Cookies

What are Cookies?

A cookie (also known as a web cookie or browser cookie) is a small piece of data a server sends to a user's web browser. The browser may store cookies, create new cookies, modify existing ones, and send them back to the same server with later requests. Cookies enable web applications to store limited amounts of data and remember state information; by default the HTTP protocol is stateless.

What Cookies are Used For

Typically, the server will use the contents of HTTP cookies to determine whether different requests come from the same browser/user and then issue a personalized or generic response as appropriate. The following describes a basic user sign-in system:

1. 1. The user sends sign-in credentials to the server, for example via a form submission.
2. 2. If the credentials are correct, the server updates the UI to indicate that the user is signed in, and responds with a cookie containing a session ID that records their sign-in status on the browser.
3. 3. At a later time, the user moves to a different page on the same site. The browser sends the cookie containing the session ID along with the corresponding request to indicate that it still thinks the user is signed in.
4. 4. The server checks the session ID and, if it is still valid, sends the user a personalized version of the new page. If it is not valid, the session ID is deleted and the user is shown a generic version of the page (or perhaps shown an "access denied" message and asked to sign in again).

Cookies are mainly used for three purposes: Session management (user sign-in status, shopping cart contents, game scores, or any other user session-related details that the server needs to remember), Personalization (user preferences such as display language and UI theme), and Tracking (recording and analyzing user behavior).