Spring Boot + MySQL Connection Pool Tuning

Spring Boot uses HikariCP as its default connection pool, and most connection pool problems in production come from two settings: maximumPoolSize and connectionTimeout.

maximumPoolSize — smaller than you think

• The common mistake is setting this to match the thread count or expected concurrent requests — but the bottleneck is the database CPU and disk, not connection slot availability
• HikariCP's own guidance: (number of CPU cores x 2) + effective spindle count — for a 4-core MySQL server, that's roughly 9-10 connections, not 100 or 200; a pool that's too large causes connection queuing at the MySQL layer, consuming memory without improving throughput
• Monitor hikaricp_pending_threads — if threads are consistently waiting for a connection, either the pool is undersized or queries are holding connections too long

connectionTimeout — fail fast

• connectionTimeout controls how long a thread waits for a pool connection before throwing SQLTimeoutException — the Spring Boot default is 30 seconds, which is far too long for production; a user's HTTP request will have timed out before that
• Set it to 3-5 seconds so that when the pool is exhausted, callers fail fast rather than hanging and consuming server threads

Key properties in application.properties

• spring.datasource.hikari.maximum-pool-size: the upper connection limit
• spring.datasource.hikari.connection-timeout: milliseconds to wait for a connection (default 30000; set to 3000-5000)
• spring.datasource.hikari.idle-timeout: how long an idle connection can stay in the pool before being retired (default 600000ms)
• spring.datasource.hikari.max-lifetime: maximum connection age before rotation (default 1800000ms; prevents stale connections after MySQL's wait_timeout closes them)

Enable HikariCP metrics with Micrometer by adding spring-boot-starter-actuator — pool wait time, active connections, and pending threads are the metrics to monitor.