Knowledge, Truth, and Lies

In a distributed system, a single node cannot trust its own judgment. The Truth is defined by the Majority (Quorum).

Fencing Tokens

If a node thinks it is the leader, but it was paused, it might try to write to storage. To prevent this, use a Fencing Token (a monotonically increasing number).

• The lock server issues token 33.
• The client writes with token 33.
• If the storage server has already seen token 34 (from a new leader), it rejects write 33.

Byzantine Faults

Nodes might "lie" (send corrupted data). This is the Byzantine Generals Problem. In trusted datacenters, we usually assume nodes are honest but unreliable (crash-recovery), not Byzantine.