Architecture

Connection pooling

Reuse a fixed pool of long-lived connections instead of opening one per request.

In plain terms

Database connections are expensive; PgBouncer, HikariCP, and the like multiplex thousands of short requests over hundreds of long connections.

Origin

Pattern as old as database connections — RPC libraries in the 1980s did similar pooling. PgBouncer (2007) and HikariCP (2013) are the modern reference implementations.

Where it shows up in production

PgBouncer External pooler in front of Postgres. Three modes — session, transaction, statement.
HikariCP JVM's standard pool. Spring Boot default since 2017.
pgbouncer + Lambda Pattern: many short-lived Lambdas multiplex over a small pool. Solves connection-storm pathology.

Sources & further reading

Docs PgBouncer docs
Docs HikariCP project

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