Database connection pool settings govern how many connections an application or pooler can hold, how long callers wait for one, and when unused connections are closed. Their meanings depend on the pooling software: HikariCP’s size is a per-application-pool limit, while PgBouncer’s default pool size applies per user/database pair. Treat the documented defaults below as version-sensitive starting values, not universal tuning advice; the official pages were accessed October 4, 2026.
What pool size actually limits
A pool-size setting is a concurrency and connection-budget limit, not a target to maximize. The critical first question is which connections the setting counts and at what scope.
| Setting | What it limits | Documented default |
|---|---|---|
HikariCP maximumPoolSize |
Total connections in that HikariCP pool, including idle and in-use connections. It is the maximum number of actual backend connections the pool can establish. | 10, according to the HikariCP project documentation. |
PgBouncer default_pool_size |
Server connections per user/database pair. Database- or user-specific pool_size values can override the default. |
20, according to the PgBouncer configuration documentation. |
PgBouncer max_db_connections |
Overall server-connection ceiling for a PgBouncer database, regardless of user. A value of zero means unlimited. | Not stated in the cited PgBouncer configuration documentation. |
PgBouncer max_client_conn |
Number of clients that may connect to the PgBouncer instance. This is a client-side limit, not a backend server-connection limit. | Not stated in the cited PgBouncer configuration documentation. |
Do not compare a HikariCP pool size directly with PgBouncer’s default: one counts a local pool’s total connections, while the other scopes server connections to a user/database pair. An application’s possible backend connections can also multiply across replicas and pools. Inventory every process and pool that reaches the database, then compare the combined potential connections with database capacity and any proxy limits before raising a cap. The cited documentation does not establish a universal sizing formula or capacity figure.
How long callers wait: acquisition timeout
In HikariCP, connectionTimeout is the maximum time a caller waits to obtain a connection when none is available. Its documented default is 30,000 milliseconds (30 seconds), and 250 milliseconds is the lowest allowed setting in the project documentation. If the pool is at its maximum and has no idle connection, acquisition fails when the wait expires.
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This is not a SQL query timeout: it limits waiting to borrow a connection, not the duration of a statement after it begins executing. A short acquisition timeout can make overload visible sooner, but it does not add capacity or explain why connections are unavailable.
Idle connections and cleanup
HikariCP: idleTimeout and minimumIdle
HikariCP’s idleTimeout applies only when minimumIdle is lower than maximumPoolSize. The documented default for minimumIdle equals maximumPoolSize, so the default relationship does not let idle connections shrink below the pool’s maximum. Set the relationship deliberately if you expect the pool to retire surplus idle connections.
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The documented idleTimeout default is 600,000 milliseconds (10 minutes); zero disables idle retirement, and the minimum accepted value is 10,000 milliseconds. HikariCP documents timing variation of up to 30 seconds, averaging 15 seconds, and says a connection is not retired before the configured timeout. The project recommends a fixed-size pool for maximum performance and responsiveness rather than setting minimumIdle without a reason.
PgBouncer: server and pool idle timeouts
PgBouncer’s server_idle_timeout closes an unused server connection after the configured idle period. Its documented default is 600 seconds. By contrast, pool_idle_timeout frees an entire user/database pool only when both client and server connections are absent. Freeing a pool also discards that pool’s statistics, so aggregate monitoring totals may decrease when a pool is removed. These settings act at different levels and are not interchangeable.
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Connection lifetime is different from idle time
HikariCP’s maxLifetime places a cap on a connection’s lifetime independently of whether it has been idle. The documented default is 30 minutes. A connection in use is not retired until it is closed and returned to the pool. HikariCP recommends setting this value a few seconds below any database or infrastructure lifetime restriction.
keepaliveTime is another separate control: it applies only to idle connections, must be below maxLifetime, and has a documented default of two minutes. Review lifetime and keepalive settings when infrastructure drops connections; changing idle-retirement timing does not address the same condition.
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PgBouncer pool mode determines when connections are reused
Pool mode changes when a server connection is released for reuse. It is an application-compatibility decision, not simply another size setting.
| PgBouncer mode | When server connection is released | Important constraint |
|---|---|---|
| Session | When the client disconnects. | Connection reuse waits for the client session to end. |
| Transaction | When the transaction completes. | Check that application behavior does not depend on retaining a server connection beyond a transaction. |
| Statement | When each query completes. | Multi-statement transactions are disallowed. |
Choose only after checking how the application uses transaction boundaries and session behavior. The documented mode differences do not show that one mode is universally better.
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How to diagnose waiting, idle counts, and dropped connections
- Callers wait while the pool is full: This can indicate saturation, connections held for a long time, or slow work. The acquisition timeout controls how long callers wait before failing; it does not identify the cause.
- Many connections appear idle: That can be expected with a fixed-size HikariCP pool or when
minimumIdleis set at the maximum. An idle count by itself does not prove a leak. - Unused connections remain open: Check whether HikariCP idle retirement is enabled and whether
minimumIdleis belowmaximumPoolSize. Separately check whether PgBouncer’s server idle timeout applies; these controls clean up different pools and connection layers. - Connections are dropped by the database or network: Review HikariCP connection lifetime and keepalive settings separately from idle cleanup.
- PgBouncer accepts many clients but backend connections are constrained: Check client limits separately from per-pair pool size and database-wide server limits. When increasing
max_client_conn, account for operating-system file descriptors: PgBouncer notes that possible descriptor use may exceed the client limit because server pools also consume descriptors.
A practical tuning sequence
- Map the connection path. List application replicas, each application-side pool, and any PgBouncer instance between the application and database. Record which caps are per process, per user/database pair, or aggregate.
- Measure the symptom. Distinguish acquisition waits and failures from query duration, idle connections from in-use connections, and client counts from backend server connections.
- Check setting interactions. For HikariCP, inspect
maximumPoolSize,minimumIdle, andidleTimeouttogether; reviewmaxLifetimeandkeepaliveTimeseparately. For PgBouncer, inspect pool-size overrides, aggregate limits, pool mode, and the relevant idle timeout. - Compare the possible connection total with capacity. Include all replicas and pools, plus PgBouncer’s per-pair and aggregate limits. Do not raise a local pool limit without considering the connections it can multiply across the deployment.
- Change one cause-matched setting at a time and observe the result. Validate against the deployed software version and workload; the documented defaults alone do not establish a suitable production value.
For exact parameter descriptions and version details, consult the HikariCP project documentation and PgBouncer configuration documentation.
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