Question about retain message management

[Nicolas Jouanin]

Hi,
I have a question concerning the management of retain messages by a broker.
[MQTT-3.3.1-5] says when a client publish a message with retains flag set, that Server MUST store the Application Message and its QoS, so that it can be delivered to future subscribers whose subscriptions match its topic name.
Should the server store this message globally, i.e.. store only the last messages received from any client on each topic, or should its store last messages received per session, ie. store last retained messages received from each client on each topic ?

[Nicholas O'Leary]

Hi, 

It is per topic, not per client session. When a client subscribes to a topic, it receives the most recent retained message on that topic regardless of which client published it.

Nick

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[Nicolas Jouanin]

OK, so each new message received with RETAIN flag replaces any previous message (may be sent from another client) received on that topic.

One more thing I don’t completely understand is the difference between [MQTT-3.3.1-7] (discard retained message on QOS_0) and [MQTT-3.3.1-10] (discard retain on empty payload)

[Nicholas O'Leary]

MQTT-3.3.1-10 describes a mechanism by which any client can clear a retained message on a topic (RETAIN flag set, 0-length payload).

MQTT-3.3.1-7 is covering the case where brokers are not required to persist QoS 0 messages and what that means for QoS 0 retained messages. 

A broker SHOULD store the message, but as is consistent with other QoS 0 messages, the broker is permitted to discard the message if it finds it needs to, in which case there will no longer be a retained message on the topic.

Hope that clarifies it.

[Nicolas Jouanin]

Hi,

Thanks for your reply. 

This is more or less what I understood and how i’ve written my broker implementation. It happens that conformance test with paho are failing and I was told (on paho-dev mailing-list) that it may be due to retain messages not correctly cleared. I needed first to be sure I understood the spec. correctly.

I’ll check my code now.

[Karl Palsson]

-----BEGIN PGP SIGNED MESSAGE-----
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"Nicholas O'Leary" <nick....@gmail.com> wrote:
> MQTT-3.3.1-7 is covering the case where brokers are not required to
> persist
> QoS 0 messages and what that means for QoS 0 retained messages.
> A broker SHOULD store the message, but as is consistent with other QoS 0
> messages, the broker is permitted to discard the message if it finds it
> needs to, in which case there will no longer be a retained message on
> the
> topic.
>
> Hope that clarifies it.

wat? I hadn't ever noticed that one. So, retain is _optional_ for qos
0? I guess this "makes sense" if you consider that qos 0 doesn't have
an ack, so a "conforming" broker could just ignore all qos0 messages too
right? "did my best, sorry that wasn't good enough"

I would consider this to be extremely surprising if I found that retain
didn't retain, are there any brokers that actually did this?

Sincerely,
Karl P

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[Nicholas O'Leary]

Strictly speaking it is optional, but the broker SHOULD store them just as any other QoS message.

I don't know of any brokers that explicitly don't retain qos0 messages. There may be brokers that, if you restart the broker, qos0 messages are forgotten about as they were only held in memory... that would be within the requirements of the spec.

Nick