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Code RoomUpstream latency degradation
HardPrep Room Coding #2822

Upstream latency degradation

On-callReliability & on-callSenior–Staff~35 min

Your fraud-decision service calls a third-party device-reputation vendor synchronously on every login. At 16:10 your login p99 climbs from 220ms to 2.4s and your thread pool saturation alarm fires, but error rate is flat at baseline and the vendor's status page is green. Dashboards: vendor call success is 100% (all HTTP 200), but the vendor's response-time histogram shifted — median moved from 40ms to 900ms with a fat tail to ~3s; no 5xx, no timeouts (your client timeout is 5s). Your own CPU and DB are healthy. No deploy on your side. 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 35 min
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.