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Subject: [OASIS Issue Tracker] Commented: (MQTT-19) How Retained publications are deleted.

    [ http://tools.oasis-open.org/issues/browse/MQTT-19?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=33603#action_33603 ] 

Dominik Obermaier commented on MQTT-19:

My thoughts on your points:

1) This would only be the case if it is a message is sent with a zero byte payload *and* the retained message flag set. So I do not see how a client could accidentally delete the retained message on the server because the client has to explicitly set the retained flag. 

2) I personally am not in complete agreement that deleting a retained message is solely an administrative task. I see many use cases for clients deleting a retained message. (let's say a client publishes a retained message to "/warning", so every new subscriber is notified that there is a warning. When there is no warning anymore the client could delete the warning message so newly connected clients do not see the warning message. Perhaps a bit constructed but this is perfectly possible with the current 3.1 specification.) 

However, I second the idea to allow administrators to delete retained messages. I think this should be vendor specific.

3. I think it is a problem of the server vendor to handle possible race conditions in such cases. In fact there are some parts of the MQTT specification which are much harder to implement (like QoS 2 handling) in cases where many servers are connected (like in a MQTT server cluster). 

> How Retained publications are deleted.
> --------------------------------------
>                 Key: MQTT-19
>                 URL: http://tools.oasis-open.org/issues/browse/MQTT-19
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: core
>            Reporter: Andrew Banks
> There specification suggests that a retained message published with a zero byte payload can be used to cause a retained message to be deleted by the server, although it
> leaves this behavior as optional. It is not clear whether the zero length message should be sent to subscribers.

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