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
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.