TL;DR — Real-Time Fraud Detection

Design a real-time fraud detection system for a payment processor handling 10K transactions per second. Every transaction must receive a risk score within 100ms to decide: approve, decline, or flag for manual review. False positives directly hurt revenue (declined legitimate transactions), while false negatives cause chargebacks and regulatory risk. Key features: Score every transaction with a fraud probability in real-time. Apply configurable rule-based checks (velocity, geo, blocklist).