CODEWITHSUNDEEP
pythondjangosqliteconcurrency
Josh Smeaton
Josh Smeaton

Reputation: 48720

Django and Sqlite Concurrency issue

I've done a bit of reading related to the concurrency issues with sqlite, but I don't see how they'd apply to Django since it's inherently single threaded. I'm not using any multiprocess modules either. I have absolutely no experience with concurrent programming either, so if someone can identify WHY the following code is causing an OperationalError: 'database is locked' I'd be grateful.

views.py 

def screening(request, ovramt=None):
errors = []
if request.method == "POST":
    form = ScreeningForm(request.POST)
    if form.is_valid():
       print "Woo valid!!"
    return HttpResponse()

else: # GET            
    if ovramt is None:
        o = Ovramt.objects.select_related(depth=1).latest("date_completed")
        print "found?"
        print o.id
    else:
        try:
            o = Ovramt.objects.select_related(depth=1).get(id=ovramt)
        except:
            errors.append("OVRAMT NOT FOUND") 


    if o.residents.count() <= 0:
        o.add_active_residents()
    residents = list(o.residents)

models.py

def add_active_residents(self):
    ssa_res = SSA_Resident.objects.select_related(depth=1).filter(ssa=self.ssa, active=True)
    for r in ssa_res:
        self.residents.add(r.resident) # Fails Here
    self.save()

The add_active_residents method works fine, until it is called from the views module. Is there an open connection to the database open in the view which prevents writing from the model? Does someone have an explanation why this code will error?

Upvotes: 5

Views: 3740

Answers (4)

monkut
monkut

Reputation: 43832

My understanding is that only write operations will result in a db-locked condition. http://www.sqlite.org/lockingv3.html

It's hard to say what the problem is without knowing how django is handling sqlite internally.

Speaking from using sqlite with standard cgi, I've noticed that in some cases it can take a 'long' time to release the lock. You may want to increase the timeout value mentioned by Matthew Christensen.

Upvotes: 2

Matthew Christensen
Matthew Christensen

Reputation: 1731

Are you using Python 2.6?

If so, this is (apparently) a known issue that can be mitigated by adding:

DATABASE_OPTIONS = {'timeout': 30}

to your settings.py

See http://code.djangoproject.com/ticket/9409

Upvotes: 2

Ali Afshar
Ali Afshar

Reputation: 41633

Sounds like you are actually running a multithreaded application, despite what you say. I am a bit clueless about Django, but I would assume that even though it might be single-threaded, whatever debugging server, or production server you run your application in won't be "inherently single threaded".

Upvotes: 1

S.Lott
S.Lott

Reputation: 391846

In the following method function

def add_active_residents(self):
    ssa_res = SSA_Resident.objects.select_related(depth=1).filter(ssa=self.ssa, active=True)
    for r in ssa_res:
        self.residents.add(r.resident) # Fails Here
    self.save()

Why is there a select_related? You only really need the FK's of ssa_res items. Why do additional queries for related items?

Upvotes: 5

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