JBoss.orgCommunity Documentation
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..