Python and file locking - NFS or MySQL?

[Christopher DeMarco]

Hi all...

...I've got a Python script running on a bunch of boxen sharing some
common NFS-exported space.  I need (not want :) to lock files for
writing, and I need (not want :) to do it safely (i.e. atomically).
I'm doing this in Linux; NFS4 is available.  As I understand it, my
options are:

1.  Python's fcntl() is an interface to the fcntl(2) system call,
which is claimed to work "mostly" over NFS v >= 3.

2.  open(2) is atomic on a local FS, I've read discussions that imply
that link(2) is atomic over NFS (is it?), so I can link from local
lockfile to remote target atomically.  I don't grok this; open(2) +
link(2) + stat(2) == 3 calls on my fingers.  HTH is this supposed to
work?

3.  Atomically update a MySQL database indicating that the file is
locked - MySQL has atomic transactions now, right?  And how good is
the Python MySQL API - IIRC Perl didn't have atomic transactions last
year; will this work in contemporary Python?

I've got a while (several weeks) to chew this problem over (my current
implementation is ``assert("Poof!  File locked")'').

What are my options for safely locking files via NFS?  I don't want to
get involved with NLM as my impression is it's being buggy and
unwieldy.  Thanks in advance!


I was present at an undersea, unexplained mass sponge migration.
-- 
Christopher DeMarco <cmd at alephant.net>
Alephant Systems (http://alephant.net)
PGP public key at http://pgp.alephant.net
+1 412 708 9660
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