In HikariCP, the properties spring.datasource.hikari.maximum-pool-size and spring.datasource.hikari.minimum-idle control how many connections the pool maintains and manages. Here’s a detailed breakdown:
🔹 spring.datasource.hikari.maximum-pool-size
Definition:
The maximum number of connections that can exist in the pool (active + idle).
Purpose:
Limits the total number of database connections your app can open at once.
Example:
spring.datasource.hikari.maximum-pool-size=10
➡️ At most 10 connections can be used concurrently.
When it’s reached:
- If all 10 are in use and a 11th request comes in, it will wait up to
connection-timeoutmilliseconds for a connection to become available. - After that, it throws a
SQLTransientConnectionException.
🔹 spring.datasource.hikari.minimum-idle
Definition:
The minimum number of idle connections to maintain in the pool.
Purpose:
Ensures a baseline of ready-to-use connections, improving response time under load.
Example:
spring.datasource.hikari.minimum-idle=3➡️ HikariCP will keep at least 3 idle connections ready, even when load is low.
Important Note:
If minimum-idle > maximum-pool-size, it will be reset to match maximum-pool-size.
✅ TL;DR Comparison
| Property | Purpose | Affects |
|---|---|---|
maximum-pool-size | Total number of connections (idle + active) | Max concurrency |
minimum-idle | Idle “standby” connections kept in the pool | Readiness/performance |