Grokking the System Design Interview, Volume II
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13. Fencing

When a leader fails, the system elects a new one, but the old leader might still be running and issuing commands. Fencing blocks that old leader from touching cluster resources, so it cannot cause damage.

Background

In a leader-follower setup, we can never be fully sure a failed leader has actually stopped. A slow network or a network partition can trigger a new election. The old leader may still be running, still believing it holds the role.

So the system elects a new leader. A real question remains: how do we stop the old one from issuing commands that conflict with the new leader's?

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