When client is receiving "long processing" topics, it will be disconnect at some point.

[markus-ja]

In my application I want to receive images, do some modifications and save it. To simulate a "worst case" scenarios I put a Thread.Sleep(500) delay in the MqttClient.ApplicationReceiveHandler.

When publishing that topic about 100 times in a for loop, the subscribing client droppes the connection at some point. The server log shows "client xxx has exceeded timeout, disconnecting."

I tried to set the .WithKeepAlivePeriode up to 5 minutes. And then it worked!

It seems, when processing a long receiving queue, the KeepAlive signal could not be processed as long as the topic queue is not dequeued (processed).

[SeppPenner]

Maybe this is related? #755.

As far as I understood it, this is a bug in the server?

[markus-ja]

Not sure if it is a server bug.

I logged the following message comming from MqttClient.TrySendKeepAliveMessagesAsync()

[2019-12-19T14:35:17.6474769Z] [] [10] [MqttClient] [Warning]: MQTT communication exception while sending/receiving keep alive packets.
MQTTnet.Exceptions.MqttCommunicationTimedOutException: Exception of type 'MQTTnet.Exceptions.MqttCommunicationTimedOutException' was thrown.
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MQTTnet.PacketDispatcher.MqttPacketAwaiter`1.<WaitOneAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MQTTnet.Client.MqttClient.<SendAndReceiveAsync>d__46`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at MQTTnet.Client.MqttClient.<TrySendKeepAliveMessagesAsync>d__47.MoveNext()

It seems the server doesn't response to the ping (keep alive) request.
I am using server mosquitto v1.6.7 (win-x86)

[SeppPenner]

I am using server mosquitto v1.6.7 (win-x86)

Maybe Mosquitto has some timeouts set or some size limits?

[ghost]

Hi, This is the same bug as mentioned in #790.

The issue is with the MqttClient. The ping is sent to the server and the server responds (I checked with wireshark) and then the Mqtt client timesout for some reason (the MqttCommunicationTimeOutException).

We resolved this issue by creating a local queue and storing the received messages there. Then mqtt is only responsible for sending and receiving messages while another task is responsible for executing the received message.

[ghc4]

@Rochlop I'm thinking on the very same approach, but... what are you doing when your local queue is full? Maybe unsubscribe the topic while the local worker consume the local queue and then resubscribe?

[ghost]

@ghc4 we currently have a background service consuming from the queue and the queued message is then processed in another thread using Task.Run(), so the background service is always able to consume from the queue. Although, in our use case, we do not have more than maybe 10 messages being run at the same time, so I am not so sure that this is the best use case for you.
Your idea about unsubscribing sounds good. Another way may be using mqtt ack messages to decline a message because the queue is too full or send a message count to the server and only have the server send messages if the current message count on the client is below so many.
Otherwise, it is kind of hard to say what would work best for your use cases as I do not really know much about it ;).