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Subject: RE: [mqtt-comment] Contradictory statements on Message Expiry and Retained messages

That is certainly a reasonable behaviour, but isnât specifically about the case of messages being queued to being delivered to a particular client. It is defining what the meaning of the Message Expiry Interval is in a PUBLISH message, which may be queued for delivery to a client immediately, or retained for future delivery to a client, or both. If what you describe is the intended Server behaviour then the description of the Message Expiry Interval property of a PUBLISH packet should be clearer as to what behaviour is expected, both when that message is immediately matched to a current client subscription, and when it is retained for future delivery.


I note that the statement in is captured as a mandatory normative statement (MQTT-3.3.2-5), whereas the statement in relating to Retain is not.


Again, I do no dispute that the behaviours you suggest are not reasonable and useful, and probably what was intended, but I do feel that the spec is weak in clearly defining those behaviours for a Server implementation.


From: Roger Light <roger@atchoo.org>
Sent: Thursday, June 4, 2020 4:51 PM
To: David Horton <David.Horton@solace.com>
Subject: Re: [mqtt-comment] Contradictory statements on Message Expiry and Retained messages


Hi David,

My understanding is that those statements cover entirely different situations. is talking about messages that are queued to be delivered to
a client. So you may have a client that connects with a large session
expiry interval, makes a QoS>0 subscription, then disconnects. If
messages are published that match the client subscription they can be
held in a queue for that client for when it reconnects. If the message
has expiry set on it, then the message may expire before the client
reconnects. Whether that message was set to be retained is of no
bearing here. is talking about a message that was set as retained, and will
hence be sent to any clients that subscribe to the topic in question.
Once the message expires, it would be deleted. If a new retained
message comes along for the same topic that also has a message expiry,
then the old message will be removed as normal, and the new message
stored as that retained message with a new expiry time.

Message expiry should apply to retained messages exactly so it is easy
for a publisher to control when a message is too old without having to
publish empty messages to the topic.



On Thu, 4 Jun 2020 at 21:30, David Horton <David.Horton@solace.com> wrote:
> The MQTT 5.0 specs has the following 2 statements, which I feel are contradictory or at least misleading:
> In Retain:
> If the current retained message for a Topic expires, it is discarded and there will be no retained message for that topic.
> In Message Expiry Interval:
> If the Message Expiry Interval has passed and the Server has not managed to start onward delivery to a matching subscriber, then it MUST delete the copy of the message for that subscriber.
> The implication from is that Message Expiry applies to retained messages. Yet in suggests messages expire only when they have been selected for delivery to a certain individual subscriber. How then can a retained message, which is held outside of the state of any particular session, expire?
> I can see the possible value in retained messages expiring when they become too old, but why would this be tied to their having been delivered to any particular Client? Does this mean that a retained messageâs expiry timer gets restarted every time it is delivered to a new Client? That doesnât make much sense. Surely it is up to the publisher of the retained message to refresh it periodically if that is what is desired, or remove it when it is too old by sending an empty message to the same topic.
> I feel the statement in is simply in error, and retained messages should not be exposed to Message Expiry.

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