12

The situation is the following:

There is a client, a publisher, it is not subscribed to any topic. This client has a single topic of its own, and publishes data reguraly to it. But there are not any other clients that are subscribed to this topic.

So this poor and lonely client might be considered as abnormal (Chapter 5.4.8).

Server implementations might monitor Client behavior to detect potential security incidents. For example:

  • [...]

  • Sending undeliverable messages (no subscribers to the topics)

It has no idea about how many clients have subscribed to its topic. So it does not know that it might be considered a client with abnormal behavior.

So what happens with such a client will depend on the server implementation? What are the practices, it should be simply disconnected, but then won't it try to reconnect then?

9

As you say, it depends on the server implementation, especially the QoS of the transmitted message if it is "at least once".

IMHO MQTT is a broadcasting system, not a end-to-end protocol between two machines so we don't absolutely need a subscriber every time we create a subject.

I can post anything (temperature,...) and two months later implement something that will read it, or even remove it and think of something else while my sensor still publishes data.

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