What Is a RESTful API and How Does It Work?

Over 90% of public APIs on the web follow REST architecture. If you’ve ever pulled data from a server, submitted a form, or used an app that talks to a cloud-based service, a RESTful API was probably behind it.

So what is a RESTful API, and why did it become the default way systems communicate over HTTP?

This guide breaks down the core concepts: how REST differs from SOAP and GraphQL, the six constraints Roy Fielding defined in his 2000 dissertation, how HTTP methods and status codes work in practice, and what most teams get wrong when building APIs they call “RESTful.” Whether you’re working on your first software development project or evaluating API design patterns for production, this covers what you actually need to know.

What is a RESTful API

A RESTful API is an application programming interface that follows the REST architectural style. REST stands for Representational State Transfer.

Roy Fielding introduced REST in his 2000 doctoral dissertation at the University of California, Irvine. He wasn’t designing APIs at the time. He was analyzing why the World Wide Web worked so well and trying to extract the architectural principles behind it.

That distinction matters. REST is a set of constraints for how networked systems should behave. A RESTful API is an API that actually conforms to those constraints. Most developers use the terms loosely, but technically, calling something “RESTful” means it follows Fielding’s original rules.

In practice, a RESTful API lets a client (like a browser, a mobile app, or another server) communicate with a back-end system over HTTP. The client sends a request to a specific URL, and the server responds with data, usually in JSON format.

Postman’s 2025 State of the API report found that REST still dominates at 93% of all API architectures in use. Webhooks, WebSockets, and GraphQL are growing, but REST remains the default for most web services.

Salesforce was technically the first company to sell an API commercially back in 2000. But eBay built an early REST-based API that showed the industry how accessible this approach could be. That pattern caught on fast.

Today, 65% of organizations generate revenue directly from their APIs, according to the same Postman report. RESTful APIs power everything from payment processing at Stripe to repository management on GitHub.

The reason REST became the standard isn’t complicated. HTTP was already everywhere. JSON is lightweight and readable. And the constraints Fielding outlined, things like statelessness and a uniform interface, happen to make APIs easier to build, test, and scale. At least in most cases.

How REST Differs from Other API Architectures

REST is not the only way to build an API. It’s just the most common one. Knowing where it fits, and where it doesn’t, is the difference between picking the right tool and fighting your own architecture for months.

REST vs. SOAP

SOAP (Simple Object Access Protocol) came first. It’s a strict messaging protocol that uses XML exclusively and relies on formal contracts called WSDL files. Every request gets wrapped in an envelope structure with headers and body elements.

REST is an architectural style, not a protocol. That’s a key distinction.

SOAP still shows up in banking, healthcare, and legacy enterprise systems where built-in WS-Security and ACID compliance matter more than flexibility. But for most web apps and mobile services, REST won that fight years ago.

REST vs. GraphQL

GraphQL, developed by Facebook in 2012, takes a different approach. Instead of multiple endpoints (like /users, /users/123/orders), GraphQL exposes a single endpoint. The client specifies exactly what data it wants in the request body.

That solves the “over-fetching” problem where REST endpoints return more data than the client needs. It also solves “under-fetching,” where a client has to hit three separate REST endpoints to assemble one screen of data.

RapidAPI’s 2024 Developer Survey shows GraphQL adoption grew 340% among Fortune 500 companies. But REST still powers 83% of public APIs. The two solve different problems, and most large organizations use both depending on the context. For a deeper comparison, check out the differences between REST API and GraphQL.

REST vs. gRPC

gRPC uses binary protocol buffers instead of text-based JSON. That makes it significantly faster for machine-to-machine communication.

Companies like Netflix, Google, and Uber use gRPC for internal microservices communication. TestDino research indicates gRPC benchmarks show up to 10x lower latency than REST in production AI workloads (25ms vs. 250ms).

The tradeoff? gRPC doesn’t work natively in browsers. It requires protocol-aware tooling for debugging and testing. REST is still the better choice for public-facing APIs that need broad client compatibility.

The Six Constraints of REST

Calling an API “RESTful” means it follows six constraints that Fielding defined in his dissertation. Most APIs labeled as REST actually violate one or more of them. Only a few meet all six.

Here’s what each constraint requires.

Client-server separation: The client and the server operate independently. The client doesn’t need to know how the server stores data. The server doesn’t care what the client’s UI looks like. This separation lets both sides change without breaking the other.

Statelessness: Every request from the client must contain all the information the server needs to process it. The server stores nothing about previous requests between calls.

