Concurrent writes to the same file

Dave Angel davea at
Thu Jul 11 07:34:48 CEST 2013

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?


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