Android Question MQTT and persistence. QOS2

JakeBullet70 · Jul 21, 2016

Here is a question on MQTT persistence. QOS2

I have my b4a shopping list app and 3 family members are using it and it is working fine so far. We use MQTT and the QoS2 so on or offline everybody syncs fine. No problems. Yesterday one of the phones died. So as I understand it, the MQTT messages are still going to be stored for the dead phone?

I have been searching the internet for the last hour trying to find a way to tell MQTT to stop storing messages for a particular client and have come up with nothing.

So any ideas?

JakeBullet70 · Jul 21, 2016

After I wrote this it sort of hit me... I need to store the last time a client was online and if after a certain amount of time passes by if they NEVER reappear then I need to unsubscribe for them.

Sound about right?

Erel · Jul 21, 2016

MQTT doesn't store messages. It store the last message per topic if you set 'retain' to true.

JakeBullet70 said:
Sound about right?

It is better to use the LastWill option. The broker will send the set message when a client is disconnected (without properly closing the connection).

coslad · Jul 21, 2016

Erel you wrong , persistent message are saved into the broker storage until they are confirmed from the client .

http://www.hivemq.com/blog/mqtt-essentials-part-7-persistent-session-queuing-messages

From that page :

How long are messages stored on the broker ?
A often asked question is how long is a session stored on the broker. The easy answer is until the clients comes back online and receives the message. But what happens if a client does not come online for a long time? The constraint for storing messages is often the memory limit of the operating system. There is no standard way on what to do in this scenario. It totally depends on the use case. In HiveMQ we will provide a possibility to manipulate queued message and purge them.

Erel · Jul 22, 2016

It is not so simple. There are many small details that will affect the behavior. The broker is not meant to persist messages for a long time.

Specifically B4A client keeps the default value of clean session which is true so every connection a new session is created.

It can be changed with JavaObject.

rboeck · Jul 22, 2016

What i miss is the feature, to see if a persistent session is waiting. In this situation i dont need to subscribe, because the last session is continued, if there is no session waiting, i have to subscripe for my messages. Can you explain the possibilities with JavaObject and MQTT client?

Erel · Jul 22, 2016

rboeck said:
In this situation i dont need to subscribe

Why does it matter? You should always subscribe after a connection is established.

You can set the clean session parameter with:

B4X:

Dim jo As JavaObject = MqttConnectionOptions
jo.RunMethod("setCleanSession", Array(False))

rboeck · Jul 22, 2016

But we can use:

B4X:

Dim mo As MqttConnectOptions
mo.Initialize(User, Password)
mo.CleanSession=False

Is there a difference?

Erel · Jul 24, 2016

No difference at all. My mistake.

rboeck · Jul 26, 2016

Replay to #7
What makes the difference when i use a pending session? I currently try to make an synchronisation based on mqtt. I publish json files, the topic contains the timestamp.
When i subscribe my topics, i get all messages from the beginning, even when i have sent them with QOS2. Only when i dont subscribe, i am in the last session and get exactly the messages i have to get. But then it could come to the situation, that i client looses completly this session, so i have to subscribe for new.
When i would see, if a session is waiting, i could use it, when no session is waiting, i could subscribe it new.
Because of the timestamps i can currently handle the situation, but i think, that the concept of mqtt with persistent sessions can only realized perfect.
Maybe someone knows, why i get all messages, even with QOS2, when i subscribe new. My client id is also the same.

Erel · Jul 27, 2016

I wouldn't rely on the broker to do the synchronization. What will happen if the broker is restarted? Set one of the clients to act as a "server" and send all the relevant messages to new clients.

JakeBullet70 · Jul 27, 2016

rboeck said:
What makes the difference when i use a pending session? I currently try to make an synchronisation based on mqtt. I publish json files, the topic contains the timestamp.
When i subscribe my topics, i get all messages from the beginning, even when i have sent them with QOS2. Only when i dont subscribe, i am in the last session and get exactly the messages i have to get. But then it could come to the situation, that i client looses completly this session, so i have to subscribe for new.
When i would see, if a session is waiting, i could use it, when no session is waiting, i could subscribe it new.
Because of the timestamps i can currently handle the situation, but i think, that the concept of mqtt with persistent sessions can only realized perfect.
Maybe someone knows, why i get all messages, even with QOS2, when i subscribe new. My client id is also the same.

Mine works fine with QOS2. What broker are you using? Erel's or?

rboeck · Jul 27, 2016

It's cloudmqtt. You never get anything twice? What can be my mistake?

JakeBullet70 · Jul 28, 2016

My GUT feeling says it is your config. But have you tried another broker?
You can use the public mosquitto.org test : tcp://test.mosquitto.org:1883

Or I have a private mosquitto broker if you would rather test there. If so let me know and I will PM you with details.
FYI: I am the only one on my MQTT broker. It is not Erel's but a full install of mosquitto.

rboeck · Jul 28, 2016

Thanks for your ideas; currently i have to check everything an make step by step. Later i will try another provider.
Thanks
Reinhard

Android Question MQTT and persistence. QOS2

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