Cacheability: Responses must define themselves as cacheable or non-cacheable. When done right, caching reduces the number of client-server interactions and directly improves performance.

Uniform interface: This is the one most developers get wrong. It means resources are identified through URIs, manipulated through representations (JSON, XML), messages are self-descriptive, and hypermedia drives application state (HATEOAS).

Layered system: A client can’t tell whether it’s connected directly to the server or going through a proxy, load balancer, or API gateway. Each layer only knows about its immediate neighbor.

Code on demand (optional): The server can send executable code to the client. Fielding included this as the only optional constraint. Almost nobody implements it.

Statelessness in Practice

This is the constraint that causes the most confusion, and the most violations.

Stateless means the server doesn’t store session data between requests. Each request travels with everything: the authentication token, the query parameters, the context. Nothing carries over from the last call.

JWT tokens and OAuth 2.0 work well here because they embed identity information inside the token itself. The server validates the token on each request without needing to look up a session store. This approach is central to how token-based authentication works in RESTful systems.

Where teams break statelessness: server-side sessions, sticky load balancing, storing user state in memory between calls. All of these create coupling between the client and a specific server instance, which kills horizontal scalability.

What Uniform Interface Actually Means

Uniform interface has four sub-constraints. Most developers know the first two. Almost nobody implements the fourth.

• Resource identification: Every resource gets a unique URI. /users/42 always refers to user 42.
• Resource manipulation through representations: The client works with a representation of the resource (JSON object), not the resource itself (database row).
• Self-descriptive messages: Each request and response carries enough metadata (content type, cache directives) that any intermediary can process it.
• HATEOAS: The server response includes links that tell the client what it can do next. This is the part Fielding says is required for true REST, but in practice, very few APIs implement it.

Fielding himself wrote in a 2008 blog post that an API must be hypertext-driven to legitimately call itself RESTful. Most production APIs skip HATEOAS entirely and still label themselves “REST.” That gap between theory and practice is real, and most teams live with it.

RESTful API HTTP Methods and Their Roles

RESTful APIs use standard HTTP methods to perform operations on resources. Each method maps to a specific type of action, and using them correctly is what makes an API predictable for any developer consuming it.

The Core Methods

GET retrieves a resource. It’s safe (doesn’t change server state) and idempotent (calling it 10 times produces the same result as calling it once). GET requests should never modify data. Period.

POST creates a new resource. Send data to /users and you get a new user back with a 201 Created status. POST is not idempotent. Hitting it twice creates two resources.

PUT replaces an entire resource at a specific URI. Send a full user object to /users/42 and whatever was there before gets completely overwritten. PUT is idempotent.

PATCH applies a partial update. Only the fields you include in the request body get changed. Took me a while to appreciate the difference between PUT and PATCH, but it matters when you’re working with large resource objects and only need to flip one field.

DELETE removes a resource. Also idempotent. Deleting the same resource twice should return a success (or a 404 the second time, depending on your implementation).

Less Common Methods

HEAD works exactly like GET but returns only headers, no body. Useful for checking if a resource exists or reading metadata without downloading the full payload.

OPTIONS returns the HTTP methods that a server supports for a given URL. This is what browsers send during CORS preflight requests.

Treblle’s 2024 Anatomy of an API report analyzed over a billion API calls and found that GET-only requests fell by 5% while POST requests grew by 4%. APIs are handling more write-heavy workloads now, which aligns with the shift toward AI-driven data exchange.

Correct method usage matters beyond semantics. GET responses can be cached by browsers and CDNs. PUT and DELETE being idempotent means retries are safe during network failures. When teams use POST for everything (and plenty do), they lose all of these built-in advantages.

RESTful API Resource Design and URL Structure

URL design is where REST either clicks or falls apart. A well-designed URI scheme makes an API feel intuitive. A bad one makes every integration a guessing game.

Resources as Nouns

REST URLs should reference things, not actions.

• /users (correct, noun-based)
• /getUsers (incorrect, verb in URL)
• /users/42/orders (correct, nested resource)
• /fetchOrdersByUser?id=42 (incorrect, action-based)

Treblle’s data shows 77% of API endpoints use nouns, but only 16% follow plural naming conventions. That’s a low number for what’s supposed to be a best practice. Old habits from RPC-style APIs are clearly sticky.

Nesting vs. Flat Structure

Nested URLs like /users/42/orders communicate parent-child relationships. They read naturally and make the data model clear.

But nesting goes wrong when you go too deep. Something like /companies/5/departments/3/teams/7/members/42 is technically valid but painful to use. After two levels, most teams flatten the structure and use query parameters instead.

