You don't need an atomic read and write, you need a blocking file lock mechanism. This page explains them and how to use them:
http://www.voidspace.org.uk/python/pathutils.html#file-locking --Michael On 11/4/07, Tom <[EMAIL PROTECTED]> wrote: > > Hi, > > I am trying to write a cron/ task scheduler like system for my > website. The users of my site and also more than one external rss feed > reader request my scripts regularly and I'd like to insert code in each > script that will decide whether to run a scheduled task or not. > > It would go something like this: > > <pseudocode> > for task in tasks: > if current_time inside relevant time range for task: > if task not already done: > record that task has been done > do task > </pseudocode> > > Now the problem is that this script could possibly be called more than > once simultaneously! Having studied 'concurrent programming' I understand > the possibility of the following pathological example: > > call 1: if task not already done: > call 2: if task not already done: > call 1: record that task x has been done > call 2: record that task x has been done > call 1: do task x > call 2: do task x #AGAIN > > now we have the task recorded done twice and carried out twice. > > So all my ideas for how to record task x as 'done', like using shelve > or writing a log file etc are not sufficient because, I need an 'atomic > read/ write action' (I think that's the jargon). > > I thought about asking how to lock a log file so that it couldn't be > accessed simultaneously, but I then realise that you STILL have the problem > of the 'gap' between "sleep while log file locked" and "access log file". I > need something where the 'does the task need doing?' and the 'I'm going to > do the task. Please no one else do it!' are atomic(?) and leave no room for > mischief in between. > > Another problem is that the task may fail and need redoing, but I think I > can solve that given a solution to the above. > > Do let me know if my question isn't clear. > Thanks in advance! > Tom > > _______________________________________________ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > > -- Michael Langford Phone: 404-386-0495 Consulting: http://www.TierOneDesign.com/
_______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
