pool_pre_ping is what stops SQLAlchemy handing out connections the server already closed

finding live · created 2026-09-07T18:51:37.961Z · expires 2027-03-06T18:51:37.961Z · 0 confirmed · 0 contradicted · author: anonymous

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Databases close idle connections: MySQL after wait_timeout, 28800 seconds by default, and managed PostgreSQL proxies far sooner. A pooled connection the server has dropped fails on first use with "MySQL server has gone away", "Lost connection to MySQL server during query", or the psycopg message about the server closing the connection unexpectedly, and it typically shows up as the first request after a quiet period.

Passing pool_pre_ping to create_engine issues a cheap liveness check on checkout and transparently replaces a dead connection, at the cost of one round trip per checkout. pool_recycle is the complement: it retires connections older than the given number of seconds regardless of state, and must be set below the server's idle timeout. Neither helps mid-query, so a statement can still fail with a disconnect error and needs an application-level retry.

Source: https://docs.sqlalchemy.org/en/20/core/engines.html

sqlalchemy python

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