Stripe’s API is a good reference here. They keep nesting shallow (/customers/{id}/subscriptions) and use query parameters for filtering: /charges?customer=cus123. GitHub does something similar with repository resources, keeping URIs clean and predictable.

Query Parameters

Filtering: /products?category=electronics∈stock=true

Sorting: /products?sort=priceℴ=asc

Pagination: /products?page=2&limit=25

These patterns are conventions, not part of the REST specification. But they’ve become so common that deviating from them creates friction for anyone consuming your API. The OpenAPI specification (formerly Swagger) helps standardize how these parameters get documented across teams.

HTTP Status Codes in RESTful APIs

Status codes are how RESTful APIs communicate what happened with a request. They’re built into HTTP, and using them correctly saves everyone time, especially the developer debugging a failed call at 2 AM.

Success Codes (2xx)

200 OK: The request worked. Used for successful GET, PUT, or PATCH responses.

201 Created: A new resource was created, typically returned after a POST.

204 No Content: The request was successful but there’s nothing to return. Common after DELETE operations.

Client Error Codes (4xx)

The 401 vs. 403 distinction trips people up constantly. 401 means “who are you?” and 403 means “I know who you are but you can’t do that.” Mixing them up confuses every client-side developer trying to handle errors properly.

Server Error Codes (5xx)

500 Internal Server Error is the generic catch-all. It means something broke on the server side but the API isn’t telling you what.

502 Bad Gateway and 503 Service Unavailable show up when the API is behind a reverse proxy or gateway and the upstream server is failing or overloaded.

Treblle’s research found that 10% of 200 OK responses actually contain an error in the response body. That’s a common anti-pattern where teams return a 200 status with an error message inside the JSON payload. It breaks client-side error handling, defeats HTTP caching, and makes monitoring tools useless for tracking real failure rates.

Getting status codes right is one of those things that separates a well-built API from one that just happens to work. When the development process includes proper status code mapping from the start, integrations go faster and defect tracking gets much simpler.

Authentication and Security for RESTful APIs

API security is the single biggest headache in the industry right now. Salt Security’s 2024 report found that 95% of organizations experienced security problems in their production APIs, with 23% suffering actual breaches.

The challenge is protecting endpoints without violating REST’s statelessness constraint. Every authentication mechanism has to work within that boundary.

API Keys

The simplest approach. You generate a random string, pass it in the request header, and the server validates it.

GitHub’s 2024 report showed they detect and revoke over 2 million exposed API keys per year. That’s roughly 5,500 per day. API keys are easy to implement but dangerous when leaked, and they leak constantly because developers hardcode them into client-side code or commit them to public repositories.

API keys work fine for server-to-server communication and internal tools. They’re a poor choice for anything user-facing.

OAuth 2.0 and JWT

OAuth 2.0 is an authorization framework, not an authentication protocol. It lets users grant third-party apps limited access to their resources without sharing passwords. Google, GitHub, and Stripe all use it.

OAuth defines several flows:

• Authorization code (most secure, used by web apps)
• Client credentials (machine-to-machine)
• Device code (smart TVs, CLI tools)

JSON Web Tokens (JWT) work well with RESTful APIs because they’re self-contained. The token carries user identity, permissions, and an expiration timestamp, all signed cryptographically. The server validates the signature without querying a database.

A 2024 security analysis found that 68% of OAuth implementations had at least one vulnerability, mostly insecure redirect URIs and improper token validation. The protocol itself is solid. Implementations are where things go wrong.

HTTPS, CORS, and Rate Limiting

HTTPS is non-negotiable. Treblle’s 2024 data shows HTTPS adoption among APIs actually dropped from 74% to 45% year over year. That’s alarming.

CORS (Cross-Origin Resource Sharing) controls which domains can call your API from a browser. Misconfigured CORS headers are a common attack vector, especially in front-end applications that consume APIs directly.

API throttling and rate limiting prevent abuse by capping how many requests a client can make in a given window. Twilio, for instance, returns a 429 status code with a Retry-After header when clients hit their limit.

Salt Security’s 2025 report escalated the numbers further: 99% of organizations encountered API security problems in the past year. Only 10% had an API posture governance strategy in place. The gap between API usage growth and security maturity keeps widening.

RESTful API Versioning and Lifecycle Management

APIs break. Features change, data models shift, and old patterns get replaced. Versioning is how you handle that without taking down every client that depends on your endpoints.

