Simple Object Access Protocol (SOAP) is a messaging protocol that describes how
messages should be encoded so that they can be delivered over the network by using a transport protocol such as HTTP.
Messages that are being sent are XML documents and must adhere to the protocol. As you can see, the WSDL is what defines the XML message that is being sent from the web services business function to the calling software application.
The Web Services Description Language (WSDL) is represented as a series of XML
statements that constitute the definition for the interfaces of each service (that is, the contract between the client and the web service provider). The WSDL has the following rules:
Fully describes the web service:
– Data types of the parameters (promotes strong data typing)
Describes endpoints to contact the web service
Web services that are based on SOAP are generally activity-oriented, where service operations are the center of attention (resources might be involved, but serve only to refine the context within which the activity is done).
Web services that are based on SOAP are based on standards and specifications that govern various areas, such as reliable messaging, security, and transaction management.