A simple HTML5 web worker pool (think thread pool) implementation for parallel execution of computationally intensive operations. The pool will lazily grow up to its given capacity. On saturation, messages will be queued up until a worker from the pool becomes available. The interface hides the Worker listener pattern and instead provides a sing…

WorkerPool.js

/**
 * A web worker pool implementation for parallel execution of
 * computationally intensive operations. The pool will lazily grow up to its
 * given capacity. On saturation, messages will be queued up until a worker
 * from the pool becomes available.
 * 
 *     var capacity = 3;
 *     var pool = new WorkerPool(capacity, "./worker.js");
 *     for (var i = 0; i < 10; i++) {
 *         pool.postMessage(msg, function(err, result) {
 *
 *         });
 *     }
 *
 * @param {number} capacity - The size of the pool. This will be the maximum
 * concurrency allowed.
 * @param {string} url - The web worker url. Workers are lazily created.
 *
 */
function WorkerPool(capacity, url) {
    this.capacity = capacity;
    this.url = url;
    this.queue = [];
    this.idle = Array(capacity);
};

WorkerPool.prototype.postMessage = function(msg, callback) {
    this.queue.push({
        msg : msg,
        callback : callback
    });
    this.dispatch();
};

WorkerPool.prototype.dispatch = function() {
    if (!this.queue.length || !this.idle.length) {
        return;
    }
    var task = this.queue.shift();
    var worker = this.idle.pop() || new Worker(this.url);

    var self = this;
    function onEvent(e) {
        this.removeEventListener('message', arguments.callee, false);
        this.removeEventListener('error', arguments.callee, false);

        self.idle.push(worker);
        self.dispatch();

        switch (e.type) {
            case 'message':
                task.callback.call(this, null, e.data, e);
                break;
            case 'error':
                task.callback.call(this, e.message, null, e);
                break;
        }
    };
    worker.addEventListener('message', onEvent, false);
    worker.addEventListener('error', onEvent, false);

    worker.postMessage(task.msg);
};