What is the mechanism of session management in web applications?

The session mechanism in web applications is based on user identification between requests, which are inherently stateless.

The process usually looks like this:

1. User's first request: The client (browser) sends a request to the server.
2. Creating a session: The server receives the request and, if it is the first request from this client or the session has not been created yet, generates a unique session identifier (Session ID).
3. Storing session data: The server stores data associated with this Session ID. This can include user data (login, role), shopping cart state, settings, etc. Storage can be on the server itself (in memory, files) or in a separate storage (database, Redis).
4. Transmitting Session ID to the client: The server sends a response to the client, including the Session ID. Common methods of transmission:
   • Cookie: The server sets a cookie with the Session ID in the user's browser. The browser automatically sends this cookie with each subsequent request to the same domain.
   • URL: The Session ID is included in URL parameters (less secure and recommended method).
   • Hidden form fields: The Session ID is placed in a hidden <input type="hidden"> field in an HTML form (used when submitting forms).
5. Subsequent requests: When the client sends the next request, it includes the Session ID (for example, from the cookie).
6. User identification: The server receives the request, extracts the Session ID, and uses it to find the corresponding session data. This way, the server "recognizes" the user and their previous state.
7. Updating session data: As the user interacts with the application, the server can modify or add data to the session storage associated with the current Session ID.
8. Ending the session: The session can be explicitly terminated (for example, when the user logs out) or automatically after a certain period of inactivity (session timeout). When the session ends, data associated with the Session ID are deleted from the server.

Main components:

• Session ID: Unique session identifier.
• Server-side session storage: The place where data associated with the Session ID is stored.
• Mechanism of transmitting Session ID to the client: Usually Cookie.

This mechanism allows web applications to maintain interaction context with each user throughout their session on the site, despite the stateless nature of the HTTP protocol.