Postgres pool pinned at capacity
A Node.js API using a 20-connection Postgres pool starts throwing 'timeout exceeded when trying to acquire a connection' and 'remaining connection slots are reserved' from Postgres at 12:00. The pool's in-use gauge is pinned at 20/20 and never drops, even during a brief traffic lull where QPS halved — used connections stayed at 20. CPU and memory are fine. A feature shipped this morning added a code path that runs a query inside a manual transaction (`BEGIN`) and, on one early-return error branch, returns without `COMMIT`/`ROLLBACK` and without releasing the client. Triage and mitigate.
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.