A REST API is an application programming interface that follows the principles of REST (Representational State Transfer) to enable structured communication between software systems. REST APIs typically organize data as resources and use standard web technologies to retrieve, create, update, or delete that data.
REST remains the dominant API architecture. According to the 2025 State of the API Report, 93% of surveyed developers use REST. At the same time, 50% use webhooks and 33% use GraphQL, showing that development teams increasingly combine REST with other API patterns for specific use cases.
This guide explains what a REST API is, how REST APIs work, what makes an API RESTful, and how standard HTTP methods are used to work with resources. You will also see a practical REST API request and response example, compare REST API vs Web API, and learn how REST is used in eCommerce integrations.
What Is a REST API?
A REST API is an application programming interface designed according to REST (Representational State Transfer) principles. It enables software systems to exchange representations of resources through a consistent interface. Resources can include products, orders, customers, users, or other data that an application needs to access or modify.
In a REST API, each resource is identified by a URI, while requests specify what the client wants to do with that resource. REST APIs commonly use HTTP methods such as GET, POST, PUT, PATCH, and DELETE. However, REST itself is an architectural style rather than a protocol, and it does not require a specific data format such as JSON.
What Is REST?
REST stands for Representational State Transfer. Software architect Roy Fielding introduced the architectural style in his 2000 doctoral dissertation. REST defines architectural constraints for distributed systems rather than a specific protocol or API specification.
The term “representational state transfer” refers to exchanging representations of resources between a client and a server. For example, an eCommerce resource can be a product or order. A server can return a representation of that resource containing fields such as its ID, status, price, or quantity.
REST is closely associated with HTTP because HTTP provides many features that work naturally with RESTful systems. However, REST and HTTP are not the same thing. Likewise, a REST API does not have to use JSON, although JSON is widely used by modern web APIs.
What Makes an API RESTful?
An API is considered RESTful when its architecture follows the constraints defined by REST. These constraints help separate client and server responsibilities, improve scalability, and provide a consistent way to interact with resources.
- Client-server: The client and server have separate responsibilities and can evolve independently.
- Stateless: Each request contains the information required to process it, without relying on stored client session state between requests.
- Cacheable: Responses indicate whether they can be cached, which can reduce repeated requests and improve performance.
- Uniform interface: Clients interact with resources through a consistent interface that separates resource identification from their representations.
- Layered system: Intermediaries such as gateways, proxies, and caches can exist between the client and server without changing the interface.
- Code on demand (optional): A server may extend client functionality by transferring executable code. This is the only optional REST constraint.
Together, these constraints distinguish RESTful architecture from APIs that simply communicate over the web. Therefore, using HTTP and returning JSON alone does not automatically make an API RESTful.
How Does a REST API Work?
A REST API works through requests and responses between a client and a server. The client identifies a resource, sends a request describing the intended operation, and receives a representation of the resource or the result of that operation.
For example, an order management system may request order data from an eCommerce platform. The request identifies the order resource and includes the information needed to process the operation. The server then processes the request and returns a response, commonly as JSON, together with information about the result.
- The client identifies a resource. A resource can represent an order, product, customer, shipment, or another entity.
- The client sends a request. The request contains the information needed to perform an operation on that resource.
- The server processes the request. Because REST is stateless, each request should contain the context required for the server to handle it.
- The server returns a response. The response can contain a resource representation, operation result, and an HTTP status code.
This request-response model keeps client and server responsibilities separate. As a result, REST APIs can support integrations between applications built with different programming languages and technology stacks.
REST API Methods
REST APIs commonly use HTTP methods to express operations on resources. GET retrieves data, POST submits data or commonly creates a resource, PUT replaces a resource representation, PATCH applies partial changes, and DELETE removes a resource. Using HTTP methods consistently makes API behavior easier for developers to understand.
| HTTP Method | Typical Purpose | Safe | Idempotent |
|---|---|---|---|
| GET | Retrieve a resource or collection | Yes | Yes |
| POST | Submit data or create a resource | No | No |
| PUT | Replace a resource representation | No | Yes |
| PATCH | Apply partial modifications to a resource | No | Not guaranteed |
| DELETE | Remove a resource | No | Yes |
Safe methods are intended only for retrieving information and should not change the requested resource state. Idempotent methods are designed so that repeating the same request has the same intended effect as making it once. These properties are especially important when developers implement retries and error handling in distributed systems.
