When you split your application into resources, you increase its surface area. Your users can construct an appropriate URI and enter your application right where they need to be. But the resources aren’t the data; they’re just the service designer’s idea of how to split up the data into “a list of open bugs” or “information about jellyfish.” A web server can’t send an idea; it has to send a series of bytes, in a specific file format, in a specific language. This is a representation of the resource.
A resource is a source of representations, and a representation is just some data about the current state of a resource. Most resources are themselves items of data (like a list of bugs), so an obvious representation of a resource is the data itself. The server might present a list of open bugs as an XML document, a web page, or as comma-separated text. The sales numbers for the last quarter of 2004 might be represented numerically or as a graphical chart. Lots of news sites make their articles available in an ad-laden format, and in a stripped-down “printer-friendly” format. These are all different representations of the same resources.
But some resources represent physical objects, or other things that can’t be reduced to information. What’s a good representation for such things? You don’t need to worry about perfect fidelity: a representation is any useful information about the state of a resource.
Consider a physical object, a soda machine, hooked up to a web service.The goal is to let the machine’s customers avoid unnecessary trips to the machine. With the service, customers know when the soda is cold, and when their favorite brand is sold out. Nobody expects the physical cans of soda to be made available through the web service, because physical objects aren’t data. But they do have data about them: metadata. Each slot in the soda machine can be instrumented with a device that knows about the flavor, price, and temperature of the next available can of soda. Each slot can be exposed as a resource, and so can the soda machine as a whole. The metadata from the instruments can be used in representations of the resources.
Even when one of an object’s representations contains the actual data, it may also have representations that contain metadata. An online bookstore may serve two representations of a book:
One containing only metadata, like a cover image and reviews, used to advertise the book.
An electronic copy of the data in the book, sent to you via HTTP when you pay for it.
Representations can flow the other way, too. You can send a representation of a new resource to the server and have the server create the resource. This is what happens when you upload a picture to Flickr. Or you can give the server a new representation of an existing resource, and have the server modify the resource to bring it in line with the new representation.
If a server provides multiple representations of a resource, how does it figure out which one the client is asking for? For instance, a press release might be put out in both English and Spanish. Which one does a given client want?
There are a number of ways to figure this out within the constraints of REST. The simplest, and the one I recommend for the Resource-Oriented Architecture, is to give a distinct URI to each representation of a resource. http://www.example.com/releases/104.en could designate the English representation of the press release, and http://www.example.com/releases/104.es could designate the Spanish representation.
I recommend this technique for ROA applications because it means the URI contains all information necessary for the server to fulfill the request. The disadvantage, as whenever you expose multiple URIs for the same resource, is dilution: people who talk about the press release in different languages appear to be talking about different things. You can mitigate this problem somewhat by exposing the URI http://www.example.com/releases/104 to mean the release as a Platonic form, independent of any language.
The alternative way is called content negotiation. In this scenario the only exposed URI is the Platonic form URI, http://www.example.com/releases/104. When a client makes a request for that URI, it provides special HTTP request headers that signal what kind of representations the client is willing to accept.
Your web browser has a setting for language preferences: which
languages you’d prefer to get web pages in. The browser submits this
information with every HTTP request, in the
header. The server usually ignores this information because most web
pages are available in only one language. But it fits with what we’re
trying to do here: expose different-language representations of the
same resource. When a client requests
http://www.example.com/releases/104, the server
can decide whether to serve the English or the Spanish representation
based on the client’s
The Google search engine is a good place to try this
out. You can get your search results in almost any language by
changing your browser language settings, or by manipulating the
hl query variable in the URI (for instance,
hl=tr for Turkish). The search
engine supports both content negotiation and different URIs for
A client can also set the
Accept header to specify which file format
it prefers for representations. A client can say it prefers XHTML to
HTML, or SVG to any other graphics format.
The server is allowed to use any of this request metadata when deciding which representation to send. Other types of request metadata include payment information, authentication credentials, the time of the request, caching directives, and even the IP address of the client. All of these might make a difference in the server’s decision of what data to include in the representation, which language and which format to use, and even whether to send a representation at all or to deny access.
It’s RESTful to keep this information in the HTTP headers, and it’s RESTful to put it in the URI. I recommend keeping as much of this information as possible in the URI, and as little as possible in request metadata. I think URIs are more useful than metadata. URIs get passed around from person to person, and from program to program. The request metadata almost always gets lost in transition.
Here’s a simple example of this dilemma: the W3C HTML validator,
a web service available at http://validator.w3.org/. Here’s a URI to a resource on
the W3C’s site, a validation report on the English version of my
hypothetical press release:
Here’s another resource: a validation report on the Spanish
version of the press release:
Every URI in your web space becomes a resource in the W3C’s web application, whether or not it designates a distinct resource on your site. If your press release has a separate URI for each representation, you can get two resources from the W3C: validation reports for the English and the Spanish versions of the press release.
But if you only expose the Platonic form URI, and serve both representations from that URI, you can only get one resource from the W3C. That would be a validation report for the default version of the press release (probably the English one). You’ve got no way of knowing whether or not the Spanish representation contains HTML formatting errors. If the server doesn’t expose the Spanish press release as its own URI, there’s no corresponding resource available on the W3C site. This doesn’t mean you can’t expose that Platonic form URI: just that it shouldn’t be the only URI you use.
Unlike humans, computer programs are very bad at dealing with representations they didn’t expect. I think an automated web client should be as explicit as possible about the representation it wants. This almost always means specifying a representation in the URL.