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An important aspect of the Uniform Interface constraint is described as Hypermedia As The Engine Of Application State, or HATEOAS. We discuss the nature and value of HATEOAS in our Why is HATEOAS important? article, but we'll provide a simple explanation of it here. HATEOAS is a design feature where information about the state of a resource should be provided to the client through hyperlinks (aka hypermedia). This avoids the need for maintaining state and it avoids the need for the client to assume (or hardcode) how state change requests are made. Incorporating HATEOAS to the design of web APIs requires a fair amount of extra work and planning. This often drives many developers to opt out of HATEOAS. This is often referred to as practical pragmatic REST whereas purist REST includes and advocates for HATEOAS. Regardless of the ongoing internal debates, Dr. Fielding himself has expressed his feelings rather clearly, "...if the engine of application state (and hence the API) is not being driven by hypertext, then it cannot be RESTful and cannot be a REST API" (emphasis added).

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