[ https://issues.apache.org/jira/browse/AMQ-9625 ]
Christopher L. Shannon deleted comment on AMQ-9625:
---------------------------------------------
was (Author: jira-bot):
Commit c06969583d9d6340674e332e9dce356fad2a314f in activemq's branch
refs/heads/6.1.x.AMF.x from Christopher L. Shannon
[ https://gitbox.apache.org/repos/asf?p=activemq.git;h=c06969583d ]
AMQ-9625 - Prevent queue messages from becoming stuck
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.
AMFU-168, AMF-3692 - cherry-pick in fix from FOSS broker
> 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: 1h
> Remaining Estimate: 0h
>
> For the last several years I have occasionally seen "stuck" messages that
> appear on queues that will not be dispatched until after a broker restart.
> The bug looks to be 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. There's been
> some other work done previously to help this issue, notably AMQ-3149 (more
> details in that issue and more below)
> 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. (Async message writes)
> 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 as described in AMQ-3149 otherwise we run into
> problems because messages won't be dispatched if not finished so this mode is
> really only useful for the cache being enabled. Furthermore, if the cache is
> not enabled messages could get stuck if no new messages come in for a while
> so that's another reason this mode has no effect if the cache is off.
> 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 written
> 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|https://github.com/apache/activemq/blob/3400983a22284a28a8989d4b0aaf762090b0911a/activemq-broker/src/main/java/org/apache/activemq/broker/region/Queue.java#L895]
> be async (AMQ-3149) because we need to have the messages written to disk to
> be dispatched and prevent stuck/delayed delivery.
> 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
> (because it was async still), so it gets missed as it's still pending.
> Normally as described earlier, if the cache is disabled, all the new store
> writes should be sync and not async messages but in this case the message
> that triggered the disabling/sync was still submitted as async but never
> cached. Furthermore, if there are several producers you can have multiple in
> flight async messages that were submitted before the cache was disabled, but
> then by the time they get added to the cache the cache is seen as disabled
> and they are not added.
> The end result is that if the consumer and producers are very fast, you might
> have one or more pending async writes that are skipped by the cache because
> it was disabled so you get into a similar situation as AMQ-3149 described,
> and when the cursor goes to page off the next messages it may not see the
> pending write(s) that haven't finished (and was not added to the cache) so
> those messages are skipped. 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. When the
> cache is disabled we just need to ensure that any messages that are in flight
> are now treated as sync and not async so they do not get missed as they will
> no longer be added to the memory in the cache.
> When a message added to the cache triggers the cache to disable, we just need
> to wait for its async task to finish so that it will be visible when reading
> in the next batch and paging off disk before we set the batch on the store.
> Also, lots of fast producers could cause other messages to be submitted as
> async just before the cache was disabled, and then by the time it reaches the
> cache they get skipped so we need to wait on those as well.
> Writing a unit test for this would be very difficult due to the race
> conditions an async going on, however these small changes are very simple and
> low risk as the only thing that is happening is we are just converting
> waiting on the async write to finish (converts them to sync) during the small
> window when the cache is toggled off which prevents the bug.
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