A particularly popular form of stateless server designs are web APIs that use the REST architectural style, also called REST APIs or RESTful Web APIs.

1 REST

REST officially an abbreviation for representational state transfer, though that longer term is rarely used. REST includes 5 properties:

Client/Server – clients are distinct from servers, interacting though a defined interface or API.
Stateless – servers do not store per-client data.
Cache – each response from the server indicates how long it may be cached.
Uniform interface – the various request and response formats use as similar format to one another as possible.
Layered system – the server is designed to be able to split into many separate computers, with clients unable to tell which computer responded or even if there are several of them.

2 RESTful Web API

In general, RESTful web APIs have the following properties:

Clients send HTTP requests; servers reply.
The HTTP request method describes if/how the request changes server state. The two most common methods used are
- GET does not change server state
- POST changes existing server state
Sometimes PUT is used to create new server state and DELETE to remove existing server state, but sometimes they are both handled using POST instead. Other HTTP request methods are relatively rarely used in web APIs.
The HTTP request path identifies what resource you are accessing.
The HTTP request body (which is not used by GET requests) describes specific changes to be implemented.
HTTP headers may be used to pick one of several equivalent response formats, but should not be used to change the meaning of the request.
All request bodies within a single API use the same format; often JSON or XML, but sometimes the simpler but less flexible application/x-www-form-urlencoded and multipart/form-data formats used by HTML forms and query strings.
All response bodies that provide structured data structure it the same way, often JSON or XML.