As I understand, MQTT QoS levels are applicable on the connection between Publisher and Broker or Broker and Subscriber. QoS levels are nothing to do with the connection between Publisher and Subscriber directly. That means, a Publisher can't be sure that if a specific Subscriber received the message or not. The only thing the Publisher know that the Broker has received the message if appropriate QoS level is provided. Is that true?

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There is nothing in the MQTT protocol for end to end delivery notification.

This is because when a message is published there may be

  • 0 clients subscribed to the topic
  • N clients subscribed to the topic
  • N offline clients with a persistent subscription to the topic

This means that end to end notification would have to handle all 3 of the use cases, where a notification could:

  • never come
  • come from 1000s of client
  • come from some clients then some more at any time in the future

If you need end to end then you need to publish a second acknowledgement message from the receiving client.

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