Retrieving published messages on subscribe after disconnect

[Sam Matthews]

HI, I'm new to MQTT and trying to understand how it works.

I have set up a simple node js chat program. I am connecting to 'test.mosca.io' over websockets.  I can run multiple copies of my browser app and can chat back and forth just fine.

What i can't understand (and I've been plodding through the docs all day) is how  client A should receive published messages from client B when it is disconnected and resubscribes to the topic.

My steps and parameters are as follows.

Client A

 - connect to broker, cleanSession = false

- subscribe to topic 'xyz' with unique client id 'AAA'

Client B

 - connect to broker, cleanSession = false

- subscribe to topic 'xyz' with unique client id 'BBB'

  

Client A send message to Client B - message received

Client B send message to Client A - message received

Client A disconnects from broker, but does not unsubscribe from 'xyz'

Client B send message to Client A

Client A

 - connect to broker, cleanSession = false

- subscribe to topic 'xyz' with the same client id 'AAA'

Message from Client B not appear in MessageArrived.

I think this should be working and I can't see what I've missed :-(

Any help much appreciated....

Sam

[Nicholas O'Leary]

Hi Sam

What QoS are you using to send the messages?

Brokers are not required to store qos0 messages for offline clients.

Nick

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[Karl P]

I'm guessing this is sam from #mqtt this afternoon, who had a pretty identical
problem. He actually fixed it, it was a mixup with client ids vs message ids,
once he had actually fixed client ids, it was working properly for him.

Cheers,
Karl P

On 03/11/2014 06:07 PM, Nicholas O'Leary wrote:
> Hi Sam
>
> What QoS are you using to send the messages?
>
> Brokers are not required to store qos0 messages for offline clients.
>
> Nick
>
> On 11 Mar 2014 18:05, "Sam Matthews" <smatth...@gmail.com

> <mailto:smatth...@gmail.com>> wrote:
>
> HI, I'm new to MQTT and trying to understand how it works.
>
> I have set up a simple node js chat program. I am connecting to

> 'test.mosca.io <http://test.mosca.io>' over websockets. I can run multiple

> <mailto:mqtt+uns...@googlegroups.com>.

> To post to this group, send email to mq...@googlegroups.com

> <mailto:mq...@googlegroups.com>.

> Visit this group at http://groups.google.com/group/mqtt.
> For more options, visit https://groups.google.com/d/optout.
>

> --
> To learn more about MQTT please visit http://mqtt.org
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> You received this message because you are subscribed to the Google Groups "MQ
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[Sam Matthews]

Yes, Hi...l and thank you very much for your help.  The QOS is 1.  

Yes the problem was that I was creating a new client id on each connection, not understanding the significance of persisting it.  After I fixed that it worked well.

I am working on a project for fun, and to learn thoroughly MQTT.  Some concepts I would like to tackle are....

 - MQTT authentication

 - connect over a secure web sockets connection.

 - other types of data that can be sent over the wire - like images.

 - MQTT clients on Android, IOS

... so hopefully I'll be back with more questions.

Thanks,

Sam