I’ve shown you the power of REST, but I haven’t shown you in any systematic way how that power is structured or how to expose it. In this chapter I outline a concrete RESTful architecture: the Resource-Oriented Architecture (ROA). The ROA is a way of turning a problem into a RESTful web service: an arrangement of URIs, HTTP, and XML that works like the rest of the Web, and that programmers will enjoy using.
In Chapter 1 I classified RESTful web services by their answers to two questions. These answers correspond to two of the four defining features of REST:
The scoping information (“why should the server send this data instead of that data?”) is kept in the URI. This is the principle of addressability.
The method information (“why should the server send this data instead of deleting it?”) is kept in the HTTP method. There are only a few HTTP methods, and everyone knows ahead of time what they do. This is the principle of the uniform interface.
In this chapter I introduce the moving parts of the Resource-Oriented Architecture: resources (of course), their names, their representations, and the links between them. I explain and promote the properties of the ROA: addressability, statelessness, connectedness, and the uniform interface. I show how the web technologies (HTTP, URIs, and XML) implement the moving parts to make the properties possible.
In the previous chapters I illustrated concepts by pointing to existing web services, like S3. I continue that tradition in this chapter, but I’ll also illustrate concepts by pointing to existing web sites. Hopefully I’ve convinced you by now that web sites are web services, and that many web applications (such as search engines) are RESTful web services. When I talk about abstract concepts like addressability, it’s useful to show you real URIs, which you can type into your web browser to see the concepts in action.
Resource-Oriented What Now?
Why come up with a new term, Resource-Oriented Architecture? Why not just say REST? Well, I do say REST, on the cover of this book, and I hold that everything in the Resource-Oriented Architecture is also RESTful. But REST is not an architecture: it’s a set of design criteria. You can say that one architecture meets those criteria better than another, but there is no one “REST architecture.”
Up to now, people have tended to mint one-off architectures as they design their services, according to their own understandings of REST. The most obvious outcome of this is the wide variety of REST-RPC hybrid web services that their creators claim are RESTful. I’m trying to put a stop to that by presenting a set of concrete rules for building web services that really will be RESTful. In the next two chapters I’ll even show simple procedures you can follow to turn requirements into resources. If you don’t like my rules, you’ll at least have an idea of what you can change and stay RESTful.
As a set of design criteria, REST is very general. In particular, it’s not tied to the Web. Nothing about REST depends on the mechanics of HTTP or the structure of URIs. But I’m talking about web services, so I explicitly tie the Resource-Oriented Architecture to the technologies of the Web. I want to talk about how to do REST with HTTP and URIs, in specific programming languages. If the future produces RESTful architectures that don’t run on top of the Web, their best practices will probably look similar to the ROA, but the details will be different. We’ll cross that bridge when we come to it.
The traditional definition of REST leaves a lot of open space, which practitioners have seeded with folklore. I deliberately go further than Roy Fielding in his dissertation, or the W3C in their standards: I want to clear some of that open space so that the folklore has room to grow into a well-defined set of best practices. Even if REST were an architecture, it wouldn’t be fair to call my architecture by the same name. I’d be tying my empirical observations and suggestions to the more general thoughts of those who built the Web.
My final reason for coming up with a new term is that “REST” is a term used in religious nerd wars. When it’s used, the implication is usually that there is one true RESTful architecture and it’s the one the speaker prefers. People who prefer another RESTful architecture disagree. The REST community fragments, despite a general agreement on basic things like the value of URIs and HTTP.
Ideally there would be no religious wars, but I’ve seen enough to know that wishing won’t end them. So I’m giving a distinctive name to my philosophy of how RESTful applications should be designed. When these ideas are, inevitably, used as fodder in wars, people who disagree with me can address aspects of the Resource-Oriented Architecture separate from other RESTful architectures, and from REST in general. Clarity is the first step toward understanding.
The phrases “resource-oriented” and “resource-oriented architecture” have been used to describe RESTful architectures in general.* I don’t claim that “Resource-Oriented Architecture” is a completely original term, but I think that my usage meshes well with preexisting uses, and that it’s better to use this term than claim to speak for REST as a whole.