What's a MessagePassingQueue? Should I Use It?

The MessagePassingQueue is the JCTools view on what should be the queue interface. It differs from the java.util.Queue in several important ways:

• It does not extend Collection. It's for message passing, not arbitrary storage. As an extension it does not define an iterator or offer other search methods.
• It expands on the offer and poll methods to provide means of different and potentially more performant interactions.

The MessagePassingQueue goes beyond the generic j.u.Queue interface, and so requires the developer to commit to this new way of interacting with a queue. This goes beyond a drop in replacement of a j.u.Queue, so implication must be considered. It does however offer some new functionality which might be worth your while:

• relaxedOffer/relaxedPoll: adds/removes an element to/from the queue without the strong requirement that failing to do so implies the queue is full/empty. This proves a win for MPSC/SPMC/MPMC queues where the relaxation allows for greatly reducing contention. For the SPSC case they delegate directly to normal offer/poll.
• fill/drain: batch offer/poll elements to/from the queue. This allows some costs to be amortised, but runs the risk of inlining behaviour changes which would remove the benefit. Worth trying, but no guaranteed benefit.
• Perpetual fill/drain: where it is desirable for the queue to be continuously filled or drained by a particular thread it is possible to use a fill/drain method which will take over the thread for that purpose.

The perpetual drain/fill methods require the user to implement a WaitStrategy which will model the intended backoff mechanism for the thread spinning on the queue. For inspiration on different ways to implement a backoff strategy see the different stategies implemented by the Disruptor library or similarly in Agrona