Aurora MySQL's Hidden Idle Close Behavior — Tracking Down HikariCP's "Failed to validate connection"

In production, HikariPool - Failed to validate connection warning logs frequently occurred along with No operations allowed after connection closed errors. This didn't lead to a service outage, but a large number of logs were piling up every minute.

Upon investigation, the cause was found to be Aurora MySQL's unusual idle close mechanism, which behaves differently from standard MySQL. Aurora MySQL operates using a dual timeout approach that utilizes a thread-pool and a monitor thread.

In this process, the DB closes the connection before the connection pool does, and resolving this requires connection pool settings that account for Aurora MySQL's non-standard behavior.