How to know the remote device is online using mqtt (as a device simulator I am using MQTTfx). After publishing to a topic I am not getting any acknowledgement message and in the callbacks I am getting the same payload data that is published.
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You can not find out is another MQTT client is online at a protocol level, in fact at a Pub/Sub pattern level a publishing client should not care is 0 or an infinite number of other clients are subscribed to the topic that message is published on.
The callbacks are only an indication that the message has been successfully delivered to the broker not to any of the subscribers. There is no end to end delivery notification (because there may be no subscribers to a given topic).
If you need to know the status of devices then you need to implement this yourself, the usual pattern is to use a status topic with a retained message and the LWT to clear this if the client crashes and setting it manually when it starts up or shuts down cleanly.
hardillb
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