[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=34496#action_34496 ] 

Nick O'Leary commented on MQTT-16:
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Following further discussion, here is an updated proposal that covers both the meaning of $-prefixed topics and how subscriptions are handled:

NB: one of the original non-normative examples has been removed: "a client that publishes or subscribes to $foo would be non-compliant". This implies a client would be required to police the topics the driving application tried to use. If, in future versions, addition $-topic names are defined, it would be overly onerous to update all client implementations to add the newly permitted topics. The policing of the topic should be a server-side function.
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Topics beginning with $
  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. 

  non-normative examples: 
  1. A server that defined and accepted subscriptions to $SYS/monitor/clients would be compliant 
  2. A server that defined and accepted publications to $SYS/control/setMaxClients would be compliant 
  3. A server that accepted subscriptions or publications on $foo would be non-compliant. 
  4. A topic name of stock/$price is compliant

Subscription handling
  1. A topic filter that starts with a wildcard character (# or +) does not match topic names that begin with a $ character

  non-normative examples:
  1. A subscription to # will not receive any messages published to a topic beginning with a $
  2. A subscription to +/monitor/clients will not receive any messages published to $SYS/monitor/clients
  3. A subscription to $SYS/# will receive messages published to topics beginning with $SYS/
  4. A subscription to $SYS/monitor/+ will receive messages published to $SYS/monitor/clients
  5. For a client to receive messages from topics that begin with $SYS/ and from topics that don't begin with a $, it must subscribe to both # and $SYS/#
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> 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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