Latency spikes with regex validation
An HTTP request-validation service (single-threaded event loop, Node.js) goes from p50 8ms / p99 30ms to p50 9ms / p99 9000ms at 14:10. Throughput collapses and the health check intermittently times out. CPU on the box pegs at 100% on one core; the others are idle. There was no deploy today, but marketing launched a campaign at 14:00 that drove a 3x traffic spike from a new partner whose user-supplied 'coupon code' field is validated with a regex. The event-loop-lag metric spiked from <1ms to multiple seconds. How do you triage and mitigate?
What a strong answer looks like
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.
0:00 of about 40 min
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.