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resource. REST depends less on vendor software and mechanisms, which
layer additional messaging frameworks on top of HTTP. It provides equiv-
alent functionality when compared to alternative approaches to communi-
cation, and it does not require a separate resource discovery mechanism,
because of the use of hyperlinks in representations. REST also provides
better long-term compatibility because of the capability of document
types such as HTML to evolve without breaking backward or forward
compatibility and the ability of resources to add support for new content
types as they are defined without dropping or reducing support for older
content types.
SOAs can be built using REST services—an approach sometimes referred
to as (ROA) REST-oriented architecture. The main advantage of ROA is ease
of implementation, agility of the design, and the lightweight approach.
The latest version of WSDL now contains HTTP verbs and is considered an
acceptable method of documenting REST services. There is also an alterna-
tive known as WADL (Web Application Description Language).
7.4.3.1 Web Application Description Language (WADL)
Most RESTful services are described using free text. Although the number
of RESTful services is increasing, there is currently no standard approach
to describe this kind of services. If, for WS-* Web Services, the Web Service
Description Language is the de facto standard, in the RESTful service domain,
all approaches to describe services are still in an early stage. More structured
approaches such as the Web Application Description Language (WADL)
are currently emerging. The Web Application Description Language is an
XML-based language for the description of RESTful services. It is designed
to provide a machine-processable protocol description format for use with
HTTP-based Web applications, especially those using XML to communicate.
A WADL document is defined using the following elements:
Application is a top-level element that contains the overall descrip-
tion of the service. It might contain grammars, resources, methods,
representation, and fault elements.
Grammars acts as a container for definitions of any XML structures
exchanged during the execution of the protocol described by the
WADL document. Using the subelement include , one or more struc-
tures can be included.
Resources act as a container for the resources provided by the Web
application.
Resource describes a single resource provided by the Web appli-
cation. Each resource is identified by a URI, and the associated
resources parent element. It can contain the following subelements:
path_variable that is used to parameterize the identifiers of the
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