This post assumes that you understand transactions at database level. Also this post has been written considering PostgreSQL as database.
To recap, every SQL statement in psql implicitly executes inside a transaction without user/client having to explicitly use transaction constructs like
We will write a view which leads to a race condition and will then fix it using Django’s F expressions and later using Django transaction decorator.
Let’s assume we have the following view.
from rest_framework.views import APIView from rest_framework.responses import Response class LikeView(APIView): def patch(self, request, *args, **kwargs): post = get_object_or_404(Post, pk=kwargs['pk']) post.likes += 1 post.save() return Response("Voted")
Assume this api is exposed at
Let’s assume the correct
likes is 0 for post with id 1.
If you make 50 concurrent calls to
http://localhost:8000/api/post/1/like, you will notice that the likes might not have increased to 50. It might show 47 or 48 or some other number less than 50.
This happened because there is a race condition in the code. One process/thread might have read a value for
likes and before it could save(), another process/thread would have read the same value. So even though
save() executed from both processes, still the value would have been incremented only once.
The fix for this is to handle increment of
likes using a database call without reading it in memory.
Modify the above code to following:
from django.db.models import F class LikeView(APIView): def patch(self, request, *args, **kwargs): Post.objects.filter(pk=kwargs['pk']).select_for_update(votes=F('votes') + 1)
We used ORM
select_for_update() which runs underlying database’s
select ... for update. As the db update runs in a transaction so each process/thread would correctly increment the value of
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