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

    [ http://tools.oasis-open.org/issues/browse/MQTT-16?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=34140#action_34140 ] 

Raphael Cohen commented on MQTT-16:

"I love that I can type it into any subscription client on any command line on any OS without worrying ..."

I can't ever see this being an issue. Any client that lets you send command messages isn't going to be the thinnest veneer over the protocol. Sure, a good design would be for a command line client might look like telnet to an user - but it can't be a veneer. Why?

Well, if you're going to start sending command messages this way then I'm going to want make damn sure as a broker vendor that they're at least signed and, ideally, encrypted.  As Alex Kritikos has pointed out elsewhere, it isn't enough to just have authentication on connection, but also to be aware that security profiles are needed around topics. Once MQTT takes off and messages are bridged, routed or forwarded, even silently, then this becomes critical.

So any client may very well have a prompt to which you type, and then type a command, but how that gets sent, and a reply comes back, would be under the covers. So arguments about 'typing', are, frankly, moot. No one bar a network engineer is going to be running raw.

If you really want a telnet like veneer, then STOMP makes a lot more sense as a choice of protocol.

"how Unicode is represented".

This isn't a problem. We're using an UTF-8 representation. The choice of special characters is deliberate to avoid the naive mistakes of XML1.0 encoding and the like - which have created issues which have stayed with us in the data integration space for a long time (where transparent encoding is nearly impossible due to a lack of ability to encode any code point in any token kind).

"Anyone dealing with an existing broker already has to deal with the fact that $SYS is special cased."

This is incorrect. $SYS isn't part of any standard at the moment, and isn't part of nearly all MQ brokers that exist today that might support MQTT.

That said, there are existing implementations that use $SYS, and so reserving $ makes some sense because it reflects the spirit of the TC's charter, even though it significantly interferes with topic hierarchies which want to use currency symbols for brevity and efficiency - stock trading comes to mind...

> 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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