Velocity rules at authorization
Design the velocity-rules engine that runs inline in card authorization and blocks patterns like 'this card was tried >5 times in 60s', 'this device touched >10 cards in 1h', 'this BIN spiked 50x above baseline'. ~25,000 auths/sec, decision budget <15ms, and the counts must be accurate enough that a real fraud burst is caught within seconds, while a legitimate retail spike doesn't false-positive everyone. How do you maintain millions of sliding-window counters at this rate and latency?
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify4:00 left
Estimate4:00 planned
Design11:30 planned
Deep dive9:30 planned
Failure6:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.