key to hold the string
key already holds a value, it is overwritten, regardless of its type.
Any previous time to live associated with the key is discarded on successful SET operation.
Starting with Redis 2.6.12 SET supports a set of options that modify its behavior:
EXseconds -- Set the specified expire time, in seconds.
PXmilliseconds -- Set the specified expire time, in milliseconds.
NX-- Only set the key if it does not already exist.
XX-- Only set the key if it already exist.
Simple string reply:
OK if SET was executed correctly.
Null reply: a Null Bulk Reply is returned if the SET operation was not performed becase the user specified the
XX option but the condition was not met.
OKredis> GET mykey
SET resource-name anystring NX EX max-lock-time is a simple way to implement a locking system with Redis.
A client can acquire the lock if the above command returns
OK (or retry after some time if the command returns Nil), and remove the lock just using DEL.
The lock will be auto-released after the expire time is reached.
It is possible to make this system more robust modifying the unlock schema as follows:
- Instead of setting a fixed string, set a non-guessable large random string, called token.
- Instead of releasing the lock with DEL, send a script that only removes the key if the value matches.
This avoids that a client will try to release the lock after the expire time deleting the key created by another client that acquired the lock later.
An example of unlock script would be similar to the following:
if redis.call("get",KEYS) == ARGV then return redis.call("del",KEYS) else return 0 end
The script should be called with
EVAL ...script... 1 resource-name token-value