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Lab: Consuming a SOAP API (Practical)

Overview

In this lab, you'll learn how to consume an existing SOAP web service. You'll read a WSDL (Web Service Description Language) file to understand the API, and make a SOAP request using curl. This lab is practical and lightweight—no enterprise SOAP internals, just enough to use a real SOAP API.

What is SOAP?

  • SOAP (Simple Object Access Protocol) is an older protocol for exchanging structured data over HTTP, often using XML.
  • Unlike REST, SOAP uses XML envelopes and a strict contract defined by a WSDL.
  • Many legacy systems and some public APIs still use SOAP.

Step 1: Find and Read the WSDL

  • A WSDL (Web Service Description Language) file describes the available operations, input/output formats, and endpoint URLs for a SOAP service.
  • Example public SOAP service: Calculator WSDL
  • Open the WSDL URL in your browser. You'll see XML describing the service, operations (like Add, Subtract), and message formats.

Note: This lab uses a public SOAP service for demonstration. If the service is unavailable, your instructor may provide a local SOAP example or pre-recorded response for inspection.

Step 2: Understand the SOAP Request Format

  • SOAP requests are sent as XML in the body of an HTTP POST.
  • The WSDL tells you the endpoint and the XML structure for each operation.
  • Example: To call the Add operation on the Calculator service, the SOAP body looks like this:
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <Add xmlns="http://tempuri.org/">
      <intA>2</intA>
      <intB>3</intB>
    </Add>
  </soap:Body>
</soap:Envelope>

Step 3: Make a SOAP Request with curl

  • Use curl to POST the SOAP XML to the service endpoint.
  • Set the Content-Type: text/xml header and the SOAPAction header (from the WSDL).
curl -s \
  -X POST \
  -H "Content-Type: text/xml; charset=utf-8" \
  -H "SOAPAction: \"http://tempuri.org/Add\"" \
  --data @add.xml \
  http://www.dneonline.com/calculator.asmx
  • Save your SOAP XML to a file called add.xml.
  • The response will be XML containing the result.
<?xml version="1.0" encoding="utf-8"?>
    <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
        <soap:Body>
            <AddResponse xmlns="http://tempuri.org">
                <AddResult>5</AddResult>
            </AddResponse>
        </soap:Body>
    </soap:Envelope>

Notice how SOAP requires a fixed XML structure and explicit operation names, while REST typically relies on HTTP verbs and simpler JSON payloads.

Step 4: Try It Yourself

  1. Open the Calculator WSDL and review the operations.
  2. Write a SOAP XML request for the Add operation (as above) and save it as add.xml.
  3. Use the curl command to send the request and view the response.
  4. Try changing the numbers or calling a different operation (like Subtract).

Key Takeaways

  • SOAP is XML-based and uses a WSDL for contract.
  • You can consume SOAP APIs with curl by sending XML POST requests.
  • No need to learn enterprise SOAP internals for basic consumption!

Calculator WSDL XML Reference

below you can see the SOAP definition that specifies how to use the Add function.

<wsdl:definitions xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" xmlns:tm="http://microsoft.com/wsdl/mime/textMatching/" xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/" xmlns:mime="http://schemas.xmlsoap.org/wsdl/mime/" xmlns:tns="http://tempuri.org/" xmlns:s="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://schemas.xmlsoap.org/wsdl/soap12/" xmlns:http="http://schemas.xmlsoap.org/wsdl/http/" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/" targetNamespace="http://tempuri.org/">

<wsdl:types>
<s:schema elementFormDefault="qualified" targetNamespace="http://tempuri.org/">
    <s:element name="Add">
        <s:complexType>
            <s:sequence>
                <s:element minOccurs="1" maxOccurs="1" name="intA" type="s:int"/>
                <s:element minOccurs="1" maxOccurs="1" name="intB" type="s:int"/>
            </s:sequence>
        </s:complexType>
    </s:element>
...