Design Challenge: Real-Time Fraud Detection at 100K Events/Sec

Difficulty: Hard · Concept: The 9-Step Framework

Same no-auto-grading, self-assessment format as the RAG design challenge — write a real end-to-end design in free text, then reveal a rubric and the site's own real walkthrough for this exact problem.

The problem: a completely different shape from a retrieval problem. You've been asked to design a system that decides, correctly, on every single transaction, in milliseconds, at a sustained 100,000 events per second — where fraud is rare (often well under 1% of transactions) and getting the decision wrong in either direction is genuinely expensive: a missed fraud case costs real money, a false alarm blocks a legitimate customer.

Design it end to end — how does a transaction actually get scored in milliseconds, what happens to a rare-class model that could otherwise cheat its way to 99% accuracy by never flagging anything, and how do you know when the model has quietly stopped working?

Next: Design Challenge: A Support Agent That Can Actually Take Actions — where the hard part shifts from infrastructure to trust.