Leader key (TTL lease)
Also called: leader lock, TTL lease, leadership key
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.
What this means
A single entry in the consensus store that names which node is primary, and it's written with a short expiry. The primary's agent has to keep renewing it every loop; if the primary dies and stops renewing, the key expires, and the surviving nodes race to grab it. Exactly one wins, and that one gets promoted.
Why it matters operationally
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.