ReaderWSDL

Complete Guide: WSDL and SOAP

Understand the fundamentals of XML-based web services

What is WSDL?

WSDL (Web Services Description Language) is an XML document that completely describes a web service: its operations, messages, data types and endpoints. It's like a "contract" that defines exactly how to interact with the service.

WSDL Structure

📋 Main Elements

  • Types: Defines data types (XSD)
  • Messages: Inputs and outputs
  • PortType: Operations interface
  • Binding: Protocol and format
  • Service: Concrete endpoints

🔗 Relationships

  • Types → Messages (uses types)
  • Messages → PortType (defines interface)
  • PortType → Binding (implements protocol)
  • Binding → Service (final endpoint)

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WSDL Example


          <?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns="http://schemas.xmlsoap.org/wsdl/"
             xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
             xmlns:xsd="http://www.w3.org/2001/XMLSchema"
             xmlns:tns="http://example.com/calculator"
             targetNamespace="http://example.com/calculator"
             elementFormDefault="qualified">
  <types>
    <xsd:schema targetNamespace="http://example.com/calculator"
                elementFormDefault="qualified">
      <xsd:element name="AddRequest">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="a" type="xsd:int"/>
            <xsd:element name="b" type="xsd:int"/>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
      <xsd:element name="AddResponse">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="result" type="xsd:int"/>
          </xsd:sequence>
        </xsd:complexType>
      </xsd:element>
    </xsd:schema>
  </types>
  <message name="AddRequestMessage">
    <part name="parameters" element="tns:AddRequest"/>
  </message>
  <message name="AddResponseMessage">
    <part name="parameters" element="tns:AddResponse"/>
  </message>
  <portType name="CalculatorPortType">
    <operation name="Add">
      <input message="tns:AddRequestMessage"/>
      <output message="tns:AddResponseMessage"/>
    </operation>
  </portType>
  <binding name="CalculatorBinding" type="tns:CalculatorPortType">
    <soap:binding transport="http://schemas.xmlsoap.org/soap/http" style="document"/>
    <operation name="Add">
      <soap:operation soapAction="http://example.com/calculator/Add"/>
      <input>
        <soap:body use="literal"/>
      </input>
      <output>
        <soap:body use="literal"/>
      </output>
    </operation>
  </binding>
  <service name="CalculatorService">
    <documentation>Calculator service for basic mathematical operations</documentation>
    <port name="CalculatorPort" binding="tns:CalculatorBinding">
      <soap:address location="http://example.com/calculator"/>
    </port>
  </service>
</definitions>

SOAP: The Protocol

SOAP (Simple Object Access Protocol) is the communication protocol that uses XML to exchange structured messages between applications.

SOAP Message Structure

SOAP Request


POST http://example.com/calculator HTTP/1.1
Content-Type: text/xml; charset=utf-8
SOAPAction: "http://example.com/calculator/Add"
Content-Length: [automatically calculated]

<?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
               xmlns:calc="http://example.com/calculator">
  <soap:Header>
    <!-- Optional metadata: Headers, authentication, transaction, etc -->
  </soap:Header>
  <soap:Body>
    <calc:AddRequest>
      <calc:a>15</calc:a>
      <calc:b>25</calc:b>
    </calc:AddRequest>
  </soap:Body>
</soap:Envelope>

SOAP Response


        <?xml version="1.0" encoding="UTF-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
               xmlns:calc="http://example.com/calculator">
  <soap:Body>
    <calc:AddResponse>
      <calc:result>40</calc:result>
    </calc:AddResponse>
  </soap:Body>
</soap:Envelope>

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SOAP vs REST: Detailed Comparison

🏢 SOAP

  • Protocol: XML over HTTP/HTTPS
  • Contract: WSDL mandatory
  • Security: WS-Security integrated
  • Transactions: Native support
  • Overhead: High (verbose XML)
  • Use cases: Enterprise systems

🌐 REST

  • Protocol: JSON/XML over HTTP
  • Contract: OpenAPI optional
  • Security: OAuth, JWT
  • Transactions: Manual implementation
  • Overhead: Low (lightweight JSON)
  • Use cases: Modern web APIs

🛠️ Essential Tools

Development

  • SoapUI: Testing and debugging
  • Postman: Universal client
  • Insomnia: Modern client
  • XMLSpy: WSDL/XSD editor

Code Generation

  • wsimport (Java): JAX-WS
  • svcutil (.NET): WCF
  • zeep (Python): Dynamic client
  • node-soap: Node.js

Validation

  • XMLLint: XML validation
  • WSDL Analyzer: Online
  • Schema Validator: XSD
  • SOAP Validator: Messages

💡 Common Problems and Solutions

🔧 Namespace Issues

Problem: Namespace errors are very common in SOAP.

Solution: Always check namespaces in WSDL and use exactly the same ones in the request.

🔒 SSL/TLS Certificates

Problem: Legacy services may use outdated TLS.

Solution: Configure client to accept specific versions or update certificates.

🌐 CORS and Proxy

Problem: Browsers block cross-origin SOAP requests.

Solution: Use a proxy server or configure CORS on the SOAP server.

📦 External Imports

Problem: WSDL with imports to inaccessible external XSD.

Solution: Download all files locally or use tools that resolve imports automatically.

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🚀 Next Steps

Now that you understand the fundamental concepts, explore: