The Case of the Missing R2DBC Connection Pool

In an environment using Spring WebFlux and R2DBC, an issue was discovered where the R2DBC connection pool was not automatically created at application startup. This behavior meant that connections were only created after the first database request came in, creating a risk of latency for initial traffic or leading to connection shortage failures.

In particular, there was an environment difference where the connection pool was created normally in IntelliJ, but connections were not created when directly running a jar file built with gradle.

To identify the cause, the internal code of r2dbc-pool was analyzed, and a solution was confirmed: explicitly implementing a warmup method through configuration to pre-create connections at the desired point in time.