the last client disconnects the table process terminates and with it - the game context and database content (not implemented in the prototype). -\end_layout - -\begin_layout Subsection -A GGS server application in JavaScript -\begin_inset CommandInset label -LatexCommand label -name "sec:Example-of-a-GGS-app" - -\end_inset - - -\end_layout - -\begin_layout Standard -Below is a concrete example of a simple chat server application written - using the GGS. - The language chosen for this chat server is JavaScript. - The GGS processes all incoming data through a protocol parser, which interprets - the data and parses it into an internal format for the GGS. -\end_layout - -\begin_layout Standard -When the GGS receives a -\emph on -Game-Command -\emph default - from a client, it is passed along to the game VM through a function called - -\emph on -playerCommand -\emph default - which is the entry point for each game and has to be implemented by the - developer; one can think of it like the -\emph on -main() -\emph default - function of a C or Java program -\emph on -. - -\emph default - Typically the -\emph on -playerCommand -\emph default - function contains conditional constructs which decide the next action to - take. - In -\begin_inset CommandInset ref -LatexCommand ref -reference "alg:A-concrete-example" - -\end_inset - - an example of the -\emph on -playerCommand -\emph default - function can be seen. -\end_layout - -\begin_layout Standard -In -\begin_inset CommandInset ref -LatexCommand ref -reference "alg:A-concrete-example" - -\end_inset - - the -\emph on -playerCommand -\emph default - function accepts two different commands. - The first command is a command which allows chat clients connected to the - chat server to change nicknames, which are used when chatting. - In order to change the nickname, a client must send a Game-Command -\begin_inset Quotes eld -\end_inset - -nick -\begin_inset Quotes erd -\end_inset - - with the actual new nickname as a argument. - When a message arrives to the GGS which has the form corresponding to the - nickname change, the -\emph on -playerCommand -\emph default - function is called with the parameters -\emph on -player_id, command, -\emph default -and -\emph on -args -\emph default - filled in appropriately. -\end_layout - -\begin_layout Standard -The -\emph on -playerCommand -\emph default - function is responsible for calling the helper functions responsibly for - carrying out the actions of each message received. - -\emph on -changeNick -\emph default - is a function which is called when the -\begin_inset Quotes eld -\end_inset - -nick -\begin_inset Quotes erd -\end_inset - - message is received. - The -\emph on -changeNick -\emph default -function uses a feature of the GGS called localstorage (see section -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Communication-with-the-GDL-VM" - -\end_inset - -), which is an interface to the database backend contained in the database - module (see -\begin_inset CommandInset ref -LatexCommand ref -reference "sub:The-database-module" - -\end_inset - -). - The database can be used as any key-value store, however the syntax for - insertions and fetch operations is tightly integrated in the GDL of the - GGS. -\end_layout - -\begin_layout Standard -Access to the localStorage is provided through the -\emph on -GGS object -\emph default -, which also can be used to communicate with the rest of the system from - the GDL. - Implementation specifics of the GGS object are provided in -\begin_inset CommandInset ref -LatexCommand ref -reference "sec:Communication-with-the-GDL-VM" - -\end_inset - -. -\end_layout - -\begin_layout Standard -\begin_inset Float algorithm -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -lstset{ -\end_layout - -\begin_layout Plain Layout - -language=JavaScript, -\end_layout - -\begin_layout Plain Layout - -backgroundcolor= -\backslash -color{white}, -\end_layout - -\begin_layout Plain Layout - -extendedchars=true, -\end_layout - -\begin_layout Plain Layout - -basicstyle= -\backslash -footnotesize -\backslash -ttfamily, -\end_layout - -\begin_layout Plain Layout - -showstringspaces=false, -\end_layout - -\begin_layout Plain Layout - -showspaces=false, -\end_layout - -\begin_layout Plain Layout - -numbers=left, -\end_layout - -\begin_layout Plain Layout - -numberstyle= -\backslash -footnotesize, -\end_layout - -\begin_layout Plain Layout - -numbersep=9pt, -\end_layout - -\begin_layout Plain Layout - -tabsize=2, -\end_layout - -\begin_layout Plain Layout - -breaklines=true, -\end_layout - -\begin_layout Plain Layout - -showtabs=false, -\end_layout - -\begin_layout Plain Layout - -captionpos=b -\end_layout - -\begin_layout Plain Layout - -} -\end_layout - -\begin_layout Plain Layout - - -\backslash -begin{lstlisting}[language=JavaScript] -\end_layout - -\begin_layout Plain Layout - -function playerCommand(player_id, command, args) { -\end_layout - -\begin_layout Plain Layout - - if(command == "nick") { -\end_layout - -\begin_layout Plain Layout - - changeNick(player_id, args); -\end_layout - -\begin_layout Plain Layout - - } else if(command == "message") { -\end_layout - -\begin_layout Plain Layout - - message(player_id, args); -\end_layout - -\begin_layout Plain Layout - - } -\end_layout - -\begin_layout Plain Layout - -} -\end_layout - -\begin_layout Plain Layout - -function changeNick(player_id, nick) { -\end_layout - -\begin_layout Plain