The old leader is cut off by a network partition, a follower is promoted, and now both accept writes. What prevents this split brain?

Options

• ✗ A shorter heartbeat timeout
• ✗ Adding more followers
• ✓ Electing leaders by majority vote and rejecting writes that carry an older term (a fencing token) Correct
• ✗ Restarting the old leader automatically when it reconnects

Why

Only one side of a partition can hold a majority, so only one leader can be elected per term. A fencing token (the term or epoch number) lets storage and followers reject an old leader that does not yet know it was replaced.

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