HTTP status codes provide additional information about the result. For example, 200 OK indicates a successful request, 201 Created can indicate that a resource was created, 400 Bad Request signals a client-side request problem, and 404 Not Found indicates that the requested resource was not found.
What Are the Benefits of REST APIs?
The main benefits of REST APIs are scalability, interoperability, cacheability, loose coupling, and a consistent resource-oriented interface. These characteristics make REST well suited to web applications, SaaS products, and integrations that need to exchange data across different systems.
| REST API Benefit | Why It Matters |
|---|---|
| Scalability | Stateless requests reduce dependencies between interactions and make it easier to distribute requests across multiple server instances. |
| Loose coupling | Client and server responsibilities remain separate, so either side can evolve without requiring the other to use the same technology stack. |
| Cacheability | Cacheable responses can reduce repeated requests, server load, and latency for frequently requested resources. |
| Interoperability | REST APIs can connect applications built with different programming languages and technologies through a shared interface. |
| Resource-oriented structure | Resources such as products, orders, and customers can be identified consistently, making API interactions easier to understand and document. |
| Broad web ecosystem | Developers can use established HTTP infrastructure, libraries, gateways, monitoring tools, and other widely available technologies. |
These benefits are particularly useful for integration-heavy SaaS products. For example, an order management system may need to retrieve orders, update products, and synchronize inventory across external services. A consistent REST API interface helps developers perform these operations without coupling their application to the implementation details of each service.
What Are the Limitations of REST APIs?
REST APIs can require multiple requests for related resources, return more or less data than a client needs, and provide no built-in mechanism for real-time event delivery. In addition, REST defines architectural constraints rather than a complete API specification. Therefore, two RESTful APIs can still differ significantly in authentication, pagination, filtering, error handling, and resource design.
- Overfetching and underfetching: Fixed resource representations may return unnecessary fields or require additional requests to retrieve related data.
- Multiple network requests: Complex workflows may require calls to several resources or endpoints.
- No built-in event delivery: REST follows a request-response model, so applications often use webhooks or other event-driven mechanisms when they need asynchronous updates.
- Implementation differences: REST constraints provide architectural guidance, but they do not standardize authentication, pagination, filtering, versioning, or error formats.
As a result, REST is not automatically the best approach for every API use case. The right architecture depends on factors such as data access patterns, real-time requirements, system complexity, and the applications that need to communicate.
REST API Design and HTTP Semantics
REST API design commonly uses HTTP semantics to make interactions with resources predictable. HTTP methods describe the intended operation, status codes communicate the result, and headers provide metadata about requests and responses.
Resource-Oriented API Design
RESTful APIs organize interactions around resources rather than individual actions. A resource can represent an order, product, customer, shipment, or another entity. Each resource is identified through a URI, while the request method describes the intended operation.
For example, an API can expose product resources that clients retrieve, create, update, or delete. This separation between resources and operations helps create a more consistent interface across an API.
Stateless Requests
REST requires client-server interactions to be stateless. Therefore, each request must contain the information the server needs to process it rather than depending on client session state stored between requests.
Statelessness can simplify scaling because requests do not have to be routed back to the same server instance solely to recover application session context. However, applications can still maintain state elsewhere, such as in databases or client-side storage.
HTTP Status Codes and Responses
Web-based REST APIs commonly use HTTP status codes to communicate request outcomes. For example, 200 OK indicates a successful request, 201 Created can indicate successful resource creation, 400 Bad Request identifies a problem with the request, and 404 Not Found indicates that the requested resource could not be found.
These conventions help clients determine whether an operation succeeded and decide how to handle errors, retries, or subsequent requests.
REST API Example: Request and Response
A REST API example typically includes a request sent by a client and a response returned by the server. The request identifies the operation and provides the required parameters, while the response contains the result, requested resource data, or information about an error.
For a real-world example, consider API2Cart, a unified REST API for eCommerce integrations. It enables software such as order management, inventory management, ERP, shipping, and other SaaS solutions to connect with 80+ eCommerce integrations through one API. Developers can work with resources such as products, orders, customers, categories, shipments, and inventory without implementing each platform API separately.
REST API Request Example
The following example uses API2Cart API v1.1 to add a product. The product.add method sends product data to a connected eCommerce store. The request can include fields such as the product name, description, price, SKU, quantity, and other product attributes.
