Features of the Resource-Oriented Architecture - Representations
(Page 4 of 4 )
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 unneccessary 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.
Please check back next week for the continuation of this series.
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 This article is excerpted from chapter four of the book RESTful Web Services, written by Leonard Richardson and Sam Ruby (O'Reilly, 2008; ISBN: 0596529260). Check it out today at your favorite bookstore. Buy this book now.
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