Representational state transfer
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Representational state transfer (REST) is a de-facto standard for a software architecture for interactive applications that typically use multiple Web services. In order to be used in a REST-based application, a Web Service needs to meet certain constraints; such a Web Service is called RESTful. A RESTful Web service is required to provide an application access to its Web resources in a textual representation and support reading and modification of them with a stateless protocol and a predefined set of operations. By being RESTfull, Web Services provide interoperability between the computer systems on the internet that provide these services. REST offers an alternative to, for instance, SOAP as method of access to a Web Service.
"Web resources" were first defined on the World Wide Web as documents or files identified by their URLs. However, today they have a much more generic and abstract definition that encompasses every thing, entity, or action that can be identified, named, addressed, handled, or performed, in any way whatsoever, on the Web. In a RESTful Web service, requests made to a resource's URI will elicit a response with a payload formatted in HTML, XML, JSON, or some other format. The response can confirm that some alteration has been made to the resource state, and the response can provide hypertext links to other related resources. When HTTP is used, as is most common, the operations (HTTP methods) available are GET, HEAD, POST, PUT, PATCH, DELETE, CONNECT, OPTIONS and TRACE.
By using a stateless protocol and standard operations, RESTful systems aim for fast performance, reliability, and the ability to grow by reusing components that can be managed and updated without affecting the system as a whole, even while it is running.
The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation. Fielding's dissertation explained the REST principles that were known as the "HTTP object model" beginning in 1994, and were used in designing the HTTP 1.1 and Uniform Resource Identifiers (URI) standards. The term is intended to evoke an image of how a well-designed Web application behaves: it is a network of Web resources (a virtual state-machine) where the user progresses through the application by selecting resource identifiers such as http://www.example.com/articles/21 and resource operations such as GET or POST (application state transitions), resulting in the next resource's representation (the next application state) being transferred to the end user for their use.
Roy Fielding defined REST in his 2000 PhD dissertation "Architectural Styles and the Design of Network-based Software Architectures" at UC Irvine. He developed the REST architectural style in parallel with HTTP 1.1 of 1996–1999, based on the existing design of HTTP 1.0 of 1996.
In a retrospective look at the development of REST, Fielding said:
Throughout the HTTP standardization process, I was called on to defend the design choices of the Web. That is an extremely difficult thing to do within a process that accepts proposals from anyone on a topic that was rapidly becoming the center of an entire industry. I had comments from well over 500 developers, many of whom were distinguished engineers with decades of experience, and I had to explain everything from the most abstract notions of Web interaction to the finest details of HTTP syntax. That process honed my model down to a core set of principles, properties, and constraints that are now called REST.
- performance in component interactions, which can be the dominant factor in user-perceived performance and network efficiency;
- scalability allowing the support of large numbers of components and interactions among components. Roy Fielding describes REST's effect on scalability as follows:
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 a uniform interface;
- 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 in the resistance to failure at the system level in the presence of failures within components, connectors, or data.
Six guiding constraints define a RESTful system. These constraints restrict the ways that the server can process and respond to client requests so that, by operating within these constraints, the system gains desirable non-functional properties, such as performance, scalability, simplicity, modifiability, visibility, portability, and reliability. If a system violates any of the required constraints, it cannot be considered RESTful.
The formal REST constraints are as follows:
The principle behind the client–server constraints is the separation of concerns. Separating the user interface concerns from the data storage concerns improves the portability of the user interfaces across multiple platforms. It also improves scalability by simplifying the server components. Perhaps most significant to the Web is that the separation allows the components to evolve independently, thus supporting the Internet-scale requirement of multiple organizational domains.
In a client–server interaction, state is made up of intrinsic state and extrinsic state. Intrinsic state, called resource state, is stored on the server and consists of information that is independent of the server’s context, thereby making it sharable to all clients of the server. Extrinsic state, called application state, is stored on each client and consists of information that is dependent on the server’s context and therefore cannot be shared. Clients are responsible for passing application state to the server when it needs it. The constraint of storing application state on the client rather than on the server makes the communication stateless.