Here is an example of a product.add request:
{
"name": "Bag",
"model": "bag_01",
"description": "Product description",
"price": 99.9,
"status": "disabled",
"categories_ids": "23,56",
"is_virtual": false,
"available_for_view": true,
"available_for_sale": true,
"old_price": 99.9,
"special_price": 56.9,
"cost_price": 65.9,
"quantity": 0,
"manage_stock": false,
"warehouse_id": "1",
"backorder_status": "true",
"weight": 0,
"weight_unit": "lb",
"barcode": "9770317847001",
"harmonized_system_code": "123456",
"country_of_origin": "123456",
"manufacturer": "Samsung",
"search_keywords": "key1,key2,key3",
"tags": "tag1,tag2",
"meta_title": "category,test",
"meta_description": "category,test",
"seo_url": "some seo url",
"taxable": true
}
Here is the response structure of product.list method:
{
"return_code": 0,
"return_message": "string",
"pagination": {
"previous": "string",
"next": "string",
"additional_fields": {},
"custom_fields": {}
},
"result": {
"products_count": 0,
"product": [
{
"id": "string",
"type": "string",
"u_model": "string",
"u_sku": "string",
"name": "string",
"description": "string",
"short_description": "string",
"price": 0,
"advanced_price": [
{
"id": "string",
"value": 0,
"avail": true,
"group_id": "string",
"quantity_from": 0,
"start_time": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"expire_time": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"additional_fields": {},
"custom_fields": {}
}
],
"cost_price": 0,
"quantity": 0,
"inventory": [
{
"warehouse_id": "string",
"quantity": 0,
"in_stock": true,
"priority": 0,
"additional_fields": {},
"custom_fields": {}
}
],
"group_items": [
{
"child_item_id": "string",
"product_id": "string",
"default_qty_in_pack": "string",
"is_qty_in_pack_fixed": true,
"price": 0,
"additional_fields": {},
"custom_fields": {}
}
],
"u_brand_id": "string",
"u_brand": "string",
"categories_ids": [
"string"
],
"stores_ids": [
"string"
],
"url": "string",
"seo_url": "string",
"meta_title": "string",
"meta_keywords": "string",
"meta_description": "string",
"avail_sale": true,
"avail_view": true,
"is_virtual": true,
"is_downloadable": true,
"weight": 0,
"weight_unit": "string",
"sort_order": 0,
"in_stock": true,
"on_sale": true,
"backorders": "string",
"manage_stock": "string",
"is_stock_managed": true,
"create_at": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"modified_at": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"tax_class_id": "string",
"special_price": {
"value": 0,
"avail": true,
"created_at": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"modified_at": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"expired_at": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"additional_fields": {},
"custom_fields": {}
},
"tier_price": [
{
"qty": 0,
"price": 0,
"additional_fields": {},
"custom_fields": {}
}
],
"group_price": [
{
"id": "string",
"group_id": "string",
"price": 0,
"store_id": "string",
"quantity": 0,
"start_time": "string",
"expire_time": "string",
"additional_fields": {},
"custom_fields": {}
}
],
"images": [
{
"id": "string",
"http_path": "string",
"file_name": "string",
"mime-type": "string",
"size": 0,
"create_at": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"modified_at": {
"value": "string",
"format": "string",
"additional_fields": {},
"custom_fields": {}
},
"alt": "string",
"avail": true,
"sort_order": 0,
"type": "string",
"additional_fields": {},
"custom_fields": {}
}
],
"product_options": [
{
"id": "string",
"product_option_id": "string",
"name": "string",
"description": "string",
"sort_order": 0,
"type": "string",
"required": true,
"available": true,
"used_in_combination": true,
"option_items": [
{
"id": "string",
"product_option_item_id": "string",
"name": "string",
"sort_order": 0,
"price": "string",
"weight": "string",
"quantity": 0,
"type_price": "string",
"sku": "string",
"is_default": true,
"additional_fields": {},
"custom_fields": {}
}
],
"additional_fields": {},
"custom_fields": {}
}
],
"u_upc": "string",
"u_mpn": "string",
"u_gtin": "string",
"u_isbn": "string",
"u_ean": "string",
"related_products_ids": [
"string"
],
"up_sell_products_ids": [
"string"
],
"cross_sell_products_ids": [
"string"
],
"dimensions_unit": "string",
"width": 0,
"height": 0,
"length": 0,
"discounts": [
{
"id": "string",
"name": "string",
"modifier_type": "string",
"value": 0,
"from_time": "string",
"to_time": "string",
"customer_group_ids": "string",
"sort_order": 0,
"additional_fields": {},
"custom_fields": {}
}
],
"additional_fields": {},
"custom_fields": {}
}
],
"additional_fields": {},
"custom_fields": {}
},
"additional_fields": {},
"custom_fields": {}
}
How This REST API Example Works
In this example, the client sends product data through the API, while API2Cart processes the request for the connected eCommerce platform. The response then indicates whether the operation succeeded and returns the corresponding result data.
