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From "Lionel Cons (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (APLO-280) Clarification about message groups
Date Tue, 18 Dec 2012 07:02:12 GMT

    [ https://issues.apache.org/jira/browse/APLO-280?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13534713#comment-13534713
] 

Lionel Cons commented on APLO-280:
----------------------------------

Let's imagine the following: two active consumers and six message groups evenly distributed.
Now comes a 3rd consumer. If you indeed rebalance the message groups, each consumer will get
messages from two groups. So some messages that were going to one of the two "old" consumers
will now go to the new one.

So rebalancing breaks the "all messages for the same message group will be sent to the same
consumer" rule.

If you imagine some permanent consumers and others that come and go frequently then the message
groups to consumers bindings will also change frequently.
                
> Clarification about message groups
> ----------------------------------
>
>                 Key: APLO-280
>                 URL: https://issues.apache.org/jira/browse/APLO-280
>             Project: ActiveMQ Apollo
>          Issue Type: Improvement
>            Reporter: Lionel Cons
>
> First, there is a typo in the added doc as it contains "When you another subscription
to a queue".
> Then, it seems there is a contradiction between "ensures that all messages for the same
message group will be sent to the same consumer" and "the broker rebalances the message groups
across consumers".
> Apollo has no way to know if new messages for an already seen message group will continue
to come or not. So, by rebalancing, it may cause new messages of the same group to go to another
consumer.
> IMHO, rebalancing can only happen safely when old consumers stop and only for their "attached"
message groups.

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