X-Git-Url: https://git.saurik.com/redis.git/blobdiff_plain/76d31044d44c7adb4af71dd273b52ec9a4768e17..49128f0b9da725de992e427fa341a837bcc2991b:/doc/SetnxCommand.html

diff --git a/doc/SetnxCommand.html b/doc/SetnxCommand.html
index 91f9206d..8c47e72d 100644
--- a/doc/SetnxCommand.html
+++ b/doc/SetnxCommand.html
@@ -41,6 +41,7 @@ SETNX lock.foo <current UNIX time + lock timeout + 1>
 <ul><li> 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.</li><li> C1 sends DEL lock.foo</li><li> C1 sends SETNX =&gt; success!</li><li> C2 sends DEL lock.foo</li><li> C2 sends SETNX =&gt; success!</li><li> ERROR: both C1 and C2 acquired the lock because of the race condition.</li></ul>
 <blockquote>Fortunately it's possible to avoid this issue using the following algorithm.Let's see how C4, our sane client, uses the good algorithm:</blockquote>
 <ul><li> C4 sends SETNX lock.foo in order to acquire the lock</li><li> The crashed C3 client still holds it, so Redis will reply with 0 to C4.</li><li> C4 GET lock.foo to check if the lock expired. If not it will sleep one second (for instance) and retry from the start.</li><li> 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 &lt;current unix timestamp + lock timeout + 1&gt;</li><li> Thanks to the <a href="GetsetCommand.html">GETSET</a> command semantic C4 can check if the old value stored at key is still an expired timestamp. If so we acquired the lock!</li><li> 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.</li></ul>
+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).
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