08003: connection_does_not_exist

What does it mean?

An operation was attempted on a database connection that does not exist (already closed). The application tried to send a query using an invalid connection handle. Belongs to Class 08 (connection_exception).

Why does it happen?

Connection timeout, closure by the server, a network outage, or a connection taken from the pool that is actually dead; or reuse of a connection that was close()d in application code.

How to fix it

Enable liveness checks in the connection pool (validation query / keepalive); configure settings that evict dead connections from the pool (e.g. connectionTestQuery/maxLifetime in HikariCP). In the application, make sure the connection is open before using it and reconnect automatically when it drops. Review TCP keepalive and idle_in_transaction_session_timeout values.