System Design · interactive

Distributed locks

A lock across machines needs a timeout, or a crashed holder would keep it forever. But a holder that merely pauses doesn't know its time ran out. Watch two clients both believe they hold the lock.

Lock held by

client 1

Storage holds

empty

t=0sclient 1Takes the lock for 10 s.

Takes the lock for 10 s.

Protection

t = 0 s

Step 1 of 4 · read left to right

1
Take the lock with a lease
The lock service grants it for 10 seconds. If the holder crashes, the lock frees itself.
SET lock client1 NX PX 10000
2
A pause outlives the lease
GC pauses, a slow disk, a laptop lid closing: the client can't notice time passing while it's frozen.
Frozen from t=1 s to t=16 s; the lease ends at t=10 s.
3
Two clients think they hold it
Client 2 takes the lock legitimately. Client 1 wakes up and carries on as if nothing happened.
The lock service did nothing wrong. Time-based locks can't stop this alone.
4
Fencing tokens: storage checks the number
Every grant comes with a number that only goes up. Storage remembers the highest it has seen and rejects anything lower.
Keep stepping.

In production

Tools and technologies

Lock services give you leases; correctness under pauses needs fencing at the storage.

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