WSDL stands for Web service Description Language.

A WSDL document is a contract between a SOAP–based web service and its consumers.

A WSDL document provides critical information such as service endpoint, service operations, data types required for operations, description of messages exchanged, and underlying service pattern (request/response, solicit/response etc).

The WSDL document is useful for both creating and executing clients against a web service.

WSDL Structure

The root element in a WSDL is named definitions as the WSDL provides definitions grouped into various sections like:

• types section

• message section

• portType section

• binding section

• service section

Example:

The types section

• The types section provides data type definitions under some data type system such as XML schema.

• The types section can hold, points to, or imports an XSD.

• If the types section is empty, then the service uses only simple data types such as xsd:string and xsd:long.

• The types section is optional.

• Although the WSDL 2.0 specification allows for alternatives to XML schema, XML schema is the default and the dominant type system used in WSDL.

Example:

The message section

• The message section defines the messages. 

• Message is an abstract, typed definition of the data being communicated.

• All input parameters for a web service operation (method) are considered together as one input message.

• At runtime each message is a SOAP document.

• The order of the messages indicates the service pattern.

  • For example the message order in/out indicates the request/response pattern.

Example:

The portType section

• The port Type section groups the operations that the web service delivers,

  • Operation is an abstract description of an action supported by the web service.

  • Each operation having one or more messages.

    • An input message means input to the web service and output message means messages from a web service.

    • All input parameters for a web service operation (method) are bundled together as one input message.

    • These messages are also defined separately in the messages section.

    • At runtime each message is a SOAP document.

  • The portType section presents the services as named operations.

    • Operations are named after methods annotated as @WebMethod.

• The WSDL portType presents the service operations abstractly but provides no implementation details.

  • A web service’s portType is similar to a java interface that, it presents the service abstractly, with no implementation details.

Example:

The binding section

• A WSDL binding provides concrete details about the service.

• Specify implementation details of a service defined abstractly in the potType section :

  • The transport protocol to be used in sending and receiving SOAP messages.

  • The style of the service – rpc or document

  • The data format to be used in the SOAP message – literal and encoded.

Example:

The service section

• The service section list one or more port elements.

  • A port consists of a portType (interface) together with a corresponding binding (implementation).

• Service section specifies one or more end points at which the service’s functionality is available.

  • A service endpoint url is contained in the location attribute of <soap:address>, which is inside the service section.   

  • The service endpoint url informs clients about where the service can be accessed.

Example:

Fault Elements

• Application exceptions can be mapped to faults in WSDL.

• Fault elements may come in places such as:

  • Within a message element as a part:

    • <part name="fault" element="tns:MissingName" />

  • As a child of operation element within portType section.

    •  <fault message="tns:MissingName" name="MissingName" /> 

  • As a child of operation element within binding section

    • <fault name="MissingName">

    • <soap:fault name="MissingName" use="literal" /> 

    • </fault>

Example: Try it out yourself in coming lab.

Advantages of WSDL

• WSDL is platform and language neutral.

  • A WSDL generated by a c# program can be used to generate a Java web service.

• With WSDL and XSD, the definition of the message can travel with the message.

Limitations of WSDL

• Though a wsdl provides details on how to invoke a service, it does not contain information about the use of the service.

  • Programmer should be aware about the use of a service.