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Representational State Transfer (REST) is a software architecture style consisting of guidelines and best practices for creating scalable web services. REST is a coordinated set of constraints applied to the design of components in a distributed hypermedia system that can lead to a more performant and maintainable architecture.
REST has gained widespread acceptance across the Web as a simpler alternative to SOAP and WSDL-based Web services. RESTful systems communicate over the Hypertext Transfer Protocol with the same HTTP verbs (GET, POST, PUT, DELETE, etc.) used by web browsers to retrieve web pages and send data to remote servers.
The REST architectural style was developed by W3C Technical Architecture Group (TAG) in parallel with HTTP 1.1, based on the existing design of HTTP 1.0. The World Wide Web represents the largest implementation of a system conforming to the REST architectural style.
REST's client–server separation of concerns simplifies component implementation, reduces the complexity of connector semantics, improves the effectiveness of performance tuning, and increases the scalability of pure server components. Layered system constraints allow intermediaries—proxies, gateways, and firewalls—to be introduced at various points in the communication without changing the interfaces between components, thus allowing them to assist in communication translation or improve performance via large-scale, shared caching. REST enables intermediate processing by constraining messages to be self-descriptive: interaction is stateless between requests, standard methods and media types are used to indicate semantics and exchange information, and responses explicitly indicate cacheability.
A uniform interface separates clients from servers. This separation of concerns means that, for example, clients are not concerned with data storage, which remains internal to each server, so that the portability of client code is improved. Servers are not concerned with the user interface or user state, so that servers can be simpler and more scalable. Servers and clients may also be replaced and developed independently, as long as the interface between them is not altered.
The client–server communication is further constrained by no client context being stored on the server between requests. Each request from any client contains all the information necessary to service the request, and session state is held in the client. The session state can be transferred by the server to another service such as a database to maintain a persistent state for a period and allow authentication. The client begins sending requests when it is ready to make the transition to a new state. While one or more requests are outstanding, the client is considered to be in transition. The representation of each application state contains links that may be used the next time the client chooses to initiate a new state-transition.
As on the World Wide Web, clients can cache responses. Responses must therefore, implicitly or explicitly, define themselves as cacheable, or not, to prevent clients from reusing stale or inappropriate data in response to further requests. Well-managed caching partially or completely eliminates some client–server interactions, further improving scalability and performance.
A client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way. Intermediary servers may improve system scalability by enabling load balancing and by providing shared caches. They may also enforce security policies.
The uniform interface constraint is fundamental to the design of any REST service. The uniform interface simplifies and decouples the architecture, which enables each part to evolve independently. The four constraints for this uniform interface are:
One can characterise applications conforming to the REST constraints described in this section as "RESTful". If a service violates any of the required constraints, it cannot be considered RESTful.
Complying with these constraints, and thus conforming to the REST architectural style, enables any kind of distributed hypermedia system to have desirable emergent properties, such as performance, scalability, simplicity, modifiability, visibility, portability, and reliability.
Web service APIs that adhere to the REST architectural constraints are called RESTful. HTTP based RESTful APIs are defined with these aspects:
A simple example of designing a web service for managing employee data using an OData REST implementation might involve several methods, each corresponding to one of the HTTP verbs. A method like "Employees/GetEmployees" would be mapped to the GET verb (or "Employees/GetEmployee/12345" in the case of retrieving details for a single specific employee), handling all requests submitted to the web service, "Employees/AddEmployee" would be mapped to the POST verb, "Employees/UpdateEmployee" would be mapped to PUT and "Employees/DeleteEmployee" would be mapped to the DELETE verb. If the service were also exposing an interface to allow remote clients to manage consumer products the API would follow a similar naming convention but obviously specific to consumer products (i.e. Products/GetProducts, Products/AddProduct etc.). Any remote client that has access and is authorized to use any of these methods would be able to execute them provided that the remote client is capable of sending and receiving data using the HTTP protocol.
The following table shows the HTTP methods that are typically used to implement a RESTful API.
|Collection URI, such as ||List the URIs and perhaps other details of the collection's members.||Replace the entire collection with another collection.||Create a new entry in the collection. The new entry's URI is assigned automatically and is usually returned by the operation.||Delete the entire collection.|
|Element URI, such as ||Retrieve a representation of the addressed member of the collection, expressed in an appropriate Internet media type.||Replace the addressed member of the collection, or if it doesn't exist, create it.||Not generally used. Treat the addressed member as a collection in its own right and create a new entry in it.||Delete the addressed member of the collection.|
The PUT and DELETE methods are referred to as idempotent, meaning that the operation will produce the same result no matter how many times it is repeated. The GET method is a safe method (or nullipotent), meaning that calling it produces no side-effects. In other words, retrieving or accessing a record doesn't change it.
Unlike SOAP-based web services, there is no "official" standard for RESTful web APIs. This is because REST is an architectural style, unlike SOAP, which is a protocol. Even though REST is not a standard per se, most RESTful implementations make use of standards like HTTP, URI, JSON, XML, etc.
The main topic of this book is the web service architectures which can be considered RESTful: those which get a good score when judged on the criteria set forth in Roy Fielding's dissertation.
POST /api/carts Content-Type:application/vnd.example.coolapp.cart-v1+xml Content-Length: 1032 <cart> <customerId>1343</customerId> <lineItems> <lineItem> <productId>12343</productId> <quantity>4</quantity> </lineItem> ... </lineItems> </cart> Response: HTTP/1.1 201 Created Location: /api/carts/323392