Implementing WSDL

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XML attempts to self-describe—to contain both data and information describing that data. To assist with this difficult job when XML is used to send Web service messages, a new language named WSDL (Web Services Description Language) was developed. WSDL is an effort to standardize descriptions of responses, formats, and protocols used during Web service messaging. WSDL can describe these aspects of a Web service message:

The address of the Web service

What kind of processing should be carried out on the data

The type of exchange (one-way, multicast, response/request, solicit/response)

The type of data being exchanged between client and server

The type of message used for input and output (procedure-style or document-style)

The protocol used to send the message(s)

Error information

WSDL descriptions are written in XML and are usually placed within an XML schema or set of schemas. The client and service employ the same schema and agree both on how the client should format its message and how the Web service should process the data it gets from the client. In this way, WSDL permits various proprietary models to easily couple, such as COM or ERP.

WSDL is yet another in the many initiatives designed to promote interoperability. It's called a "contract" between client and service, and ideally should describe all the information necessary to permit successful Web service consumption without the need for human intervention. Alas, this remains more a dream than a practical reality, like other aspects of the XML program. Nonetheless, WSDL can improve the readability (by humans) of the interaction between client and service.

Simply put: WSDL describes what kinds of messages a given server accepts, specifying the format required and the types permitted.

Here are the specific elements of WSDL:

Message Optional, can appear in various places within the document.

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