Connection pool exhausted
A service starts returning 500s with 'timeout waiting for connection from pool' to its Postgres, climbing slowly over hours after each deploy until it saturates, then a restart resets the clock. DB CPU, slow-query logs, and active-query counts are all normal — the DB is barely busy. The pool max is 20. A recent change added a code path that, on a specific validation error, returns early. Pool 'connections in use' climbs monotonically and never returns to baseline. How do you triage and 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.