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\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 system with 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 such as 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 \begin_inset space ~ \end_inset 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 is 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 was not 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 this 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 the Erlang language provides. \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, 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, 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 collapsed \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 or \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 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 \begin_inset ERT status open \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 is 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 do not need to participate in the same instance of the game, games such as chess are possible candidates for the testing sessions. \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 are deemed more demanding than the turn based games. Tests are 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 of 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 is 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 have been 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 have been created to test the server successfully. \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 many 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. 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 in Erlang. Threads in a Linux system, for example, are treated much like operating system processes in different systems. Due to the size of the 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 prototype 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 permanent processes running at all times. The constantly running processes in the GGS prototype 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 represent 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 from which these messages can be retrieved. \end_layout \begin_layout Standard Processes in Erlang are also called \emph on Light Weight Processes \emph default because they 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 makes 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 not pose a big problem. \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 GDL VMs \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 interact 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. This behaviour is the most widely used one in the GGS prototype. 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 In order to understand 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 fail 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. In order 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, however 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. 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. \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, namely 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 in order 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 headers section. The headers section is followed by a data section. In the headers 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 headers section, however the data section must always follow directly after the headers 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 in case 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 a 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 headers 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 relations between each player and table, therefore it is used to perform lookups on tables and players to find out which ones that 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 before 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 an 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 are potentially harmful is not stored in the backup process. \end_layout \begin_layout Standard On 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 piece in the game. \end_layout \begin_layout Standard Each table is associated with a game VM. The actions sent to a table is 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 it was associated with before 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. The game VM is started by the table module. The table module hands over a token used for identification to the game VM during initialization. During initialization a new VM instance and various objects associated to the VM instance are created. Callbacks to Erlang are registered into the VM and the source code of a game is loaded into the VM, finally 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. 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 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 \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 before 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 is 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 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. 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 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 Game data will not be lost when a game is stopped or has gone down for any reason. 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 \noun on world \noun default and \noun on Localstorage \noun default . The \noun on World \noun default 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 \noun on Localstorage \noun default contains data independent of the game state. Game resources, constants and global variables are all examples of data that reside within the \noun on Localstorage \noun default . To store a value within the database, not only is the table token and the name of the namespace required, but an unique key so that the value can be successfully retrieved or modified later. The key is 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 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" \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 which 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 \noun on world \noun default , \noun on players \emph default \noun default and \emph on \noun on localStorage \emph default \noun 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 \begin_inset ERT status open \begin_layout Plain Layout { \backslash tt GGS.sendCommand(player \backslash _id, command, args)} \end_layout \end_inset \emph default and \emph on \begin_inset ERT status open \begin_layout Plain Layout { \backslash tt GGS.sendCommandToAll(command, args)} \end_layout \end_inset \emph default . The localStorage is a convenient way to store global data and other variables separated from the game state. Unique ids called game tokens are generated for hosted games so that they are not mixed up. \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 \noun on localStorage \noun default and \noun on world \noun default 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 \noun on localStorage \noun default is provided, thus the \noun on localStorage \noun default must be connected to the GGS object, this can be seen in line 5. \end_layout \begin_layout Standard Both the \noun on GGS \noun default and \noun on localStorage \noun default 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 \noun on localStorage \noun default 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 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 'reliable system' is defined by the IEEE as \end_layout \begin_layout Quotation A system with the ability of a system or component to perform its required functions under stated conditions for a specified period of time \begin_inset CommandInset citation LatexCommand citet key "ieee_90" \end_inset . \end_layout \begin_layout Standard There are some tools for creating reliable applications built in to Erlang: \begin_inset ERT status collapsed \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 \series bold Links between processes \series default . 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 \series bold Transparent distribution over a network of processors \series default . 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 \series bold Hot code replacements \series default . 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 open \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 \begin_inset CommandInset label LatexCommand label name "fig:The-supervisor-structure" \end_inset 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. In the case of the GGS, a process misbehaving most commonly triggers a restart of the faulting process. \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, this is how it makes decisions. \end_layout \begin_layout Standard In Erlang, there is a simple version of supervisors. No state of the processes being supervised is inspected. There is, however 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, etc. \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, the system has been separated into two large supervised parts. An attempt to restart a crashing child separately is made, 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, the nearest supervisor of this child is restarted. This ensures separation of the subsystems so that a crash is as isolated as possible. \end_layout \begin_layout Standard Figure \begin_inset CommandInset ref LatexCommand vref reference "fig:The-supervisor-structure" \end_inset 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 A choice has been made 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. This was not chosen since 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, the backup process is queried 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 This type of redundancy is only implemented in the coordinator process, similar configurations should however be possible for all modules of the GGS. \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 is 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 exists in Erlang which means that with some work it could be implemented into the GGS. \end_layout \begin_layout Section 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 functions 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. \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 Erlang provides a module for unit testing called eunit. Eunit, being a part of OTP, is rich in functionality and well documented, it doesn't however allow any means of testing asynchronous behaviours as opposed to other means of software testing. \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