[SOLVED] How does persistence session work in MQTT?

How does persistence session work in MQTT?

I use MQTT with QOS 1.

I have a consumer and a producer. They communicate with each other (HiveMQ as a client). They use VerneMQ as a broker. I use a persistence session.

If a consumer is offline and he goes later online, he should become all messages where he was offline. But that works only if consumer is on the other computer and I don't stop a micro service, but I turn off a WiFi and then turn it on. But if I shut down a micro service and then start it again that doesn't work with offline messages.

I think that's why it subscribes again on that topic, if I start a micro service again. Is it the reason? Or not?

UPDATE: I've just tested it without subscribing at the second start of the consumer. That doesn't work either. So subscribing is not the reason why the consumer doesn't get the messages.

Solution

For persistence session, so that you become all messages when you were offline, you need some conditions to be satisfied:

1) turn off CleanStart on connect:

Mqtt5Connect.builder()
            .cleanStart(false)
            .noSessionExpiry()
            .build()

2) collect remaining messages on connect with publishes

  mqttClient.publishes(MqttGlobalPublishFilter.REMAINING) {
        mqtt5Publish -> handleMessage(mqtt5Publish.topic.toString(), mqtt5Publish.payload.decodeContent())
    }

3) QOS 1+

That goes like a Swiss watch.