Distributed locks are a very useful primitive in many environments where different processes must operate with shared resources in a mutually exclusive way.
Mutual exclusion. At any given moment, only one client can hold a lock.
Deadlock free. Eventually it is always possible to acquire a lock, even if the client that locked a resource crashes or gets partitioned.
Fault tolerance. As long as the majority of Redis nodes are up, clients are able to acquire and release locks.
Before trying to overcome the limitation of the single instance setup described above, let’s check how to do it correctly in this simple case, since this is actually a viable solution in applications where a race condition from time to time is acceptable, and because locking into a single instance is the foundation we’ll use for the distributed algorithm described here.
To acquire the lock, the way to go is the following:
SET resource_name my_random_value NX PX 30000
The command will set the key only if it does not already exist (NX option), with an expire of 30000 milliseconds (PX option). The key is set to a value “myrandomvalue”. This value must be unique across all clients and all lock requests.
Basically the random value is used in order to release the lock in a safe way, with a script that tells Redis: remove the key only if it exists and the value stored at the key is exactly the one I expect to be. This is accomplished by the following Lua script:
if redis.call("get",KEYS[1]) == ARGV[1] then
return redis.call("del",KEYS[1])
else
return 0
end