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From "Hiram Chirino (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (APLO-280) Clarification about message groups
Date Mon, 17 Dec 2012 16:45:31 GMT

    [ https://issues.apache.org/jira/browse/APLO-280?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13534058#comment-13534058
] 

Hiram Chirino commented on APLO-280:
------------------------------------

So what apollo does when it 're-balances the groups' is that it:

1) stops dispatching messages for the groups that are being 're-balanced'
2) waits until acks are received for message sent to consumer that are part of the 're-balanced
groups'
3) Enables dispatch of messages to the new consumers.

So I guess it should be "ensures that all messages for the same message group will be sent
to only 1 consumer at time".   

As for 'rebalancing can only happen safely when old consumers stop' I'd like to get more info
about that case.  Without a good rebalancing strategy, it can be hard to spread the load after
consumers get restarted.


                
> 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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