Representational state transfer

"REST" redirects here. For other uses, see Rest.

In computing, representational state transfer (REST) is an architectural style consisting of a coordinated set of components, connectors, and data elements within a distributed hypermedia system, where the focus is on component roles and a specific set of interactions between data elements rather than implementation details. Its purpose is to induce performance, scalability, simplicity, modifiability, visibility, portability, and reliability. REST is the software architectural style of the World Wide Web.

The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation at UC Irvine. REST has been applied to describe desired web architecture, to identify existing problems, to compare alternative solutions and to ensure that protocol extensions would not violate the core constraints that make the web successful. Fielding used REST to design HTTP 1.1 and Uniform Resource Identifiers (URI).

To the extent that systems conform to the constraints of REST they can be called RESTful. RESTful systems typically, but not always, communicate over Hypertext Transfer Protocol (HTTP) with the same HTTP verbs (GET, POST, PUT, DELETE, etc.) that web browsers use to retrieve web pages and to send data to remote servers. REST systems interface with external systems as web resources identified by Uniform Resource Identifiers (URIs), for example /people/tom, which can be operated upon using standard verbs such as GET /people/tom.

The name "Representational State" is intended to evoke an image of how a well-designed Web application behaves: a network of web pages (a virtual state-machine), where the user progresses through the application by selecting links (state transitions), resulting in the next page (representing the next state of the application) being transferred to the user and rendered for their use.

History

REST was defined by Roy Thomas Fielding in his 2000 PhD dissertation "Architectural Styles and the Design of Network-based Software Architectures". Fielding developed the REST architectural style in parallel with HTTP 1.1 of 1996-1999, based on the existing design of HTTP 1.0 of 1996.

In a retrospective look at the development of REST Roy Fielding said:

Throughout the HTTP standardization process, I was called on to defend the design choices of the Web. That is an extremely difficult thing to do within a process that accepts proposals from anyone on a topic that was rapidly becoming the center of an entire industry. I had comments from well over 500 developers, many of whom were distinguished engineers with decades of experience, and I had to explain everything from the most abstract notions of Web interaction to the finest details of HTTP syntax. That process honed my model down to a core set of principles, properties, and constraints that are now called REST.

Architectural properties

The architectural properties affected by the constraints of the REST architectural style are

• Performance - component interactions can be the dominant factor in user-perceived performance and network efficiency
• Scalability to support large numbers of components and interactions among components. Roy Fielding, one of the principal authors of the HTTP specification, describes REST's effect on scalability as follows:

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.

• Simplicity of interfaces
• Modifiability of components to meet changing needs (even while the application is running)
• Visibility of communication between components by service agents
• Portability of components by moving program code with the data
• Reliability is the resistance to failure at the system level in the presence of failures within components, connectors, or data

Architectural constraints

The architectural properties of REST are realized by applying specific interaction constraints to components, connectors, and data elements. 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 non-functional properties, such as performance, scalability, simplicity, modifiability, visibility, portability, and reliability.

The formal REST constraints are

Client–server

See also: Client–server model

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.

Stateless

See also: Stateless protocol

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.

Cacheable

See also: Web cache

As on the World Wide Web, clients and intermediaries 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.

Layered system

See also: Layered system

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.

Code on demand (optional)

See also: Client-side scripting

Servers can temporarily extend or customize the functionality of a client by the transfer of executable code. Examples of this may include compiled components such as Java applets and client-side scripts such as JavaScript.

Uniform interface

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

Identification of resources

Individual resources are identified in requests, for example using URIs in web-based REST systems. The resources themselves are conceptually separate from the representations that are returned to the client. For example, the server may send data from its database as HTML, XML or JSON, none of which are the server's internal representation.

Manipulation of resources through these representations

When a client holds a representation of a resource, including any metadata attached, it has enough information to modify or delete the resource.

Self-descriptive messages

Each message includes enough information to describe how to process the message. For example, which parser to invoke may be specified by an Internet media type (previously known as a MIME type).

Hypermedia as the engine of application state (HATEOAS)

Clients make state transitions only through actions that are dynamically identified within hypermedia by the server (e.g., by hyperlinks within hypertext). Except for simple fixed entry points to the application, a client does not assume that any particular action is available for any particular resources beyond those described in representations previously received from the server. There is no universally accepted format for representing links between two resources. RFC 5988 and JSON Hypermedia API Language (proposed) are two popular formats for specifying REST hypermedia links.

Applied to web services

Web service APIs that adhere to the REST architectural constraints are called RESTful APIs. HTTP-based RESTful APIs are defined with the following aspects:

• base URI, such as http://example.com/resources/
• an Internet media type for the data. This is often JSON but can be any other valid Internet media type (e.g., XML, Atom, microformats, application/vnd.collection+json, etc.)
• standard HTTP methods (e.g., OPTIONS, GET, PUT, POST, and DELETE)
• hypertext links to reference state
• hypertext links to reference-related resources

Relationship between URI and HTTP Methods

The following table shows how HTTP methods are typically used in a RESTful API:

HTTP methods

Uniform Resource Identifier (URI)	GET	PUT	POST	DELETE
Collection, such as http://api.example.com/resources/	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, such as http://api.example.com/resources/item17	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 does not 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 does not change it. The distinction between PUT/DELETE and GET are roughly analogous to the notion of Command-Query Separation (CQS). For example: A query operation (like GET) promises no side-effects (e.g. changes) in data being queried. Commands (like PUT/DELETE) answer no questions about the data, but compute changes applied to the data (e.g. UPDATE or INSERT to use database terms).

Unlike SOAP-based web services, there is no "official" standard for RESTful web APIs. This is because REST is an architectural style, while SOAP is a protocol. Even though REST is not a standard per se, most RESTful implementations make use of standards such as HTTP, URI, JSON, and XML.

See also

• Create, read, update and delete (CRUD)
• Domain Application Protocol (DAP)
• Microservices
• Resource-oriented architecture (ROA)
• Resource-oriented computing (ROC)
• Semantic URLs
• Service-oriented architecture (SOA)
• Web-oriented architecture (WOA)
• Overview of RESTful API Description Languages
  • OData – Protocol for REST APIs
  • RAML (software)
  • RSDL (RESTful Service Description Language)
  • OpenAPI Specification — specification for defining interfaces

Further reading

• Pautasso, Cesare; Wilde, Erik; Alarcon, Rosa (2014), REST: Advanced Research Topics and Practical Applications 
• Pautasso, Cesare; Zimmermann, Olaf; Leymann, Frank (April 2008), "RESTful Web Services vs. Big Web Services: Making the Right Architectural Decision", 17th International World Wide Web Conference (WWW2008) (Beijing, China) 
• Ferreira, Otavio (Nov 2009), Semantic Web Services: A RESTful Approach, IADIS, ISBN 978-972-8924-93-5