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https://issues.apache.org/jira/browse/AMQ-9625?focusedWorklogId=944624&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-944624
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ASF GitHub Bot logged work on AMQ-9625:
---------------------------------------
Author: ASF GitHub Bot
Created on: 20/Nov/24 17:26
Start Date: 20/Nov/24 17:26
Worklog Time Spent: 10m
Work Description: cshannon opened a new pull request, #1355:
URL: https://github.com/apache/activemq/pull/1355
Fixes a race condition bug that can lead to a message being missed on
dispatch and stuck on a Queue until restart when caching and
concurrentStoreAndDispatch are enabled on a Queue and the cache becomes
disabled.
This is pretty much impossible to reproduce or test in a unit test so
there's no test for this but the risk of breaking anything is very low because
all this is doing is just waiting for the future to complete to ensure the task
finishes before we continue.
Issue Time Tracking
-------------------
Worklog Id: (was: 944624)
Remaining Estimate: 0h
Time Spent: 10m
> Messages can become stuck on Queues
> -----------------------------------
>
> Key: AMQ-9625
> URL: https://issues.apache.org/jira/browse/AMQ-9625
> Project: ActiveMQ Classic
> Issue Type: Bug
> Affects Versions: 5.18.6, 6.1.4
> Reporter: Christopher L. Shannon
> Assignee: Christopher L. Shannon
> Priority: Major
> Fix For: 6.2.0, 5.18.7, 6.1.5
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> For the last several years I have been occasionaly seeing "stuck" messages
> that appear on queues that will not be dispatched until after a broker
> restart. The bug is the same as described in
> https://issues.apache.org/jira/browse/AMQ-2955 (the root cause was never
> figured out, it was just closed as they couldn't reproduce it). The resulting
> behavior seen is that KahaDB has the batch cursor set to a point after a
> message that is stored so that message will never dispatch.
> I recently figured out how to reproduce it in a test environment and finally
> tracked down what the root cause and a fix. Not surprisingly, there are a few
> things at play here and the bug is a race condition so it won't be seen
> unless a bunch of things hold true (and if the broker is configured a certain
> way)
> h3. Background:
> There are 2 optimizations that the broker uses that are playing into this and
> both must be enabled for the issue to happen.
> # {{useCache=true}} , The normal flow for incoming messages is that they get
> written to the store and then they get paged off disk (same thread or another
> thread) to be dispatched to consumers. However, there's also a message cache
> and if enabled and if there's free memory, the message will be added to the
> the cache after sending to disk so we don't need to re-read it off disk again
> later when dispatching.
> # {{concurrentStoreAndDispatchQueues=true}} The broker also has an
> optimization for queues where it it will try and dispatch incoming messages
> concurrently to consumers while also writing to disk. if the consumers are
> fast enough to ack, we can cancel the disk write which saves disk IO and this
> obviously is a benefit for slow disks. This requires the cache to be enabled
> to be beneficial as without the cache enabled the message would not be
> visible in the cursor to dispatch to a consumer until the write finished.
> The two settings work together and in practice this means the flow ends up
> being that the message is submitted to the store to be added as part of an
> async task that is queued up in the background by the store. While the task
> is in the queue, the message is then concurrently added to the in memory
> cache and the broker will proceed to dispatch to consumers, who may or may
> not acknowledge dispatched messages before the disk write is finished if the
> consumers are fast and keeping up. Messages that were already written are
> removed like normal but if the async task was not finished it gets cancelled
> and saves a disk write.
> h3. Bug description:
> When the broker runs out of memory to cache messages, the cache has to be
> [disabled|https://github.com/apache/activemq/blob/3400983a22284a28a8989d4b0aaf762090b0911a/activemq-broker/src/main/java/org/apache/activemq/broker/region/cursors/AbstractStoreCursor.java#L258].
> As part of this process the cache has to
> [tell|https://github.com/apache/activemq/blob/3400983a22284a28a8989d4b0aaf762090b0911a/activemq-broker/src/main/java/org/apache/activemq/broker/region/cursors/AbstractStoreCursor.java#L336]
> the store what the last message is that was cached so that when the cache is
> exhausted we can resume paging off disk and dispatching in the correct spot.
> The process for disabling the cache starts when a new incoming message is
> attempted to be added to the cache and it detects that memory is full. When
> this happens the process for disabling and syncing to the store starts and
> the cache goes through and makes sure any previously cached messages that may
> be pending to be written are completed (either acked and cancelled or writen
> to disk and completed) and after that will tell the store where to resume,
> which would be after the last cached message. When the cache is disabled, new
> writes should no longer be async because we need to have the messages written
> to disk to be dispatched.
> In theory, because the store was told the last cached message, the new
> incoming message that triggered the disabling/sync would be eventually paged
> off disk and dispatched. However, there is a race condition bug and what is
> actually happening is sometimes the new incoming message has not completed
> the write to the store when the queue goes to fill the next batch to
> dispatch, so it gets missed as it's still pending. In this case the message
> that triggered the disabling/sync was submitted to the cache but never
> actually cached because memory was full, and then the dispatch continues and
> proceeds before guaranteeing the write.
> The end result is that if the consumer is very fast, when the store goes to
> page off the next messages it may not see the pending write that hasn't
> finished (and was not added to the cache) so the store skips ahead before the
> incoming message completes. By the time the incoming message is finished
> writing to the store, the disk cursor has moved past it and the message will
> be skipped and gets stuck until the broker restarts to reset the batch.
> This can happen repeatedly for cache enables/disables which is why you might
> see 1 stuck message or more if it repeatedly happens.
> h3. Solution:
> The solution is actually rather simple and a couple lines of code. Because
> the incoming message that was attempted to be added to the cache was not
> added to the cache (because memory was full), we just need to wait for that
> message that triggered the disable and sync to finish its task of writing to
> the store so that it will be visible when reading in the next batch and
> paging off disk. This guarantees the message won't be missed and no more
> stuck message.
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