Publish from MessageReceivedHandler doesn't work

When I Publish message from handler

client.UseApplicationMessageReceivedHandler(async e => { await client.PublishAsync(message); });

I am getting MqttCommunicationTimedOutException from MqttPacketAwaiter before receiving acknowledgement that message was Published.

Despite I have set very long communication timeout - it is throwed much earlier. I am using QOS 1.

Workaround that we’ve currently decided to use - to publish asynchronously:

client.UseApplicationMessageReceivedHandler(e => { Task.Run(async () => await client.PublishAsync(message)); });

[chkr1011]

You can use a second client if that is OK in your UseCase (new connection, new client ID etc.). The problem is that there are different use cases out there. If you specify that a message is only processed successfully if a another message was delivered (including ACK) then there is a dead lock.

I personally have a separate queue in my apps (BlockingCollection) and a different worker thread. So I am never blocked but I always send ACK even if the real operation was not performed yet. To detect that I have some more topics or payloads which contain some custom data which indicates a success.

I also thought about adding an internal queue for PUBLISH packets so that other packages like some ACK will be still processed but the problem is that this queue will grow and grow and maybe the receive speed is higher than processing speed so that the queue will grow forever. Adding a limit for that queue may lead to the same dead lock at some point.

[chkr1011]

From MQTT spec:

1 The receiver is not required to complete delivery of the Application Message before sending the PUBACK. When its original sender receives the PUBACK packet, ownership of the Application Message is transferred to the receiver.

It just mentioned “ownership”. So it does not include a full blown business logic. This makes sense to me because we are talking about a telemetry transport protocol only. Not a full blown RPC protocol like SOAP etc.