XML in REST API response and SOAP XML.

The difference between XML that we get in response to any REST API and XML response from SOAP API lies in the underlying protocol, design, and structure of the API. Here’s a breakdown of the differences:

Definition and Nature:

• REST with XML:
  • REST (Representational State Transfer) is an architectural style that works over HTTP/HTTPS.
  • Data can be sent in multiple formats like JSON, XML, or even plain text, depending on the requirements.
  • When REST sends data as XML, it simply uses XML as a data format, but the API still follows REST principles.
  • Lightweight and focuses on simplicity.
• SOAP (Simple Object Access Protocol):
  • SOAP is a protocol with strict rules for sending and receiving data.
  • It exclusively uses XML as its messaging format.
  • SOAP URLs usually point to WSDL (Web Services Description Language), which defines the web service, its operations, and message structure.
  • SOAP has a heavyweight and standardized design.

Data Format:

• REST API with XML:
  • Uses XML purely as a data format (alternative to JSON).
  • REST does not care about how the data is formatted, as long as it conforms to HTTP standards.

Example XML response:
<record>
  <id>1</id>
  <title>Record 1</title>
</record>

• SOAP:
  • Requires a strict XML schema for requests and responses.

The entire SOAP envelope follows a predefined structure:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetRecordResponse>
      <Record>
        <Id>1</Id>
        <Title>Record 1</Title>
      </Record>
    </GetRecordResponse>
  </soap:Body>
</soap:Envelope>

Structure:

• REST with XML:
  • XML is simple and usually represents the data payload without much additional structure.
  • Lightweight compared to SOAP.
• SOAP:
  • Follows a strict envelope-body structure.
  • Contains additional elements like headers, fault handling, and metadata.
  • Includes namespaces to avoid naming conflicts.

Transport Protocol:

• REST with XML:
  • Can use multiple transport protocols like HTTP, HTTPS, FTP, etc.
  • Most commonly works over HTTP methods: GET, POST, PUT, DELETE, PATCH.
• SOAP:
  • Primarily works with HTTP/HTTPS.
  • Can also use protocols like SMTP or JMS.

Complexity:

• REST with XML:
  • Simpler and more flexible.
  • Developers can choose to send lightweight XML responses or JSON depending on client needs.
• SOAP:
  • More complex due to strict standards, XML schema validation, and WSDL files.
  • Adds overhead because of the mandatory envelope and headers.

Performance:

• REST with XML:
  • Faster because XML is used in a straightforward way.
  • If performance is critical, JSON is preferred as it’s lighter than XML.
• SOAP:
  • Slower due to the larger size of the XML payload and additional processing overhead (like parsing the envelope).

Error Handling:

• REST with XML:
  • Typically uses standard HTTP status codes (e.g., 200 OK, 400 Bad Request, 500 Internal Server Error).
• SOAP:
  • SOAP defines a structured fault element for error handling within the message:

<soap:Fault>
  <faultcode>soap:Server</faultcode>
  <faultstring>Internal Server Error</faultstring>
</soap:Fault>

Usage Scenarios:

• REST with XML:
  • Used in modern web APIs where flexibility, simplicity, and performance are key.
  • REST APIs typically prefer JSON today, but XML is still supported when required.
• SOAP:
  • Used in enterprise applications, especially where strict standards, security, and ACID compliance are needed.
  • Examples include banking, financial systems, and legacy services.

Which One to Use?

• Use REST API with XML if you need simplicity, flexibility, and compatibility with older systems.
• Use SOAP if you require strict standards, built-in security (WS-Security), transactions, and advanced error handling.

If you are starting fresh, REST (preferably with JSON) is usually the better choice due to its simplicity and performance benefits.