A SOAP (Simple Object Access Protocol) message is defined by a specific XML structure that facilitates information exchange. The root element, known as the Envelope, encapsulates the entire message and serves as the mandatory container. Within the Envelope, an optional Header section may be included to hold metadata such as authentication credentials or transaction identifiers. The Body element is required and contains the primary data, including the method to be invoked and its parameters. In the event of an error during processing, the server may return a Fault element inside the Body, providing details such as error codes and descriptions.

Below is a standard representation of a SOAP message structure:

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
    <soap:Header>
        <!-- Optional metadata goes here -->
    </soap:Header>
    <soap:Body>
        <!-- Operational data and parameters -->
    </soap:Body>
</soap:Envelope>

The Body typically encapsulates specific action elements, such as RetrieveInventory or ProcessPayment, which include the necessary input arguments. These messages are transported over HTTP using the POST method. The request headers must specify the content type, often set to application/soap+xml or text/xml, to ensure the server correctly interprets the payload. Once the server parses the Envelope and executes the defined operation, it returns a corresponding SOAP message containing the results or a fault structure.

Practical Implementation Example

Consider the following scenario using a hypothetical weather API endpoint: http://service.weather.net/WeatherService.asmx. The goal is to retrieve weather information for a specific city.

HTTP Request Payload:

POST /WeatherService.asmx HTTP/1.1
Host: service.weather.net
Content-Type: text/xml; charset=utf-8
Content-Length: length
SOAPAction: "http://weather.net/GetLocalWeather"

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetLocalWeather xmlns="http://weather.net/">
      <city>New York</city>
      <apiKey>YOUR_API_KEY</apiKey>
    </GetLocalWeather>
  </soap:Body>
</soap:Envelope>

Server Response:

HTTP/1.1 200 OK
Content-Type: text/xml; charset=utf-8
Content-Length: length

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetLocalWeatherResponse xmlns="http://weather.net/">
      <GetLocalWeatherResult>Sunny, 25°C</GetLocalWeatherResult>
    </GetLocalWeatherResponse>
  </soap:Body>
</soap:Envelope>