Web Workers makes it possible to run a script operation in a background thread separate from the main execution thread of a web application. The advantage of this is that laborious processing can be performed in a separate thread, allowing the main (usually the UI) thread to run without being blocked/slowed down. A worker is an object created using a constructor (e.g. Worker()) that runs a named JavaScript file - this file contains the code that will run in the worker thread; workers run in another global context that is different from the current window. This context is represented by either a DedicatedWorkerGlobalScope object (in the case of dedicated workers - workers that are utilized by a single script), or a SharedWorkerGlobalScope (in the case of shared workers - workers that are shared between multiple scripts). You can run whatever code you like inside the worker thread, with some exceptions. For example, you can't directly manipulate the DOM from inside a worker, or use some default methods and properties of the window object.

But you can use a large number of items available under window, including WebSockets, and data storage mechanisms like IndexedDB. See Functions and classes available to workers for more details. Data is sent between workers and the main thread via a system of messages - both sides send their messages using the postMessage() method, and respond to messages via the onmessage event handler (the message is contained within the Message event's data property). The data is copied rather than shared. Workers may in turn spawn new workers, as long as those workers are hosted within the same origin as the parent page. In addition, workers may use XMLHttpRequest for network I/O, with the exception that the responseXML and channel attributes on XMLHttpRequest always return null. Shared workers are workers that can be utilized by multiple scripts running in different windows, IFrames, etc., as long as they are in the same domain as the worker. They are a little more complex than dedicated workers - scripts must communicate via an active port. See SharedWorker for more details. ServiceWorkers essentially act as proxy servers that sit between web applications, the browser, and the network (when available). They are intended, among other things, to enable the creation of effective offline experiences, intercept network requests and take appropriate action based on whether the network is available, and update assets residing on the server. They will also allow access to push notifications and background sync APIs. Chrome Workers are a Firefox-only type of worker that you can use if you are developing add-ons and want to use workers in extensions and have access to js-ctypes in your worker. See ChromeWorker for more details. Audio Workers provide the ability for direct scripted audio processing to be done inside a web worker context. Basic dedicated worker example (run dedicated worker). Basic shared worker example (run shared worker).

Please beware that as of 18th November the W3C is no longer actively working on the Web SQL Database specification. The Web SQL database API isn’t actually part of the HTML5 specification, but it is part of the suite of specifications that allows us developers to build fully fledged web applications, so it’s about time we dig in and check it out. What’s in the box? If you haven’t guessed from the overly verbose specification title, Web SQL Databases is a spec that brings SQL to the client side. If you have a back-end developer’s background, then you’ll probably be familiar with SQL and happy as a pig in muck. If not, you might want to learn some SQL before you start hacking around, Google’s your friend here. The specification is based around SQLite (3.1.19), but having come from MySQL myself, it’s all pretty much the same (sorry for the sweeping statement!). For an example of Web SQL Databases working, have a look at the Twitter HTML5 chatter demo I put together.

Support is a little patchy at the moment. Only Webkit (Safari, SafariMobile and Chrome) and Opera 10.50 (ATOW alpha on Mac) support web databases. Fellow Doctor Bruce Lawson has told me that Firefox are holding off as they feel there’s a better implementation than SQLite (though I hope it’s similar, whatever they pick). Either way, I’d definitely recommend checking out the SQLite documentation for the functions that are available. Because of this patchy support and the simple fact that Webkit had implemented the database spec some time ago, the spec on the W3C is now slightly ahead of the implementations in Safari, while Webkit is still catching up. On the other hand, since Opera has only just added support, it’s closer to the spec (I’ll mention the differences as we go along). Nonetheless, it’s fun to play with, so let’s get playing! If you try to open a database that doesn’t exist, the API will create it on the fly for you.

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