REST vs SOAP  Differences

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Quick Answer

  • REST is an architectural style: resources are identified by URLs, standard HTTP methods are used, and JSON is common. It is lightweight, flexible and widely used for web, mobile and modern API development.
  • SOAP is a protocol with strict rules: messages use XML envelopes, services can be described through WSDL contracts, and standards exist for security and reliability. It remains common in banking, insurance and enterprise integrations.

REST gives you conventions; SOAP gives you a contract.

Comparison at a Glance

Aspect REST SOAP
Type Architectural style Protocol
Message format Usually JSON; also XML, plain text and others XML
Transport HTTP/HTTPS Usually HTTP/HTTPS; can also use transports such as SMTP or JMS
Operations HTTP methods on resources — GET, POST, PUT, PATCH, DELETE Named operations such as GetUser, usually sent through POST
Contract Optional — often OpenAPI/Swagger WSDL, a formal machine-readable contract
Errors HTTP status codes plus an error body SOAP Fault element in the response
State Stateless by design Stateless by default; WS-* extensions can support stateful patterns
Security HTTPS, OAuth 2.0, JWT, API keys WS-Security plus HTTPS
Caching HTTP caching can be used, particularly with GET No equivalent built-in HTTP-style caching model
Message size and processing Generally lightweight Generally more verbose and processing-intensive
Typical use Web, mobile, microservices, public APIs Banking, payments, insurance, telecom and enterprise systems

The Same Request, Both Ways

Suppose we want to fetch user 42.

REST

GET /users/42 HTTP/1.1
Host: api.example.com
Accept: application/json

HTTP/1.1 200 OK
Content-Type: application/json

{ "id": 42, "name": "Asha", "email": "asha@example.com" }

SOAP

POST /UserService HTTP/1.1
Host: api.example.com
Content-Type: text/xml; charset=utf-8
SOAPAction: "GetUser"

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
                  xmlns:usr="http://example.com/users">
  <soapenv:Header/>
  <soapenv:Body>
    <usr:GetUser>
      <usr:id>42</usr:id>
    </usr:GetUser>
  </soapenv:Body>
</soapenv:Envelope>

The REST request is essentially a URL plus an HTTP method, while the SOAP request contains an XML envelope with a named operation inside the body.

What Is REST?

REST (Representational State Transfer) is a set of architectural constraints for designing networked applications and web APIs.

Common REST principles include:

  • Resources identified by URLs, such as /users/42 and /orders.
  • Uniform interface using standard HTTP methods with consistent meanings.
  • Stateless communication, where each request contains the information needed to process it.
  • Client-server separation.
  • Cacheable responses where appropriate.
  • Layered system architecture.

For example:

GET /users/42
POST /users
PUT /users/42
PATCH /users/42
DELETE /users/42

Because REST works naturally with HTTP and commonly uses lightweight formats such as JSON, it is widely used for web applications, mobile applications, microservices and public APIs.

More: HTTP Methods & Status Codes.

What Is SOAP?

SOAP (Simple Object Access Protocol) is a messaging protocol used to exchange structured XML messages.

A SOAP message is built around an Envelope that can contain:

  • Header — optional information such as security tokens or transaction-related data.
  • Body — the operation request and associated data.
  • Fault — structured error information when processing fails.

A simplified SOAP structure looks like this:

<soap:Envelope>
    <soap:Header>
        <!-- Optional metadata -->
    </soap:Header>

    <soap:Body>
        <!-- Operation and data -->
    </soap:Body>
</soap:Envelope>

WSDL

WSDL (Web Services Description Language) provides a formal description of a SOAP service.

It can describe:

  • Available operations
  • Input messages
  • Output messages
  • Data types
  • Service endpoints
  • Communication details

Because the contract is machine-readable, tooling can use a WSDL to generate client-side code or service bindings.

WS-* Standards

SOAP-based ecosystems can use additional standards for capabilities such as:

  • Message-level security
  • Reliable messaging
  • Transactions
  • Other enterprise integration requirements

WS-Security, for example, provides mechanisms for message-level security such as signing and encryption.

This combination of formal contracts and enterprise standards is one reason SOAP continues to appear in banking, insurance, payments and other enterprise integrations.

