A REST API lets software communicate over a network using resources, requests, and responses. In a typical web API, a client sends an HTTP request to an address for a resource; the request’s method expresses the intended action, and the server replies with a status code and, sometimes, content. REST is an architectural style, while HTTP is a protocol often used to build web APIs—using HTTP alone does not make an API RESTful.
What is a REST API?
REST stands for Representational State Transfer. It is an architectural style for networked systems, not a programming language, data format, or protocol. A REST API is an interface designed around that style so clients can interact with resources.
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HTTP is a common protocol for web APIs and supplies familiar concepts such as methods, request and response messages, and status codes. The distinction matters: HTTP defines how those messages work, while REST provides architectural guidance. An API can use HTTP without necessarily conforming to REST. For the standards background, see RFC 9110, HTTP Semantics, and Roy Fielding’s dissertation on architectural styles and network-based software architectures.
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Endpoint: the address a client targets
An endpoint is commonly the URI a client uses to address a resource through an API. In HTTP, the request target identifies a resource, but the server determines how that identifier maps to its implementation. A path such as /products/42 is a useful illustrative example, not a required REST naming convention.
Request: the client’s message
An HTTP request includes a method and a request target. It may also include headers, which carry metadata, and content, often called a body, which can contain data to be processed. The method is the primary source of the request’s standardized semantics. As RFC 9110 puts it: “Each message is either a request or a response.”
Response: the server’s reply
An HTTP response includes a status code and may include headers and content. The status helps describe the result; the response content, when present, provides a representation or other information. Not every response has a body: 204 No Content, for example, has no content.
What do the common HTTP methods mean?
HTTP defines method semantics, but whether a particular API endpoint supports a method depends on that resource and the server. The following descriptions give the standardized intent, not a guarantee about every API:
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| Method | Standardized intent | Illustrative use |
|---|---|---|
GET |
Request a current representation of the target resource. | Retrieve a product record. |
POST |
Ask the target resource to process the submitted content according to its own semantics; this may create a resource. | Submit an order for processing. |
PUT |
Request replacement of the target resource’s current representations with the submitted content. | Replace a stored profile representation. |
DELETE |
Request removal of the target resource’s current representations. | Request removal of a saved item. |
For a first comparison of two endpoints, check the resource or URI, the allowed method and its effect, required headers and content, expected success and error codes, and the response format and fields. These details are API-specific, so confirm them in that API’s documentation rather than assuming a method or field is available.
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How do you read an API response?
Start with the status code
HTTP status codes are three-digit numbers in the range 100–599. Their first digit indicates the broad class: 1xx informational, 2xx successful, 3xx redirection, 4xx client error, and 5xx server error. The specific code is more useful than the class alone. For example, 202 Accepted means the request was accepted for processing, but processing is not complete.
Read the status together with the response content, if any, and the API’s documentation. A 2xx response does not necessarily include a body; 204 No Content explicitly indicates that there is no response content.
Check the representation format
REST does not require JSON. An API might return JSON, another format, or different representations depending on its design. Look at the relevant response metadata and the API documentation to determine how to interpret the content. JSON in an example is only illustrative, not a requirement for REST.
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Understand error details
Some HTTP APIs use the Problem Details format defined by RFC 9457. Its common members include type, title, status, detail, and instance. APIs are not required to use this format. If a Problem Details body includes a status member, treat it as advisory: the actual HTTP response status remains important to software interpreting the response.
What varies from one REST API to another?
- Which methods each resource allows and what the API does with them.
- Which headers and request content are required.
- Which status codes the API returns for success, errors, or incomplete processing.
- What formats its representations use and which fields they contain.
- Whether errors follow Problem Details or a different structure.
These are reasons to consult the API’s own documentation before building a request. HTTP provides shared message and method semantics, but it does not tell you every API’s available resources, required fields, or application-specific behavior.
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