Development
08 January 2023

What is WSDL?

WSDL (Web Services Description Language) is an XML-based language that is used to describe the interface of a web service. It defines the operations that the web service provides, the parameters that the operations accept and return, and the binding information that specifies how to access the web service.

WSDL is used to enable communication between web services and clients, such as web browsers and software applications. It allows clients to discover the capabilities of a web service, and to invoke its operations in a standardized way.

A WSDL document is made up of several elements that define the web service and its interface. These elements include:

  • types: A section that defines the data types that are used in the web service.
  • message: A section that defines the data structures that are used in the web service.
  • portType: A section that defines the operations that the web service provides, and the input and output parameters of those operations.
  • binding: A section that specifies how the web service can be accessed, such as the protocol (e.g., HTTP) and the encoding (e.g., XML) that are used.
  • service: A section that defines the endpoint of the web service, and the binding information that is used to access it.

WSDL is an important part of the web service ecosystem, and is used to enable interoperability between different systems and technologies. It is often used in conjunction with other standards, such as SOAP and UDDI, to create a complete web service solution.

Here is a simple WSDL document that defines a web service that provides a single operation called "greet" that takes a name as an input parameter and returns a greeting message as an output parameter:

<?xml version="1.0"?>
<definitions name="GreetingService"
             targetNamespace="http://example.com/greeting"
             xmlns="http://schemas.xmlsoap.org/wsdl/"
             xmlns:tns="http://example.com/greeting"
             xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <!-- Data types -->
  <types>
    <xsd:schema targetNamespace="http://example.com/greeting">
      <xsd:element name="greet" type="xsd:string"/>
      <xsd:element name="greetResponse" type="xsd:string"/>
    </xsd:schema>
  </types>
 
  <!-- Messages -->
  <message name="greetRequest">
    <part name="name" type="xsd:string"/>
  </message>
  <message name="greetResponse">
    <part name="message" type="xsd:string"/>
  </message>
 
  <!-- Port type -->
  <portType name="GreetingPortType">
    <operation name="greet">
      <input message="tns:greetRequest"/>
      <output message="tns:greetResponse"/>
    </operation>
  </portType>
 
  <!-- Binding -->
  <binding name="GreetingBinding" type="tns:GreetingPortType">
    <soap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http"/>
    <operation name="greet">
      <soap:operation soapAction="greet"/>
      <input>
        <soap:body use="encoded" encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
      </input>
      <output>
        <soap:body use="encoded" encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
      </output>
    </operation>
  </binding>
 
  <!-- Service -->
  <service name="GreetingService">
    <port name="GreetingPort" binding="tns:GreetingBinding">
      <soap:address location="http://example.com/greeting"/>
    </port>
  </service>
 
</definitions>

Let's break down this WSDL document and see what each element does:

    • The definitions element is the root element of the WSDL (Web Services Description Language) file, and contains all of the other elements that define the web service. It has several attributes that specify metadata about the web service, such as its name and target namespace.
        • The name attribute inside the definitions element specifies the name of the web service. In this example, the web service is called "GreetingService".
        • The targetNamespace attribute inside the definitions element specifies the namespace of the web service. In this example, the namespace is "http://example.com/greeting". This is used to uniquely identify the web service and to prevent naming conflicts with other web services.
        • The xmlns attribute specifies the default namespace of the WSDL document. It is used to specify the namespace of elements that do not have a prefix.
        • The xmlns:tns attribute specifies inside the definitions element a namespace prefix that is used to refer to the web service's namespace. In this example, the prefix "tns" stands for "target namespace".
        • The xmlns:xsd attribute specifies inside the definitions element a namespace prefix that is used to refer to the XML Schema namespace. This is used to define data types that are used in the web service.
    • The types element defines the data types that are used in the web service. In this example, we have defined two elements: greet, which is a string that represents the name of the person being greeted, and greetResponse, which is a string that represents the greeting message.
    • The message element defines the data structures that are used in the web service. In this example, we have defined two messages: greetRequest, which represents the input parameter for the greet operation, and greetResponse, which represents the output parameter for the greet operation. Each message has a single part element that specifies the name and data type of the parameter.
    • The portType element defines the operations that the web service provides, and the input and output parameters of those operations. In this example, we have defined a single operation called greet, which takes a single input parameter (the name) and returns a single output parameter (the message).
    • The binding element specifies how the web service can be accessed, such as the protocol (e.g., HTTP) and the encoding (e.g., XML) that are used. In this example, we are using the SOAP protocol and the RPC encoding style.
    • The service element defines the endpoint of the web service, and the binding information that is used to access it. In this example, we have defined a single service called GreetingService, which has a single endpoint called GreetingPort that is accessed using the GreetingBinding binding.

I hope this helps to clarify the elements of a WSDL document! Let me know if you have any questions or need further assistance.

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