Hypermedia as the Engine of Application State is a component of the REST application architecture that distinguishes it from other network application architectures. With HATEOAS, a client interacts with a network application whose application servers provide information dynamically through hypermedia. A REST client needs little to no prior knowledge about how to interact with an application or server beyond a generic understanding of hypermedia. By contrast, clients and servers in CORBA interact through a fixed interface shared through documentation or an interface description language. The restrictions imposed by HATEOAS decouple client and server. This enables server functionality to evolve independently.
Example
A REST client enters a REST application through a simple fixed URL. All future actions the client may take are discovered within resource representations returned from the server. The media types used for these representations, and the link relations they may contain, are standardized. The client transitions through application states by selecting from the links within a representation or by manipulating the representation in other ways afforded by its media type. In this way, RESTful interaction is driven by hypermedia, rather than out-of-band information. For example, this GET requestfetches an account resource, requesting details in a JSON representation: GET /accounts/12345 HTTP/1.1 Host: bank.example.com Accept: application/vnd.acme.account+json ...
The response is: HTTP/1.1 200 OK Content-Type: application/vnd.acme.account+json Content-Length:...
The response contains these possible follow-up links: make a deposit, withdrawal, or transfer, or close the account. When the account information is retrieved later, the account is overdrawn: HTTP/1.1 200 OK Content-Type: application/vnd.acme.account+json Content-Length:...
Now only one link is available: to deposit more money. In its current state, the other links are not available. Hence the term Engine of Application State. What actions are possible varies as the state of the resource varies. A client does not need to understand every media type and communication mechanism offered by the server. The ability to understand new media types can be acquired at run-time through "code-on-demand" provided to the client by the server.
Origins
The HATEOAS constraint is an essential part of the "uniform interface" feature of REST, as defined in Roy Fielding's doctoral dissertation. Fielding has further described the concept on his blog. The purpose of some of the strictness of this and other REST constraints, Fielding explains, is "software design on the scale of decades: every detail is intended to promote software longevity and independent evolution. Many of the constraints are directly opposed to short-term efficiency. Unfortunately, people are fairly good at short-term design, and usually awful at long-term design".