As on the World Wide Web, clients and intermediaries can cache responses. Responses must, implicitly or explicitly, define themselves as either cacheable or non-cacheable to prevent clients from providing 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. If a proxy or load balancer is placed between the client and server, it won't affect their communications, and there won't be a need to update the client or server code. Intermediary servers can improve system scalability by enabling load balancing and by providing shared caches. Also, security can be added as a layer on top of the web services, separating business logic from security logic. Adding security as a separate layer enforces security policies. Finally, intermediary servers can call multiple other servers to generate a response to the client.
Code on demand (optional)
The uniform interface constraint is fundamental to the design of any RESTful system. It simplifies and decouples the architecture, which enables each part to evolve independently. The four constraints for this uniform interface are:
- Resource identification in requests
- Individual resources are identified in requests, for example using URIs in RESTful Web services. The resources themselves are conceptually separate from the representations that are returned to the client. For example, the server could send data from its database as HTML, XML or as JSON—none of which are the server's internal representation.
- Resource manipulation through representations
- When a client holds a representation of a resource, including any metadata attached, it has enough information to modify or delete the resource's state.
- Self-descriptive messages
- Each message includes enough information to describe how to process the message. For example, which parser to invoke can be specified by a media type.
- Hypermedia as the engine of application state (HATEOAS)
- Having accessed an initial URI for the REST application—analogous to a human Web user accessing the home page of a website—a REST client should then be able to use server-provided links dynamically to discover all the available resources it needs. As access proceeds, the server responds with text that includes hyperlinks to other resources that are currently available. There is no need for the client to be hard-coded with information regarding the structure or dynamics of the application.
Applied to web services
- a base URI, such as
- standard HTTP methods (e.g., GET, POST, PUT, and DELETE);
- a media type that defines state transition data elements (e.g., Atom, microformats, application/vnd.collection+json,:91–99 etc.). The current representation tells the client how to compose requests for transitions to all the next available application states. This could be as simple as a URI or as complex as a Java applet.
Semantics of HTTP methods
The following table shows how HTTP methods are intended to be used in HTTP APIs, including RESTful ones.
|GET:§4.3.1||Get a representation of the target resource’s state.|
|POST:§4.3.3||Let the target resource process the representation enclosed in the request.|
|PUT:§4.3.4||Set the target resource’s state to the state defined by the representation enclosed in the request.|
|DELETE:§4.3.5||Delete the target resource’s state.|
The GET method is safe, meaning that applying it to a resource does not result in a state change of the resource (read-only semantics).:§4.2.1 The GET, PUT, and DELETE methods are idempotent, meaning that applying them multiple times to a resource results in the same state change of the resource as applying them once, though the response might differ.:§4.2.2 The GET and POST methods are cacheable, meaning that responses to them are allowed to be stored for future reuse.:§4.2.3
The GET (read), PUT (create and update), and DELETE (delete) methods are CRUD operations as they have storage management semantics, meaning that they let user agents directly manipulate the states of target resources. The POST method is not a CRUD operation but a process operation that has target-resource-specific semantics excluding storage management semantics, so it does not let user agents directly manipulate the states of target resources.:§4.3.3
Unlike SOAP-based web services, there is no "official" standard for RESTful web APIs. This is because REST is an architectural style, while SOAP is a protocol. REST is not a standard in itself, but RESTful implementations make use of standards, such as HTTP, URI, JSON, and XML. Many developers also describe their APIs as being RESTful, even though these APIs actually don't fulfil all of the architectural constraints described above (especially the uniform interface constraint).
- Atomicity, consistency, isolation, durability (ACID)
- Clean URLs
- Create, read, update and delete (CRUD)
- Domain Application Protocol (DAP)
- Overview of RESTful API Description Languages
- Resource-oriented architecture (ROA)
- Resource-oriented computing (ROC)
- Service-oriented architecture (SOA)
- Web-oriented architecture (WOA)
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This chapter introduced the Representational State Transfer (REST) architectural style for distributed hypermedia systems. REST provides a set of architectural constraints that, when applied as a whole, emphasizes scalability of component interactions, the generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems.
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Since 1994, the REST architectural style has been used to guide the design and development of the architecture for the modern Web. This chapter describes the experience and lessons learned from applying REST while authoring the Internet standards for the Hypertext Transfer Protocol (HTTP) and Uniform Resource Identifiers (URI), the two specifications that define the generic interface used by all component interactions on the Web, as well as from the deployment of these technologies in the form of the libwww-perl client library, the Apache HTTP Server Project, and other implementations of the protocol standards.
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