Intrigued by the Web, researchers studied its rapid growth and sought to understand the reasons for its success. In that spirit, the Web’s architectural underpinnings were investigated in a seminal work that supports much of our thinking around contemporary web-based systems.
As part of his doctoral work, Roy Fielding generalized the Web’s architectural principles and presented them as a framework of constraints, or an architectural style. Through this framework, Fielding described how distributed information systems such as the Web are built and operated. He described the interplay between resources, and the role of unique identifiers in such systems. He also talked about using a limited set of operations with uniform semantics to build a ubiquitous infrastructure that can support any type of application. Fielding referred to this architectural style as REpresentational State Transfer, or REST. REST describes the Web as a distributed hypermedia application whose linked resources communicate by exchanging representations of resource state.
The description of the Web, as captured in W3C’s “Architecture of the World Wide Web” and other IETF RFC documents, was heavily influenced by Fielding’s work. The architectural abstractions and constraints he established led to the introduction of hypermedia as the engine of application state. The latter has given us a new perspective on how the Web can be used for tasks other than information storage and retrieval. His work on REST demonstrated that the Web is an application platform, with the REST architectural style providing guiding principles for building distributed applications that scale well, exhibit loose coupling, and compose functionality across service boundaries.
The idea is simple, and yet very powerful. A distributed application makes forward progress by transitioning from one state to another, just like a state machine. The difference from traditional state machines, however, is that the possible states and the transitions between them are not known in advance. Instead, as the application reaches a new state, the next possible transitions are discovered. It’s like a treasure hunt.
We’re used to this notion on the human Web. In a typical e-commerce solution such as Amazon.com, the server generates web pages with links on them that corral the user through the process of selecting goods, purchasing, and arranging delivery.
In a hypermedia system, application states are communicated through representations of uniquely identifiable resources. The identifiers of the states to which the application can transition are embedded in the representation of the current state in the form of links. Figure 1-7 illustrates such a hypermedia state machine.
This, in simple terms, is what the famous hypermedia as the engine of application state or HATEOAS constraint is all about. We see it in action every day on the Web, when we follow the links to other pages within our browsers. In this book, we show how the same principles can be used to enable computer-to-computer interactions.
While REST captures the fundamental principles that underlie the Web, there are still occasions where practice sidesteps theoretical guidance. Even so, the term REST has become so popular that it is almost impossible to disassociate it from any approach that uses HTTP. It’s no surprise that the term REST is treated as a buzzword these days rather than as an accurate description of the Web’s blueprints.
The pervasiveness of HTTP sets it aside as being special among all the Internet protocols. The Web has become a universal “on ramp,” providing near-ubiquitous connectivity for billions of software agents across the planet. Correspondingly, the focus of this book is on the Web as it is used in practice—as a distributed application platform rather than as a single large hypermedia system. Although we are highly appreciative of Fielding’s research, and of much subsequent work in understanding web-scale systems, we’ll use the term web throughout this book to depict a warts-’n-all view, reserving the REST terminology to describe solutions that embrace the REST architectural style. We do this because many of today’s distributed applications on the Web do not follow the REST architectural tenets, even though many still refer to these applications as “RESTful.”