A cookie is a small piece of data sent from a website and stored on the user’s computer by the user’s web browser while the user is browsing. They will have the following properties:
Cookies were designed to be a reliable mechanism for websites to remember stateful information (such as items added in the shopping cart in an online store) or to record the user’s browsing activity (including clicking particular buttons, logging in etc.). They can also be used to remember arbitrary pieces of information that the user previously entered into form fields such as names, addresses, passwords, and credit card numbers.
Session cookies exist only in the temporary memory while the user navigates the website. Web browsers normally delete these when the user closes the browser.
Instead of expiring when the web browser is closed as session cookies do, a persistent cookie expires at a specific date or after a specific length of time. These are however reset if the expiration time is reached or the user manually deletes the cookie.
Normally, a cookie’s domain attribute will match the domain that is shown in the web browser’s address bar. This is called a first-party cookie. A third-party cookie, however, belongs to a domain different from the one shown in the address bar.
This is browser dependent, however in practice a limit of 4096 bytes is considered practical – making them suitable for storing small data strings. HTML 5 local storage may be preferable where larger data sets are required.
Cookies can read and be used as flags for segmenting visitors into explicit test or personalisation scenarios. For example, new vs returning visitors.
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