[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=62950#comment-62950 ] 

Jonathan Levell commented on MQTT-285:
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Ken: If the bridge set both flags (as I understand it), retain messages stored in the source system prior to the bridge subscription would not be propagated down the the bridge.

In a system with a static collection of retained messages that I wanted to bridge to a new system I'd expect one of two behaviours:
   a) The bridge copies this static, pre-existing collection of retained messages across the bridge and republishes them at the destination
OR
  b) The bridge wants to copy this static, pre-existing collection of retained messages and populate the retained message "slots" on the destination without publishing them to matching subscriptions.

Option (b) I think is currently done by existing software (like RSMB) but the action at the destination requires special actions outside the current MQTT spec.

As I understand the current two flag proposal, the static pre-existing collection of retained messages would NOT be sent down the bridge? If we change the NoSendRetain flag to NoRetainOnExistingSub then we allow bridges to have behaviour (a) (and still remake their durable subscriptions on each connection) - a separate JIRA would be needed to allow the (b) behaviour...



> 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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