Representational state transfer

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Representational state transfer (REST) is a software 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.[1][2]

The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation at UC Irvine.[1][3] 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).[4][5] The REST architectural style is also applied to the development of web services[6] as an alternative to other distributed-computing specifications such as SOAP.

History[edit]

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.[7] The World Wide Web represents the largest implementation of a system conforming to the REST architectural style.

Software architecture[edit]

A software architecture is defined by a configuration of components, connectors, and data constrained in their relationships in order to achieve a desired set of architectural properties.[8]

Components[edit]

A component is an abstract unit of software instructions and internal state that provides a transformation of data via its interface.[8]

Connectors[edit]

A connector is an abstract mechanism that mediates communication, coordination, or cooperation among components.[8]

Data[edit]

A datum is an element of information that is transferred from a component, or received by a component, via a connector.[8]

Architectural properties[edit]

The architectural properties induced by the architectural constraints of the REST architectural style are:[9][1]

  • Performance - component interactions can be the dominant factor in user-perceived performance and network efficiency.[10]
  • Scalability to support large numbers of components and interactions among component
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.[11]

  • 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 partial failures within components, connectors, or data[10]

Architectural constraints[edit]

The architectural properties of REST are realized by applying specific interaction constraints to components, connectors, and data elements.[1][9] The formal REST constraints are:

Client–server[edit]

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[edit]

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.[12]

Cacheable[edit]

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[edit]

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)[edit]

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. "Code on demand" is the only optional constraint of the REST architecture.

Uniform interface[edit]

The uniform interface constraint is fundamental to the design of any REST service.[13] 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.
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.[1]
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".[14] 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.

Concept[edit]

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.[15]

REST was initially described in the context of HTTP, but it is not limited to that protocol.[16] 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.

Applied to web services[edit]

Web service APIs that adhere to the REST constraints are called RESTful. RESTful APIs are defined with these 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, images, etc.)
  • standard HTTP methods (e.g., GET, PUT, POST, or DELETE)
  • hypertext links to reference state
  • hypertext links to reference related resources[17]

The following table shows how the HTTP methods are typically used to implement a RESTful API.

RESTful API HTTP methods
Resource GET PUT POST DELETE
Collection URI, such as http://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 URI, such as http://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 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 idempotent methods. The GET method is a safe method (or nullipotent), meaning that calling it produces no side-effects.

Unlike SOAP-based web services, there is no "official" standard for RESTful web APIs.[18] 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.

See also[edit]

References[edit]

Notes[edit]

  1. ^ a b c d e Chapter 5 of Fielding's dissertation is "Representational State Transfer (REST)".
  2. ^ 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 
  3. ^ "Fielding discussing the definition of the REST term". Tech.groups.yahoo.com. Retrieved 2013-11-28. 
  4. ^ RFC 1945
  5. ^ RFC 2616
  6. ^ Fielding, R. T.; Taylor, R. N. (2000). Principled design of the modern Web architecture. pp. 407–416. doi:10.1145/337180.337228.  edit
  7. ^ "Fielding discusses the development of the REST style". Tech.groups.yahoo.com. Retrieved 2013-02-07. 
  8. ^ a b c d Chapter 1 of Fielding's dissertation is "Software Architecture".
  9. ^ a b 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. 
  10. ^ a b Chapter 2 of Fielding's dissertation is "Network-based Application Architectures".
  11. ^ (Fielding 2000, §5.3.1)
  12. ^ "Fielding talks about application states". Tech.groups.yahoo.com. Retrieved 2013-02-07. 
  13. ^ (Fielding 2000, §5.1.5)
  14. ^ 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." 
  15. ^ "Fielding's PhD thesis, section 6.1". Ics.uci.edu. Retrieved 2013-02-07. 
  16. ^ 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. 
  17. ^ "REST APIs must be hypertext driven by Roy Fielding". Roy.gbiv.com. 2008-10-20. Retrieved 2013-02-07. 
  18. ^ Elkstein, M. What is REST?. Retrieved on 2009-07-04.