[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=34369#action_34369 ] 

Nick O'Leary commented on MQTT-71:
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> But if you're a broker receiving a QoS2, you get the message, send an ack, get an ack back. At that point, can't the acks from the client be in a different order? 

Only if the client has gone out of its way to reorder the acks it sends. In the QoS2 flow, when the client receives the PUBREC in response to the PUBLISH, the _only_ action it needs to take is to send the PUBREL. There is no good reason for a client to randomly delay responding to the PUBREC.

On the server side, it does need to store some amount of state about the pending messages. But this does not need to be an ordered list on the principle that the acks flow in order so messages are released/delivered in order.

> 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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