mqtt message delivery , after connection lost

[titov]

I am just getting started with mqtt , I have an application which sends out data asynchronously.I am using

the asynchronous mqtt paho client for this. I have set qos as 2 and clean session is true. The device in which

I am running my client application is a very limited device which gets disconnected from the data connection

most of the times due to intermittent network outages.So I am testing the reliability of the device to send data once

the connection resumes without ANY data loss.

I have handled connection lost aync callback and it tries to reconnect till it succeeds.

Another thread runs always and keep publishing data , the thread do not care whether the connection 

is ON or not , it just calls the publish method of my mqtt instance.

Here are my connection options.

library used =  mqtt paho client for java ( mqtt-client-0.4.0.jar )

clean session = true

qos = 2

keepalive = 60

My problem is Situation B.

Situation A ( Success ) ( A thread is relentessly publishing data to my async mqtt client instance , it do not care whether the client is connected or not  )

1) Good connectivity , published data is delivered , all good :)

2) NO data connectivity , connect lost callback is not called yet , because the keep alive limit not reached.

3) Thread is still pushing data to the mqtt client instance publish method.

4) I get back connectivity , before keep alive limit , all the pending messages are delivered ! all good.

Situation B ( Failure ) ( A thread is relentessly publishing data to my async mqtt client instance , it do not care whether the client is connected or not  )

1) Good connectivity , published data is delivered , all good :)

2) NO data connectivity , connect lost callback is not called yet , because the keep alive limit not reached.

3) Thread is still pushing data to the mqtt client instance publish method.

4) Still no connectivity , keep alive reached. connection lost is called. 

5) Thread is still pushing data to the mqtt client instance  publish method , only to get exception that "client is not connected"

6) I get back connectivity , the messages after data connectivity loss and before "connection lost" is delivered , good !

Problem --->

7) But the messages after "connection lost" till the connection regained are not anywhere to be seen.Its not delivered.

Is this the way the mqtt spec says  , or is there a technical problem. What I understood from the spec is Qos = 2 ensures data delivery , no matter what , even after the connection is lost , all the pending messages is 

supposed to be delivered once the connection comes back.

I would really appreciate any help on this. Situation B , scenario 7.

[Nicholas O'Leary]

Hi,

when you use clean session = true, then all session state is wiped when the client loses its connection.

From what you describe, you should be using clean session = false to ensure message state is saved.

Regards,

Nick

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[Pablo Lopez]

Hi Titov (and Nicholas)

I'm also a newby in Mqtt, but I haven't even been able to reproduce what you described is the A) case.

I have the same initial parameters as you had (Paho Java, Async client...), but as soon as I have a network outage, the publish method I call throw an exception :

Caused by: Client is not connected (32104)

at org.eclipse.paho.client.mqttv3.internal.ExceptionHelper.createMqttException(ExceptionHelper.java:27)

at org.eclipse.paho.client.mqttv3.internal.ClientComms.sendNoWait(ClientComms.java:132)

at org.eclipse.paho.client.mqttv3.MqttTopic.publish(MqttTopic.java:65)

Could you please help me by providing your (working) code snippet, making the 'ignore if client is connected, publish anyway' work ? Thanks a lot.

Pablo

[Pablo Lopez]

Hi Titov (and Nicholas)

I'm a noob in the Mqtt and Paho world. I have exact same problematic as you had, unreliable network with a set of sensors linked to a Raspberry Pi. The Pis connect to a Mosquitto broker, when they can.

I used Paho with the MqttAsyncClient, but I was not able to reach the A) state you described. When the network goes down,  the publish method ends abruptly with a "Client is not connected (32104)" exception.

Going through the source, I saw that in the end, publish calls ClientComms#sendNoWait(MqttWireMessage message, MqttToken token) which first instruction is to check if isConnected() and throw the exception otherwise.

What am I missing ? Can you please provide your working code snippet, handling network outage ?

Thanks.

Pablo.