Question

I implemented in Java what I called a “foldable queue”, i.e., a LinkedBlockingQueue used by an ExecutorService. The idea is that each task as a unique id that if is in the queue while another task is submitted via that same id, it is not added to the queue. The Java code looks like this:

public final class FoldablePricingQueue extends LinkedBlockingQueue<Runnable> {
    @Override
    public boolean offer(final Runnable runnable) {
        if (contains(runnable)) {
            return true; // rejected, but true not to throw an exception
        } else {
            return super.offer(runnable);
        }
    }
}

Threads have to be pre-started but this is a minor detail. I have an Abstract class that implements Runnable that takes a unique id… this is the one passed in

I would like to implement the same logic using Scala and Akka (Actors).
I would need to have access to the mailbox, and I think I would need to override the ! method and check the mailbox for the event.. has anyone done this before?

Answer 1

This is exactly how the Akka mailbox works. The Akka mailbox can only exist once in the task-queue.

Look at:

https://github.com/jboner/akka/blob/master/akka-actor/src/main/scala/akka/dispatch/Dispatcher.scala#L143

https://github.com/jboner/akka/blob/master/akka-actor/src/main/scala/akka/dispatch/Dispatcher.scala#L198

Very cheaply implemented using an atomic boolean, so no need to traverse the queue.

Also, by the way, your Queue in Java is broken since it doesn’t override put, add or offer(E, long, TimeUnit).