Publish-subscribe in Apache Camel

If you'd like to use publish-subscribe pattern in Apache Camel application, you should not use the suggested seda:queue?multipleConsumers=true approach, described here. There is a "by design" flaw that may cost you a lot.

  from("seda:queue?multipleConsumers=true")
    .bean(Processor1)

  from("seda:queue?multipleConsumers=true")
    .bean(Processor2)

multipleConsumers implementation makes use of multicast and parallelProcessing under the hood and this combination utilizes the default thread pool to dispatch exchanges to the underlying endpoints. The problem here is that every subscriber becomes parallel implicitly. You didn't expect it, but there will be several threads running Processor1 concurrently and several threads running Processor2 concurrently as well.

Hence, if you have inner state inside either of them which relies on the order of incoming exchanges – you are in trouble.

By the way, this could be a bigger surprise if you had only one subscriber for some time and everything was working. It may (suddenly) break when you add another one.

Demonstration

This project is a simple demonstration of this behavior - review it, look into the comments and try to run:

$ sbt
> run -ac routes.xml

Bug report

The bug report is here.

The Camel team will not fix it, they claimed this behavior is "as designed".

Workarounds

threads(1) – NO

You would probably want to make execution sequential:

  from("seda:queue?multipleConsumers=true")
    .threads(1)
    .bean(Processor1)

This will not help, b/c this whole fragment gets called concurrently.

direct:seq – NO

  from("seda:queue?multipleConsumers=true")
    .to("direct:seq")

  from("direct:seq")
    .bean(Processor1)

This will make execution sequential, but it cannot guarantee the order of exchanges is intact.

Disruptor – YES

What seems the real fix is using Disruptor component, see the PR for required changes. Fetch the branch and try running it.