[OASIS Issue Tracker] Commented: (MQTT-71) Are messages order within Topic+Qos or within Qos

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    [ http://tools.oasis-open.org/issues/browse/MQTT-71?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=34342#action_34342 ] 

Raphael Cohen commented on MQTT-71:
-----------------------------------

Good call.

What's the weakest we can get away with? I think it's close to your 'or' - messages with the same QoS SHOULD be ordered. The input spec is correct, but an example or 2 might help.

It's very much going to depend on implementation as to how this is honoured. A typical broker might operate an in-flight window of, say, 4000, messages; it message 5 is delayed, it might then decide to delay acceptance of messages 6-3999 because of that. Or it might release for deliver messages as and when the acks occur, and simply use the in-flight window for buffering. I suspect implementations may be limited in what they can practically do. Most will probably go for the latter design, as it's memory efficient and keeps up a flow of messages.

In the past, the broker implementations I've written would simply release messages to downstream as and when they were settled (ack'ed) upstream. This design makes delivery order entirely at the client's discretion - if it wants ordering preserved, it acks in the order that it sent things... If QoS is zero, then its receipt order.

So I suppose my suggestion is (a) give an example to the input spec language and (b) order solely by QoS, with non-normative caveats (c) that within a QoS 1 or 2 level, messages are ordered as per ack and (d) within QoS 0, ordering is receipt ordering.



> Are messages order within Topic+Qos or within Qos
> -------------------------------------------------
>
>                 Key: MQTT-71
>                 URL: http://tools.oasis-open.org/issues/browse/MQTT-71
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: core
>            Reporter: Andrew Banks
>
> The input specification states 
> •	With an in-flight window greater than 1, message ordering can only be achieved within the QoS level.
> Should we state that 
> messages with the same Qos and Topic Name are delivered in order
> or
> messages with the same Qos regardless of Topic Name are delivered in order

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