When to Use Each

Choose REST When

REST is generally a good fit when:

  • Building web, mobile or public APIs.
  • Developing microservices.
  • You want lightweight communication.
  • JSON is convenient for clients.
  • HTTP caching is useful.
  • Multiple clients such as browsers, mobile apps and partner systems will consume the API.
  • You want an API that follows standard HTTP semantics.

Choose or Keep SOAP When

SOAP can be appropriate when:

  • A formal, machine-readable contract is required between organisations.
  • Message-level security is important.
  • Reliable messaging capabilities are required.
  • Distributed transaction requirements influence the architecture.
  • You are integrating with existing enterprise systems.
  • Banking, insurance, government or other legacy systems already expose SOAP services.

In practice, many organisations use both.

A company may expose REST APIs for newer applications while continuing to use SOAP for existing enterprise integrations.

GraphQL is another API approach for flexible data queries, but it is different from both REST and SOAP.

How Testing Differs

Area Testing REST Testing SOAP
Tools Postman, REST Assured, Playwright API testing SoapUI/ReadyAPI, Postman, REST Assured with XML
Status checks Validate HTTP status codes such as 200, 201, 400 and 404 Status codes plus SOAP Fault information in the response
Body validation JSONPath, JSON Schema XPath, XSD and WSDL-related validation
Contract testing OpenAPI specification WSDL/XSD
Negative tests Invalid JSON, missing fields, incorrect methods Malformed XML, missing elements, incorrect namespaces

Testing SOAP with REST Assured

REST Assured can also be used to test SOAP services.

You can send the SOAP envelope as the request body and validate the XML response using XPath.

given()
    .contentType("text/xml; charset=utf-8")
    .header("SOAPAction", "GetUser")
    .body(soapEnvelope)
.when()
    .post("/UserService")
.then()
    .statusCode(200)
    .body(hasXPath("//*[local-name()='name']", equalTo("Asha")));

When testing SOAP, do not rely only on the HTTP status code.

A successful HTTP response can still contain application-level information that indicates a SOAP Fault or business failure, depending on the service implementation.

Practise designing test cases for real endpoints in the API Testing Scenario Lab.

Also see JSON & XML Data Formats.

From Real Projects

My API testing experience covers CRUD operations, HTTP methods, JSON path, and validating JSON and XML data. On Testsigma, a platform that itself automates API tests, understanding requests, responses and status codes was part of understanding the product. Whatever tool you use, validate the status code, the response structure and the values — not just that the call returned something. Most modern APIs are REST, but many enterprise and banking systems still expose SOAP services.

📖 Related tutorials

📚 Official documentation: GraphQL: Learn · MDN: HTTP

Frequently Asked Questions

What is the difference between REST and SOAP?

REST is an architectural style that commonly uses URLs, HTTP methods and JSON.

SOAP is a protocol based around structured XML messages and can use formal WSDL contracts and standards such as WS-Security.

Which data formats do REST and SOAP use?

REST commonly uses JSON, but it can also work with XML, plain text and other representations.

SOAP messages use XML for the SOAP envelope and message structure.

Is REST better than SOAP?

Neither is universally better.

REST is commonly used for modern web, mobile, microservice and public APIs, while SOAP remains useful for enterprise integrations where formal contracts, message-level security or established SOAP infrastructure are important.

The appropriate choice depends on the system's requirements and existing architecture.

Is REST stateless?

Yes.

Statelessness is one of REST's architectural constraints. Each request should contain the information the server needs to process it rather than relying on stored client session state on the server.

What is a WSDL?

WSDL (Web Services Description Language) is an XML-based description of a web service.

It can define the service's operations, messages, data types and endpoints, providing a formal contract between the client and service.

How do you test SOAP APIs?

SOAP APIs can be tested using tools such as:

  • SoapUI / ReadyAPI
  • Postman
  • REST Assured with XML request bodies

Testing should include:

  • SOAP request validation
  • Response validation
  • XPath assertions
  • XSD validation where applicable
  • WSDL contract validation
  • SOAP Fault validation
  • Positive and negative scenarios
  • Authentication and security scenarios

Do not validate only the HTTP status code.

Why is SOAP still used?

SOAP remains in use because some enterprise systems require formal service contracts, established security mechanisms, reliable messaging capabilities or existing SOAP-based integrations.

Banking, payments, insurance, government and other enterprise environments can therefore continue to have SOAP services alongside newer REST APIs.