Re: Object Lookup after Save

On Tue, May 21, 2013 at 4:23 PM, Chris Conover <clcono@gmail.com> wrote:
> Hello,
>
> I'm having an issue looking up objects immediately after they are saved and
> am wondering if anyone has any advice on how to address the problem.
> Specifically, I'm saving an object in a view (nothing fancy, a simple
> save()) and then kicking off a Gearman task to do some operations on that
> saved object in the background. I pass the newly created object's PK as data
> to the Gearman worker which then does a simple Objects.objects.get(pk=PK).
> However, almost all of the time this lookup operation fails with an
> DoesNotExist exception. I believe the problem has to do with transactions.
> Namely, the read in the Gearman worker is happening before the write from
> the view is committed to the database. I've tried several things including
> refactoring the saving code to be wrapped in a
> @transaction.commit_on_success block, moving the worker submission to
> post_save and adding a long delay before the Gearman worker does the lookup.
> Nothing really seems to solve the problem completely. Even with a 60 second
> delay, the lookup fails with some frequency. Am I missing something here? Is
> there some Django query cache that I can clear? Or should I just rewrite all
> this to just to use a look-back perspective.
>
> The stack is Django 1.4 connecting to MySQL 5.5.27. Django is handling
> 200-1000 requests per second and the database is handling about double that.
>
> Thanks,
> Chris

from django import transaction
…
obj.save()
transaction.commit()
task.submit(obj.id)

You will also need to make sure that gearman is doing things correctly
as well. You haven't mentioned what database you are using, but if
gearman's DB connection is in a read repeated mode, you can do
whatever you like in django but you won't see new data in gearman
until gearman's current transaction is committed.

Cheers

Tom

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