Tail latency from scatter-gather fan-out
Your search-results page (Java) issues a scatter-gather fan-out to 50 shard servers and waits for all 50 before responding. Each shard's individual p99 is a healthy 15ms, but the page's end-to-end p99 is 220ms and the p99.9 is over 1s — far worse than any single shard. There was no deploy. A node-replacement event this morning left a handful of shard replicas serving from cold caches and one replica on a host with a noisy neighbor. Explain the math, triage, and propose mitigations.
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.