[SOLVED] In NATS MQTT, how do I publish a retained message?

In NATS MQTT, how do I publish a retained message?

MQTT has retained messages: i.e. when someone subscribes to a topic — they immediately receive a message "pinned" to this topic.

I'm using NATS as MQTT server, and my app connects to it as a NATS client. How do I "pin" a retained message?

Solution

In short: you need to add a message to the $MQTT_rmsgs jetstream:

$ nats pub '$MQTT.rmsgs.topic.to.pin.message.to' \
  -H 'Nmqtt-RTopic: topic/to/pin/message/to' \
  -H 'Nmqtt-RFlags: 1' \
  -H 'Nmqtt-ROrigin: fmB75D1M' \
  -H 'Nmqtt-RSource: username' \
  -J message-body

I discovered this by running nats sub '>' while publishing a retained message with an MQTT client.

It revealed a stream of retained messages and a bunch of headers:

$ nats sub '>'
[#7] Received on "$MQTT.rmsgs.topic.to.pin.message.to" with reply "$MQTT.JSA.fmB75D1K.MS.ixCNAR8K06mZh3lnNPegV1"
Nmqtt-RTopic: topic/to/pin/message/to
Nmqtt-RFlags: 1
Nmqtt-ROrigin: fmB75D1K
Nmqtt-RSource: username

message-body


[#10] Received JetStream message: consumer: $MQTT_rmsgs > $MQTT_rmsgs_sfgG3wpEFNySpmlDwqaGGn / subject: $MQTT.rmsgs.device.007.to.retain / delivered: 1 / consumer seq: 12 / stream seq: 28
Nmqtt-RTopic: topic/to/pin/message/to
Nmqtt-RFlags: 1
Nmqtt-ROrigin: fmB75D1K
Nmqtt-RSource: username

message-body

What you see here is someone publishing a message to $MQTT.rmsgs.topic.to.pin.message.to which becomes a pinned message for the topic.to.pin.message.to MQTT topic.

More details of how it works:

P.S. I noticed some lag when I manually pin a message. Maybe this "undocumented" approach has some quirks; not sure yet