Web Services:

Using XML in Web Services:

Web services allow communication between applications over the internet or a network. XML is key in web services as it provides a standard format for data exchange.

One of the most common protocols that uses XML for this purpose is SOAP (Simple Object Access Protocol).

SOAP (Simple Object Access Protocol):

XML-Based: Uses XML to encode messages.
Communication: Between web services and clients.
Transport: Usually HTTP or HTTPS.
Message Structure: Envelope, header and body.

Example of a SOAP Message:

                    
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:example="http://www.example.com">
   <soapenv:Header/>
   <soapenv:Body>
      <example:getBook>
         <example:bookId>001</example:bookId>
      </example:getBook>
   </soapenv:Body>
</soapenv:Envelope>

In this example:

Envelope: Defines the start and end of the message.
Header: Contains optional metadata.
Body: Contains the actual data being transmitted, such as a request for a book's information.

Understanding WSDL (Web Services Description Language)

WSDL (Web Services Description Language) is an XML based language to describe the functionality of a web service. It provides a standard way to specify the operations of the service, the messages and how to access the service.

Key Components of WSDL:

Types: Define the data types used by the service.
Messages: Describe the messages between client and server.
Port Types: Define the operations (methods) of the service.
Bindings: Specify the protocol and message format.
Service: Define the service endpoint (URL).

Example of a WSDL Document:

                    
<definitions xmlns="http://schemas.xmlsoap.org/wsdl/"
  xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
  xmlns:tns="http://www.example.com"
  targetNamespace="http://www.example.com">
  <types>
        <xsd:schema targetNamespace="http://www.example.com">
         <xsd:element name="getBookRequest" type="xsd:string"/>
         <xsd:element name="getBookResponse" type="tns:Book"/>
         <xsd:complexType name="Book">
            <xsd:sequence>
               <xsd:element name="title" type="xsd:string"/>
               <xsd:element name="author" type="xsd:string"/>
               <xsd:element name="price" type="xsd:decimal"/>
            </xsd:sequence>
         </xsd:complexType>
        </xsd:schema>
    </types>
    <message name="getBookRequest">
      <part name="bookId" element="xsd:string"/>
    </message>
    <message name="getBookResponse">
      <part name="book" element="tns:Book"/>
    </message>
    <portType name="BookServicePortType">
      <operation name="getBook">
        <input message="tns:getBookRequest"/>
        <output message="tns:getBookResponse"/>
      </operation>
    </portType>
    <binding name="BookServiceBinding" type="tns:BookServicePortType">
      <soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http"/>
      <operation name="getBook">
         <soap:operation soapAction="getBook"/>
         <input>
            <soap:body use="literal"/>
         </input>
         <output>
            <soap:body use="literal"/>
          </output>
      </operation>
    </binding>
    <service name="BookService">
      <port name="BookServicePort" binding="tns:BookServiceBinding">
        <soap:address location="http://www.example.com/BookService"/>
      </port>
    </service>
</definitions>

Interacting with XML-Based Web Services:

XML web services require requests and responses in XML, usually via SOAP. Here’s how to do it in different languages.

JavaScript (using fetch):

                    
const soapRequest = `
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:example="http://www.example.com">
  <soapenv:Header/>
  <soapenv:Body>
    <example:getBook>
      <example:bookId>001</example:bookId>
    </example:getBook>
  </soapenv:Body>
</soapenv:Envelope>
`;

fetch("http://www.example.com/BookService", {
  method: "POST",
  headers: {
    "Content-Type": "text/xml"
  },
  body: soapRequest
})
.then(response => response.text())
.then(responseXML => {
  console.log(responseXML);
})
.catch(error => {
  console.error("Error:", error);
});

Python (using requests):

                    
import requests

soap_request = """
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:example="http://www.example.com">
  <soapenv:Header/>
  <soapenv:Body>
    <example:getBook>
      <example:bookId>001</example:bookId>
    </example:getBook>
  </soapenv:Body>
</soapenv:Envelope>
"""

headers = {"Content-Type": "text/xml"}
response = requests.post("http://www.example.com/BookService", data=soap_request, headers=headers)

print(response.text)

Java (using HttpURLConnection):

                    
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;

public class SOAPClient {
    public static void main(String[] args) throws Exception {
        String soapRequest = """
        <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:example="http://www.example.com">
          <soapenv:Header/>
          <soapenv:Body>
            <example:getBook>
              <example:bookId>001</example:bookId>
            </example:getBook>
          </soapenv:Body>
        </soapenv:Envelope>
        """;

        URL url = new URL("http://www.example.com/BookService");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("POST");
        conn.setRequestProperty("Content-Type", "text/xml; charset=utf-8");
        conn.setDoOutput(true);

        try (OutputStream os = conn.getOutputStream()) {
            os.write(soapRequest.getBytes(StandardCharsets.UTF_8));
        }

        int responseCode = conn.getResponseCode();
        System.out.println("Response Code: " + responseCode);
    }
}