Count subscriptions and make unsubscribe only stop if it matches the amount of subscriptions

[dzek69]

Is your feature request related to a problem? Please describe.
Currently, when I subscribe to a topic more than once it requires only one unsubscribe call to actually stop receiving messages on given topic.
In a large app, that multiple modules may want to listen to some topics on demand, but stop subscription whenever that's not needed anymore.

Describe the solution you'd like
If I subscribe to given topic more than once, it should require exactly the same amount of unsubscribe calls to actually stop for listening to given topic.

Describe alternatives you've considered
Alternative library, if one exists.

Additional context
This is slightly related to #1951 as well, which is another pain point of mqtt.js. If I'd like to create such a wrapper described in the issue - I would bump into problem described here if I'd want to unsubscribe. I'd need to count subscriptions myself and basically wrap the whole library (overridding functions) to make it safe to use with any code.

[robertsLando]

In my opinion such logic should not be handled inside mqtt itself as it's out of the scope of mqtt protocol. My approach when using mqttjs is to always create a wrapper around the client, in this way you can easily keep track of the subscriptions of each topic and implement the logic described above.

[dzek69]

If that's the case I think it should be stated in the README. It's perfectly fine to have a "raw" protocol client, but it must clear that there is a lot of real work to do before this can be used in an app without falling into pitfalls like that.

BTW: Is there any well-known client that wraps mqtt.js?

[robertsLando]

It's perfectly fine to have a "raw" protocol client, but it must clear that there is a lot of real work to do before this can be used in an app without falling into pitfalls like that.

I'm not aware of other clients libraries that work different then this one does. Some allow to attach a subscribe callback but that's the only thing, the one related to subscribe/unsubscribe is really something that is out of the scope of the library, some may want exactly the opposite you are asking and introducing this behaviour would be a breaking change.

BTW: Is there any well-known client that wraps mqtt.js?

I'm not aware of any wrapper I wrote some on my personal projects but it's not a big deal to build one that fits your needs, it's quite easy

[dzek69]

It's quite easy if you know the protocol and the client library so well that you are actually aware of all the limitations. How many details like that are not obvious - I don't know.

[robertsLando]

I use many protocol libraries out there (just to mention some Modbus, Z-Wave, Opcua, M-Bus, S7) and I always had to create a wrapper around all of them and MQTT is the easier one, try to see ZwaveClient one as an example to understand what I mean. I cannot ask z-wave driver author to implement all those features as its focus must be to support all protocol requests then users should develop on top of it.

[robertsLando]

Here an example client: https://github.com/woifes/node-monorepo/blob/develop/packages/mqtt-client/src/Client.ts