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https://issues.apache.org/jira/browse/AMQ-7129?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Christopher L. Shannon resolved AMQ-7129.
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Resolution: Fixed
> Durable subscription messages can be orphaned when using individual ack mode
> with KahaDB
> ----------------------------------------------------------------------------------------
>
> Key: AMQ-7129
> URL: https://issues.apache.org/jira/browse/AMQ-7129
> Project: ActiveMQ
> Issue Type: Bug
> Components: KahaDB
> Affects Versions: 5.15.8
> Reporter: Christopher L. Shannon
> Assignee: Christopher L. Shannon
> Priority: Major
> Fix For: 5.16.0, 5.15.9
>
>
> While not a common use case it is possible for a client to use individual
> acknowledge mode on a durable subscription. This allows the following
> scenario:
> # durable subscription is created on a topic
> # 10 messages are published to the topic
> # durable subscription consumes the messages but only acks the 5th message
> # Broker is restarted
> What should happen is there should be 9 messages left that are recovered when
> the subscription comes back online. What actually happens though is that
> messages 1-4 are stuck forever because the KahaDB index will not load them on
> activation. The problem is that the lackAck value that is stored in the index
> is for message 5 so it starts on message 5 for recovery and ignores messages
> 1-4 even though those values are still tracked as part of the ackPositions
> sequence set.
> The solution is that when the subscription is loaded to check if there are
> any ackPositions for that subscription that are earlier than the lastAck
> value and if they are we need to reset the cursor to the first ackPosition.
> Then we need to verify on recovery that any message iterated over actually
> exists as part of the sequence set so we don't load duplicates (this scenario
> happens if multiple subscriptions exist on the topic)
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