Sending an initial message after client subscribed

Question

Is there a way to check if a client has initially connected. I want to send a message to a topic if a client has "logged in".

Answer 1

There is no client presence detection built into MQTT at the protocol level.

But there is nothing to stop you building it yourself.

Have the client publish a retained message on a known topic e.g. presence/<client-id> and then set up a Last Will and Testament to either clear the message or publish a different value (e.g. online vs offline). You will also need to explicitly clear it if the client shuts down cleanly.

Other clients can subscribe to presense/+ in order to keep track of which clients are online.