Representational state transfer (REST) is an architectural style consisting of a coordinated set of architectural constraints applied to components, connectors, and data elements, within a distributed hypermedia system. REST ignores the details of component implementation and protocol syntax in order to focus on the roles of components, the constraints upon their interaction with other components, and their interpretation of significant data elements.
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).
The REST architectural style is also applied to the development of Web services as an alternative to other distributed-computing specifications such as SOAP.
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.
The Architectural properties induced by the architectural constraints of the REST architectural style are:
Fielding describes REST's effect on scalability thus:
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 of the information necessary to service the request, and session state is held in the client. Important to note is that the session state can be transferred by the server to another service such as a database to maintain a persistent state for a period of time 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 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 guiding principles of this 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 does not send its database, but rather, perhaps, some HTML, XML or JSON that represents some database records expressed, for instance, in Swahili and encoded in UTF-8, depending on the details of the request and the server implementation.
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.
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). Responses also explicitly indicate their cacheability.
Hypermedia as the engine of application state (A.K.A. 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.
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.
Representational State Transfer is intended to evoke an image of how a well-designed Web application behaves: presented with a network of Web pages (a virtual state-machine), the user progresses through an 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.
REST was initially described in the context of HTTP, but it is not limited to that protocol. RESTful architectures may be based on other Application Layer protocols if they already provide a rich and uniform vocabulary for applications based on the transfer of meaningful representational state. RESTful applications maximize the use of the existing, well-defined interface and other built-in capabilities provided by the chosen network protocol, and minimize the addition of new application-specific features on top of it.
Vocabulary re-use vs. its arbitrary extension: HTTP and SOAP
REST uses these operations and other existing features of the HTTP protocol. For example, layered proxy and gateway components perform additional functions on the network, such as HTTP caching and security enforcement.
SOAPRPC over HTTP, on the other hand, encourages each application designer to define new, application-specific operations that supplant HTTP operations. An example could be:
This additive vocabulary disregards many of HTTP's existing capabilities, such as authentication, caching, and content-type negotiation. The advantage SOAP has over REST is that SOAP, not following HTTP conventions, works equally well over raw TCP, named pipes, message queues, etc.
An important concept in REST is the existence of resources (sources of specific information), each of which is referenced with a global identifier (e.g., a URI in HTTP). In order to manipulate these resources, components of the network (user agents and origin servers) communicate via a standardized interface (e.g., HTTP) and exchange representations of these resources (the actual documents conveying the information). For example, a resource that represents a circle (as a logical object) may accept and return a representation that specifies a center point and radius, formatted in SVG, but may also accept and return a representation that specifies any three distinct points along the curve (since this also uniquely identifies a circle) as a comma-separated list.
Any number of connectors (e.g., clients, servers, caches, tunnels, etc.) can mediate the request, but each does so without "seeing past" its own request (referred to as "layering", another constraint of REST and a common principle in many other parts of information and networking architecture). Thus, an application can interact with a resource by knowing two things: the identifier of the resource and the action required—it does not need to know whether there are caches, proxies, gateways, firewalls, tunnels, or anything else between it and the server actually holding the information. The application does, however, need to understand the format of the information (representation) returned, which is typically an HTML, XML, or JSON document of some kind, although it may be an image, plain text, or any other content.
Applied to Web Services
Web Service APIs that adhere to the REST constraints are called RESTful. RESTful APIs are defined with these aspects:
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, a RESTful implementation such as the Web can use standards like HTTP, URI, XML, etc.
RESTful Web Services can be found in a number of places on the public Web:
^Richardson, Leonard; Sam Ruby (2007), RESTful web service, O'Reilly Media, ISBN978-0-596-52926-0, retrieved 18 January 2011, "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."Cite uses deprecated parameters (help)