Also called: Patroni agent, HA cluster manager, patronictl
In plain English
Patroni is an open-source agent that runs beside each PostgreSQL node and automates high availability. It stores cluster state in a distributed consensus store (etcd, Consul, ZooKeeper, or Kubernetes), elects a single primary via a leader key, promotes a standby on failure, and rejoins old primaries with pg_rewind. PostgreSQL provides the failover primitives; Patroni decides when to use them.
Why it matters
PostgreSQL has no built-in idea of “which node should be primary.” Patroni supplies that decision safely, using a consensus store and a watchdog to prevent two nodes from ever being primary at once. patronictl is its operator CLI.