It is necessary to count the number of active users, session_id is reset. How to do it better?

Question

PHP

Hello everyone)
There is a book reader module to calculate the number of active readers per week, I write each one to the database, then I count them. I check the uniqueness by session_id, but the session is often reset, especially if it is a search engine robot, the same situation with cookies. Perhaps there are some other solutions for organizing the count of active readers?