Proposal to change subscription administration to prevent missing messaging while not overloading plugin

[avanlievenoogen]

See comment at other pull request: https://github.com/iuriaranda/signalk-mqtt-bridge/pull/4#issuecomment-1485344917

[iuriaranda]

Hey @avanlievenoogen , first of all thanks for the contribution.

About your changes, I see two problems:

• the initial idea was to have a very flexible subscription mechanism, that mimics how the Venus MQTT integration works (as explained in the README). As of now this mechanism allows users to subscribe to topics like "vessels/self/electrical/batteries/+/current" or "vessels/+/navigation/#", which is not possible at all with the .getBus(topic) method, so we would be loosing an important feature there.
• the "delta overload issue" (for lack of a better term) could potentially cause problems in the connection between the plugin and the MQTT broker, rather than in the plugin itself, and it would only occur with the deltas on topics already subscribed, since the non-subscribed deltas won't hit MQTT anyways. And this specific situation is not prevented in the proposed implementation.

As I mentioned in the other PR, I ported the debounceImmediate function in particular from the signalk-mqtt-gw plugin, and I'm not 100% sure that the original intention still applies here. So with that in mind, and considering it's currently doing more harm than good, I'm going to remove the debounceImmediate.

If we then have overload problems, we could add throttling as an option in the plugin configuration page, or find a buffering mechanism to store deltas and send them in batches to MQTT. But I wouldn't waste efforts right now on a problem that might not even exist.

[avanlievenoogen]

Hi @iuriaranda , thanks for taking the time to look at my proposal. I agree with your rational and I recognize that we would loose flexibility. It's too bad that the getBus doesn't allow wildcards which forces you to make subscription administration functionality in the plugin itself which I think would be better placed inside the server. If all the active plugins have to check all the Delta's themselves it feels like a waste.