What's the Difference Between SOAP and REST?

Ever wondered how two completely different systems talk to each other over the Internet? That’s what APIs (Application Programming Interfaces) do. Two of the most common API styles are SOAP and REST. You’ll see both around sometimes side by side so let’s unpack what makes each tick

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SOAP: The Strict XML Protocol

SOAP (Simple Object Access Protocol) is a formal protocol, with its own envelope, header, body, and lots of built in standards:

• Message format: Pure XML wrapped in a “SOAP envelope.”
• Contract: Defined in a WSDL (Web Services Description Language) file.
• Security: WS Security( Web Services Security) tokens, signatures, and encryption.
• Reliability: WS ReliableMessaging ensures messages aren’t lost.
• Transport: Not just HTTP you can use SMTP, TCP, even JMS.

Here’s how a typical SOAP call flows:

sequenceDiagram participant Client participant Server Client->>Server: POST /UserService (SOAP Envelope with CreateUser request) Server-->>Server: Validate envelope, check WS Security token Server-->>Server: Invoke CreateUser in backend Server-->>Client: 200 OK (SOAP Envelope with response XML)

You can see the extra steps envelope parsing, security token checks, WSDL driven binding. It’s heavyweight but gives you a rigid, enterprise grade contract.

REST: The Flexible Architectural Style

REST (Representational State Transfer) isn’t a protocol but an architecture a set of six guiding principles:

1. Client server: Separation of concerns.
2. Stateless: Each request carries all needed info.
3. Cacheable: Responses can be stored to speed up repeat calls.
4. Layered system: Clients don’t know whether they talk directly to the server or through proxies.
5. Uniform interface: Standard methods (HTTP verbs) and media types (JSON, XML, HTML).
6. Code on demand (optional): Servers can send executable code.

A typical RESTful interaction:

flowchart TD A[Client: GET /users/123] --> B[Server receives HTTP request] B --> C[Lookup user#123 in DB] C --> D[Format as JSON] D --> E[Return 200 OK + JSON body]

It’s lean no envelopes or extra standards. You talk over plain HTTP and exchange JSON (most often), XML, or whatever format you choose.

Side by Side Comparison

Aspect	SOAP	REST
Nature	Protocol	Architectural style
Format	Only XML	Any (JSON, XML, HTML, plain text…)
Contract	Strict Web Services Description Language(WSDL)	Informal (URI + docs)
Transport	HTTP, SMTP, TCP, JMS	Primarily HTTP(S)
State	Can be stateful	Stateless
Security	WS Security (tokens, signatures)	TLS/HTTPS, OAuth, JWT
Error Handling	Built in (fault elements)	HTTP status codes + custom payloads
Performance	Heavier messages, more overhead	Lightweight, cacheable
Scalability	Harder needs to track state	Easier stateless, can load balance transparently
Best for	Enterprise, ACID style transactions, legacy apps	Modern web/mobile, microservices, public APIs

When to Use SOAP vs REST

SOAP shines when you need:

• ACID compliance and transactional integrity.
• Enterprise security via WS Security(Web Services Security)
• Guaranteed delivery (WS ReliableMessaging).
• Integration with existing legacy SOAP services.

REST is your pick when you want:

• Rapid, lightweight web or mobile APIs.
• Stateless interactions that scale horizontally.
• Flexible payloads (JSON) easy for browsers and apps.
• Public endpoints where standard HTTP caching, OAuth, and CORS rules apply.

How SOAP Works Under the Hood

1. Client builds a SOAP envelope with the method call.
2. Server validates the envelope, checks WS Security tokens.
3. Server runs the operation and wraps the result in another envelope.
4. Client parses the response envelope.

sequenceDiagram participant C as Client participant S as SOAP Server C->>S: CreateOrder… S-->>S: Authenticate token, validate schema S-->>S: Execute CreateOrder logic S-->>C: OrderCreated

How REST Works Under the Hood

1. Client issues an HTTP verb (GET, POST, PUT, DELETE) on a URI.
2. Server routes to a controller or handler.
3. Server performs business logic, accesses databases, etc.
4. Server returns an HTTP status and a payload (usually JSON).

sequenceDiagram participant Client participant Server participant Controller as OrderController participant Service participant Repo as Repository Client->>Server: POST /orders Server->>Controller: OrderController.create() Controller->>Service: process order Service->>Repo: save to DB Repo-->>Service: confirmation Service-->>Controller: order processed Controller-->>Server: build 201 Created + Location header Server-->>Client: 201 Created (Location + JSON payload)

Similarities

• Both use HTTP under the hood.
• Both can work securely over TLS/SSL.
• Both define standards for request/response semantics.

Summary

flowchart LR SOAP --> Protocol REST --> Style Protocol --> XMLOnly Style --> AnyFormat XMLOnly --> Envelope AnyFormat --> HTTPVerbs