XML :
XML Web services are the basic building blocks among the distributed computing on the Internet. Open standards and the focus on communication and collaboration among people and applications have shape the location where XML Web services are becoming the platform for application integration.
Some of the key aspects why XML was mostly used for their Web Service:

• XML Web Services interpretation useful functionality to Web users through a standard Web protocol in most cases SOAP was used.
• XML Web services offer a way to explain their interfaces in enough detail to allow a user to build a client application to talk to them.
• XML Web services are registered so all the potential users can find them easily.

SOAP:

Today SOAP is essential for application development to allow Internet communication between programs. SOAP provides a way to converse among the applications running on different operating systems, with different technologies and programming languages.

A SOAP message is ordinary XML documents contain the following elements:

• Envelope element that identify the XML document as a SOAP message.
• Header element that contain header information.
• Body element that contain call and response information.
• Fault element that provide information about errors that occurred while processing the message.

WSDL:

WSDL stands for Web Services Description Language it is an XML-based language that describes Web services and how to access them.

WSDL express four critical pieces of data:

• Interface information describing all publicly accessible functions.
• Data type information for every message requests and message responses.
• Binding information about the transport protocol to be use.
• Address information for locating the specific service.
  

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