Connection Pooling

A connection pool keeps a bounded set of open connections you borrow and return, turning a slow per-request open into a fast grab-and-release.

The naive pool is a `BlockingQueue` you `take` from and `offer` back; HikariCP replaces the locking queue with a lock-free `ConcurrentBag` and adds validation, lifetime recycling, and leak detection.

Size the pool to the database's capacity, not to app load: if N instances times pool size exceeds `max_connections`, new connections are refused and the whole system can stall.

The pool empties because connections are held inside open transactions waiting on slow non-database work; `idle_in_transaction_session_timeout` kills those stuck transactions and frees the connections.

PgBouncer multiplexes thousands of client connections onto a few dozen real database connections at one central choke point, which an application-side pool cannot do across the whole fleet.

The pool is a shared finite resource, so one slow query that hogs a connection starves every other request behind it.