News You Can Bruise for 2008 August 19

It means that you operate the web service by following links and filling out forms. (For definitions of "link" and "form" that depend on the media type.) It means you can use a generic "web service browser" that doesn't have a bunch of hard-coded knowledge about one particular web service, the way PyAmazon does. When the web service changes, user behavior may need to change with it, but the client itself isn't instantly rendered useless, the way PyAmazon was. I called this "connectedness" in RWS, and Roy Fielding himself slammed me for it, but when Tim Bray, after years of real-world experience, isn't sure what "hypermedia as the engine of application state" means, I think that's a sign it needs to be explained in different words.

Saying that RESTful services "work like the web" is not just saying that things have URIs or you can cache GET requests. Ideas as confusing as hypermedia-as-the-engine-of-application-state become understandable when you think about the way you use the web.

If a website is well-designed, you don't need to mess around with the URL bar to get where you want to be: you fill out forms and follow links. You don't need a custom web browser to use crummy.com. My website is distinguished from all other websites by the HTML documents I serve. If I change my weblog software, it'll serve you different documents, and you may have to change the way you use the website, but your browser doesn't crash.

You know what to do (how to drive the crummy.com "application" into the desired state) because the document I send you lays out your options. And because the document represents the options as hypermedia links and forms, you know how to build an HTTP request that will carry out your desires.