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retained message

A message the broker keeps and hands to anyone who subscribes later, so they get the last known value at once.

A publish with the retain flag set replaces the broker's stored message for that topic; every new matching subscription immediately receives it with retain=1, and a retained publish with an empty payload deletes it.

Examples

  • The sensor publishes the fan state on retained to gh1/dev-a1/state/fan at 8:20; a dashboard subscribing at 9:00 gets on at once.

  • Clearing a retained value needs a null payload, mosquitto_pub -t gh1/dev-a1/status -r -n, not the text "".

See also

last will state topic
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