Middle

Πώς λειτουργεί το πρωτόκολλο HTTP;

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HTTP (Hypertext Transfer Protocol) — a layer of application protocol used to retrieve resources such as web pages. It operates on the "client-server" model.

The client (usually a browser) sends a request to the server, which processes the request and sends a response.

Main elements of an HTTP message:

Start line: Defines the message type (request or response) and main parameters.
- Request: Method (GET, POST, PUT, DELETE, etc.), path to the resource, protocol version.
- Response: Protocol version, status code (200 OK, 404 Not Found, 500 Internal Server Error, etc.), textual description of the status.
```
GET /index.html HTTP/1.1 // Example of a request start line
```
```
HTTP/1.1 200 OK // Example of a response start line
```
Headers: Contain meta-information about the message and the message body. Each header is represented as "HeaderName: Value".
- Request headers: Host, User-Agent, Accept, Content-Type, Content-Length, etc.
- Response headers: Content-Type, Content-Length, Server, Set-Cookie, etc.
```
Host: example.com // Example of Host header
User-Agent: Mozilla/5.0 // Example of User-Agent header
```
Empty line: Separates headers from the message body.
Message body (Body): Contains the transmitted data. It may be absent in some requests (e.g., GET) and responses (e.g., 204 No Content). In POST or PUT requests, the body usually contains the data sent. In responses, the body contains the requested resource (e.g., HTML code, images, JSON).
```
<!DOCTYPE html>
<html>
<head>
    <title>Example</title>
</head>
<body>
    <h1>Hello!</h1>
</body>
</html>
```

Main HTTP methods:

Method	Description	Usage
GET	Requests a resource at the specified URL.	Fetching data without changing server state.
POST	Sends data to be processed on the server.	Sending forms, uploading files, creating resources.
PUT	Updates or creates a resource at the specified URL.	Updating existing resources.
DELETE	Deletes a resource at the specified URL.	Removing resources.
HEAD	Requests headers of the resource without its body.	Getting metadata about the resource.

HTTP is a stateless protocol. Each request is independent. Mechanisms like cookies and sessions are used to maintain state.

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