Versioning Strategies

URI versioning is the most visible and easiest to test. Header-based versioning keeps URLs clean but makes debugging trickier. Stripe’s date-based system lets users pin to a specific version and upgrade when ready, which is the most developer-friendly approach out there.

Treblle reports that 71% of APIs now use some form of versioning. That still leaves nearly a third of APIs with no versioning at all, which creates real problems when breaking changes ship.

Deprecation and Sunset Policies

Twilio gives developers a full 12-month notice before deprecating any API version. Salesforce rolls out new versions three times per year and supports versions going back to Spring 2014 (v30).

Both approaches follow a similar pattern: announce, overlap, retire.

Typical deprecation timeline:

• 6-month announcement period
• 12 months of active migration support
• 18-24 months total before removal

The HTTP Sunset header is a standard way to signal upcoming deprecation in the response itself. Not enough APIs use it, but it’s the cleanest way to communicate lifecycle changes programmatically. Using semantic versioning alongside clear deprecation policies helps teams plan their upgrade cycles without surprises.

Tools for Building and Testing RESTful APIs

The tooling around REST APIs has matured significantly. Picking the right framework and testing stack depends on your language, team size, and what you’re actually building.

Frameworks by Language

Node.js: Express.js remains the default for JavaScript APIs. It’s minimal, flexible, and has the largest middleware ecosystem. Most tutorials and starter projects still use it.

Python: FastAPI has been gaining ground fast. It generates OpenAPI docs automatically from type hints and handles async natively. Flask is still around for simpler projects, but FastAPI is where the momentum is.

Java: Spring Boot dominates enterprise API development. Heavier to set up, but once running, it handles complex business logic and security at a level most lightweight frameworks can’t match.

PHP: Laravel is the most popular choice for PHP-based RESTful APIs, with built-in support for authentication (Sanctum), ORM (Eloquent), and testing.

Postman’s 2025 report notes that 75% of teams use CI/CD pipelines for API deployment. The framework you choose matters less than how you test and ship it.

Testing and Documentation Tools

Postman grew from 25 million to over 35 million users by 2026, according to SQ Magazine data. It handles API testing, documentation, and collaboration in one platform.

Swagger/OpenAPI is the standard for describing RESTful APIs. Version 3 is in widespread use, with Version 4 on the way. The 2024 State of SaaS APIs report found that OpenAPI and Swagger are the most supported API specification formats across the industry.

Other tools worth knowing: Insomnia for lightweight testing, build pipelines integrated with API contract tests, and mock servers for prototyping before the back-end is ready.

API Gateways and Monitoring

Kong, AWS API Gateway, and Apigee sit between clients and your API servers. They handle authentication, rate limiting, request routing, and analytics at the infrastructure level.

Monitoring tools like Datadog and New Relic track API latency, error rates, and throughput. SQ Magazine reports that 93% of developers now prioritize REST API performance metrics as part of their workflow.

The tech stack you choose for web development affects how smoothly these tools integrate. Most modern gateways and monitors plug into continuous integration pipelines with minimal configuration.

Common Mistakes That Make an API Not Truly RESTful

Most APIs that call themselves RESTful aren’t. That’s not a controversial statement. Treblle’s analysis of over a billion API calls confirms it: the average API scores 57 out of 100 on quality.

Here’s what goes wrong the most.

Verbs in URLs and Wrong HTTP Methods

This is the classic one. URLs like /getUsers, /deleteOrder/123, or /createProduct mix actions into the resource identifier. REST URLs should be nouns. The HTTP method tells you what’s happening.

And then there’s the “POST for everything” anti-pattern. Zuplo research calls it one of the most common API design mistakes. When POST handles reads, deletes, and updates, you lose caching, idempotency, and any predictability the HTTP spec was designed to give you.

Treblle data backs this up: only 16% of endpoints follow plural noun naming conventions, even though it’s been a standard recommendation for over a decade.

Breaking Statelessness

Server-side sessions are the most common violation. Storing user context between requests on the server creates tight coupling between a client and a specific server instance.

This kills horizontal scaling. If a request has to hit the same server every time, your load balancer can’t distribute traffic evenly. Sticky sessions are a band-aid, not a fix.

The correct approach: put everything the server needs into each request. JWT tokens, query parameters, request headers. Nothing should carry over from the previous call.

Ignoring Status Codes and Error Handling

Returning 200 OK with an error message buried in the JSON body is still disturbingly common. Treblle found 10% of 200 responses actually contained errors.

This breaks client-side error handling, confuses monitoring dashboards, and makes your API unpredictable for anyone building on top of it.

A well-designed API uses the full range of HTTP status codes: 201 for resource creation, 204 for successful deletes, 400 for bad input, 401/403 for auth issues, 422 for validation failures. Each code tells the client exactly what happened without parsing the response body.

The HATEOAS Debate

Fielding’s original dissertation says an API must include hypermedia links (HATEOAS) to be truly RESTful. The server response should tell the client what actions are available next.

Almost nobody does this.

Most production APIs skip HATEOAS entirely. The debate over whether it’s worth implementing has been ongoing since the mid-2000s. Strict REST purists say you can’t call it RESTful without it. Practically, most teams treat REST as a set of guidelines rather than a strict rulebook, and they ship APIs that work well without HATEOAS.

Treblle’s data shows that only 15% of APIs use rate limiting, and endpoint security scores average 40 out of 100. The industry has bigger problems to solve than hypermedia compliance. Getting software development best practices right across the release cycle would move the needle a lot further than chasing theoretical purity.

FAQ on What Is A Restful Api

What does RESTful API stand for?

RESTful API stands for Representational State Transfer Application Programming Interface. It’s an API that follows the REST architectural style defined by Roy Fielding in his 2000 dissertation, using standard HTTP methods to exchange data between client and server.

How does a RESTful API work?

A client sends an HTTP request to a specific URL (endpoint) on the server. The server processes it and returns a response, usually in JSON format. Each request is stateless, meaning the server doesn’t store anything about previous interactions.

What is the difference between REST and RESTful?

REST is the architectural style, a set of constraints for networked systems. RESTful describes an API that actually implements those constraints. Many APIs claim to be RESTful but violate core principles like statelessness or uniform interface.

What are the main HTTP methods used in RESTful APIs?

The five primary methods are GET (read), POST (create), PUT (full update), PATCH (partial update), and DELETE (remove). Each maps to a CRUD operation. Correct usage enables caching, idempotency, and predictable behavior across API endpoints.

Is REST the same as HTTP?

No. HTTP is a protocol for transferring data. REST is an architectural style that typically uses HTTP as its transport layer. You could theoretically implement REST over other protocols, though HTTP is the standard in practice.

What makes RESTful APIs better than SOAP?

REST is lighter, faster, and supports multiple data formats like JSON and XML. SOAP is limited to XML and requires strict contracts. REST works better for progressive web apps and mobile clients where bandwidth and speed matter.

What is statelessness in a RESTful API?

Statelessness means every request contains all information the server needs to process it. No session data gets stored between calls. Authentication tokens like JWT travel with each request, keeping the server free from client-specific memory.

What is HATEOAS in REST?

HATEOAS stands for Hypermedia as the Engine of Application State. It means server responses include links telling the client what actions are available next. It’s part of Fielding’s original spec, but most production APIs skip it entirely.

What tools are used to build and test RESTful APIs?

Common frameworks include Express.js, FastAPI, and Spring Boot. For testing and documentation, Postman and Swagger (OpenAPI) are industry standards. API gateways like Kong and AWS API Gateway handle routing, security, and deployment at scale.

Can RESTful APIs be used with microservices?

Yes. REST is the most common protocol for communication between microservices. A CNCF 2024 survey found 78% of microservices architectures primarily use REST for inter-service calls, though gRPC is gaining ground for latency-sensitive internal traffic.

Conclusion

Understanding what is a RESTful API comes down to grasping a few core ideas: stateless client-server communication, resource-based URL design, correct HTTP method usage, and proper status code handling. The theory is straightforward. Execution is where most teams stumble.

REST dominates because it maps naturally to how the web already works. JSON payloads are lightweight. HTTP caching is built in. And the learning curve is low enough that any developer can start building endpoints within hours.

But “RESTful” is a spectrum. Getting authentication right with OAuth 2.0 or JWT, choosing a versioning strategy, and avoiding anti-patterns like verbs in URLs or server-side sessions takes deliberate effort.

Start with clean resource design. Pick a solid framework like Express.js, FastAPI, or Spring Boot. Write integration tests early. Document your endpoints with OpenAPI. And version your API before the first breaking change ships, not after.

The architects and teams that treat their API as a product, not just a technical layer, are the ones who build services that last.

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Bogdan Sandu

Bogdan Sandu specializes in web design, focusing on creating user-friendly websites, and innovative UI kits.

Many of his resources are available on various design marketplaces and for free on Codepen.

Over the years, he's worked with a range of clients and contributed to design publications like Design Your Way, Designmodo, WebDesignerDepot, WPDean, Speckyboy, and Slider Revolution among others.

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