Meaning, they don't have to be distributed. I'm thinking about using memcached
or redis
for that. Probably the latter one. What I'm concerned about is "we've got to free some memory, so we'll delete this key/value before it expired" thing. But I'm open to other suggestions as well.
How to implement non-distributed locks which expire?
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1
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tl;dr Use ready-made solution, suggested by developers.
So, I decided not to use
memcached
for the purpose. Since it's a caching server. I don't see a way to ensure that it doesn't delete my keys because it's out of memory. With,redis
that's not an issue as long asmaxmemory-policy = noeviction
.There are 3 links I want to share with you. They are basically 3 ways, that I now know, to solve the issue. As long as you have
redis >= 2.6.0
that is.redis >= 2.6.12
If you've got
redis >= 2.6.12
, you're lucky and can simply usesetnx
command with its new optionsex
andnx
:But we can't just delete the lock in the end, if we are to allow for critical section taking longer then we expected (
>= ttl
). Consider the following situation:For that not to happen we are going to store current timestamp as a value of the lock. Then, knowing that Lua scripts are atomic (see Atomicity of scripts):
However, is it possible for two clients to have equal
now
values? For that all the above actions should happen within one second andttl
must be equal to0
.Resulting code:
expire
The other solution I found here. You simply make the value expire with
expire
command:So, only
acquire_lock
function changes:getset
And the last one is described again in documentation. Marked with "left for historical reasons" note.
This time we store timestamp of the moment when the lock is to expire. We store it with
setnx
command. If it succeeds, we've acquired the lock. Otherwise, either someone else's holding the lock, or the lock has expired. Be it the latter, we usegetset
to set new value and if the old value hasn't changed, we've acquired the lock:What makes me uncomfortable here is that we seem to have changed someone else's
expires_at
value, don't we?On a side note, you can check which
redis
is it that you're using this way:Some debugging functions: