I have a workflow that I want to looks something like this:

                                         / Worker 1 \
=Request Channel= -> [Holding Queue|||] -> Worker 2 -> =Response Channel=
                                         \ Worker 3 /

That is:

• Requests come in and they enter a FIFO queue
• Identical workers then pick up tasks from the queue
• At any given time any worker may work only one task
• When a worker is free and the holding queue is non-empty the worker should immediately pick up another task
• When tasks are complete, a worker places the result on the Response Channel

I know there are QueueChannels in Spring Integration, but these channels require polling (which seems suboptimal). In particular, if a worker can be busy, I'd like the worker to be busy.

Also, I've considered avoiding the queue altogether and simply letting tasks round-robin to all workers, but it's preferable to have a single waiting line as some tasks may be accomplished faster than others. Furthermore, I'd like insight into how many jobs are remaining (which I can get from the queue) and the ability to cancel all or particular jobs.

How can I implement this message queuing/work distribution pattern while avoiding a polling?

Edit: It appears I'm looking for the Message Dispatcher pattern -- how can I implement this using Spring/Spring Integration?