Also called: hardware watchdog, /dev/watchdog, self-fencing, STONITH
In plain English
A watchdog is a timer (often /dev/watchdog) that reboots a node unless it is regularly “pet” by a healthy process. On a Patroni primary, the agent pets the watchdog; if the agent hangs and can’t demote PostgreSQL, the watchdog fires and reboots the node — forcibly removing the old primary before a new one can rise.
Why it matters
This is the fencing mechanism that upgrades split-brain prevention from “probably safe” to “guaranteed.” Without a watchdog, a wedged-but-alive old primary could in theory keep accepting writes after a new primary is elected.