Saturday, August 21, 2004

Turning 90 degrees

An interesting problem: you have a depth-first process and need to turn it into a breadth-first one. How to proceed? A simple publish-subscribe message bus works single-threaded so when a message receiver publishes a new message in response, the new message is processed before the rest of the receivers of the first message have a chance to run. This leads to some confusing situations where message order is counter-intuitive.

Since everything is plain Java, I cannot use continuations for the activation records else I would just reorder them. Ah, but I can come close enough for this simple system. Consider these two data structures (in reality, they are immutable classes, but I shortened them to C-style structs for conciseness):

public class Binding {
    public Receiver recipient; // the target object for Method.invoke(Object...)
    public Method onReceive; // the method
}

public class ActivationSet {
    public Set<Binding> bindings; // JDK 5.0, plain Set for JDK 1.3 or JDK 1.4
    public Message message; // the argument for on Method.invoke(Object...)
}

Now the publish algorithm is quite simple given an instance variable of Queue<ActivationSet> for the channel:

public void publish(Message message) {
    Set<Binding> bindings = findSubscribers(message);

    if (bindings.isEmtpy()) return;

    // Check if we're first to publish
    boolean topContext = activations.isEmpty();
    // Queue our recipients
    activations.offer(new ActivationSet(bindings, message));
    // Let the top context handle recipients in queue-order
    if (!topContext) return;

    while (!activations.isEmpty()) {
        // activate() invokes onReceive for each binding in the set
        activations.peek().activate();
        // Be careful not to remove our own activation set prematurely;
        // otherwise the topContext check above won't work right
        activations.remove();
    }
}

The effect is to queue up the depth-first nature of recursive method calls and handle them in breadth-first order. The code is quite simple once the concept is clear.

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