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Representational State Transfer (REST) is an architectural style that abstracts the architectural elements within a distributed hypermedia system.[1] 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.[2] REST has emerged as a predominant web API design model.[citation needed]

The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation at UC Irvine.[1][3] Fielding is one of the principal authors of HTTP 1.0 and 1.1.[4][5]

About

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.[6] 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.[7]

Key goals

Key goals of REST include:

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 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.[8]

Constraints

The REST architectural style describes the following six constraints applied to the architecture, while leaving the implementation of the individual components free to design:

Client–server
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
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.
Cacheable
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)
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
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.

One can characterise applications conforming to the REST constraints described in this section as "RESTful".[9] 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

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.[10]

REST was initially described in the context of HTTP, but it is not limited to that protocol.[11] 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

In addition to URIs; Internet media types; request and response codes; etc., HTTP has a vocabulary of operations called request methods, most notably:

GET
POST
PUT
PATCH
DELETE

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.

SOAP RPC over HTTP, on the other hand, encourages each application designer to define new, application-specific operations that supplant HTTP operations. An example could be:

getUsers()
getNewUsersSince(date SinceDate)
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.[12] 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.[13]

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.[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.

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 REST principles. It is a collection of resources, with four defined aspects:

  • the base URI for the web API, such as http://example.com/resources/
  • 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.[14]

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

RESTful web 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.[15] 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.

Objections

REST invites the client to specify the unique ID of a new item to be created in a collection managed by the server. History has shown[according to whom?] it to be less problematic if the client's new-item request specifies only the content data, and the server returns a unique "handle" for the new item. Several techniques[which?] for handling ensuing complications are known[by whom?], but the REST architectural style does not specify which of these are to be employed.

Outside the Web

Software that may interact with a number of different kinds of objects or devices can do so by virtue of a uniform, agreed interface.

CMIP

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:

See also

References

  • Fielding, Roy T.; Taylor, Richard N. (2002-05), "Principled Design of the Modern Web Architecture" (PDF), ACM Transactions on Internet Technology (TOIT), 2 (2), New York: Association for Computing Machinery: 115–150, doi:10.1145/514183.514185, ISSN 1533-5399 {{citation}}: Check date values in: |date= (help)
  • 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 {{citation}}: Check date values in: |date= (help); Unknown parameter |month= ignored (help)
  • Richardson, Leonard; Ruby, Sam (2007-05), RESTful Web Services, O'Reilly, ISBN 978-0-596-52926-0 {{citation}}: Check date values in: |date= (help)

Notes

  1. ^ a b c Chapter 5 of Fielding's dissertation is "Representational State Transfer (REST)".
  2. ^ Fielding, Roy T.; Taylor, Richard N. (2002-05), "Principled Design of the Modern Web Architecture" (PDF), ACM Transactions on Internet Technology (TOIT), 2 (2), New York: Association for Computing Machinery: 115–150, doi:10.1145/514183.514185, ISSN 1533-5399 {{citation}}: Check date values in: |date= (help)
  3. ^ "Fielding discussing the definition of the REST term". Tech.groups.yahoo.com. Retrieved 2013-02-07.
  4. ^ RFC 1945
  5. ^ RFC 2616
  6. ^ "Fielding discusses the development of the REST style". Tech.groups.yahoo.com. Retrieved 2013-02-07.
  7. ^ "Fielding talks about application states". Tech.groups.yahoo.com. Retrieved 2013-02-07.
  8. ^ (Fielding 2000, §5.3.1)
  9. ^ Richardson, Leonard (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. {{citation}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ "Fielding's PhD thesis, section 6.1". Ics.uci.edu. Retrieved 2013-02-07.
  11. ^ 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.
  12. ^ Scribner, Kenn; Seely, Scott (2009), Effective REST Services via .NET, Boston: Addison-Wesley, ISBN 978-0-321-61325-7
  13. ^ (Fielding 2000, §5.1.5)
  14. ^ "REST APIs must be hypertext driven by Roy Fielding". Roy.gbiv.com. 2008-10-20. Retrieved 2013-02-07.
  15. ^ Elkstein, M. What is REST?. Retrieved on 2009-07-04.
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