Connection pool doesn't maintain minIdle connections - all idle connections get closed after maxIdleTime

[vavisha]

Bug Report

Versions

r2dbc-pool-1.0.2.RELEASE
r2dbc-postgresql-1.0.7.RELEASE
spring-boot-3.4.3
postgres:15.7

Current Behavior

Connection pool doesn't maintain minIdle connections - all idle connections get closed after maxIdleTime

Steps to reproduce

1. Configure the settings

spring.r2dbc.pool.min-idle: 20
spring.r2dbc.pool.initial-size: 20
spring.r2dbc.pool.max-size: 100
spring.r2dbc.pool.max-idle-time: 10s

2. Run the application
3. Wait 10 seconds

All connections were closed (due to database inactivity)

Expected behavior/code

Idle connections should never drop below the value specified in spring.r2dbc.pool.min-idle"

[chi-chauson]

I am facing the same issue as described above. Could it be

• Eviction Mechanism: r2dbc-pool uses a background eviction mechanism that periodically checks for idle connections and closes them if they exceed maxIdleTime.
• minIdle Implementation: Ideally, the eviction process should maintain at least minIdle connections, closing those that exceed maxIdleTime while maintaining the current idle count is greater than minIdle.
• Potential Issue in Eviction Logic: When the backgroundEvictionInterval runs, it might be applying maxIdleTime universally to all idle connections without respecting the minIdle threshold.

Here is what I found:

• r2dbc-pool configures the eviction interval in ConnectionPool.java
• The eviction process is scheduled and run by reactor-pool's SimpleDequePool
• During eviction, idle connections exceeding the maxIdleTime are closed
• I think r2dbc may have missed the comment here (note that usually a manually released ref will put a new ref in the queue, so we expect refs to be pristine here)
• But crucially: there's no check to ensure at least minIdle connections remain

Does it make sense to do something like

BiPredicate<Connection, PooledRefMetadata> evictionPredicate = (connection, metadata) -> {
    // Standard eviction check - remove connections that exceed max idle time
    boolean shouldEvict = metadata.idleTime() >= maxIdleTime.toMillis() || 
                          (maxLifeTime > 0 && metadata.lifeTime() >= maxLifeTime.toMillis());
    
    // If we're evicting, trigger pool replenishment asynchronously if needed
    if (shouldEvict && pool.metrics().idleSize() <= configuration.getMinIdle()) {
        // Schedule warmup on a different thread to avoid blocking the eviction
        Schedulers.parallel().schedule(() -> pool.warmup().subscribe(), 0, TimeUnit.MILLISECONDS);
    }
    
    return shouldEvict;
};