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Counting Semaphore?
As described by RedisLabs one can implement a counting semaphore using Redis. Is there any plan to add a counting semaphore implementation to this package? I think it would be a good fit since it already support locking. What do you think?
We'll visit this in the v2 pass. Not promises, but we're doing a full API evaluation and seeing what to add or change there.
Nick it looks like this didn't make it into the 2.0 milestone unless I'm missing it.
It appears that the "RedisLabs" link is broken. Can anyone point me in the right direction of how go about implementing a semaphore on top of SE.Redis?
My first idea was to use INCR and DECR, but this wouldn't support expiration/release very well in the event that a client disconnects unexpectedly. If latency wasn't an issue, I could designate n distinct keys and have clients cycle through them until an available one is found.
Perhaps something more efficient could be designed using a list or set to store the expirations? But I'm starting to feel like I am reinventing the wheel...
I think you might be. FWIW Redis now has a page about the RedLock algorithm that includes numerous implementations of it.
I don't remember when exactly, but at some point over the 7 years since I started this thread I started to use one of the RedLock implementations and it's been working for my needs.
A speculative unary lock: is pretty trivial to do these says - SET {key} {token} NX EX {timeout} - which is available via StringLock or LockTake; a speculative semaphore is usually implemented via Lua, so that you can check the current availability and optionally increment it (whether via a counter or a lpush/sadd) all in a single operation. However, both of these have the "speculative" problem, by which I mean it isn't a "and wait (with timeout) to be able to successfully take the lock/semaphore"; for that we need blocking operation support, and that is something we're currently talking to the redis folks about (blocking, multiplexing, protocol details, etc)
Thanks, since posting I stumbled across @madelson's implementation in DistributedLock.Redis, which as you say leverages a lua script, and am quite happy with it. That library is a nice piece of work 🙇♂️.