Web Services Description Language

The Web Services Description Language (WSDL, pronounced 'wiz-dəl' or spelled out, 'W-S-D-L') is an XML-based language that provides a model for describing Web services. The meaning of the acronym has changed from version 1.1 where the D was standing for Definition.

Description
The current version of the specification is 2.0; version 1.1 has not been endorsed by the W3C but version 2.0 is a W3C recommendation. WSDL 1.2 was renamed WSDL 2.0 because of its substantial differences from WSDL 1.1. By accepting binding to all the HTTP request methods (not only GET and POST as in version 1.1) WSDL 2.0 specification offers better support for RESTful web services, and is much simpler to implement. However support for this specification is still poor in software development kits for Web Services which often offer tools only for WSDL 1.1.



The WSDL defines services as collections of network endpoints, or ports. The WSDL specification provides an XML format for documents for this purpose. The abstract definition of ports and messages are separated from their concrete use or instance, allowing the reuse of these definitions. A port is defined by associating a network address with a reusable binding, and a collection of ports defines a service. Messages are abstract descriptions of the data being exchanged, and port types are abstract collections of supported operations. The concrete protocol and data format specifications for a particular port type constitutes a reusable binding, where the operations and messages are then bound to a concrete network protocol and message format. In this way, WSDL describes the public interface to the web service.

WSDL is often used in combination with SOAP and an XML Schema to provide web services over the Internet. A client program connecting to a web service can read the WSDL to determine what operations are available on the server. Any special datatypes used are embedded in the WSDL file in the form of XML Schema. The client can then use SOAP to actually call one of the operations listed in the WSDL.

Objects in a WSDL 1.1/WSDL 2.0
Service/Service: The service can be thought of as a container for a set of system functions that have been exposed to the web based protocols.

Port/Endpoint: The port does nothing more than defining the address or connection point to a web service. It is typically represented by a simple http url string.

Binding/Binding: Specifies the interface, defines the SOAP binding style (RPC/Document) and transport (SOAP Protocol). The binding section also defines the operations.

PortType/Interface: The  element, which has been renamed to in WSDL 2.0, defines a web service, the operations that can be performed, and the messages that are used to perform the operation.

Operation/Operation: Each operation can be compared to a method or function call in a traditional programming language. Here the soap actions are defined and the way the message is encoded for example, "literal."

Message/N.A.: Typically, a message corresponds to an operation. The message contains the information needed to perform the operation. Each message consists of one or more logical parts. Each part is associated with a message-typing attribute. The message name attribute provides a unique name among all messages. The part name attribute provides a unique name among all the parts of the enclosing message. Parts are a description of the logical content of a message. In RPC binding, a binding may reference the name of a part in order to specify binding-specific information about the part. A part may represent a parameter in the message, the bindings define the actual meaning of the part. Messages had been removed in WSDL 2.0, where you simply and directly refer to XML schema types for defining bodies of inputs, outputs and faults.

Types/Types: The purpose of the types in WSDL is to describe the data. XML Schema is used (inline or referenced) for this purpose.

Example WSDL file
Here is an example of a structured WSDL 2.0 document.

History
WSDL 1.0 (Sept. 2000) has been developed by IBM, Microsoft and Ariba to describe Web Services for their SOAP toolkit. They built this by combining two(2) service description languages: NASSL(Network Application Service Specification Language) from IBM and SDL(Service Description Language) from Microsoft.

WSDL 1.1, published on March 2001, is the formalization of WSDL 1.0. No major changes were introduced between 1.0 and 1.1.

WSDL 1.2 (June 2003) is still a working draft at W3C. According to W3C: WSDL 1.2 is easier and more flexible for developers than the previous version. WSDL 1.2 attempts to remove non-interoperable features and also defined the better HTTP 1.1 binding. WSDL 1.2 was not supported by most of the SOAP servers/vendors.

WSDL 2.0 became a W3C recommendation on June 2007. WSDL 1.2 was renamed to WSDL 2.0 because it has substantial differences from WSDL 1.1. The changes are:


 * Adding further semantics to the description language
 * Removal of message constructs
 * No support for operator overloading
 * PortTypes renamed to interfaces
 * Ports renamed to endpoints.