When building or integrating web services, choosing between SOAP and REST is a critical architectural decision. While REST has become dominant in modern web development, SOAP still maintains relevance in specific enterprise-level use cases. In this article, we'll explore the differences between SOAP and REST, examine when to use each, provide message format examples, and explain the concept of statefulness versus statelessness. Practical examples are also included to help clarify these concepts.

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Is SOAP Getting Deprecated?

SOAP is not officially deprecated, but it is less commonly used in modern web and mobile application development. The rise of RESTful APIs is due to their simplicity, performance, and broad compatibility with lightweight data formats like JSON.

However, SOAP is still actively used in:

• Enterprise systems (e.g., banking, government, healthcare)

• Legacy applications

• Scenarios where high security, reliable messaging, and transaction compliance (ACID) are mandatory

SOAP is supported by major platforms like .NET, Java (JAX-WS), and enterprise middleware solutions. It’s maintained by the W3C and continues to evolve with protocols such as WS-Security and WS-ReliableMessaging.

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When to Use SOAP and When to Use REST

Use SOAP When:

• You require formal contracts (via WSDL)

• The communication needs to be highly secure

• ACID-compliant transactions are a must (e.g., financial services)

• The system is stateful

• You work in enterprise environments using legacy systems

Example Use Cases:

• Bank-to-bank money transfers

• B2B enterprise integrations

• Identity verification systems

Use REST When:

• You need scalable, lightweight, and stateless communication

• Data needs to be easily consumed by web/mobile applications

• Your system architecture is based on microservices

• You want to use JSON or XML flexibly

• You need fast response times

Example Use Cases:

• Social media APIs (e.g., Twitter, Facebook)

• Public data APIs (e.g., weather, maps)

• E-commerce storefronts and mobile apps

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Example of SOAP and REST Message Formats

SOAP Request (XML)

POST /WeatherService HTTP/1.1
Host: www.example.com
Content-Type: text/xml; charset=utf-8
SOAPAction: "http://example.com/GetWeather"

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

REST Request (JSON over HTTP)

GET /weather?city=London HTTP/1.1
Host: www.example.com
Accept: application/json

Response:

{
  "city": "London",
  "temperature": "18°C",
  "condition": "Cloudy"
}

Key Differences:

• SOAP messages are strictly XML-based, verbose, and require a defined schema.

• REST can use JSON, XML, or other formats, and is typically leaner and easier to parse.

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Understanding Stateful and Stateless Communication

What is Statelessness?

A stateless service does not store any information about the client's session. Each request from the client contains all the information needed to process it.

Example (REST):
A REST API for flight search does not remember previous searches. Each request is self-contained:

GET /flights?from=NYC&to=LAX&date=2025-08-01

Even if the user made a similar request before, the server processes it as new.

What is Statefulness?

A stateful service maintains session information between client requests. This is useful when multiple steps in a transaction depend on prior ones.

Example (SOAP):
A SOAP-based bank transaction might involve:

1. Login and authentication

2. Session maintained on the server

3. Money transfer using session token

4. Logout to close the session

This allows for consistent, secure interactions across multiple requests.

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Example APIs: SOAP vs REST

SOAP Example – Bank Transfer API (Simplified)

Request:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
                  xmlns:bank="http://example.com/bank">
   <soapenv:Header/>
   <soapenv:Body>
      <bank:TransferFunds>
         <bank:FromAccount>123456</bank:FromAccount>
         <bank:ToAccount>654321</bank:ToAccount>
         <bank:Amount>500.00</bank:Amount>
         <bank:Currency>USD</bank:Currency>
      </bank:TransferFunds>
   </soapenv:Body>
</soapenv:Envelope>

REST Example – User Profile API

Request:

GET /api/users/789 HTTP/1.1
Host: api.example.com
Accept: application/json

Response:

{
  "id": 789,
  "name": "Jane Doe",
  "email": "jane@example.com"
}

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Conclusion

SOAP and REST serve different purposes and suit different application architectures. SOAP is ideal for complex, secure, and stateful enterprise applications that require strict compliance and reliability. REST, on the other hand, is better suited for lightweight, scalable, and flexible applications, particularly those targeting the web and mobile platforms.

Choosing between SOAP and REST should be guided by business needs, technical requirements, and system constraints. Understanding their core differences ensures better API design, easier maintenance, and improved user experience.

If you're building for the web or mobile, REST is usually the right choice. For legacy systems or high-security transactions, SOAP still holds strong.

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