The Web Services Description Language (WSDL) is a standard, structured way of describing SOAP messages and Web services. It is an XML format for describing the network services offered by a service provider. The provider will publish a WSDL file that contains details about the services provided, and the set of operations within each service that the provider supports. For each of the operations, the WSDL file also describes the format that the client must follow in requesting an operation.

Since the WSDL file sets up requirements for both the provider and service requester, this file is like a contract between the two. The provider agrees to provide certain services if the client sends a properly formatted SOAP request. Suppose that we have a WSDL file defining a service called StockQuoteService. This service describes operations such as getTradePrice, getLowestTradePrice, and getHighestTradePrice. You place this WSDL file on the service provider server. A client who wishes to send a SOAP request to this server must first obtain a copy of the WSDL file from the provider, and then use it to format a suitable SOAP request. The client sends this request to the provider. The provider executes the requested operation and sends the results back to the client requester as a SOAP response.

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The specification of WSDL and its file structures can be found on the W3C site .