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

[OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org>]

    [ https://issues.oasis-open.org/browse/MQTT-285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=63246#comment-63246 ] 

Jonathan Levell commented on MQTT-285:
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How about having a 2-bit value on the subscribe that says which retained messages the subscribe should deliver:
0 - always send retained
1 - never send retained
2 - only send retained if new subscription 
3 - error (reserved for future use)

Separately we have a extra metadata field on subscribe that if specified means the subscriber wants to know whether the message was originally published as retained. If the subscriber indicates that it wants to know the original value of the retain setting specified by the publisher then when the broker forwards publications that were sent as retained, a new metadata field is added to the publication that indicates "PublishedAsRetained"

> 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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