R4R WSDL

Introduction of WSDL:

1. WSDL stands for Web Services Description Language.
2. WSDL is written in XML - it is an XML-based language.
3. WSDL document defines services as collections of network endpoints, or ports.
4. WSDL combines the best of IBM’s NASSL (Network Accessible Services Language) and Microsoft SOAP Contract Language.
5. WSDL is an XML based protocol for information exchange in decentralized and distributed environments.
6. WSDL is the standard format for describing a web service.
7. WSDL definition describes how to access a web service and what operations it will perform.
8. WSDL is a language for describing how to interface with XML-based services.
9. WSDL is an integral part of UDDI, an XML-based worldwide business registry.
10. WSDL is the language that UDDI uses.
11. WSDL was developed jointly by Microsoft and IBM.
     

History of WSDL

WSDL 1.1 was submitted as a W3C Note by Ariba, IBM and Microsoft for describing services for the W3C XML Activity on XML Protocols in March 2001.WSDL 1.1 has not been endorsed by the World Wide Web Consortium (W3C), however it has just (May 11th, 2005) released a draft for version 2.0, that will be a recommendation (an official standard), and thus endorsed by the W3C.

The main parts of a WSDL document:

WSDL document uses the following elements in the definition of network services:

Types– The <types> element defines the data types that are used by the web service. For maximum platform neutrality, WSDL uses XML Schema syntax  to define data types
Message– an abstract, typed definition of the data being communicated.
Operation– an abstract description of an action supported by the service.
Port Type–an abstract set of operations supported by one or more endpoints.
Binding– The <binding> element defines the message format and communication protocol details for each port. It links the port type to a transport method eg. HTTP.

Port– a single endpoint defined as a combination of a binding and a network address.
Service– a collection of related endpoints.

How to Write  WSDL Document :

WSDL is a language that allows you to write WSDL documents, so that:

A WSDL document can be defined as a set of Web services.
A Web service can be defined as an interface and a set of end points.
An end point can be defined as an address and a binding.
An address can be defined to tell where the service is provided.
A binding can be defined to tell how messages should be formatted and transmitted.
An interface can be defined as a set of operations.
An operation can be defined as a sequence of input and output messages.
 

This diagram Define how WSDL can be used to define Web services in a WSDL document:

The following diagram illustrate Where WSDL is used.

This diagram Define how a web service is registered, discovered and then invoked .
In this diagram, the web service is registered in a UDDI registry, where a business partner or other system can find the service. The registry information from UDDI is used to locate WSDL document that details the call semantics for the web service.

How WSDL Works in the World of Java Technology.

In This Diagram how a Web service is registered, found and called in a scenario based on Java technology. In this diagram, the Web service is registered in a UDDI repository using the Java API for XML Registries (JAXR), where a business partner or other system can find the service. The registry information from UDDI is used to locate a WSDL document that details the call semantics for the Web service. With the WSDL document in hand, the Java programmer can then feed it to a tool that can generate a Java object proxy to the Web service, or simply use it as a reference document along with a lower-level SOAP API.

Advantages:

WSDL to Java:

1. Convenient when WSDL document already exists
2. More powerful
3. Requires WSDL and XML Schema knowledge
4. Easy to stray outside of WS-I BP 1.0, harming interoperability

Java to WSDL:

1. Easy to use
2. No need to learn WSDL or XML Schema
3. Less control over published service contract

Simple Structure Of WSDL Program:

<definitions>

<types>

   definition of types........

</types>

<message>

   definition of a message....

</message>

<portType>

   definition of a port.......

</portType>

<binding>

   definition of a binding....

</binding>

</definitions>

Disadvantage of WSDL:

1. Only one web service operation per WSDL.
2. Embedded schemas don't allow modularization of WSDL by importing XML schemas into the types section.
3. Because of 1 and 2, no ability to reuse XML schema definitions across operations.
4. No control over generated "container" message type names.