WSDL, or Web Services Description Language, is an XML-based interface description language used to describe the functionality offered by web services. It provides a machine-readable description of how services can be called, the parameters they expect, and the data structures they return. Here is a detailed explanation of WSDL:
Overview of WSDL
WSDL is a standard format for describing a web service. It was developed jointly by Microsoft and IBM and is now maintained by the World Wide Web Consortium (W3C). The latest version, WSDL 2.0, became a W3C recommendation in 2007[3][5].
Structure of a WSDL Document
A WSDL document is composed of several key elements, each serving a specific purpose in defining the web service:
Definitions: This is the root element of a WSDL document. It defines the name of the web service, declares multiple namespaces used throughout the document, and contains all the service elements[15][18].
Types: This element defines the data types used by the web service. It typically uses XML Schema to define complex data types that are used in the messages[14][18].
Message: This element defines the data elements for each operation. A message can consist of one or more parts, each part being an element or type defined in the schema[12][18].
PortType: This element describes the operations that can be performed and the messages involved. It is an abstract set of operations supported by one or more endpoints[12][18].
Binding: This element defines the protocol and data format for each port type. It specifies how the operations and messages are bound to a concrete network protocol and message format[12][18].
Service: This element groups a set of related endpoints. It specifies the address for invoking the web service[12][18].
How WSDL Works
WSDL serves as a contract between the service provider and the user of the service. It describes the public interface to the web service, detailing how to access the service and what operations it will perform. The abstract definitions of ports and messages are separated from their concrete use or instance, allowing their definitions to be reused[2][5][6].
Abstract and Concrete Descriptions
Abstract Descriptions: These include the definitions of messages and port types. Messages are abstract descriptions of the data being exchanged, and port types are abstract collections of supported operations[2][5][10].
Concrete Descriptions: These include the bindings and services. Bindings specify the concrete protocol and data format for a particular port type, while services group related endpoints[2][5][10].
Usage of WSDL
WSDL is often used in combination with SOAP (Simple Object Access Protocol) and XML Schema to provide web services over the Internet. A client program connecting to a web service can read the WSDL file to determine what operations are available on the server. Any special data types used are embedded in the WSDL file as XML Schema. The client can then use SOAP to call one of the operations listed in the WSDL file, using XML over HTTP[1][2][3][5].
History and Versions
WSDL 1.0: Developed in 2000 by IBM, Microsoft, and Ariba.
WSDL 1.1: Published in March 2001, it became widely used but was not endorsed by the W3C.
WSDL 2.0: Became a W3C recommendation in 2007. It introduced significant changes, such as better support for RESTful web services an...
