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## Chapter 72. Introduction to the Representational State Transfer (REST)

## Introduction

Representational State Transfer (REST) is a style of software architecture for distributed hypermedia systems such as the World Wide Web. The term was introduced in the doctoral dissertation in 2000 by Roy Fielding, one of the principal authors of the Hypertext Transfer Protocol (HTTP) specification, and has come into widespread use in the networking community.

REST strictly refers to a collection of network architecture principles that outline how resources are defined and addressed. The term is often used in a looser sense to describe any simple interface that transmits domain-specific data over HTTP without an additional messaging layer such as SOAP or session tracking via HTTP cookies.

The key abstraction of information in REST is a resource. Any information that can be named can be a resource: a document or image, a temporal service (e.g. "today's weather in Los Angeles"), a collection of other resources, a non-virtual object (e.g. a person), and so on. In other words, any concept that might be the target of an author's hypertext reference must fit within the definition of a resource. A resource is a conceptual mapping to a set of entities, not the entity that corresponds to the mapping at any particular point in time.

REST uses a resource identifier to identify the particular resource involved in an interaction between components. REST connectors provide a generic interface for accessing and manipulating the value set of a resource, regardless of how the membership function is defined or the type of software that is handling the request. URL or URN are the examples of a resource identifier.

REST components perform actions with a resource by using a representation to capture the current or intended state of that resource and transferring that representation between components. A representation is a sequence of bytes, plus representation metadata to describe those bytes. Other commonly used but less precise names for a representation include: document, file, and HTTP message entity, instance, or variant. A representation consists of data, metadata describing the data, and, on occasion, metadata to describe the metadata (usually for the purpose of verifying message integrity). Metadata are in the form of name-value pairs, where the name corresponds to a standard that defines the value's structure and semantics. The data format of a representation is known as a media type.

Table 72.1. REST Data Elements

Data Element | Modern Web Examples
--- | ---
resource | the intended conceptual target of a hypertext reference
resource identifier | URL, URN
representation | HTML document, JPEG image
representation metadata | media type, last-modified time
resource metadata | source link, alternates, vary
control data | if-modified-since, cache-control

REST uses various connector types to encapsulate the activities of accessing resources and transferring resource representations. The connectors present an abstract interface for component communication, enhancing simplicity by providing a complete separation of concepts and hiding the underlying implementation of resources and communication mechanisms.

Table 72.2. REST Connectors

Connector | Modern Web Examples
--- | ---
client | libwww, libwww-perl
server | libwww, Apache API, NSAPI
cache | browser cache, Akamai cache network
resolver | bind (DNS lookup library)
tunnel | SOCKS, SSL after HTTP CONNECT

The primary connector types are client and server. The essential difference between the two is that a client initiates communication by making a request, whereas a server listens for connections and responds to requests in order to supply access to its services. A component may include both client and server connectors.

An important part of RESTful architecture is a well-defined interface to communicate, in particular it is a set of HTTP methods such as POST, GET, PUT and DELETE. These methods are often compared with the CREATE, READ, UPDATE, DELETE (CRUD) operations associated with database technologies. An analogy can also be made:

- PUT is analogous to CREATE or PASTE OVER,

- GET to READ or COPY,

- POST to UPDATE or PASTE AFTER, and

- DELETE to DELETE or CUT.

Note: RESTful architecture is not limited to those methods, one of good examples of extension is the WebDAV protocol.

The CRUD (Create, Read, Update and Delete) verbs are designed to operate with atomic data within the context of a database transaction. REST is designed around the atomic transfer of a more complex state and can be viewed as a mechanism for transferring structured information from one application to another.

HTTP separates the notions of a web server and a web browser. This allows the implementation of each to vary from the other based on the client/server principle. When used RESTfully, HTTP is stateless. Each message contains all the information necessary to understand the request.

As a result, neither the client nor the server needs to remember any communication-state between messages. Any state retained by the server must be modeled as a resource..

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