Where exactly are session data stored, and how does the server determine which session data corresponds to a specific HTTP request?

Session data is stored on the server side (in memory, files, database, or specialized storage like Redis).

The server determines the correspondence of session data to an HTTP request using a session identifier. This identifier is unique for each session.

The process looks like this:

1. On the first request from the client, the server creates a session, generates a unique session identifier (SID), and saves the session data.
2. The SID is sent back to the client in the response, most often in Cookies.
3. On subsequent requests, the client sends this SID back to the server (also in Cookies or as a URL parameter).
4. The server retrieves the SID from the request, finds the corresponding session data by this identifier, and uses it to process the request.

Example of using SID in Cookies:

GET /profile HTTP/1.1
Host: example.com
Cookie: PHPSESSID=abc123def456; other_cookie=value