How to remember information about user actions?

Question

Programming

How do they remember unviewed topics, unread comments on large sites, for example, on the same habrahabr? Cookies or something else?

Answer 1

View statistics, in most cases self-written and highly adaptive to the target task. If global.

Answer 2

The same Habr remembers only the topics that you watched recently. This data is stored on the server. This is not a cache, because is not reset when the server is restarted. That is, these are either files or a database.

Answer 3

You can look in the direction of data storage on the client, - local storage and analogues.