SOAP - How SOAP works

1. SOAP Workflow (How it works)

SOAP works in a request–response model:

  1. Client prepares a SOAP request

    • The client (e.g., an application) creates an XML message inside a SOAP envelope.

    • This message describes what service or method it wants to call, and what data it’s sending.

  2. Request sent over a protocol (HTTP/SMTP/etc.)

    • The message is sent to the server (usually over HTTP).

  3. Server processes the request

    • The server receives the SOAP message, extracts the data, and executes the required operation.

  4. Server sends a SOAP response

    • The server prepares another XML message with the result (or an error in a <Fault> block).

  5. Client receives and interprets the response

    • The client parses the XML and uses the returned data.

2. SOAP Structure Reminder

Every SOAP message is an XML document with four main parts:

  • <Envelope> → Root element (mandatory).

  • <Header> → Metadata like authentication, routing (optional).

  • <Body> → Actual request/response data (mandatory).

  • <Fault> → Error handling (optional).

3. Simple Example: Get Temperature of a City

Step A: Client sends request

Suppose a client wants to get the temperature for London from a weather web service.

SOAP request message:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
   <soap:Header/>
   <soap:Body>
      <GetTemperature xmlns="http://www.example.com/weather">
         <City>London</City>
      </GetTemperature>
   </soap:Body>
</soap:Envelope>

  • Envelope: Defines the SOAP message.

  • Body: Contains the method (GetTemperature) and its parameter (City = London).

Step B: Server processes request

  • The server sees that the client is calling the method GetTemperature.

  • It looks up London’s temperature (say 18°C).

Step C: Server sends response

SOAP response message:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
   <soap:Header/>
   <soap:Body>
      <GetTemperatureResponse xmlns="http://www.example.com/weather">
         <City>London</City>
         <Temperature>18</Temperature>
         <Unit>Celsius</Unit>
      </GetTemperatureResponse>
   </soap:Body>
</soap:Envelope>

  • The server wraps the response data (temperature = 18°C) inside the SOAP Body.

Step D: Client receives response

  • The client application reads the XML, extracts Temperature = 18, Unit = Celsius, and displays it.

4. Error Handling Example (Fault)

If something goes wrong (e.g., invalid city name), the server responds with a SOAP Fault:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
   <soap:Body>
      <soap:Fault>
         <faultcode>soap:Client</faultcode>
         <faultstring>Invalid city name</faultstring>
      </soap:Fault>
   </soap:Body>
</soap:Envelope>

5. Summary of How SOAP Works

  1. Client creates a SOAP XML request.

  2. Request sent to the server over HTTP (or other protocol).

  3. Server parses XML, executes the service.

  4. Server sends back a SOAP XML response.

  5. Client reads and processes the result.