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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=60726#comment-60726 ] 

Ken Borgendale commented on MQTT-276:

The other feature of MQTT which has significant problems for distributed servers is retained messages.  This is especially true if an attempt is made to make the retained messages sent at subscribe be those which existed just before the first message is sent on the subscription. 

In the case of both QoS=2 and retained messages, there are two problems:
- Complexity of server implementation
- Performance cost of using the function

The performance cost of both of these is insignificant if they are not used by clients.  Thus the "Common alternatives" suggested are basically to use QoS=1 rather than QoS=2.  That would put the requirement on the client to handle duplication, and a mismatch of client and server state will not cause future message loss.

You state that for QoS=2 the server state must be replicated across all nodes.  In the common case that a TCP connection stays in affinity with a server for its lifetime, you only need to migrate the server state associated with a clientID (including the inflight messages) at the point that that clientID connects on another server.  Either stable network connections or a bias to reconnect on the same server will therefore decrease the effect of this on server performance.

> 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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