Lock contention limits throughput
Your inventory-reservation service scaled its stateless workers from 10 to 50 to handle a sale, but throughput on the hot SKUs plateaued and latency rose. Every reservation takes a per-SKU distributed lock (Redis Redlock-style) to prevent overselling. Tracing shows workers spending most of their time waiting to acquire the lock for a handful of popular SKUs, and lock wait time grew roughly linearly as you added workers. CPU is low everywhere. How do you triage and what's the durable fix?
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 30 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.