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+ #sidebar
StringCommandsSidebar
Time complexity: O(1)SETNX works exactly like SET with the only difference thatif the key already exists no operation is performed.SETNX actually means "SET if Not eXists".
Integer reply, specifically:
1 if the key was set
0 if the key was not set
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SETNX can also be seen as a locking primitive. For instance to acquirethe lock of the key foo, the client could try the following:
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+SETNX lock.foo <current UNIX time + lock timeout + 1>
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If SETNX returns 1 the client acquired the lock, setting the lock.fookey to the UNIX time at witch the lock should no longer be considered valid.The client will later use DEL lock.foo in order to release the lock.
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If SETNX returns 0 the key is already locked by some other client. We caneither return to the caller if it's a non blocking lock, or enter aloop retrying to hold the lock until we succeed or some kind of timeoutexpires.
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In the above locking algorithm there is a problem: what happens if a clientfails, crashes, or is otherwise not able to release the lock?It's possible to detect this condition because the lock key contains aUNIX timestamp. If such a timestamp is <= the current Unix time the lockis no longer valid.
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When this happens we can't just call DEL against the key to remove the lockand then try to issue a SETNX, as there is a race condition here, whenmultiple clients detected an expired lock and are trying to release it.
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- C1 and C2 read lock.foo to check the timestamp, because SETNX returned 0 to both C1 and C2, as the lock is still hold by C3 that crashed after holding the lock.
- C1 sends DEL lock.foo
- C1 sends SETNX => success!
- C2 sends DEL lock.foo
- C2 sends SETNX => success!
- ERROR: both C1 and C2 acquired the lock because of the race condition.
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Fortunately it's possible to avoid this issue using the following algorithm.Let's see how C4, our sane client, uses the good algorithm:
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- C4 sends SETNX lock.foo in order to acquire the lock
- The crashed C3 client still holds it, so Redis will reply with 0 to C4.
- C4 GET lock.foo to check if the lock expired. If not it will sleep one second (for instance) and retry from the start.
- If instead the lock is expired because the UNIX time at lock.foo is older than the current UNIX time, C4 tries to perform GETSET lock.foo <current unix timestamp + lock timeout + 1>
- Thanks to the GETSET command semantic C4 can check if the old value stored at key is still an expired timestamp. If so we acquired the lock!
- Otherwise if another client, for instance C5, was faster than C4 and acquired the lock with the GETSET operation, C4 GETSET operation will return a non expired timestamp. C4 will simply restart from the first step. Note that even if C4 set the key a bit a few seconds in the future this is not a problem.
+IMPORTANT NOTE: In order to make this locking algorithm more robust, a client holding a lock should always check the timeout didn't expired before to unlock the key with DEL because client failures can be complex, not just crashing but also blocking a lot of time against some operation and trying to issue DEL after a lot of time (when the LOCK is already hold by some other client).