Redis

SET key value [EX seconds] [PX milliseconds] [NX|XX]

Set key to hold the string value. If 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.

Options

Starting with Redis 2.6.12 SET supports a set of options that modify its behavior:

Note: Since the SET command options can replace SETNX, SETEX, PSETEX, it is possible that in future versions of Redis these three commands will be deprecated and finally removed.

Return value

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 NX or XX option but the condition was not met.

Examples

redis>  SET mykey "Hello"
OK
redis>  GET mykey
"Hello"
redis> 

Patterns

The command 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:

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[1]) == ARGV[1]
then
    return redis.call("del",KEYS[1])
else
    return 0
end

The script should be called with EVAL ...script... 1 resource-name token-value

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