From: Daniel Berger <djberg96@...>
Date: 2008-06-07T00:10:20+09:00
Subject: Re: Ensuring only one instance of a script is running



On May 28, 5:48 pm, "ara.t.howard" <ara.t.how...@gmail.com> wrote:
> On May 28, 2008, at 4:35 PM, Mike Kasick wrote:
>
> > Cute trick.
>
> > However, I wouldn't recommend it's use in any distributed code.  
> > flocks
> > are intended to serve a purpose other than what's achieved as a side
> > effect here, and although it's generally safe, the behavior is unknown
> > at best if the script file resides in a remote file system like NFS.
>
> > For example, in Linux versions prior to 2.6.12 flocks are local  
> > only, so
> > this would have the intended effect.  However, in Linux versions  
> > 2.6.12
> > and newer, flocks are emulated by POSIX locks, which should result in
> > only a single instance running across a set of machines attempting to
> > run the same script located in an NFS share.
>
> you can use the same trick even on NFS
>
>    require 'posixlock' # gem installposixlock
>
>    DATA.posixlock( File::LOCK_EX | File::LOCK_NB )
>
> and this will work for both NFS and local fs.  of course you'd need to  
> degrade to flock ifposixlockis not installed....
>
> my lockfile gem is also NFS safe and can be used for this purpose.  
> actually it can make any program run one instance without modification
>
> rlock lockifle run_this_once.rb

Good to know, thanks Ara.

BTW, I don't see posixlock on the main codeforpeople file listing at
http://rubyforge.org/frs/?group_id=1024

I was able to find 0.0.1 via the RAA, though. Is that the latest
version? I ask because I want to build from source.

Thanks,

Dan