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Diffstat (limited to 'iredis/data/commands/setnx.md')
| -rw-r--r-- | iredis/data/commands/setnx.md | 103 |
1 files changed, 50 insertions, 53 deletions
diff --git a/iredis/data/commands/setnx.md b/iredis/data/commands/setnx.md index acd77df..833573c 100644 --- a/iredis/data/commands/setnx.md +++ b/iredis/data/commands/setnx.md @@ -1,13 +1,14 @@ -Set `key` to hold string `value` if `key` does not exist. In that case, it is -equal to `SET`. When `key` already holds a value, no operation is performed. +Set `key` to hold string `value` if `key` does not exist. +In that case, it is equal to `SET`. +When `key` already holds a value, no operation is performed. `SETNX` is short for "**SET** if **N**ot e**X**ists". @return @integer-reply, specifically: -- `1` if the key was set -- `0` if the key was not set +* `1` if the key was set +* `0` if the key was not set @examples @@ -21,82 +22,78 @@ GET mykey **Please note that:** -1. The following pattern is discouraged in favor of - [the Redlock algorithm](https://redis.io/topics/distlock) which is only a bit - more complex to implement, but offers better guarantees and is fault - tolerant. -2. We document the old pattern anyway because certain existing implementations - link to this page as a reference. Moreover it is an interesting example of - how Redis commands can be used in order to mount programming primitives. -3. Anyway even assuming a single-instance locking primitive, starting with - 2.6.12 it is possible to create a much simpler locking primitive, equivalent - to the one discussed here, using the `SET` command to acquire the lock, and a - simple Lua script to release the lock. The pattern is documented in the `SET` - command page. - -That said, `SETNX` can be used, and was historically used, as a locking -primitive. For example, to acquire the lock of the key `foo`, the client could -try the following: +1. The following pattern is discouraged in favor of [the Redlock algorithm](https://redis.io/topics/distlock) which is only a bit more complex to implement, but offers better guarantees and is fault tolerant. +2. We document the old pattern anyway because certain existing implementations link to this page as a reference. Moreover it is an interesting example of how Redis commands can be used in order to mount programming primitives. +3. Anyway even assuming a single-instance locking primitive, starting with 2.6.12 it is possible to create a much simpler locking primitive, equivalent to the one discussed here, using the `SET` command to acquire the lock, and a simple Lua script to release the lock. The pattern is documented in the `SET` command page. + +That said, `SETNX` can be used, and was historically used, as a locking primitive. For example, to acquire the lock of the key `foo`, the client could try the +following: ``` SETNX lock.foo <current Unix time + lock timeout + 1> ``` If `SETNX` returns `1` the client acquired the lock, setting the `lock.foo` key -to the Unix time at which the lock should no longer be considered valid. The -client will later use `DEL lock.foo` in order to release the lock. +to the Unix time at which the lock should no longer be considered valid. +The client will later use `DEL lock.foo` in order to release the lock. -If `SETNX` returns `0` the key is already locked by some other client. We can -either return to the caller if it's a non blocking lock, or enter a loop +If `SETNX` returns `0` the key is already locked by some other client. +We can either return to the caller if it's a non blocking lock, or enter a loop retrying to hold the lock until we succeed or some kind of timeout expires. ### Handling deadlocks In the above locking algorithm there is a problem: what happens if a client -fails, crashes, or is otherwise not able to release the lock? It's possible to -detect this condition because the lock key contains a UNIX timestamp. If such a -timestamp is equal to the current Unix time the lock is no longer valid. +fails, crashes, or is otherwise not able to release the lock? +It's possible to detect this condition because the lock key contains a UNIX +timestamp. +If such a timestamp is equal to the current Unix time the lock is no longer +valid. When this happens we can't just call `DEL` against the key to remove the lock and then try to issue a `SETNX`, as there is a race condition here, when multiple clients detected an expired lock and are trying to release it. -- C1 and C2 read `lock.foo` to check the timestamp, because they both received +* 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. -- C1 sends `DEL lock.foo` -- C1 sends `SETNX lock.foo` and it succeeds -- C2 sends `DEL lock.foo` -- C2 sends `SETNX lock.foo` and it succeeds -- **ERROR**: both C1 and C2 acquired the lock because of the race condition. +* C1 sends `DEL lock.foo` +* C1 sends `SETNX lock.foo` and it succeeds +* C2 sends `DEL lock.foo` +* C2 sends `SETNX lock.foo` and it succeeds +* **ERROR**: both C1 and C2 acquired the lock because of the race condition. Fortunately, it's possible to avoid this issue using the following algorithm. Let's see how C4, our sane client, uses the good algorithm: -- C4 sends `SETNX lock.foo` in order to acquire the lock +* C4 sends `SETNX lock.foo` in order to acquire the lock -- The crashed client C3 still holds it, so Redis will reply with `0` to C4. +* The crashed client C3 still holds it, so Redis will reply with `0` to C4. -- C4 sends `GET lock.foo` to check if the lock expired. If it is not, it will - sleep for some time and retry from the start. +* C4 sends `GET lock.foo` to check if the lock expired. + If it is not, it will sleep for some time and retry from the start. -- Instead, if the lock is expired because the Unix time at `lock.foo` is older - than the current Unix time, C4 tries to perform: +* Instead, if 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> - ``` + ``` + GETSET lock.foo <current Unix timestamp + lock timeout + 1> + ``` -- Because of the `GETSET` semantic, C4 can check if the old value stored at - `key` is still an expired timestamp. If it is, the lock was acquired. +* Because of the `GETSET` semantic, C4 can check if the old value stored at + `key` is still an expired timestamp. + If it is, the lock was acquired. -- If another client, for instance C5, was faster than C4 and acquired the lock - with the `GETSET` operation, the 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. +* If another client, for instance C5, was faster than C4 and acquired the lock + with the `GETSET` operation, the 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. -In order to make this locking algorithm more robust, a client holding a lock -should always check the timeout didn't expire before unlocking the key with -`DEL` because client failures can be complex, not just crashing but also -blocking a lot of time against some operations and trying to issue `DEL` after a -lot of time (when the LOCK is already held by another client). +In order to make this locking algorithm more robust, a +client holding a lock should always check the timeout didn't expire before +unlocking the key with `DEL` because client failures can be complex, not just +crashing but also blocking a lot of time against some operations and trying +to issue `DEL` after a lot of time (when the LOCK is already held by another +client). |