Adding Web Services to Your Applications - Java EE 7 Development with WildFly - page 245

Developing SOAP-based web services
As stated, web services are based on the exchange of messages using nonproprietary pro-
tocol messages. The messages themselves are not sufficient to define the web service plat-
form. We actually need a list of standard components, including the following:
• A language used to define the interfaces provided by a web service in a manner
that is not dependent on the platform on which it is running or the programming
language used to implement it
• A common standard format to exchange messages between web service providers
and web service consumers
• A registry within which service definitions can be placed
The Web Service Description Language , also known as WSDL , ( http://www.w3.org/TR/
wsdl ) is the de facto standard to provide a description of a web service contract exposed to
clients. In particular, a WSDL document describes a web service in terms of the operations
that it provides, and the data types that each operation requires as inputs and can return in
the form of results.
Communication between the service provider and service consumer happens by means of
XML messages that rely on the SOAP specification.
A basic SOAP message consists of an envelope that may contain any number of headers
and a body. These parts are delimited by XML elements called envelope , header , and
body , which belong to a namespace defined by the SOAP specification. The following fig-
ure depicts the basic structure of a SOAP message: