The Web Services Description Language (WSDL) is an XML schema format that defines an extensible framework for describing Web services interfaces. WSDL was developed primarily by Microsoft and IBM and was submitted to W3C by 25 companies. WSDL is at the heart of the Web services framework, providing a common way in which to represent the data types passed in messages, the operations to be performed on the messages, and the mapping of the messages onto network transports.

WSDL is, like the rest of the Web services framework, designed for use with both procedureoriented and document-oriented interactions. As with the rest of the XML technologies, WSDL is so extensible and has so many options that ensuring compatibility and interoperability across differing implementations may be difficult. If the sender and the receiver of a message can share and understand the same WSDL file the same way, however, interoperability can be ensured.

WSDL is divided into three major elements:

• Data type definitions
• Abstract operations
• Service bindings

Each major element can be specified in a separate XML document and imported in various combinations to create a final Web services description, or they can all be defined together in a single document. The data type definitions determine the structure and the content of the messages. Abstract operations determine the operations performed on the message content, and service bindings determine the network transport that will carry the message to its destination.

Figure 2.3 WSDL consists of three major elements and seven parts.

The definition parts include data type definitions, messages, and abstract operations, which are similar to interface definitions in CORBA or DCOM. Messages can have multiple parts and can be defined for use with the procedure-oriented interaction style, the document-oriented interaction style, or both. Through the abstraction layers, the same messages can be defined and used for multiple port types. Like the other parts of WSDL, messages also include extensibility components—for example, for including other message attributes.

WSDL data type definitions are based on XML schemas, but another, equivalent or similar type definition system can be substituted. For example, CORBA Interface Definition Language (IDL) data types could be used instead of XML schema data types. (If another type definition system is used, however, both parties to a Web services interaction must be able to understand it.)

The service bindings map the abstract messages and operations onto specific transports, such as SOAP. The binding extensibility components are used to include information specific to SOAP and other mappings. Abstract definitions can be mapped to a variety of physical transports. The WSDL specification includes examples of SOAP one-way mappings for SMTP (Simple Mail Transfer Protocol), SOAP RPC mappings for HTTP, SOAP mappings to HTTP GET and POST, and a mapping example for the MIME (multipurpose Internet messaging extensions) multipart
binding for SOAP.

XML namespaces are used to ensure the uniqueness of the XML element names used in each of the three major WSDL elements. Of course, when the WSDL elements are developed separately and imported into a single complete file, the namespaces used in the separate files must not overlap. Associated schemas are used to validate both the WSDL file and the messages and operations defined within the WSDL file.

It's safe to say that WSDL is likely to include many extensions, changes, and additions as Web services mature. Like SOAP, WSDL is designed as an extensible XML framework that can easily be adapted to multiple data type mappings, message type definitions, operations, and transports. For example, IETF (Internet Engineering Task Force) working groups are proposing a new protocol standard—Blocks Extensible Exchange Protocol (BEEP)—to define a useful connection - oriented transport. (HTTP, by contrast, is inherently connectionless, making it difficult to resolve quality-of-service problems at the transport level.) Companies interested in using Web services for internal application or integration may choose to extend WSDL to map to more traditional protocols, such as DCOM or IIOP (Internet Inter-Orb Protocol).

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