SOAP vs REST
Updated 2026-11-13 · 2 min read
Compare a messaging protocol with an architectural style, including contracts, payloads, faults and testing priorities. Includes a comparison table, concrete examples and an interview answer.
The difference in one table
| Aspect | SOAP | REST |
|---|---|---|
| What it is | An XML-based messaging protocol framework | An architectural style for distributed systems |
| Message shape | SOAP envelope with defined message structure | Representations; JSON is common but not required |
| Contract | Services often publish WSDL and XML schemas | HTTP APIs may publish OpenAPI or another contract |
| Transport | Can bind to different underlying protocols | Web APIs commonly use HTTP semantics |
| Failure evidence | Inspect SOAP faults and applicable transport status | Inspect HTTP status, headers and error representation |
Examples
SOAP: a hypothetical booking service receives a structured booking message in an XML envelope. Test required fields, namespaces, schema constraints and the specified fault for an unavailable seat. Validate the operation's actual contract rather than inventing an envelope from memory.
REST-style HTTP API: a booking resource is read or changed through its interface. Check resource representations, method behaviour, status codes and access rules. A JSON body over HTTP does not by itself establish that the API follows REST constraints.
Both designs still need tests for business correctness, duplicate requests, permissions and failure handling. Payload format alone does not decide which system is easier to test.
How to answer in an interview
SOAP is an XML-based messaging protocol, whereas REST is an architectural style. SOAP testing emphasises the message contract, envelope and faults; REST-style HTTP testing emphasises resources, representations and HTTP semantics. JSON is common with REST APIs but is not required, and SOAP is not restricted to HTTP. I choose test assertions from the actual service contract rather than assuming one approach is automatically faster or safer.
Common confusion
Do not claim SOAP always requires WSDL or REST cannot use XML. Security depends on the deployed controls, not the label. Many teams call any JSON-over-HTTP API REST, so describe what the interface actually does before judging its design.
Likely follow-up: Can both describe the same business capability? Yes: a booking operation can be exposed through different interfaces, but each interface needs its own contract checks.
Quick answers
Is REST a replacement version of SOAP?
No. They describe different kinds of design choices. Migration depends on consumers, contracts and operating constraints.
Which is better for automation?
Both can be automated. Choose tools that can express the required contract, authentication, data setup and assertions for the service you have.
Sources
Want a project you can show in interviews?
The portfolio project kit walks you through testing a deliberately buggy app with Selenium or Playwright and API tests, then explaining it. The app itself is free.