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home-assistant.github.io/developers/asyncio/index.html
Travis CI 024eeb853f Site updated at 2017-05-20 16:16:06 UTC
2017-05-20 16:16:07 +00:00

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Asynchronous Programming
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<p>On September 29, 2016 we released <a href="https://home-assistant.io/blog/2016/09/29/async-sleepiq-emoncms-stocks/">Home Assistant 0.29</a> as part of our bi-weekly release schedule. This release introduced a complete overhaul of the core spearheaded by <a href="https://github.com/bbangert/">Ben Bangert</a>.</p>
<p>The old core was set up like a “traditional” threaded application. Each resource that was not thread safe (ie. the state of entities) would be protected by a lock. This caused a lot of waiting and potential inconsistency because a task could now end up waiting halfway through its job until some resource got freed.</p>
<p>Our new core is based on an Python’s built-in asyncio module. Instead of having all threads have access to the core API objects, access is now limited to a special thread called the event loop. All components will now schedule themselves as a task to be executed by the event loop. This gives us the guarantee that only one task is executed at once, meaning we no longer need any locks.</p>
<p>The only problem with running everything inside the event loop is when a task is doing blocking I/O, what most third-party Python libraries are doing. For example while requesting new information from a device, the core will stop running until we get a response from the device. To handle this, a task is able to suspend itself until the response is available after which it will be enqueued for the event loop to process the result.</p>
<p>For a task to be able to suspend itself, all code that it calls has to have this capability added. This means in practice that each device integration will need a full rewrite of the library that offers the integration! As this is not something that can be achieved, ever, a 100% backwards compatible API has been added so that no platform will require updating.</p>
<p>The backwards compatible API works by scheduling a task from a different thread and blocking that thread until the task has been processed by the event loop.</p>
<h3><a href="/developers/asyncio_101/">Next step: asyncio 101 »</a></h3>
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