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Jason began his career working on EAI systems that covered a breadth of technologies ranging from Java Script, Java, C++, C#, to Silverlight. He has also spent a considerable amount of time working on systems which used WebSphere MQ as the middleware layer. Jason is now fortunate enough to work full time with products such as Apache ActiveMQ, Apache Camel, and Apache ServiceMix. Jason is a DZone MVB and is not an employee of DZone and has posted 10 posts at DZone. You can read more from them at their website. View Full User Profile

ActiveMQ: KahaDB Journal Files - More Than Just Message Content Bits

01.18.2013
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I recently came across an issue where the ActiveMQ KahaDB journal files were continually rolling despite the fact that only a small number of small persistent messages were occasionally being stored by the broker. This behavior seemed very strange being that the message sizes being persisted were only a couple of kilobytes and there was a relatively small amount of messages actually on a queue.  In this scenario something was filling up the 32MB journal files, but I wasn't quite sure what it could be?  Were there other messages somewhere in the broker? Did an index get corrupted that was actually causing messages to be written across multiple journal files?  It was pretty strange behavior but it can be explained fairly easily.  This post describes the actual cause of this behavior and I have created it to remind myself in the future that there is more in the journal file than just the message content bits.

The KahaDB journal files are used to store persistent messages that have been sent to the broker.  In addition to storing the message content, the journal files also store information on KahaDB commands and transactional information.  There are several commands for which information is stored;

KahaAddMessageCommand, KahaCommitCommand, KahaPrepareCommand, KahaProducerAuditCommand, KahaRemoveDestinationCommand, KahaRemoveMessageCommand, KahaRollbackCommand, KahaSubscriptionCommand, and KahaTraceCommand.

In this particular case, it was the KahaProducerAuditCommand which was responsible for the behavior that was observed.  This command stores information about producer ids and message ids which is used for duplicate detection.  In this case information is stored in a map object which over time grows.  This information is then stored in the journal file each time a checkpoint is run, which by default is every 5 seconds.  Over time, this can begin to use up the space allocated by the journal file causing low volume smaller messages to roll to the next journal file which in turn prevents the broker from cleaning up journal files which still have referenced messages.  Eventually this situation can lead to Producer Flow Control being trigger by the broker's store limit which prevents producers from sending new messages into the broker.

This behavior can occur under the following conditions:

  • Persistent messages are being sent to a queue
  • The messages are not being consumed on a regular basis
  • The rate of messages being sent to the broker is low
These conditions allow for this behavior to be observed fairly easily.  As the persistent messages do not get consumed they remain referenced and prevent the journal files from being cleaned up.  The low message rate allows time to pass between each new message being stored in the journal file and in the meantime checkpoints are being run which cause KahaProducerAuditCommand information to space out the actual messages within the journal file.
For this use case you can disable the duplicate detection by essentially limiting the growth of the KahaProducerAuditCommand using the following configuration on the persistent adapter in the broker configuration:
 <persistenceAdapter>  
       <kahaDB directory="${activemq.base}/data/kahadb" failoverProducersAuditDepth="0" maxFailoverProducersToTrack="0"/>  
 </persistenceAdapter>  
This is something to think about when designing your system.  Under normal circumstances, if you have consumers available to consume the persistent messages, this condition would probably never occur as the journal files roll and messages are consumed, the broker can begin to clean up old journal files.

There is currently an enhancement request at Apache which will also help resolve this issue.  AMQ-3833 has been opened to enhance the broker so it will only write the KahaProducerAuditCommand if a change has occurred since the last checkpoint.  This will help reduce the amount of data that is written to the journal files in between message storage.
Published at DZone with permission of Jason Sherman, author and DZone MVB. (source)

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)