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To: python-list@python.org
From: Neal Becker <ndbecker2@gmail.com>
Subject: Re: Concurrent writes to the same file
Date: Thu, 11 Jul 2013 11:58:57 -0400
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Xref: csiph.com comp.lang.python:50460

Dave Angel wrote:

> On 07/11/2013 12:57 AM, Jason Friedman wrote:
>> Other than using a database, what are my options for allowing two processes
>> to edit the same file at the same time?  When I say same time, I can accept
>> delays.  I considered lock files, but I cannot conceive of how I avoid race
>> conditions.
>>
> 
> In general, no.  That's what a database is for.
> 
> Now, you presumably have some reason to avoid database, but in its stead
> you have to specify some other limitations.  To start with, what do you
> mean by "the same time"?  If each process can modify the entire file,
> then there's no point in one process reading the file at all until it
> has the lock.  So the mechanism would be
>    1) wait till you can acquire the lock
>    2) open the file, read it, modify it, flush and close
>    3) release the lock
> 
> To come up with an appropriate lock, it'd be nice to start by specifying
> the entire environment.  Which Python, which OS?  Are the two processes
> on the same CPU, what's the file system, and is it locally mounted?
> 
> 
> 
> 

You can use a seperate server process to do file I/O, taking multiple inputs 
from clients.  Like syslogd.