The Tool Desk
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What the main connection-pool settings control
A connection pool reuses database connections instead of creating a new connection for every operation. Its settings govern different things: capacity, how long callers wait, and when connections can be removed or reused. Changing one does not automatically solve problems controlled by another.
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- Pool size: a cap on connections, with scope defined by the implementation.
- Acquisition timeout: how long a caller waits to get a connection from the pool. It is not a SQL query timeout.
- Idle timeout: when an unused connection may be closed, if the pool’s other settings permit it.
- Connection lifetime: how long a connection may remain in service, which is separate from how long it has been idle.
The examples below cover HikariCP, an application-side JDBC pool, and PgBouncer, a PostgreSQL connection pooler. Their settings are implementation-specific, not universal database-pool rules.
HikariCP: application-side pool settings
HikariCP’s documented defaults are version-sensitive configuration values, not recommended targets for every application. Verify them against the deployed version in the HikariCP documentation.
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| Setting | What it controls | Documented default and qualification |
|---|---|---|
maximumPoolSize |
Maximum total of idle and in-use connections in this pool. If all are in use, new callers wait according to connectionTimeout. |
10, as documented by HikariCP; not a universal sizing recommendation. |
connectionTimeout |
Maximum time a caller waits to acquire a connection when none is available. When the wait expires, acquisition fails; this does not stop or limit a query already running. | 30,000 milliseconds (30 seconds); 250 milliseconds is the lowest allowed value in the documentation. |
minimumIdle |
Minimum idle connections the pool tries to maintain. Its relationship to the maximum affects whether idle connections can be retired. | Defaults to the same value as maximumPoolSize. |
idleTimeout |
How long an idle connection may remain before retirement, when minimumIdle is below maximumPoolSize. It will not retire connections below the minimum. |
600,000 milliseconds (10 minutes); zero disables idle retirement, and the minimum accepted value is 10,000 milliseconds. Retirement timing can vary by up to 30 seconds, with a 15-second average variation; a connection is not retired before the configured timeout. |
maxLifetime |
Maximum connection lifetime. An in-use connection is removed only after it is returned to the pool. | 30 minutes. HikariCP recommends setting it a few seconds below any database or infrastructure lifetime limit. |
keepaliveTime |
Keepalive checks apply only to idle connections; this is distinct from retiring them. | 2 minutes; it must be less than maxLifetime. |
Why idle connections may not shrink
HikariCP’s documented minimumIdle default equals maximumPoolSize. With that relationship, the pool has no room to retire idle connections below its minimum. If you want idle connections to shrink, check that minimumIdle is lower than maximumPoolSize and that idleTimeout is nonzero. HikariCP recommends a fixed-size pool for maximum performance and responsiveness, so do not lower the minimum without a reason.
Why lifetime is not idle cleanup
idleTimeout addresses an unused connection when the pool can go below its minimum. maxLifetime sets a separate age limit, and an actively used connection is not removed mid-use: it is removed after being returned. If a database or network component imposes its own connection lifetime, HikariCP recommends configuring maxLifetime a few seconds shorter.
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PgBouncer: backend limits, client limits, and pool modes
PgBouncer’s settings distinguish client connections to the pooler from server connections to PostgreSQL. The scope of each limit matters. Consult the PgBouncer configuration documentation and verify the version in use.
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|---|---|---|
default_pool_size |
Maximum server connections per user/database pair; database- or user-specific pool_size settings can override it. |
20. |
min_pool_size |
Asks PgBouncer to add connections below a floor, subject to applicability conditions and the pool-size cap. | 0, disabled. |
reserve_pool_size and reserve_pool_timeout |
Allows extra server connections after a client has waited for the configured reserve-pool delay. | Reserve size 0 (disabled); wait 5 seconds. |
max_db_connections |
Ceiling on server connections for a PgBouncer database, regardless of user. | 0 means unlimited. |
max_client_conn |
Limit on clients connecting to the PgBouncer instance, not the number of PostgreSQL server connections. | Account for operating-system file descriptors when raising it: possible descriptor use can exceed the client limit because server pools also consume descriptors. |
server_idle_timeout |
Closes an unused server connection after its configured idle period. | 600 seconds. |
pool_idle_timeout |
Frees an entire user/database pool only after both client and server connections are absent. | When a pool is freed, its statistics are discarded, so aggregate monitoring totals can decrease. |
Pool mode determines when a server connection is reusable
| Mode | When the server connection is released | Application consideration |
|---|---|---|
| Session | When the client disconnects. | Reuse follows the client session. |
| Transaction | When the transaction ends. | Check that application behavior does not depend on a server connection remaining assigned between transactions. |
| Statement | When a query completes. | Multi-statement transactions are disallowed. |
Choose a mode based on the application’s transaction boundaries and session behavior; it is not simply another pool-size option.
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How to reason about pool size across an application
A local pool limit is not necessarily the database’s total possible connection count. If an application runs several replicas, each replica may have its own pool. A PgBouncer deployment can then apply per-user, per-database, and aggregate server-connection limits of its own. Inventory every process and pool that can reach the database, then compare their combined possible backend connections with the database’s capacity before increasing a limit.
There is no universal pool-sizing formula or database-capacity figure established by these product settings. The appropriate values depend on the workload and deployment. Treat HikariCP’s default of 10 and PgBouncer’s default of 20 as product defaults with different scopes—not as numbers to copy or compare directly.
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Troubleshoot waits, idle connections, and dropped connections
Callers wait and then time out
If the pool is full and has no idle connection, callers wait until the acquisition timeout expires. That observation is consistent with saturation, but does not by itself identify the cause: connections may be held for a long time, or work using them may be slow. First identify which pool is full and which connection count it reports; then investigate how long connections remain checked out before raising the cap.
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The pool shows many idle connections
A high idle count alone does not prove a leak. It may be expected when a pool is fixed-size or when its minimum idle count equals its maximum. If unused connections persist and you expect them to shrink, check whether retirement is enabled and whether the pool’s minimum allows it.
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Unused PgBouncer connections remain open
Distinguish closing one idle server connection (server_idle_timeout) from freeing an entirely unused user/database pool (pool_idle_timeout). The latter also removes that pool’s statistics.
Connections are dropped by a database or network component
Review lifetime and keepalive controls separately from idle cleanup. In HikariCP, maxLifetime addresses connection age, while keepaliveTime applies only to idle connections. Neither is the same as the acquisition wait controlled by connectionTimeout.
Quick Recap
Before changing a limit
- Identify the pool implementation and the exact scope of its setting: application pool, PgBouncer user/database pair, PgBouncer database, or PgBouncer client instance.
- Count all application replicas and pools that can reach the database, not just one process.
- Separate caller acquisition waits from query execution time.
- Check idle-minimum and idle-timeout behavior before diagnosing excess idle connections.
- Check transaction and session assumptions before changing PgBouncer pooling mode.
- Verify documented defaults against the deployed software version; defaults are not sizing advice.
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