MQTT binding why publish and subscribe of the same topic

[jvermillard]

According to https://www.openmobilealliance.org/release/LightweightM2M/V1_2_1-20221209-A/HTML-Version/OMA-TS-LightweightM2M_Transport-V1_2_1-20221209-A.html#8-0-8-MQTT-Transport-Binding

A LwM2M Server subscribes to "{PREFIX}/lwm2m/rd/#" to receive messages from LwM2M Clients. A LwM2M Server publishes responses to a specific endpoint, {ENDPOINT}, via "{PREFIX}/lwm2m/rd/{ENDPOINT}".

If you structure your topic like this, the server will receive an echo the response message it publishes to the client

Server subscribe to /lwm2m/rd/#
Device A publishes a registration on /lwm2m/rd/A, the server receives the request, and publishes the response on /lwm2m/rd/A

Since the server is subscribed to the wildcard/lwm2m/rd/# it will receive an echo of it's published message.

Is it expected? This is inefficient, because the server will need to discard this message and also the broker will deliver 2 messages in place of 1

[itroniot]

You are correct on the wildcard subscription /lwm2m/rd/# and the echo and inefficiency. There are many MQTT broker implementations that can set the subscriber to ignore the messages they publish, but this is not in their spec. If you have a proposal for a generic pub/sub topic structure without the echo, we would be happy to consider it for the spec.