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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=60876#comment-60876 ] 

Ken Borgendale commented on MQTT-276:

A server is allowed to reject a PUBLISH for any reason.  The most common reason for this is that the client is not authorized to publish this message, on this topic, with this QoS, which this retain setting, and at this time. You can classify almost anything as not authorized.  I can say the client is not authorized to use any feature which the server has not implemented.

If you can determine at CONNECT time that the server will never accept a publish of certain types, then we could return this information on a CONNACK, but we cannot assume that a client waits for the CONNACK.  This could only also deal with a very small set of possible reasons for rejections such as QoS, retain, or maximum message length.  Specifying something like "the second section of the topic name must equal your clientID" seems like a lot harder thing to communicate.

The original issue raised in this issue was that the server might not want to implement some portion of the spec for servers with complex internal structure.  On the other hand, it might just think that these features were just expensive when used, and therefore limit their usage.  Either of these is possible today, and the only real problem is that there is no good way to communicate back to the client that we have rejected the publish, let alone why we have rejected the 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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