September 12th, 2012

RESTful or Not?

As the leader of Layer 7’s North American API Architecture & Design Practice, I often get asked to review Web solutions. Rarely do people ask me if the implementation is appropriate for the intended use. Instead they want to know if the work fits a label invented over a decade ago by a PhD candidate in his dissertation. They want to know if what they’ve come up with is “RESTful”.

Essentially, REST (representational state transfer) is a style. Specifically, it’s a style of network-based software architecture. This style was first defined in 2000 by Roy Fielding. Fielding stated that “an architectural style is a coordinated set of architectural constraints that has been given a name for ease of reference”.

The set of architectural constraints Fielding defined in his dissertation remain the key criteria by which we judge whether or not a service is RESTful. Back in 2000, Fielding did a very good job of defining the six primary constraints: client-server; stateless; cache; uniform interface; layered system; code-on-demand.

However, REST is also defined by four “interface constraints” that are only partially defined in the dissertation: identification of resources; manipulation of resources through representations; self-descriptive messages; hypermedia as the engine of application state. In particular, the definitions of self-descriptive messages and hypermedia are still debated.

Assuming you can decide on clear definitions of all 10 constraints, all that remains is to identify each of them within the target design. If the implementation does not exhibit all ten (well nine, since code-on-demand is optional), then it is not RESTful. This last step is not difficult. It is the previous step (agreeing on definitions) that causes problems.

Still not sure if your service is RESTful? Well, I originally published this post, in expanded form, on my personal blog. If you want to dig deeper, take a look over there.

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