HTTP Fundamentals

HTTP is a family of stateless, application-level request and response protocols. A client sends a request about a resource. A server returns a response that reports the outcome and can carry a representation of that resource.

That small pattern supports web pages, APIs, file transfers, media delivery, and communication between services. The same semantics also let browsers, command-line clients, origin servers, proxies, gateways, and caches cooperate.

HTTP gives these systems a uniform interface. The client does not need to know how the server stores or computes a resource. The server does not need to know why the client wants it. Both sides agree on a request method, a target, fields, content, and a response status.

Start with resources and representations

A resource is the target of a request. A resource can represent a document, an image, a customer record, a search operation, or something else a server exposes. Its Uniform Resource Identifier, or URI, identifies it.

A resource is not the bytes sent across the network. A representation is information that reflects the current or intended state of a resource. One resource might have an HTML representation for a browser and a JSON representation for a program.

HTTP separates resource identification from request semantics. The URI identifies the target. The request method states what the client intends to do with that target.

One exchange has two messages

An HTTP request communicates the client's intent. Its main parts are:

• a method, such as GET or POST;
• a target URI;
• fields that carry metadata or modify the request;
• optional content.

An HTTP response communicates the result. Its main parts are:

• a status code;
• fields that describe the response or its content;
• optional content.

In HTTP/1.1, you can see this structure as text: