#25432: Django ORM race condition
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Reporter: yuvadm | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.8
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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I've hit an interesting problem that isn't covered by the current Django
documentation, and might even be a bug that Django can handle better. It
started off as a SO question at http://stackoverflow.com/q/32661885/24545
but here's the gist of it.
After creating a new object {{{MyModel.objects.create(foo=goo)}}} and
inserting it into the database, it is possible that immediate subsequent
calls to fetch that object might fail (i.e.
{{{MyModel.objects.get(foo=goo)}}} will throw {{{DoesNotExist}}}). I have
seen this happen in a test case where I make two subsequent API calls that
do exactly this and got a ~5% failure rate.
In most cases this might not be a problem, but I am using this query to
make sure I'm not creating two duplicate objects. This is essentially an
UPSERT problem. In my case, my solution was to set {{{unique=True}}} on my
{{{foo}}} field and attempt to create the object in any case, which will
naturally fail on a duplicate, then I just catch the {{{IntegrityError}}}
and fail gracefully. In this we we use DB semantics which guarantees no
duplicates.
The relevant settings for this test case are: PostgreSQL, default Django
transaction settings and no specific caching.
So 2 questions here:
1. What happens if my application '''requires''' that any two transactions
{{{a}}} and {{{b}}} behave such that {{{b}}} always sees fresh data that
was written in {{{a}}}? How can I enforce this in Django?
2. How do we document this behavior in a better way? If this is possible
or impossible, Django must be clearer on how such transactions are
handled.
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Ticket URL: <https://code.djangoproject.com/ticket/25432>
Django <https://code.djangoproject.com/>
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