Mqtt when I publish the first message, it does not stay on queue

Question

I publish the message, it does not stay on queue. Once subscribe, the message queue starts to stay. I want to keep the message in the queue even if the user is not subscribed at all. I am using qos = 1.

Answer 1

It is important to remmber that MQTT is a Pub/Sub system not a Message Queuing system.

With MQTT messages will only be queued for offline clients that already have a subscription (at QOS 1 or 2), a new client subscribing to a topic will only receive new messages.

You can use the retained flag to ensure the last message (with a retained flag set) is always delivered to a client when it subscribes to a topic before new messages, but this is a single message.