Also called: leader lock, TTL lease, leadership key
In plain English
The leader key is a single entry in the consensus store that marks which node is currently primary. It carries a TTL (time-to-live); the primary’s agent renews it on every loop. If the key is not renewed before the TTL expires, it disappears and the surviving nodes race to recreate it with an atomic operation — exactly one wins and promotes.
Why it matters
The TTL is the heartbeat of the whole cluster: it defines how long a silent primary is tolerated before a new election. The atomic, quorum-backed nature of acquiring the key is what guarantees only one primary at a time.