Representational State Transfer - Web Api Tutorial

Representational state transfer (REST) or RESTful Web services are one way of providing interoperability between computer systems on the Internet. REST-compliant Web services allow requesting systems to access and manipulate textual representations of Web resources using a uniform and predefined set of stateless operations. Other forms of Web service exist, which expose their own arbitrary sets of operations such as WSDL and SOAP. "Web resources" were first defined on the World Wide Web as documents or files identified by their URLs, but today they have a much more generic and abstract definition encompassing every thing or entity that can be identified, named, addressed or handle d, in any way whatsoever, on the Web. In a RESTful Web service, requests made to a resource's URI will elicit a response that may be in XML, HTML, JSON or some other defined format. The response may confirm that some alteration has been made to the stored resource, and it may provide hypertext links to other related resources or collections of resources. Using HTTP, as is most common, the kind of operations available include those predefined by the HTTP verbs GET, POST, PUT, DELETE and so on. By making use of a stateless protocol and standard operations, REST systems aim for fast performance, reliability, and the ability to grow, by re-using 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 used REST to design HTTP 1.1 and Uniform Resource Identifiers (URI). 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 links, such as /user/tom, and operations such as GET or DELETE (state transitions), resulting in the next resource (representing the next state of the application) being transferred to the user for their use.

History

REST was defined by Roy Fielding in his 2000 PhD dissertation "Architectural Styles and the Design of Network-based Software Architectures" at UC Irvine. Fielding developed the REST architectural style in parallel with HTTP 1.1 of 1996â€"1999, based on the existing design of HTTP 1.0 of 1996.

In a retrospective look at the development of REST, Roy 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.

Architectural properties

The architectural properties affected by the constraints of the REST architectural style are:

• 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. Roy Fielding, one of the principal authors of the HTTP specification, describes REST's effect on scalability as follows:

REST's clientâ€"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 intermediariesâ€"proxies, gateways, and firewallsâ€"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 is the resistance to failure at the system level in the presence of failures within components, connectors, or data

Architectural constraints

There are six guiding constraints that define a RESTful system. These constraints restrict the ways that the server may process and respond to client requests so that, by operating within these constraints, the service gains desirable non-functional properties, such as performance, scalability, simplicity, modifiability, visibility, portability, and reliability. If a service violates any of the required constraints, it cannot be considered RESTful.

The formal REST constraints are as follows:

Client-Server

The first constraints added to our hybrid style are those of the client-server architectural style, described in Section 3.4.1. Separation of concerns is the principle behind the client-server constraints. By separating the user interface concerns from the data storage concerns, we improve the portability of the user interface across multiple platforms and improve scalability by simplifying the server components. Perhaps most significant to the Web, however, is that the separation allows the components to evolve independently, thus supporting the Internet-scale requirement of multiple organizational domains.

Stateless

The clientâ€"server communication is 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. 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.

Cacheable

As on the World Wide Web, clients and intermediaries 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 clientâ€"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.

Uniform interface

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.

Manipulation of resources 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.

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).

Hypermedia as the engine of application state (HATEOAS)

Having accessed an initial URI for the REST applicationâ€"analogous to a human Web user accessing the home page of a websiteâ€"a REST client should then be able to use server-provided links dynamically to discover all the available actions and resources it needs. As access proceeds, the server responds with text that includes hyperlinks to other actions that are currently available. There is no need for the client to be hard-coded with information regarding the structure or dynamics of the REST service.

Applied to Web services

Web service APIs that adhere to the REST architectural constraints are called RESTful APIs. HTTP-based RESTful APIs are defined with the following aspects:

• base URL, such as http://api.example.com/resources/
• an internet media type that defines state transition data elements (e.g., Atom, microformats, application/vnd.collection+json, 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 URL or as complex as a Java applet.
• standard HTTP methods (e.g., OPTIONS, GET, PUT, POST, and DELETE)

Relationship between URL and HTTP methods

The following table shows how HTTP methods are typically used in a RESTful API:

The GET method is a safe method (or nullipotent), meaning that calling it produces no side-effects: retrieving or accessing a record does not change it. The PUT and DELETE methods are idempotent, meaning that the state of the system exposed by the API is unchanged no matter how many times the same request is repeated.

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.