[OASIS Issue Tracker] Commented: (MQTT-16) Reserve $SYS topic name

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    [ http://tools.oasis-open.org/issues/browse/MQTT-16?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=34475#action_34475 ] 

Richard Coppen commented on MQTT-16:
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I discussed the revised proposal with Peter following the scrub call and think we need to rework this to keep within the scope of our charter.

In the "out of scope" section, the charter states "The TC will not identify MQTT topics nor prescribe any mechanism or convention for classification of MQTT topics or topic spaces."

This probably rules out delivering an external 'flexible' reference to elaborate the structure of $ in 3.1.1. However, I think we can still reserve $ and provide clarity on implementation specific adoption of $SYS/. This pretty much brings us back to the original proposal.

1. Applications MUST not define any topic names that start with a leading $ character
2. MQTT server implementations MAY define topic names that start with the the string "$SYS/"
3. MQTT server implementations MUST NOT define any other topics whose names start with a leading $ character.

The non-normative examples also hold, but we still need to agree on the correct behavior for subscriptions starting with #

This has an impact for subscribe operations. There are at least two options:
(1) If a topic starts with a $, then it's effectively a different topic tree and subscriptions to # do not include $.
(2) Add non-normative content to subscribe suggesting that a suitable authorization policy may apply to the $ component of the topic tree

> Reserve $SYS topic name
> -----------------------
>
>                 Key: MQTT-16
>                 URL: http://tools.oasis-open.org/issues/browse/MQTT-16
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: core
>    Affects Versions: 3.1.1
>            Reporter: David Locke
>             Fix For: 3.1.1
>
>
> Many MQTT implementations today use $SYS as the head of a topic string for use in administration  (configuration, operations, monitoring).   
> Can the $SYS or a similar topic name be reserved for future use ?   As $SYS is already used by a number of servers using a different top level topic would avoid conflicts. 
> Would be good to collect ideas for a $SYS topic space.  These ideas could initially go into a non-normative document but with potential for inclusion in a future spec.  The intent being servers can implement  some aspects of administration and monitoring in a consistent way
> Some examples:
> - Admin action like "unretain" a retained topic   e.g. $SYS/topic/unretain    (payload is the topic to unretain)
> - Client wishes to query what subscriptions it has made    e.g. $SYS/client/getsubs  (topic is clientid)
> - Client wishes to query how many messages are queued up awaiting delivery e.g. $SYS/client/getNoQueuedMessages  
> - Query the  state of the MQTT server e.g. $SYS/server/getStatus 
> These examples should not be taken literally they are purely to show the type of things a $SYS topic space could be used for.

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