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Re: [paho-dev] When to send publish acknowledgements

Hi Al,

if I understand you correctly, this is what I do in the C client.  I send the PUBACK or PUBCOMP (old style QoS 2) after the message handler callback has been invoked, and has returned a good result.

I read that spec statement as referring to the receive maximum limit, making it clear that it applies to PUBLISH packets only, not PUBACKs, PUBRECs, PUBRELs or PUBCOMPs. I do think however there is a question of ordering, that the ack for a message shouldn't be delayed so long that the ack for a subsequently received PUBLISH is sent first, at least not routinely.

(An additional note: I have a request to have two callbacks for the receipt of QoS 2 messages, one for the PUBLISH and one for the PUBREL, to allow a 2-phase commit interaction with, for instance, a database.  I haven't implemented it yet, but I intend to.)

On 30/03/2021 16:55, Allan Stockdill-Mander wrote:
Hi All,
I've had a PR for the v5 golang client that changes the behaviour so that Publish messages are only acknowledged (the puback or pubrec sent) after the associated message handlers have been invoked and returned. I can see a benefit to doing this for the application as you effectively don't need to have incoming message persistence, until you have processed the message if the client dies and reconnects the server should resend any unacked messages. However the spec says in 3.3.4
> The Client MUST NOT delay the sending of any packets other than PUBLISH packets due to having sent Receive Maximum PUBLISH packets without receiving acknowledgements for them [MQTT-3.3.4-8]. The value of Receive Maximum applies only to the current Network Connection.
On the one hand I think this method counts as a delay, on the other I can see the benefit and how can a server determine this behaviour and what impact does it have? If you're on a constrained device with no persistence this is how it would look anyway. Thoughts?
Al Stockdill-Mander
Squad Lead - Network Intelligence - Alchemy
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