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Subject: [OASIS Issue Tracker] (MQTT-276) MQTT enhancements for large scale distributed systems
[ https://issues.oasis-open.org/browse/MQTT-276?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=60739#comment-60739 ]
Ken Borgendale commented on MQTT-276:
-------------------------------------
A server is already allowed to not support a given QoS, in any situation, for this client, for this user, for this topic, or any combination of the above. I understand that the two possible responses when doing so (sending an ACK and not delivering the message, or closing the connection) are not ideal responses. But that is the common problem of a lack of NACK on PUBLISH.
The problem with anything returned on a CONNACK is that the client is not required to wait for the CONNACK before sending other control packets. Therefore any requirement on the client would assume that it is has waited.
Sending a maximum QoS assumes that the server would decide to only preclude high QoS. The server could decide to not support QoS=0 (although admittedly not as likely). It also assumes that the decision is uniform for the connection, and does not vary based on the topic.
It seems sufficient to have a return code on the PUBLISH of something like "QoS not supported". This has the advantage that it can be used either when the server does not support the specified QoS ever, or when for this user and this topic the server does not support this QoS.
We should include a similar return code for retained messages not supported.
We really do need to wait for more work on NACK for publish before we can write the normative text on what error handling a server MUST do. For the error bit added in subscribe, we make the action on error optional, and we might want to do the same for PUBLISH.
> MQTT enhancements for large scale distributed systems
> -----------------------------------------------------
>
> Key: MQTT-276
> URL: https://issues.oasis-open.org/browse/MQTT-276
> Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
> Issue Type: New Feature
> Components: futures
> Affects Versions: 3.1.1
> Reporter: Konstantin Dotchkoff
> Assignee: Konstantin Dotchkoff
> Fix For: 3.1.1
>
>
> As presented during the TC call on 8/6/2015, there are challenges in implementing MQTT in large scale distributed systems with the specific case being QoS 2:
> https://www.oasis-open.org/apps/org/workgroup/mqtt/download.php/56237/MQTT%20QoS2%20Considerations.pptx
> Retained messages were also noted during the call.
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