How to tell MQTT to keep messages even if there is no subscriber right now?

3
votes

In MQTT, if you publish to a topic where there is no subscriber for, the message gets dropped.

While this is fine for classic pub/sub messaging, it is not so great for shared subscriptions (which have been introduced in MQTT 5), since this pattern is typically used for some kind of job queue, and you usually don't want to drop jobs just because there is no worker there right now (maybe it just crashed and is restarting).

Is it possible to tell MQTT servers not to drop messages, at least for shared subscriptions, even if there are no subscribers right now? If so, how?

PS: This is not just a persistent session, since I do not want to keep the subscriptions per client. It's more like a "persistent session" that spans multiple clients.

1
mqtt
Did you find a solution for your problem ? If so, pls share it with us. In my case, I am facing this same question and this is a crucial factor for me to choose between MQTT and Kafka. – 2020
Unfortunately, no … in fact, we dropped MQTT, as it had other downsides as well, and instead we built a custom solution for this. – Golo Roden

1 Answers

-1
votes

I don't know if any of the brokers supporting MQTT v5 shared subscriptions support this, but I can foresee a way it could work in a way that is line with the spec and spirit of pub/sub messaging.

A MQTT broker will queue messages for topics subscribed to at QOS 1 or 2 for a client that is currently offline, with a persistent session. So I see no reason why shared subscriptions should be any different. I can see it might be a little bit more technically complicated to implement but should be possible (You would need to treat the shared group as a single session).

That said I think the main focus for shared subscriptions is load balancing, followed by HA. So unless you are running all your shared subscribers on the same machine it should be unlikely that they all fail at the same time.