2.9 Bitmaps and Daily Active Users

One hundred million users, one bit each: 100,000,000 bits divided by 8 is 12,500,000 bytes, or 12.5 MB, to record whether every user was active today. The same information as a set of user IDs costs tens of bytes per member, which adds up to several gigabytes.

That is the case for bitmaps. Counting the active users is a single command, and "who was active on all seven days" is one more. This lesson follows one analytics example throughout: a daily activity bitmap per date, such as active:2026-10-30, where bit N records whether user N was active that day.