2.10 Feature Flags and Bitfield Counters

Bits can hold more than daily activity. The previous lesson used a bitmap, an ordinary string treated as an array of bits, to record which of 100 million users were active each day in 12.5 MB, and it combined days with BITOP and BITCOUNT.

This lesson changes what a position means. A bit can mark whether a user has a feature, and a group of bits can hold a small integer, which BITFIELD reads and increments atomically. The lesson ends with how to choose between bitmaps, bitfields, sets, and HyperLogLog.