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From "Torsten Mielke (JIRA)" <j...@apache.org>
Subject [jira] [Created] (AMQ-4465) Rethink replayWhenNoConsumers solution
Date Wed, 10 Apr 2013 13:30:17 GMT
Torsten Mielke created AMQ-4465:
-----------------------------------

             Summary: Rethink replayWhenNoConsumers solution
                 Key: AMQ-4465
                 URL: https://issues.apache.org/jira/browse/AMQ-4465
             Project: ActiveMQ
          Issue Type: Improvement
          Components: Broker
    Affects Versions: 5.8.0
            Reporter: Torsten Mielke


I would like to start a discussion about the way we allow messages to be replayed back to
the original broker in a broker network, i.e. setting replayWhenNoConsumers=true.

This discussion is based on the blog post 
http://tmielke.blogspot.de/2012/03/i-have-messages-on-queue-but-they-dont.html
but I will outline the full story here again. 


Consider a network of two brokers A and B. 
Broker A has a producer that sends one msg to queue Test.in. Broker B has a consumer connected
so the msg is transferred to broker B. Lets assume the consumer disconnects from B *before*
it consumes the msg and reconnects to broker A. If broker B has replayWhenNoConsumers=true,
the message will be replayed back to broker A. 
If that replay happens in a short time frame, the cursor will mark the replayed msgs as a
duplicate and won't dispatch it. To overcome this, one needs to set enableAudit=false on the
policyEntry for the destination. 

This has a consequence as it disables duplicate detection in the cursor. External JMS producers
will still be blocked from sending duplicates thanks to the duplicate detection built into
the persistence adapter. 
However you can still get duplicate messages over the network bridge now. With enableAudit=false
these duplicates will be happily added to the cursor now. If the same consumer receives the
duplicate message, it will likely detect the duplicate. However if the duplicate message is
dispatched to a different consumer, it won't be detected but will be processed by the application.

For many use cases its important not to receive duplicate messages so the above setup replayWhenNoConsumers=true
and enableAudit=false becomes a problem.

There is the additional option of specifying auditNetworkProducers="true" on the transport
connector but that's very likely going to have consequences as well. With auditNetworkProducers="true"
we will now detect duplicates over the network bridge, so if there is a network glitch while
the message is replayed back on the bridge to broker A and broker B tries to resend the message
again, it will be detected as a duplicate on broker A. This is good.

However lets assume the consumer now disconnects from broker A *after* the message was replayed
back from broker B to broker A but *before* the consumer actually received the message. The
consumer then reconnects to broker B again. 
The replayed message is on broker A now. Broker B registers a new demand for this message
(due to the consumer reconnecting) and broker A will pass on the message to broker B again.
However due to auditNetworkProducers="true" broker B will treat the resent message as a duplicate
and very likely not accept it (or even worse simply drop the message - not sure how exactly
it will behave). 

So the message is stuck again and won't be dispatched to the consumer on broker B. 
The networkTTL setting will further have an effect on this scenario and so will have other
broker topologies like a full mesh.

It seems to me that 
- When allowing replayWhenNoConsumers=true you may receive duplicate messages unless you also
set auditNetworkProducers="true" which has consequences as well.
- If consumers are reconnecting to a different broker each time that you may end up with msgs
stuck on a broker that won't get dispatched. 
- Ideally you want sticky consumers, i.e. they reconnect to the same broker if possible in
order to avoid replaying back messages. This implies that you don't want to use randomize=true
on failover urls. I don't think we recommend this in any docs.
- The network ttl will potentially never be high enough and the message may be stuck on a
particular broker as the consumer may have reconnected to another broker in the network.

I am sure there are more sides to this discussion. I just wanted to capture what gtully and
I found when discussing this problem. 



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