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Message from discussion Compare-and-swap
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Josiah Carlson  
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 More options Oct 4 2012, 12:01 pm
From: Josiah Carlson <josiah.carl...@gmail.com>
Date: Thu, 4 Oct 2012 09:01:15 -0700
Local: Thurs, Oct 4 2012 12:01 pm
Subject: Re: Compare-and-swap
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This has been discussed before.

Prior answers:
1. Use a lock (written in the standard watch/multi/exec (this can be
tricky) or Lua)
2. Use Lua to write your compare-and-swap

Either of these lets you express *exactly* what you want without
having to guess.

If you have a limitation that you need to use a version of Redis
before scripting, or need it *now* and can't run one of the Redis
release candidates, then your option is basically to use a lock. Read
my article from January about building a correct lock:
http://dr-josiah.blogspot.com/2012/01/creating-lock-with-redis.html
... because incorrect locks can destroy data.

Regards,
 - Josiah

On Thu, Oct 4, 2012 at 8:21 AM, Garry Shutler

<ga...@robustsoftware.co.uk> wrote:
> Hi,

> I'm been looking at using redis to synchronize work between multiple
> processes and I was surprised to see there wasn't a compare-and-swap
> http://en.wikipedia.org/wiki/Compare-and-swap method available.

> It's a well understood mechanism for synchronization and would make the
> locking process described in http://redis.io/commands/setnx a bit better as
> taking an expired lock would be done via CAS rather than GETSET which may
> replace a lock someone else has put in place (and avoid two people taking
> the same lock if they took it in the same millisecond). Also, it would make
> expiring a lock you hold safer as you wouldn't delete a newly created lock
> (had your lock expired), instead you could CAS the expiry date you set with
> one in the past.

> Does that make sense? Is this a desirable feature? Is there something I'm
> overlooking?

> I can look into creating a PR for this if needed but I don't know C so it
> might take me a while.

> Cheers,

> Garry Shutler

> www.robustsoftware.co.uk | @gshutler

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