Layout - - var old_nick = GGS.localStorage.getItem("nick_" + player_id); -\end_layout - -\begin_layout Plain Layout - - GGS.localStorage.setItem("nick_" + player_id, nick); -\end_layout - -\begin_layout Plain Layout - - if (!old_nick) { -\end_layout - -\begin_layout Plain Layout - - GGS.sendCommandToAll("notice", nick + " joined"); -\end_layout - -\begin_layout Plain Layout - - } else { -\end_layout - -\begin_layout Plain Layout - - GGS.sendCommandToAll("notice", old_nick + " is now called " + nick); -\end_layout - -\begin_layout Plain Layout - - } -\end_layout - -\begin_layout Plain Layout - -} -\end_layout - -\begin_layout Plain Layout - -function message(player_id, message) { -\end_layout - -\begin_layout Plain Layout - - var nick = GGS.localStorage.getItem("nick_" + player_id); -\end_layout - -\begin_layout Plain Layout - - GGS.sendCommandToAll('message', nick + "> " + message); -\end_layout - -\begin_layout Plain Layout - -} -\end_layout - -\begin_layout Plain Layout - - -\backslash -end{lstlisting} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "alg:A-concrete-example" - -\end_inset - -A concrete example of a simple chat server written in JavaScript, running - on the GGS -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Chapter -Problems of implementation -\begin_inset CommandInset label -LatexCommand label -name "cha:Problems-of-implementation" - -\end_inset - - -\end_layout - -\begin_layout Standard -This chapter contains specific problems encountered when implementing the - GGS prototype. - Some of the problems described have solutions attached, however some problems - were not solved, therefore only ideas for solutions have been attached. -\end_layout - -\begin_layout Standard -The integration of JavaScript as a GDL in the GGS prototype was particularly - difficult, and is handled in this section and so is the protocol design. -\end_layout - -\begin_layout Section -JavaScript engine -\end_layout - -\begin_layout Standard -The GGS prototype uses a virtual machine to sandbox each game. - JavaScript was chosen for the prototype due to its commonality in web applicati -ons and the flexibility of the language. - Any language with the proper bindings to Erlang could have been used in - theory. -\end_layout - -\begin_layout Standard -There are two JavaScript virtual machines, or -\emph on -engines, -\emph default - with suitable bindings to erlang available at the time of the writing of - this thesis. - There is a group of machines developed by Mozilla called -\emph on -TraceMonkey, JaegerMonkey, SpiderMonkey -\emph default -and -\emph on -IonMonkey -\emph default -, and also there is Googles -\emph on -V8 -\emph default -. - The members in the group of Mozilla machines are largely the same, and - are referred to as the same machine for simplicity. -\end_layout - -\begin_layout Standard -For the Mozilla machines, there exists a Erlang binding called erlang_js, - and for the V8 machine a binding called erlv8 exists. -\end_layout - -\begin_layout Subsection -erlang_js -\end_layout - -\begin_layout Standard -erlang_js provides direct communication with the JavaScript VM. - Which is exactly what is desired, however also required is the possibility - to communicate from JavaScript to Erlang. - The ability to communicate from JavaScript to Erlang is not yet implemented - in erlang_js, due to lack of time of the erlang_js developers. -\end_layout - -\begin_layout Standard -There were two possible solutions to the problem, either one would implement - the missing functionality, or a switch from erlang_js to some other JavaScript - engine with better bindings could be made. - -\end_layout - -\begin_layout Standard -Attempts at implementing the missing functionality were initially made but - never became stable enough for usage in the GGS and the erlang_js software - was abandoned. -\end_layout - -\begin_layout Subsection -erlv8 -\end_layout - -\begin_layout Standard -erlv8 is powered by the V8 engine developed by Google. - The ability to communicate from JavaScript to Erlang using callbacks (aka - NIF) is available in the erlv8 bindings and can be used within the GGS. -\end_layout - -\begin_layout Standard -Initial releases of the erlv8 bindings had stability issues, these however - were resolved by the erlv8 developers during the development GGS. - At this point erlv8 is the JavaScript engine powering JavaScript as a GDL - in the GGS. -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{V8}}{JavaScript engine developed by Google} -\end_layout - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{SpiderMonkey}}{JavaScript engine developed by Mozilla} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Protocol design -\end_layout - -\begin_layout Standard -Initially the GGS protocol was planed to use the UDP protocol for transport. - Due to the lack of error checking in the UDP protocol, the UDP protocol - is faster than the TCP protocol, this was a main reason in the desire to - use UDP. - The GGS does however need error checking for some of it parts to be as - reliable as possible. - Therefore an error checking layer would have to be placed on top of UDP. -\end_layout - -\begin_layout Standard -The development of an error checking layer was weighed against the implementatio -n of TCP instead of UDP, thus losing some speed. - Even though speed was lost, TCP was chosen due to the relative ease of - implementation compared to UDP. - Due