mosquitto remap local topic (no bridge)

Question

I am using eclipse-mosquitto:1.6.9 and would like to remap incoming messages to topic /registration_app to /$share/registration_app.

In this case IoT devices will be publishing registration messages to /registration_app & backend ap will be listening and processing each request. To scale backend process horizontally, I want to switch to /$share/registration_app (MQTT V5.0) for backend process, but don't want to change original incoming message topic /registration_app.

SO far I can see remapping is available in case of bridging. So would like to know if I can do remapping without bridge.

Answer 1

First, it is really bad practice to start topics with leading /. It adds a null entry to the start of the topic tree and causes problems with things like shared subscriptions (we will get to that next).

Second, I think you have miss understood how shared subscriptions work. Topics that start with $share/ (note the lack of leading /) are used to set up shared subscription groups, so groups of clients can load balance the consuming of messages published to a give topic pattern. You don't do any topic remapping yourself.

To set up a shared subscription group you subscribe to a topic as follows:

$share/<groupname>/<topic>

So in your case assuming a group called backend and messages published to 'registration_app' (again note the lack of leading /) it would be

$share/backend/registration_app

If you use a leading / on the original topic you have to insert a double / in the shared subscription topic to insert the null, so just don't do it.

$share/backend//registration_app