dhmgit commented on pull request #571:
URL: https://github.com/apache/activemq/pull/571#issuecomment-1082266184


   @ncasaux , apologies for not seeing your comment earlier -- it was buried in 
a list of several hundred notifications.
   
   This PR is from a while back and was itself based on work that I had done at 
least a couple years prior. This is to say that I might not be remembering all 
the details 100% correctly. 
   
   We run .NET applications in on-premises customer environments that we do not 
control. Those applications use the NMS client library to connect to AMQ, and 
consume topics using durable subscriptions. In certain customer environments, 
these client applications can experience sustained unreliable network 
connectivity. The reasons for the flaky connectivity were varied, but the 
common result was that some clients would get into a state where they would 
fail to acknowledge messages. On the other hand, those same clients managed to 
reconnect (via the failover transport) often enough to keep their durable 
subscriptions active. Without the setting introduced by this PR, those unACKed 
messages continue to be referenced by those active subscriptions well after the 
message TTL expires. This was ultimately leading to quota exhaustion as those 
expired messages could never be purged.
   
   I hope that helps!


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