Scope it first: every endpoint or one, all users or one tenant, and exactly when did it start. Line that timestamp up against deploys, flag flips and traffic changes. Then work down with telemetry you already have: traces to find the span that grew, database slow query logs, connection pool saturation, host CPU and memory. Mitigate first, whether that is a rollback or shedding load, and find root cause second.
Why interviewers ask this
Debugging under pressure is most of senior engineering, and the interviewer wants method rather than guesses. They are listening for scoping questions before hypotheses, for use of telemetry you should already have in place, for correlating with changes, and for the judgment to mitigate before investigating. Candidates who immediately start guessing at causes tend to do the same during a real incident.
How to structure your answer
- Scope the problem before forming a theory.
- Correlate the start time with deploys and traffic.
- Follow the trace to the slowest span.
- Mitigate first, then chase root cause.
Example answer
First I want boundaries, because they cut the search space fast. Is it one endpoint or all of them, one region, one tenant, and what time did it start? Then I line that timestamp up against deploys, feature flag flips and config changes, because most of the time something changed and the answer is right there. If nothing changed on our side, I go to traces and find which span grew, which immediately tells me whether it is us, the database or a downstream call. The database is the usual suspect: a query that was fine at a million rows stops being fine at ten million, or an index is missing, or the pool is saturated and the time is spent waiting for a connection rather than running the query. That last one is sneaky because every query still looks fast in isolation. And while I am doing all that, if there is an obvious mitigation like rolling back the last deploy, I take it first. Customers care about the latency, not my explanation.
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See how it worksFollow-up questions to expect
- What if traces show the time is spent waiting on a connection?
- How would you tell a slow query from lock contention?
- What would you add so this is faster to diagnose next time?
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