System Design · Senior · Question 71 of 75

Why does a distributed lock need fencing tokens?

✓Correct answer: A client may pause (GC, network) after its lock expires and still write; a monotonically increasing token lets storage reject stale writers
2To encrypt the lock
3Fencing tokens make locks faster

Explanation

Lock expiry alone doesn't guarantee mutual exclusion in the presence of pauses and clock issues.

Further reading

Martin Kleppmann: How to do distributed locking ↗

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