CODEWITHSUNDEEP
redis
MengCheng Wei
MengCheng Wei

Reputation: 727

Why Redis SETNX succeeds for two clients

I am studying the locking method on Redis, and found this official documents https://redis.io/commands/setnx#handling-deadlocks

My understanding is that SETNX is an atomic operation and ONLY ONE client can do it successful (get 1) and the others failed (get 0)

However in the section Handling deadlocks on this document, it said:

1) C1 and C2 read lock.foo to check the timestamp, because they both received 0 after executing SETNX, as the lock is still held by C3 that crashed after holding the lock.
2) C1 sends DEL lock.foo
3) C1 sends SETNX lock.foo and it succeeds
4) C2 sends DEL lock.foo
5) C2 sends SETNX lock.foo and it succeeds
ERROR: both C1 and C2 acquired the lock because of the race condition.

My question is that why 3) and 5) can be succeed? I think only C1 or C2 can be succeed but not both.

Please correct my understanding, thank you,.

Upvotes: 1

Views: 2678

Answers (1)

Kevin Christopher Henry
Kevin Christopher Henry

Reputation: 48952

SETNX only sets the key if it doesn't already exist. So, indeed, if multiple clients try to SETNX at the same time, only one will succeed.

But in this case (see steps 2 and 4), the clients are deleting the key before calling SETNX. Since the key no longer exists, there is nothing to prevent SETNX from succeeding.

Upvotes: 4

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