Watching sensors

While the low-level interface can be used to subscribe to updates of specific sensors, it is non-trivial to monitor all sensors of a specific remote server and react appropriately to changes in the sensor list, disconnections, reconnections and so on. Use cases include proxying a remote server (and hence mirroring its sensors) and pushing sensor values into another monitoring system.

To facilitate these use cases, it is possible to add one or more sensor watchers to a client, using add_sensor_watcher(). Sensor watchers can be freely added and removed, but once a sensor watcher has been added to a client (even if it is later removed), one should not manually subscribe to any sensors as this will conflict with the automatic subscription logic.

The sensor watcher must be an instance of AbstractSensorWatcher. This is a low-level base class which should be subclassed to implement callbacks. SensorWatcher is a concrete implementation that processes the callbacks into a stored SensorSet with the mirrored sensors. It can also (via subclassing and overriding rewrite_name()) modify the names of the mirrored sensors as they are created, which may be useful for proxying.

Sensor monitoring operates on a state machine, and state changes are reported via the state_updated() callback, which takes a SyncState.

One can also use synced (an asyncio.Event) to determine the sync status and to wait for sync to be achieved.

By default, a watcher will receive updates for all sensors of the server. However, one can override filter() to select only sensors of interest. Doing so can improve efficiency, since the client will only subscribe to sensors that are of interest to at least one watcher.

There is an example of using a SensorWatcher to proxy the sensors of one another in examples/mirror_sensors.py in the aiokatcp source distribution.