5.8 Retention Cohorts and Streaks

Daily active users is a count for one day, but a product team also asks how the same users behave across many days. The previous lesson built the tools for both kinds of count on an app with 100 million registered users: HyperLogLog for approximate unique counts, and one bitmap per day, such as active:2026-11-01, with one bit per user and BITOP to combine days.

This lesson uses those daily bitmaps to measure the retention of signup cohorts, and then flips the layout to one bitmap per user to track streaks. It ends with a guide to choosing among HyperLogLog, bitmaps and sets.