Representational state transfer
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The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation. Fielding is one of the principal authors of the Hypertext Transfer Protocol (HTTP) specification versions 1.0 and 1.1.
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-style architectures conventionally consist of clients and servers. Clients initiate requests to servers; servers process requests and return appropriate responses. Requests and responses are built around the transfer of representations of resources. A resource can be essentially any coherent and meaningful concept that may be addressed. A representation of a resource is typically a document that captures the current or intended state of a resource.
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.
Key goals 
Key goals of REST include:
- Scalability of component interactions
- Generality of interfaces
- Independent deployment of components
- Intermediary components to reduce latency, enforce security and encapsulate legacy systems
REST has been applied to describe the desired web architecture, to help 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 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 lets 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.
The REST architectural style describes the following six constraints applied to the architecture, while leaving the implementation of the individual components free to design:
- 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 any session state is held in the client.
- 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.
- 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)
- Uniform interface
- The uniform interface between clients and servers, discussed below, simplifies and decouples the architecture, which enables each part to evolve independently. The four guiding principles of this interface are detailed below.
The only optional constraint of REST architecture is "code on demand". 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.
savePurchaseOrder(string CustomerID, string PurchaseOrderID)
This additive, "re-invention of the wheel" vocabulary — defined on the spot and subject to individual judgment or preference — disregards many of HTTP's existing capabilities, such as authentication, caching, and content-type negotiation. The advantage of SOAP over REST comes from this same limitation: since it does not take advantage of HTTP conventions, SOAP works equally well over raw TCP, named pipes, message queues, etc.
Guiding principles of the interface 
The uniform interface that any REST interface must provide is considered fundamental to the design of any REST service.
- 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 on the server, provided it has permission to do so.
- 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). Responses also explicitly indicate their cacheability.
- Hypermedia as the engine of application state (aka 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.
Central principle 
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.
RESTful web APIs 
A RESTful web API (also called a RESTful web service) is a web API implemented using HTTP and the principles of REST. It is a collection of resources, with four defined aspects:
- the base URI for the web API, such as
- the Internet media type of the data supported by the web API. This is often JSON but can be any other valid Internet media type provided that it is a valid hypertext standard.
- the set of operations supported by the web API using HTTP methods (e.g., GET, PUT, POST, or DELETE).
- The API must be hypertext driven.
The following table shows how the HTTP methods are typically used to implement a web 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.|
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.
Outside the Web 
|This section does not cite any references or sources. (July 2012)|
Software that may interact with a number of different kinds of objects or devices can do so by virtue of a uniform, agreed interface.
The Common Management Information Protocol (CMIP) was designed to allow the control of network resources by presenting their manageable characteristics as object attributes. The objects have parent-child relationships that are identified using distinguished names and attributes, which are read and modified by a set of CRUD (Create, Read, Update, Delete) operations. The notable non-restful aspect of CMIP is the M_ACTION operation although, wherever possible, designers of management information bases (MIBs) would typically endeavour to represent controllable and stateful aspects of network equipment through attributes.
Public implementations 
REST can be found in a number of places on the public Web:
- The Atom Publishing Protocol for publishing to blogs is considered a canonical RESTful protocol.
- Sun Microsystems' Cloud API is a good example of resource media type documentation.
- The Open Services for Lifecycle Collaboration (OSLC) initiative is establishing a RESTful approach to integrating software development artifacts.
- CouchDB is a document-oriented database written in Erlang that provides a RESTful JSON API that can be accessed from any environment that allows HTTP requests.
- MySQL Cluster is a write-scalable auto-sharded database also accessible through a native REST/JSON interface as an Apache module.
- Microsoft's Canonical REST Entity Service.
- Nuxeo, an open source document manager, implements a Content Automation interface via a REST API
- Restful Objects, a public specification for a generic RESTful API to any domain object model
- Sones GraphDB is a graph-oriented database written in C# that provides a RESTful interface
- Google Fusion Tables support includes a RESTful API
See also 
- Clean URLs
- Create, read, update and delete (CRUD)
- HATEOAS (Hypermedia as the Engine of Application State)
- Resource-oriented architecture (ROA)
- Service-oriented architecture (SOA)
- Resource-oriented computing (ROC)
- Fielding, Roy T.; Taylor, Richard N. (2002-05), "Principled Design of the Modern Web Architecture" (PDF), ACM Transactions on Internet Technology (TOIT) (New York: Association for Computing Machinery) 2 (2): 115–150, doi:10.1145/514183.514185, ISSN 1533-5399
- Fielding, Roy Thomas (2000), Architectural Styles and the Design of Network-based Software Architectures, Doctoral dissertation, University of California, Irvine
- Pautasso, Cesare; Zimmermann, Olaf; Leymann, Frank (2008-04), "RESTful Web Services vs. Big Web Services: Making the Right Architectural Decision", 17th International World Wide Web Conference (WWW2008) (Beijing, China)
- Richardson, Leonard; Ruby, Sam (2007-05), RESTful Web Services, O'Reilly, ISBN 978-0-596-52926-0
- Chapter 5 of Fielding's dissertation is "Representational State Transfer (REST)".
- "Fielding discussing the definition of the REST term". Tech.groups.yahoo.com. Retrieved 2013-02-07.
- RFC 1945
- RFC 2616
- "Fielding discusses the development of the REST style". Tech.groups.yahoo.com. Retrieved 2013-02-07.
- "Fielding talks about application states". Tech.groups.yahoo.com. Retrieved 2013-02-07.
- (Fielding 2000, §5.3.1)
- Richardson, Leonard; Sam Ruby (2007), RESTful web service, O'Reilly Media, ISBN 978-0-596-52926-0, 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."
- "Fielding's PhD thesis, section 6.1". Ics.uci.edu. Retrieved 2013-02-07.
- Sandeep Chanda; Damien Foggon (20 February 2013). Beginning ASP.NET 4.5 Databases. Apress. pp. 158–. ISBN 978-1-4302-4380-9. Retrieved 30 April 2013.
- Scribner, Kenn; Seely, Scott (2009), Effective REST Services via .NET, Boston: Addison-Wesley, ISBN 978-0-321-61325-7
- (Fielding 2000, §5.1.5)
- "REST APIs must be hypertext driven by Roy Fielding". Roy.gbiv.com. 2008-10-20. Retrieved 2013-02-07.
- Elkstein, M. What is REST?. Retrieved on 2009-07-04.