JavaScript
Article
By Craig Buckler

How to Use JavaScript Shared Web Workers in HTML5

By Craig Buckler

We recently discussed JavaScript web workers with reference to the “dedicated” variety. If you’ve not read it yet, I suggest you do that first — this article builds on the same concepts.

Web Workers in a Nutshell

A web worker is a single JavaScript file loaded and executed in the background on a separate thread. Dedicated web workers are linked to their creator (the script which loaded the worker). Shared web workers allow any number of scripts to communicate with a single worker.

Shared web workers adhere to the same rules as their dedicated counterparts: no DOM, document or page script access, and limited read-only permission to most window properties. In addition, page scripts may only communicate with shared web workers from the same origin/domain (somesite.com).

Currently, shared web workers are supported in Chrome, Safari and Opera. Firefox 4 and IE9 support may arrive, but don’t bet on it. Shared workers may save resources but they add an extra level of complexity. Expect a few issues, e.g.,

  • DOM2 events (addEventListener) handlers appear to be the most reliable implementation.
  • You’ll almost certainly encounter browser-specific quirks and debugging is difficult. The following code works in the latest version of Chrome, but don’t assume it’ll work in Safari or Opera.
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Creating a Shared Web Worker

To create a shared web worker, you pass a JavaScript file name to a new instance of the SharedWorker object:


var worker = new SharedWorker("jsworker.js");

Communicating with a Shared Web Worker

Any of your page scripts can communicate with the shared web worker. Unlike dedicated web workers, you must communicate via a ‘port’ object and attach a message event handler. In addition, you must call the port’s start() method before using the first postMessage():

pagescript.js:


var worker = new SharedWorker("jsworker.js");

worker.port.addEventListener("message", function(e) {
	alert(e.data);
}, false);

worker.port.start();

// post a message to the shared web worker
worker.port.postMessage("Alyssa");

When the web worker script receives the first message from a script, it must attach an event handler to the active port. Under most circumstances, the handler will run its own postMessage() method to return a message to the calling code. Finally, the port’s start() method must also be executed to enable messaging:

jsworker.js:


var connections = 0; // count active connections

self.addEventListener("connect", function (e) {

	var port = e.ports[0];
	connections++;

	port.addEventListener("message", function (e) {
		port.postMessage("Hello " + e.data + " (port #" + connections + ")");
	}, false);

	port.start();

}, false);

Like their dedicated siblings, shared web workers can:

  • load further scripts with importScripts()
  • attach error handlers, and
  • run the port.close() method to prevent further communication on a specific port.

Shared web workers probably won’t be a viable technology for a couple of years, but they raise exciting opportunities for the future of JavaScript development. Let’s hope browser vendors can supply a few decent tracing and debugging tools!

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