Machine Learning System Design Interview, issue 21, Dec 6, 2025
The Silent Feature Death
Senior ML System Desgin Engineer interview at Meta, and the interviewer asks:
“Your new pricing model has 99.9% availability, 15ms latency, and zero exceptions. Yet, the A/B test results are complete garbage. What happened?”
Why a perfectly healthy model can produce garbage A/B results when your feature freshness quietly collapses.
The full answer, with the mechanism and the arithmetic, is for paid subscribers on Substack.