The example also demonstrates several common REST API concepts discussed above: a resource-oriented operation, a structured client request, a server response, and a representation of data exchanged between systems. In production integrations, authentication credentials are also required to authorize API requests.
API2Cart applies this unified approach to products, orders, customers, inventory, shipments, categories, and other eCommerce data. As a result, SaaS developers can use one API interface instead of implementing and maintaining separate integrations with each supported platform.
REST API vs Web API: What Is the Difference?
The main difference between a REST API and a Web API is that REST defines a specific architectural style, while Web API is a broader term for APIs that enable communication between applications over the web. In common web development, REST APIs are Web APIs, but not every Web API follows REST principles.
This distinction matters for integration developers because a Web API can follow different architectural models and conventions. A RESTful API, by contrast, follows REST constraints such as statelessness, client-server separation, cacheability, a uniform interface, and a layered system.
REST vs Web API Comparison
| Aspect | REST API | Web API |
|---|---|---|
| Definition | An API designed according to REST architectural constraints | A broader term for APIs that communicate through web technologies |
| Architecture | Follows REST constraints | Can follow REST or another API architecture |
| State management | Stateless client-server communication | Can be stateless or stateful, depending on the design |
| Interface | Requires a uniform interface between components | No single interface model is required by the term itself |
| Data format | REST does not require a specific format; JSON is common for web APIs | Depends on the API architecture and implementation |
| Typical use | Web applications, SaaS integrations, mobile applications, and distributed systems | Broad range of application-to-application communication over the web |
Why Does the Difference Matter for Integration Developers?
For integration developers, the architectural model affects how much platform-specific logic an application must handle. Different APIs can use different authentication flows, resource structures, request patterns, data representations, pagination models, and error formats. Therefore, connecting software to many third-party APIs can require substantial development and ongoing maintenance.
This challenge is particularly relevant for eCommerce SaaS applications. An order management system, shipping solution, ERP, PIM, or inventory management platform may need to work with multiple eCommerce APIs, each with its own implementation details.
How API2Cart Uses REST APIs for eCommerce Integrations
API2Cart provides a unified REST API that enables software vendors to connect their applications with 80+ eCommerce integrations through a single integration layer. SaaS developers can work with common eCommerce data such as products, orders, customers, inventory, categories, and shipments without building separate connections for every supported platform.
For example, an order management system can use the same API2Cart interface to retrieve orders from multiple eCommerce platforms. An inventory management solution can retrieve and update product quantities through a consistent set of API methods. API2Cart handles platform-specific differences behind the unified interface, which reduces the amount of integration logic that development teams need to build and maintain.
This approach applies the REST concepts covered in this guide to a practical eCommerce integration scenario: resources are accessed through a consistent interface, requests are processed independently, and structured responses allow connected applications to work with commerce data programmatically.
Ready to build eCommerce integrations with a unified REST API? Sign up for API2Cart and connect your software with 80+ eCommerce integrations through one API.
REST API FAQs
What does REST stand for?
REST stands for Representational State Transfer. It is an architectural style for distributed systems rather than a protocol or data format.
REST defines constraints such as client-server separation, statelessness, cacheability, a uniform interface, and a layered system. Modern REST APIs commonly apply these principles when enabling communication between applications over the web.
What is the difference between REST and REST API?
REST is an architectural style, while a REST API is an API designed according to REST principles. REST defines architectural constraints for communication between components, whereas a REST API applies those constraints to an interface that software can use to interact with resources.
Therefore, REST describes the architecture rather than a specific API implementation. A REST API is one practical way developers apply that architecture to application-to-application communication.
Is REST API the same as Web API?
No. A REST API is a type of Web API, while Web API is a broader term. REST APIs follow REST architectural constraints, including stateless communication and a uniform interface.
A Web API does not necessarily follow REST principles and can use a different architectural model. Therefore, the terms REST API and Web API should not be used interchangeably.
Does a REST API have to use HTTP and JSON?
No. REST does not require HTTP or JSON. REST is an architectural style and does not prescribe a specific network protocol or representation format.
However, modern web-based REST APIs commonly use HTTP and frequently exchange JSON representations because these technologies work well with REST principles. Other representation formats can also be used when an API requires them.
