Django locking between clean() and save()

Question

Let's say I have a model with a unique field email:

class MyModel:

    email = models.EmailField(unique=True)

    def save(self):
        .... # save model

    def clean(self):
        .... # validate model, make sure email doesn't already exist.

Usually if a browser submits a form with email being a value that already exists, it would raise a ValidationError, due to the model form field validation.

If two browsers submits the same email at the same time, with email being a value that doesn't exist yet, at least one of the requests will succeed by saving a row into the database. The other request, if it arrives long enough after the first one, will be treated normally - with a ValidationError raised saying the email already exists. But if it arrives almost at the same time as the first one, then the clean() will succeed - the email doesn't exist yet, but by the time the save() method is executed, the row from the first request would have been saved. In this latter case, a IntegrityError will be raised instead and the server would return an Internal Server 500 error, which is undesirable.

How can this last scenario be prevented? Database transactions?

Answer 1

Just to use Django database transactions in this case will not be enough. What you want to do when creating a new model is to use database table lock together with transaction. But Django has no API for locking tables yet, therefore you will have to use raw SQL to perform a table lock for your database regarding of its type.

If you use PostgreSQL, here is a perfect example: http://www.caktusgroup.com/blog/2009/05/26/explicit-table-locking-with-postgresql-and-django/

If you are under some other db, you will have to investigate how to perform table lock.

Answer 2

While the use case is slightly different, you might want to read this old answer from Alex Martelli about "optimistic concurrency" approach.