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+---
+layout: page
+title: "architecture"
+date: 2014-12-18 21:49
+sidebar: false
+comments: true
+sharing: true
+footer: true
+---
+
+<img src='{{ root_url }}/images/ha_architecture.png' style='background-color: white'/>
+
+The core of Home Assistant exists of the following parts.
+
+The <b>Event Bus</b> facilitates the firing and listening of events. This is the beating heart of Home Assistant.
+
+The <b>State Machine</b> keeps track of the states of things. Fires a state_changed event when a state has been changed.
+
+The <b>Service Registry</b> listens on the event bus for call_service events and allows other code to register services.
+
+The <b>Timer</b> 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 the state of device 'Paulus Nexus 5' changes to the 'Home' state:
+      If the sun has set and the lights are not on:
+        Turn on the lights
+
+    In the event that the combined state of all tracked devices changes to 'Not Home':
+      If the lights are on:
+        Turn off the lights
+
+    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.