Impact of Long Processing Time in messageArrived Callback (MQTTAsync) Can It Cause Disconnects?

[Tidalikk]

Hi all,

I’m using the Eclipse Paho MQTTAsync client in C, and I have a question regarding callback behavior, specifically the messageArrived callback.

In my application, the messageArrived callback can sometimes take a long time to process, could even be minutes, depending on the work being performed. I want to confirm whether this can cause any undesired side effects at the MQTT client level.

More precisely:
1. Does a long-running messageArrived callback block the internal network thread?
2. If the callback takes too long, can this delay keep-alive pings or acknowledgements long enough to trigger a disconnect?
3. Is there a recommended maximum processing time for callbacks?
4. If long processing is expected, is the correct pattern to offload work to a separate thread and return quickly from the callback?

Thanks guys, your library is amazing.

[fpagliughi]

1. Does a long-running messageArrived callback block the internal network thread?

Yes.

2. If the callback takes too long, can this delay keep-alive pings or acknowledgements long enough to trigger a disconnect?

Yes.

3. Is there a recommended maximum processing time for callbacks?

Not really; it depends on the requirements for your application, but the general recommendation is: finish as quickly as possible and don't block. In particular, it's ok to initiate another asynchronous MQTT operation from within a callback, but don't block waiting for it to complete. That will result in a deadlock.

But also keep in mind the answer to the previous question.

4. If long processing is expected, is the correct pattern to offload work to a separate thread and return quickly from the callback?

Yes.