HikariCP Connection Pool — Configuration and Tuning in Spring Boot

HikariCP is Spring Boot's default connection pool — chosen for low overhead and consistent throughput. Misconfiguring pool size is one of the most common sources of database performance problems.

Pool size — the counter-intuitive formula:

• More connections != better performance
• Each connection requires a thread on the DB server; too many connections increase CPU context switching
• HikariCP's recommended formula: (2 x CPU cores) + effective spindle count
• For a 4-core server with SSD storage: ~9 connections

Key timeouts:

• connectionTimeout (30s default) — how long a thread waits for an available connection
• maxLifetime (30 min default) — maximum age before HikariCP recycles a connection
• idleTimeout (10 min default) — how long idle connections sit before being closed down to minimumIdle

Diagnosing problems:

• leakDetectionThreshold=2000 — logs stack trace for connections held > 2 seconds
• Expose HikariCP metrics via Actuator/Micrometer: hikaricp.connections.active, pending — alert when pending spikes
• Look for long @Transactional methods that make HTTP calls; move I/O outside the transaction