The Unexpected DB Connection Pool Leak in Our Rails App

Everything looked fine — until it wasn’t. Out of nowhere, our worker tier started throwing this error: could not obtain a connection from the pool within 5.000 seconds (waited 5.000 seconds); all pooled connections were in use We weren’t doing anything wild. Our RAILS_MAX_THREADS matched the DB pool size. No rogue threads. No unclosed connections. And yet, we were stuck. Here’s how we tracked it down and what we learned from it.