Ordering MQTT messages across topics

Question

I would like read messages from a MQTT broker ordered (chronologically as written) from more than one topic. For example I have topics (which are published to independently by clients at different rates all with QoS 2):

/foo/a
/foo/b
/foo/c

The messages are in a Persistent Session for a long period using Message Expiry Interval and the subscriber could come and go, on and offline, with any number of messages on each topic not yet read.

When I subscribe to: /foo/#, will I receive messages from topics /foo/a interleaved with messages from /foo/b and /foo/c in the order they were received by the broker ?

The specification on Message Ordering says:

... When a Server processes a message that has been published to an Ordered Topic, it MUST send PUBLISH packets to consumers (for the same Topic and QoS) in the order that they were received from any given Client [MQTT-4.6.0-5 ...

"(for the same Topic and QoS)" suggests ordering can only be guaranteed for the same Topic and QoS. So the answer my question of ordering across topics seems to be undefined.. ?

ralight ralight · Accepted Answer · 2021-04-19T20:15:15

From the Mosquitto broker point of view, if a client has disconnected but has a long session expiry time and has active subscriptions with QoS>0, that is no different to a client that is connected - the session remains open. That means the messages will be delivered according to the ordering requirements in the spec.

This part of the answer covers retained messages only:

My understanding is that message ordering rules only apply for active sessions. That is to say, a client publishes messages and they must be delivered to current consumers only in the same order they were received.

It does not, however, apply to the situation when a client subscribes to a topic filter and receives retained messages. You can get a clue to the intent of the spec there, because the concept of messages being out of order for the same topic and QoS is nonsensical when there is only a single retained message per topic.

Ordering of delivery of retained messages that match a wildcard subscription is undefined. In Mosquitto is it roughly in order of delivery, breadth then depth. This is likely to change in the future to being sorted though.

Ordering MQTT messages across topics

1 Answers