[OASIS Issue Tracker] (MQTT-234) Shared Subscriptions

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

    [ https://issues.oasis-open.org/browse/MQTT-234?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=61792#comment-61792 ] 

Ken Borgendale commented on MQTT-234:
-------------------------------------

If we want to keep the current charter restriction which does not allow us to define the meaning of the topic space, I would propose an alternate encoding:

A shared subscription is indicated by one of the currently unused bits in the SUBSCRIBE packet (along with the QoS).  If this bit is set, the topic filter is the share name, and the actual topic starts after the first slash in the topic filter.

A few more comments about shared subscriptions:
-  Shared subscriptions should be considered relative to MQTT-276 (large scale systems) and we should look at whether this is optional to implement.  It is probably not appropriate to either small server or highly distributed servers.
- We need to document the behavior relative to MQTT-235 (noLocal).  JMS for instance states that noLocal is ignored for shared subscriptions.
- While I would much rather never bring up MQTT-217 (overlapping subscriptions) again, any guidance about overlapping subscriptions will need to take shared subscriptions into account and creates even more murk in this area.
- I am concerned that in the description above we imply an implementation of shared subscriptions (how messages are distributed).  We should be careful not to do this in the spec.

> Shared Subscriptions
> --------------------
>
>                 Key: MQTT-234
>                 URL: https://issues.oasis-open.org/browse/MQTT-234
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: New Feature
>          Components: futures
>    Affects Versions: 3.1.1
>            Reporter: Andrew Banks
>            Priority: Critical
>
> Shared Subscriptions
> --------------------
> Shared subscriptions allow a set of clients to share the consumption of messages
> produced in response to a subscription. The set of subscribers is created by the use
> of a common share name. Shared subscriptions provide a facility similar to point to 
> point messaging in that the processing of messages is not limited to the availability 
> or capacity of a single consumer, the share name is analogous to the queue name in 
> that it names the list of messages to be processed.
> I suggest that a shared subscription be defined by the topic filter syntax:
>   $share:shareName:topicFilter
> - This specialised topic filter takes the place of the normal topic filter in the 
>   subscribe packet. 
> - The "$share:" prefix uses a reserved $ name and will therefore not cause a conflict
>   with other topic filters.
> - shareName is a string which labels the shared subscription and must not contain any 
>   ":" characters.
> - TopicFilter follows the existing topic filter rules.
> - Subscribing clients will be a member of the share only if both the shareName and
>   topicFilter are identical. So, a different shareName with the same topicFiler 
>   will create a different share. Similarly a shareName followed by a different topic
>   filter will create separate share.
> - Servers may decline to support shared subscriptions by not accepting subscribe
>   packets containing topic filters starting with the "$share:" prefix. However, if 
>   they do accept the "$share:" prefix they must follow this definition.
> - The subscribing clients may make both non durable and durable subscriptions by 
>   setting clean session true or false. The shared subscription ceases to exist if    
>   the number of clients subscribed to it reaches zero. If the shared subscription
>   ceases to exist the messages queued for processing are deleted by the server as 
>   with an non shared subscription.
> - A good server implementation will avoid allocating messages to subscribers with 
>   durable subscriptions while the client is disconnected. Instead the messages will be 
>   allocated when a suitable client connects. 
> - A Qos=1 message should be reallocated to another client sharing the same subscription
>   if the network connection to the client is lost before the PUBACK is received.
>   Qos=0 and Qos=2 messages must not be reallocated. 



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