Service tasks

It is common for a katcp device server to do more than passively respond to requests. This work might be encapsulated in an asyncio.Task, which needs to be started up when the server starts and shut down when the server is stopped.

It’s possible to do this manually (and indeed this was the only option up to version 1.1). However, handling exceptions from these tasks is tricky, and aiokatcp provides some extra support for this. In DeviceServer.start() you can create the task and pass it to DeviceServer.add_service_task(). The server will then do the following:

1. When the server is stopped, the task will be cancelled then awaited. This happens after DeviceServer.on_stop(), so if you need to shut down the task in some other way, you can do so there.

2. If the task throws an exception, the server will immediately be halted. The exception will be re-thrown from DeviceServer.stop() or DeviceServer.join(). Note that a asyncio.CancelledError will not be re-thrown.

3. If the task exits without raising an exception, or by raising asyncio.CancelledError, it is removed from the list of service tasks. It can thus be used for tasks that don’t need to run for the full lifetime of the server, without causing the list of tasks to grow without bound.

The list of active service tasks can be retrieved from DeviceServer.service_tasks.

When using Python 3.8 or later, it is recommended to pass the name parameter to asyncio.create_task(). This will be used in a log message if the task raises an exception.