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


    [ https://issues.oasis-open.org/browse/MQTT-234?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=62452#comment-62452 ] 

Andrew Banks commented on MQTT-234:
-----------------------------------

I prefer the spelling of the share to be $share/group/topicfilter because it is obvious and simpler. Both $share and $$share could in principle clash with an existing usage..

One justification for the nolocal  flag is for bridging, it has the same use with shared subscriptions. A broker would need to make a shared subscription to its peers and specify the nolocal behaviour if it wishes to offload messages to its peers. Incidentally, the nolocal flag does not prevent looping messages in brokers with multiple interconnections, but this is true for both shared and non shared proxy subscriptions.

As with two non shared subscriptions that overlay each other, it is likely to be an application error if a client creates a shared subscription that overlays an non shared one. This is hard to detect when wildcards are used and hence hard to prohibit, so I suggest that we don't prohibit it.

> 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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