to the modularity of the GGS, a UDP extension is easily possible by - replacing the network parts of the GGS. -\end_layout - -\begin_layout Standard -The Apache Thrift -\begin_inset CommandInset citation -LatexCommand citep -key "Slee2007" - -\end_inset - - was also an alternative. - Using Thrift would mean the GGS would feature a standard protocol for network - communication. - Before finding out about Thrift during a lecture of Joe Armstrong (one - of the inventors of Erlang), an implementation of the GGS protocol had - already been implemented, moving to Thrift would mean too much efford for - a prototype during the short amount of time. -\end_layout - -\begin_layout Standard -The use of Thrift, Google protocol buffers - which is a different approach - to that implemented by Google - or other protocols can be supported quite - easily by developing protocol modules for each the protocols. - No protocol modules for these protocols have however been developed during - the writing of this thesis. -\end_layout - -\begin_layout Section -Operating system limitations -\begin_inset CommandInset label -LatexCommand label -name "sec:Operating-system-limitations" - -\end_inset - - -\end_layout - -\begin_layout Standard -The operating systems on the computers which were used to run the bots when - testing the GGS prototype had some limitations. - The operating systems used were Linux and Mac OS X, since these systems - are quite similar on a lower level they exhibited the same limitations.. -\end_layout - -\begin_layout Standard -The most notable limitation was a limit set on the number of simultaneously - open files. - Due to the implementation of sockets in UNIX-like systems such as Mac OS - X and Linux, a limit on the number of open files is a limit on the number - of open sockets. - In order to simulate many connections to the GGS, many sockets needed to - be opened. - Each socket had a bot connected on one end and the GGS on the other end. - On each test machine several thousand sockets needed to be open while testing - the GGS, therefore the limit on open files had to be removed. -\end_layout - -\begin_layout Standard -On the Linux machines the limit of open files is configured in -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt /etc/security/security.conf} -\end_layout - -\end_inset - -. -\end_layout - -\begin_layout Standard -On the Mac OS X machine the limit of open files is configured in -\begin_inset ERT -status open - -\begin_layout Plain Layout - -{ -\backslash -tt /etc/launchd.conf } -\end_layout - -\end_inset - -. -\end_layout - -\begin_layout Chapter -Results and discussion -\begin_inset CommandInset label -LatexCommand label -name "chap:Results-and-discussion" - -\end_inset - - -\end_layout - -\begin_layout Standard -In this chapter the results of the GGS prototype are presented and discussed. - The results of the ing are presented with both graphical and textual content. - Finally thoughts about how future improvements to the prototype could look - like are given. -\end_layout - -\begin_layout Section -Statistics -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Mention the hardware which the GGS was run on; A Thinkpad T410 with a core - i5 and 4GB of ram. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -The testing of the GGS prototype occurred in two sessions -\end_layout - -\end_inset - -Testing of the GGS took place in two separate sessions. - The first session simulates a highly demanding application, the second - session simulated a less demanding application. - The highly demanding application is a real time game which does several - asynchronous database writes each second. - The less demanding application does not perform any database reads or writes. -\end_layout - -\begin_layout Standard -Each of the two simulations use JavaScript as the GDL. - The JavaScript is run through Google V8. - The database module uses Mnesia. -\end_layout - -\begin_layout Standard -During the sessions two measurements were recorded. -\end_layout - -\begin_layout Itemize - -\series bold -Messages per second -\series default - is used to see how many incoming and outgoing messages the server can process - each second. - The results of the messages per second testing are shown for a high demanding - application in figure -\begin_inset CommandInset ref -LatexCommand ref -reference "fig:msg-per-sec-MNESIA" - -\end_inset - -, and for a low demanding application in -\begin_inset CommandInset ref -LatexCommand ref -reference "fig:msg-per-sec-NOMNESIA" - -\end_inset - -. -\end_layout - -\begin_layout Itemize - -\series bold -Latency between server and client -\series default - is used to measure the round-trip time for a message travelling between - the client and server. - This measurement is used to determine how many players the server can handle - while still providing a playable gaming experience. - The results of the latency test can be seen in figure -\begin_inset CommandInset ref -LatexCommand ref -reference "fig:latency-graph" - -\end_inset - -. -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status open - -\begin_layout Plain Layout -There was also a testing session where the number of clients were measured, - however this was not a good measurement of performance and therefore these - numbers will not be included in the report. - -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Since we donät include this.. - should we