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From "Gary Tully (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (AMQ-4485) Skipped message dispatch with concurrent transacted sends at cursor memory limit
Date Thu, 25 Apr 2013 12:56:15 GMT

     [ https://issues.apache.org/jira/browse/AMQ-4485?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Gary Tully resolved AMQ-4485.
-----------------------------

    Resolution: Fixed

fix in http://svn.apache.org/r1475734

idea is to have a beforeCompletion in a per destination sync that is called with the store
index exclusive lock. This tracks the required cursor update oder. The first afterCompletion
thread that is in order executes all pending updates in the correct order. Out of order completions
queue their work or see it already complete. The batch update of pending work prevents further
contention/race on the queue sendlock.
We may want to introduce blocking for an ordered execution slot if multiple sync after completions
have strict ordered dependencies. For the moment in the broker, after completions are only
 used for cursor updates.
                
> Skipped message dispatch with concurrent transacted sends at cursor memory limit
> --------------------------------------------------------------------------------
>
>                 Key: AMQ-4485
>                 URL: https://issues.apache.org/jira/browse/AMQ-4485
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: Broker
>    Affects Versions: 5.8.0
>            Reporter: Gary Tully
>            Assignee: Gary Tully
>              Labels: dispatch, limit, message, missing, order, skipped, usecache
>             Fix For: 5.9.0
>
>
> With multiple concurrent transacted sends, transaction synchronisation after completions
are used to update the cursors.
> These happen independent of the order that the store is updated, and hence the store
order index.
> When the cache is exhausted, a callback to the store to mark the end of caching assumes
matching order. If scheduling has swapped the order, it is possible to mark the order index
past what is cached and it is possible to skip a dispatch. Alternatively it is possible to
mark too early which results in duplicate dispatch if the audit is disabled or exhausted.
> The senario that exposed this occurrence used concurrent transacted sends to 100 destinations
with slow consumers. Leaving scope for out of order processing and ensuring that the cache
is exhausted.
> Using a large destination memory limit or systemUsage limit or useCache=false policy
entry will avoid this problem. The order is only important when the cache is exhausted.
> In the skipped case, the message appears on the queue but is not consumable, however
it is consumable after a restart.
> The proper fix is to ensure cursors are updated in the same order as the store.

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