messaging - How to target specific workers with shared subscriptions in MQTT 5?

Question

One of the new features of MQTT 5 is the shared subscriptions feature, which allows client-side load balancing between multiple workers, so that multiple workers can be responsible for handling messages, but every message is only ever sent to a single server.

By default, this works with a round-robin approach, but I am in the need of a slightly more advanced scenario:

What I want is some kind of routing, so that one of the messages' properties gets used as some kind of routing key. I.e., I want multiple workers to be responsible for the messages, but all messages with value X in their routing key property should always go to the same worker, and all messages with Y should do as well. The workers for X and Y may be different, but all messages with X should always go to the same one.

Question 1: Is this even possible with MQTT 5? If so, what is the term I need to look for? I tried googling for this, but wasn't really successful (mainly, I guess, because I don't know exactly what to look for).

Now, supposed this is possible: How can I then handle cases where nodes join or leave? Then I still want only a single node to be responsible, so it would be great if the assignment was not statically, but could be adjusted dynamically (or even better, would adjust itself automatically). However, what I strictly need to avoid is that two messages with X ever go to different servers at the same time.

Question 2: Supposed, this is not possible – what alternatives do I have to MQTT 5?

Answer 1

You don't at a protocol level. That is the whole point of a shared subscription to distribute the incoming messages evenly across all the subscribers.

This also goes against the pub/sub paradigm, that messages are published to a topic not an individual subscriber.

If you want to route messages differently publish them to different topics. There is nothing to stop you republishing a message on a separate topic based on it's meta data once it's been received by a client if needed.