Asynchronous Programming

On September 29, 2016 we released Home Assistant 0.29 as part of our bi-weekly release schedule. This release introduced a complete overhaul of the core spearheaded by Ben Bangert.

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.

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.

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.

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.

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.

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