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PgBouncer Pool Size: Configuring 100 Connections for 500 App Servers

Example of configuring PgBouncer pool size to handle 500 application connections with only 100 Postgres server connections. Free tool.

Worked example

Input
App servers: 500 (each opens 10 connections) Total app connections: 5,000 Postgres max_connections: 200 Goal: use PgBouncer to limit backend connections to 100
Output
PgBouncer configuration: [databases] appdb = host=postgres port=5432 dbname=appdb [pgbouncer] pool_mode = transaction max_client_conn = 5000 # app-side connections default_pool_size = 100 # postgres backend connections min_pool_size = 10 max_db_connections = 100 server_idle_timeout = 600 Math: 5,000 app connections share 100 backend connections Multiplexing ratio: 50:1 Works because: transaction pooling returns connections immediately Mode comparison: Session pooling: 1:1 (no benefit for connection limits) Transaction pooling: 50:1 ← recommended for most apps Statement pooling: highest ratio, breaks multi-statement txns
PgBouncer in transaction mode returns the backend connection to the pool after each transaction commits — allowing 5,000 app connections to share 100 backend connections. Session mode holds the backend connection for the entire client session (no multiplexing benefit). Statement mode breaks any application that uses multi-statement transactions or prepared statements. Transaction mode is the default recommendation unless your app uses session-level features (SET, advisory locks).
What to do next Set default_pool_size = max_connections / 2 (leave headroom for direct admin connections). Monitor pgbouncer SHOW POOLS to watch pool saturation. Alert if cl_waiting > 0 for more than 5 seconds.

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Frequently asked questions

Does PgBouncer support prepared statements in transaction mode?

Protocol-level prepared statements (PREPARE/EXECUTE) are not supported in transaction mode because they are session-scoped — a prepared statement prepared on one backend may not be available when the next transaction gets a different backend. Use server-side parameters or ORM query building instead. Named prepared statements work in session mode only.

How do I monitor PgBouncer pool exhaustion?

Connect to PgBouncer's admin console and run SHOW POOLS. The cl_waiting column shows clients waiting for a backend connection. If cl_waiting consistently exceeds zero, increase default_pool_size or optimize transaction duration. Also check sv_idle (idle backends in pool) — if always zero with cl_waiting > 0, you are pool-exhausted.