[OASIS Issue Tracker] (MQTT-285) Subscribe options for retained messages

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    [ https://issues.oasis-open.org/browse/MQTT-285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=62921#comment-62921 ] 

Ken Borgendale commented on MQTT-285:
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In general a bridge would want to set both the NoSendRetain and RetainAsPublished.  This means that we do not send the retained messages from a retained source, but we do send them when they match an existing subscription, but we leave the RETAIN flag set as published.  I agree there is some ambiguity in my suggested text about this.  I proposed separate flags as we also have a use case (brought to us by a customer) where they want to monitor changes but do not want to get the whole set of retained messages when the subscribe.

I do see the case for a condition which says send retained messages if a subscription is created, but not if the subscription already exists.  There is a problem with the current Session Present flag in that while it tells you if a session exists, it does not tell you that all of the subscriptions in the session exist.  For instance if the connection was lost while in the process of subscribing in a new session, the session would exist but not all of the subscriptions. 

I agree it would be good to change the SHOULD into a MUST for storing QoS=0 retained messages.  The issue is that as they are QoS=0 we cannot guarantee that we ever got them, we are allowed to discard them at any time, and they will be delivered to the consumer at QoS=0.  Thus I do not see what kind of enforcement we could have of this MUST.




> Subscribe options for retained messages
> ---------------------------------------
>
>                 Key: MQTT-285
>                 URL: https://issues.oasis-open.org/browse/MQTT-285
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: New Feature
>          Components: futures
>    Affects Versions: 5
>            Reporter: Ken Borgendale
>            Assignee: Ken Borgendale
>
> This issue came up when reviewing MQTT-235 (nolocal) as it affects the bridge use case along with others.
> The current setting of the retain flag on PUBLISH and the handling of retained messages on SUBSCRIBE is normally correct when the client really is a subscribing client, it is not correct behavior when the client is going to forward the message on in a bridge or filter use case.
> There are two issues here
> 1. In some case the subscriber does not want to get any messages which are retained at the time of the subscription.
> 2. If the subscriber is going to re-publish the message it needs to know if the messages was published with the retain flag, rather than that it came from a retained source



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