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Is MqttClientPersistence ever needed for subscriber ? [message #1705434]

Sat, 15 August 2015 06:25

Eclipse User

The java doc page for MqttClientPersistance, reads "Represents a persistent data store, used to store outbound and inbound messages while they are in flight". Why is it needed for an inbound message or more specifically why is MqttClientPersistence needed for MQTT subscriber ?

For a MQTT publisher, we need to wait for the PUBACK packet and so usage of MqttClientPersistence is clear.

Assuming, my client is always an MQTT subscriber, independent of QOS, can I always instantiate it as below ?

//instantiate subscriber client with null persistance 
MqttClient subcriberClient = new MqttClient( uri, cientId, null );

Re: Is MqttClientPersistence ever needed for subscriber ? [message #1705548 is a reply to message #1705434]

Mon, 17 August 2015 08:56

Eclipse User

When a client receives a QoS 1 message, it also need to answer with a PUBACK. If you lose Internet connectivity just after receiving a qos1 PUBLISH, the store can be used to later know that there's something that still needs to be PUBACK'd.

Similarly, QoS 2 requires some acks to be sent, and you want the store to make sure you do not incorrectly make the broker assume that something hasn't been received while it actually has.

HTH!

Re: Is MqttClientPersistence ever needed for subscriber ? [message #1705559 is a reply to message #1705548]

Mon, 17 August 2015 09:32

Eclipse User

Considerations:

1) persistence is only needed for QoS 1 and 2 messages

2) if the client connects clean session=true, then persistence does not matter because all state is deleted if/when the connection is broken

3) disk persistence is only needed if a) you connect clean session=false and b) if you want to be able to delete the client object (by stopping the application for instance) and restart the application in the same state by recreating the client object. In that case, the client object state information is read from disk.

It doesn't matter whether you are publishing or subscribing, these hold true both ways.

Re: Is MqttClientPersistence ever needed for subscriber ? [message #1705632 is a reply to message #1705434]

Tue, 18 August 2015 01:02

Eclipse User

Thanks for both replies. It helps.

A probable related query - please tell if this has to go in a separate post.
Does the clean_session flag matter for a mqtt publisher? In a publisher, the only time, I should set clean_session to false will be to receive the acknowledgments like PUBACK when a connection gets broken just after publishing a message ?

Re: Is MqttClientPersistence ever needed for subscriber ? [message #1705707 is a reply to message #1705559]

Tue, 18 August 2015 09:03

Eclipse User

Thanks for the additional point on disk persistence. I was wondering why it was needed.

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