Representational state transfer
||This article may be too technical for most readers to understand. (October 2014)|
Representational state transfer (REST) is an abstraction of the architecture of the World Wide Web; more precisely, 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 developed REST in collaboration with his colleagues during the same period he worked on HTTP 1.1 and Uniform Resource Identifiers (URI).
The REST architectural style is also applied to the development of web services. One can characterize web services as "RESTful" if they conform to the constraints described in the architectural constraints section. See the applied to web services section if you are only interested in the application of REST to web APIs.
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.
- A component is an abstract unit of software instructions and internal state that provides a transformation of data via its interface.
- A connector is an abstract mechanism that mediates communication, coordination, or cooperation among components.
- The information that is transferred from a component, or received by a component, via a connector. In REST the data elements are:
- Resource Identifier
- Identifies the particular resource involved in an interaction between components. Typically a URL or URI.
- Representation metadata
- Resource metadata
- Control data
- 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
- 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.
- 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
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. 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 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.
Code on demand (optional)
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, and it is the same one resource regardless.
- 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). Responses also explicitly indicate their cacheability.
- 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.
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.
Applied to web services
Web service APIs that adhere to the REST architectural constraints are called RESTful. HTTP based RESTful APIs are defined with these aspects:
- base URI, such as
- 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, images, etc.)
- standard HTTP methods (e.g., GET, PUT, POST, or DELETE)
- hypertext links to reference state
- hypertext links to reference related resources
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.
- OData – The Protocol for REST APIs
- RSDL (RESTful Service Description Language)
- Clean URLs
- Create, read, update and delete (CRUD)
- Resource-oriented architecture (ROA)
- Service-oriented architecture (SOA)
- Resource-oriented computing (ROC)
- Domain Application Protocol (DAP)
- RAML (software)
- Swagger Specification for defining interfaces
- Hypermedia as the Engine of Application State
- Chapter 5 of Fielding's dissertation is "Representational State Transfer (REST)".
- Fielding, Roy T.; Taylor, Richard N. (May 2002), 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 discussing the definition of the REST term". Tech.groups.yahoo.com. Retrieved 2013-11-28.
- "Fielding discusses the development of the REST style". Tech.groups.yahoo.com. Retrieved 2014-09-14.
- RFC 1945
- RFC 2616
- Fielding, R. T.; Taylor, R. N. (2000). "Principled design of the modern Web architecture". pp. 407–416. doi:10.1145/337180.337228.
- 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."
- Chapter 1 of Fielding's dissertation is "Software Architecture".
- Thomas Erl, Benjamin Carlyle, Cesare Pautasso, Raj Balasubramanian (2013). "5.1". In Thomas Erl. SOA with REST. Prentice Hall. ISBN 978-0-13-701251-0.
- Chapter 2 of Fielding's dissertation is "Network-based Application Architectures".
- (Fielding 2000, §5.3.1)
- "Fielding talks about application states". Tech.groups.yahoo.com. Retrieved 2013-02-07.
- (Fielding 2000, §5.1.5)
- "REST APIs must be hypertext driven by Roy Fielding". Roy.gbiv.com. 2008-10-20. Retrieved 2013-02-07.
- H, Jeremy (16 May 2012). "API Example Using REST". There Is No Right Way. Retrieved 31 July 2014. "
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"
- Elkstein, M. What is REST?. Retrieved on 2009-07-04.
- 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