mention it? -\end_layout - -\end_inset - - -\end_layout - -\end_inset - -The hardware that the GGS was running on was a Thinkpad T410, with a Intel - i5 processor and 4GB of RAM. -\end_layout - -\begin_layout Standard -In the first test, where Mnesia was used, the server had a peak value of - nearly 6000 messages per second. - When this number was reached Mnesia warned that it was overloaded and shortly - after that Mnesia failed to serve requests. - This result was not unexpected as this test put the database under heavy - load. - In the next testing session, the test was conducted with another client - that did not use Mnesia. - Without mnesia the server peaked at 60000 messages per second, however - this was only for a very short time. - The average throughput was around 25000 messages per second, five times - more than what the server was able to process with Mnesia in place. - -\end_layout - -\begin_layout Standard -In the second testing session the delay between the server and clients was - also measured. - A connection can be seen between those values, as long as the server is - under moderate load the delay is low and stable. - When the load on the server increases heavily the delay does the same, - this is because the server cannot process all incoming messages and therefore - messages are put in a queue within the system. -\end_layout - -\begin_layout Standard -\begin_inset Note Note -status collapsed - -\begin_layout Plain Layout -Important things to note are that the number of clients is not a good way - of measuring the performance of the server because the server is possible - to have a large number of clients on the server but it cannot handle all - the information. - Instead the performance of the server should be measured in the number - of messages it can handle per second. -\end_layout - -\begin_layout Plain Layout -We were able to reach 6000 messages per second on the server, which corresponds - to around 350 clients. - However soon after this mnesia printed some warnings and the clients started - to lag. - With this in mind one thing to investigate is if mnesia is the bottleneck - in the system. - Current game servers do not use databases to save their state and maybe - we can see the reason here. - Other possible bottlenecks may be the protocol, but this seems less likely - than mnesia. - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Float figure -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Graphics - filename graphics/msg_per_sec.eps - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "fig:msg-per-sec-MNESIA" - -\end_inset - -The graph shows messages per second for intervals of clients connected. - Each client performs 3 asynchronous writes to the Mnesia database each - second. -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Float figure -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Graphics - filename graphics/ping.eps - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "fig:latency-graph" - -\end_inset - -This graph shows the latency in a low-demand application. - The ping is measured in milliseconds for a message to make a round-trip - between client and server. -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset Float figure -wide false -sideways false -status open - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -begin{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Graphics - filename graphics/msg_per_secoutput.eps - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -end{centering} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Plain Layout -\begin_inset Caption - -\begin_layout Plain Layout -\begin_inset CommandInset label -LatexCommand label -name "fig:msg-per-sec-NOMNESIA" - -\end_inset - -The graph shows messages per second for intervals of clients connected. - No database is connected. -\end_layout - -\end_inset - - -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Section -Future improvements -\end_layout - -\begin_layout Standard -There are several things in the GGS that can be improved. - In this section the most important additions to the GGS are described, - along with a motivation as to why these additions are not found in the - GGS prototype. -\end_layout - -\begin_layout Subsection -Distribution -\end_layout - -\begin_layout Standard -The GGS was originally intended to be a distributed application, running - on several machines at once. - The design of the GGS should support this, it has however not been tested. - The technologies, such as supervisor trees and the servers supplied by - the OTP which are used in the GGS all support the development of distributed - applications. -\end_layout - -\begin_layout Standard -Distribution was however not implemented in the GGS. - Other parts of the GGS were prioritized. - A futute improvement is therefore to implement distribution in the GGS. - A simple way to achieve this is to keep one GGS instance as a coordinating - instance, and to keep clients on other instances of the GGS, which can - be dynamically added as new clients connect. -\end_layout - -\begin_layout Subsection -Performance -\end_layout - -\begin_layout Standard -The GGS prototype was not developed for maximum performance. - Performance optimizations were considered, many were however not implemented - in the prorotype. - There are several performance optimizations which can be included in future - versions of the GGS, below are some of