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

 <persistenceAdapter>  
       <kahaDB directory="${activemq.base}/data/kahadb" failoverProducersAuditDepth="0" maxFailoverProducersToTrack="0"/>  
 </persistenceAdapter>