Django app goes haywire when deployed. Race conditions? - django

Question

I wrote a django app for quizzing, and it checks the user's answers and updates scores as soon as the user submits an answer. Here is the corresponding view to do this -
current_question_key = 0 #This is a global variable.
def check_answer(request):
current_user = request.user
current_team = Team.objects.get(user = current_user)
current_score = current_team.score
if request.method == "POST":
answer = request.POST.get('answer')
question = Question.objects.get(id = current_question_key)
if answer == question.answer:
if question in current_team.questions_answered.all(): #This is required to prevent the score from increasing if the somebody submits a correct answer to the same question more than once
pass
else:
current_team.score = current_score + question.score_increment
current_team.questions_answered.add(question)
current_team.save()
else:
# This is required to prevent the score from decreasing if someone has answered it correctly earlier
if question in current_team.questions_answered.all():
pass
else :
current_team.score = current_score - question.score_increment//negative_marking_factor
current_team.save()
return HttpResponse(status=204) #This means that the server has successfully processed the request and is not going to return any data.
else:
return HttpResponse("Error404")
The value of current_question_key is changed from the view used to send the question to the front end -
def game(request):
if request.method == "POST":
key = request.POST.get('questionKey')
global current_question_key
current_question_key = key
question = Question.objects.get(id = key)
question_text = question.question_text
data = {
'question_text':question_text
}
return JsonResponse(data)
else:
current_user = request.user
current_team = Team.objects.get(user = current_user)
score = current_team.score
name = current_user.username
return render(request, 'Base/main.html', {'teamname':name, 'score':score})
When tested on django's development server, this worked perfectly fine even when around 10 people were using it simultaneously. But, as soon as I tried to serve it with nginx (hosted on my laptop, with 5 simultaneous users), the app went totally haywire and even correct answers were evaluated as wrong.
I tried apache too and had the same problem with it. Almost all requests were handled incorrectly. Could this be related to race conditions? What exactly might be going on here?

Answer 1

You cannot use a global variable like this in Django. A Django application usually runs in multiple server processes which do not share memory. Calling the game view would only set the global variable current_question_key in one of the processes. All other processes would still have old values. As a request can be served by any process, you get more or less random results.
The Django development server uses multi-threading instead of multi-processing. Threads, as opposed to processes, share the same memory, so all request see the same value for current_question_key.
You have to store current_question_key for each user in a way that is accessible to all processes. The most obvious solution would be to store this information in the user's session:
request.session['current_question_key'] = ...
Alternatively, you could store it in the database, e.g. with ForeignKey in a customer user model, or if you want to keep track of games any in a separate table like this:
from django.contrib.auth import get_user_model
from django.db import models
class Game(model.Model)
user = models.ForeignKey(
get_user_model(),
on_delete=models.CASCADE
)
question = models.ForeignKey(Question, on_delete=models.CASCADE)
created_at = models.DateTimeField(auto_now_add=True, db_index=True)
You can then get the current game for a user by sorting by creation date:
Game.objects.filter(user=request.user).order_by('-created_at').first()
Depending on how often the current question changes, you could also consider using a key-value like Redis, although that complicates things a bit.