Updated the developer documentation

source/developers/architecture.markdown

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+---
+layout: page
+title: "Architecture"
+date: 2014-12-18 21:49
+sidebar: false
+comments: true
+sharing: true
+footer: true
+---
+
+<a href='{{ root_url }}/images/ha_architecture.png'>
+  <img src='{{ root_url }}/images/ha_architecture.png' class='no-shadow' />
+</a>
+
+The core of Home Assistant exists of the following parts.
+
+The **Event Bus** facilitates the firing and listening of events. This is the beating heart of Home Assistant.
+
+The **State Machine** keeps track of the states of things. Fires a state_changed event when a state has been changed.
+
+The **Service Registry** listens on the event bus for call_service events and allows other code to register services.
+
+The **Timer** will send every 10 seconds a time_changed event on the event bus.
+
+Take for example the device_tracker component. This component is responsible for keeping track which devices are home. It checks which devices are home every time a time_changed event is fired on the event bus. It will then update the state machine with the information for each device.
+
+This setup allows us to create simple yet powerful logic for controlling your home:
+
+    In the event that device 'Paulus Nexus 5' changes to the 'Home' state:
+      If the sun has set and the lights are not on:
+        Turn on the lights
+
+<!-- comment to seperate markdown blockquotes -->
+
+    In the event that the combined state of all tracked devices changes to 'Not Home':
+      If the lights are on:
+        Turn off the lights
+
+<!-- comment to seperate markdown blockquotes -->
+
+    In the event of the sun setting:
+      If the lights are off and the combined state of all tracked device equals 'Home':
+        Turn on the lights
+
+By using the Bus as a central communication hub between components it is easy to replace components or add functionality. If you would want to change the way devices are detected you only have to write a component that updates the device states in the State Machine.
+
+## Multiple connected instances
+
+Home Assistant supports running multiple synchronzied instances using a master-slave model. Slaves forward all local events fired and states set to the master instance which will then replicate it to each slave.
+
+<a href='{{ root_url }}/images/architecture-remote.png'>
+  <img src='{{ root_url }}/images/architecture-remote.png' class='no-shadow' />
+</a>
+
+A slave instance can be started with the following code and has the same support for components as a master-instance.
+
+```python
+import homeassistant.remote as remote
+import homeassistant.components.http as http
+
+remote_api = remote.API("remote_host_or_ip", "remote_api_password")
+
+hass = remote.HomeAssistant(remote_api)
+
+http.setup(hass, "my_local_api_password")
+
+hass.start()
+hass.block_till_stopped()
+```
+
+<div class='note'><p class='title'>Note</p><p class='content'>
+Because each slave maintains its own ServiceRegistry it is possible to have multiple slaves respond to one service call.
+</p></div>