PostgreSQL connection pool exhaustion -- too many clients

Application stops responding with 'too many clients already' or connection timeouts. Database has hit max_connections limit. Adding more app instances makes it worse.

(1) Use a connection pooler: PgBouncer (most common) or pgcat in front of PostgreSQL. Transaction pooling mode reuses connections across requests. (2) Reduce pool size per app instance: 5-10 connections is usually enough per instance, not 20-50. (3) Close idle connections: set idle_in_transaction_session_timeout. (4) Monitor: SELECT count(*) FROM pg_stat_activity. (5) For serverless: use Neon pooler, Supabase pooler, or RDS Proxy. Default max_connections is 100 -- shared across ALL clients.

Each PostgreSQL connection uses ~10MB of memory and a process. Connection pooling multiplexes many app connections over few database connections.