the most important performance optimizat -ions identified. -\end_layout - -\begin_layout Subsubsection -Protocols -\begin_inset Note Note -status open - -\begin_layout Plain Layout -Need references for assertions about UDP being nicer on the CPU. - Motivate why UDP is not implemented. -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -Because of TCP being a connection oriented protocol, it is not suited for - all types of game data transfers. - Each transmission will consume more network bandwidth than connectionless - protocols like UDP and cause unnecessary load on the processor. - Therefore support for UDP would mean that more games could be run simultaneousl -y on the GGS. - Another advantage of UDP is latency being reduced. - Without having to setup a connection for each group packets of data being - sent, they will be sent instantly and therefore arrive earlier. - Latency is of highest importance in real-time games as it improves realism - and fairness in gameplay and many game developers require the freedom to - take care of safety issues as packet losses themselves. - This concludes that UDP would be a benefit for the GGS, game developers - and players alike. -\end_layout - -\begin_layout Subsubsection -Database -\end_layout - -\begin_layout Standard -Currently Mnesia is used for game data storage. - During stress tests, Mnesia has turned out to be the bottleneck due to - data losses when too many games are played on the GGS simultaneously. - -\end_layout - -\begin_layout Standard -The usage of Mnesia in the GGS is not the usage originally intended. - Originally a cache was to be placed before Mnesia. - The cache could be either Erlang Term Storage (ETS) or a Erlang process - which keeps track of all database actions. - The cache periodically flushes its contents to Mnesia, thereby reducing - the Mnesia transactions overall. -\end_layout - -\begin_layout Standard -The cache was never implemented in the prototype due to other parts of the - GGS being prioritized. - The current implementation of the database backend is not optimal, however - it functions reliably, therefore it was deemed sufficient for the prototype. -\end_layout - -\begin_layout Standard -A possible future addition to the GGS could be to add this cache in the - database module. - The API would not need to change, as this could be implemented internally - in the database module. - -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -nomenclature{ -\backslash -textbf{ETS}}{Erlang Term Storage} -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Subsection -Documentation -\end_layout - -\begin_layout Standard -To start the GGS is not self explanatory. - This together with overall usage of GGS should be documented. - The interface for usage of game developers are also in need of documentation. - Features and requirements with respect to the GGS would assist users to - know what they need to use the GGS and how they would benefit of it. - The GGS does not support many programming languages nor does it have a - complete documentation. - This needs to be taken care of in future versions. -\end_layout - -\begin_layout Chapter -Conclusion -\end_layout - -\begin_layout Standard -This thesis describes a method to create a reliable and generic game server - with help of the techniques used in the telecom industry. -\end_layout - -\begin_layout Standard -To make the GGS as generic as possible seperation of game and server logic - is necessary. - Designing a good API is vital in order to allow game developers to interact - with the server in a easy manner and with minimal overhead. - Furthermore every game should be isalated so that games can not interfare - with each other. - Isolation can be achived by introducing a context for each game which leads - to the fact that each game runs in its own sandbox. - To be able to use different game development languages virtual machines - should be used. - Each virtual machine instance evaluates game source code safely. -\end_layout - -\begin_layout Standard -This thesis concludes that it is reasonable to use the same tools as those - used by the telecom industry for creating reliable systems when developing - games for computers. - A typical game can be split up in to several parts, and using the GGS, - the parts not directly related to the actual gameplay can be implemented - in Erlang, while keeping the actual game software in a virtual machine. - It has been demonstrated in this thesis that games can be developed for - the GGS in JavaScript, while still benefiting from the features offered - by Erlang and the OTP. - -\end_layout - -\begin_layout Standard -In the current state, the GGS prototype is not scalable. - The GGS is however prepared for scaling due to its overall structure. - The implementation of scalability could be performed in two different ways, - either to scale one instance of the GGS or to scale by creating new instances - and support communication among them. -\end_layout - -\begin_layout Standard -\begin_inset ERT -status open - -\begin_layout Plain Layout - - -\backslash -addcontentsline{toc}{section}{Glossary} -\end_layout - -\begin_layout Plain Layout - - -\backslash -printnomenclature -\end_layout - -\end_inset - - -\end_layout - -\begin_layout Standard -\begin_inset CommandInset bibtex -LatexCommand bibtex -bibfiles "bibliography" -options "plainnat" - -\end_inset - - -\end_layout - -\end_body -\end_document