What is the mechanism of session management in web applications?

The session mechanism is based on maintaining the user's state between HTTP requests, which are inherently stateless. This is achieved by using a unique session identifier.

Main steps:

1. User's first request: The user sends a request to the web server.
2. Creating a session: If the request is from an unknown user (no session identifier), the server creates a new session on its side (in memory, database, file system) and generates a unique session identifier.
3. Sending the identifier to the client: The server sends this session identifier to the client as a cookie (Set-Cookie: SESSIONID=xyz; path=/).
4. Saving on the client: The browser saves this cookie and sends it with all subsequent requests to the same domain (Cookie: SESSIONID=xyz).
5. Identification on the server: Upon receiving each subsequent request with a cookie containing the session identifier, the server uses this identifier to find the corresponding session and retrieve stored user data.
6. Ending the session: The session can end due to timeout inactivity, explicit user logout, or browser closure (if the session cookie has no expiration date).

Example of using a session to store authorization information:

// Example in Java Servlets
HttpSession session = request.getSession(); // Get or create a session

// Check if the user is authorized
if (session.getAttribute("loggedInUser") == null) {
    // User is not authorized, redirect to login page
    response.sendRedirect("/login");
} else {
    // User is authorized, retrieve data
    User user = (User) session.getAttribute("loggedInUser");
    // ... work with user data
}

Session stores:

• In server memory (suitable for small applications).
• In server file system.
• In a database (classic, scalable option).
• In distributed caches (Redis, Memcached) for scaling across multiple servers.

Security:

• Using "secure" and "HttpOnly" flags for session cookies.
• Regenerating session ID after successful authentication.
• Setting an appropriate session timeout.
• Protecting against Session Hijacking (session ID interception).