Machine Learning System Design Interview, issue 24, Dec 8, 2025
The Silent Graveyard Effect
Machine Learning Engineer interview at Walmart, and the interviewer asks:
“We have 5 petabytes of transaction history spanning 5 years. Train a model to predict next month’s purchases.”
Why 5 years of “big data” at Walmart hides the customers your model most needs to learn from.
The full answer, with the mechanism and the arithmetic, is for paid subscribers on Substack.