I've started a few threads before on object persistence in medium to
high end server apps. This one is about low end apps, for example, a
simple cgi on a personal web site that might get a dozen hits a day.
The idea is you just want to keep a few pieces of data around that the
cgi can update.
Immediately, typical strategies like using a MySQL database become too
big a pain. Any kind of compiled and installed 3rd party module (e.g.
Metakit) is also too big a pain. But there still has to be some kind
of concurrency strategy, even if it's something like crude file
locking, or else two people running the cgi simultaneously can wipe
out the data store. But you don't want crashing the app to leave a
lock around if you can help it.
Anyway, something like dbm or shelve coupled with flock-style file
locking and a version of dbmopen that automatically retries after 1
second if the file is locked would do the job nicely, plus there could
be a cleanup mechanism for detecting stale locks.
Is there a standard approach to something like that, or should I just
code it the obvious way?
one easy way would be something along the lines of
from ConfigParser import ConfigParser
from fcntl import flock, LOCK_SH, LOCK_EX, LOCK_UN
class LockedParser(ConfigParser):
def _read(self, fp, fpname):
flock(fp, LOCK_SH) # block until can read
try:
rv = super(LockedParser, self)._read(fp, fpname)
finally:
flock(fp, LOCK_UN)
return rv
def write(self, fp):
flock(fp, LOCK_EX) # block until can write
try:
rv = super(LockedParser, self).write(fp)
finally:
flock(fp, LOCK_UN)
return rv
although you could do the same kind of stuff with csv, or even
Pickle. Of course this doesn't work if what you're wanting to
implement is a hit counter, but that is much easier: just grab a
LOCK_EX, read in, write out, LOCK_UN. If you care about (not)
overwriting changes, but fear you'll hold the lock for too long with
the simple 'grab the lock and run' approach, you could save a version
of the original file and compare before writing out. Complexity grows
a lot, and you suddenly would be better off using pybsddb or somesuch.
Of course I'm probably overlooking something, because it really can't
be this easy, can it?
--
John Lenton (
[email protected]) -- Random fortune:
BOFH excuse #44:
bank holiday - system operating credits not recharged
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