[Django]-Django: How can I protect against concurrent modification of database entries

This is how I do optimistic locking in Django:

updated = Entry.objects.filter(Q(id=e.id) && Q(version=e.version))\
          .update(updated_field=new_value, version=e.version+1)
if not updated:
    raise ConcurrentModificationException()

The code listed above can be implemented as a method in Custom Manager.

I am making the following assumptions:

• filter().update() will result in a single database query because filter is lazy
• a database query is atomic

These assumptions are enough to ensure that no one else has updated the entry before. If multiple rows are updated this way you should use transactions.

WARNING Django Doc:

Be aware that the update() method is
converted directly to an SQL
statement. It is a bulk operation for
direct updates. It doesn’t run any
save() methods on your models, or emit
the pre_save or post_save signals

This question is a bit old and my answer a bit late, but after what I understand this has been fixed in Django 1.4 using:

select_for_update(nowait=True)

see the docs

Returns a queryset that will lock rows until the end of the transaction, generating a SELECT … FOR UPDATE SQL statement on supported databases.

Usually, if another transaction has already acquired a lock on one of the selected rows, the query will block until the lock is released. If this is not the behavior you want, call select_for_update(nowait=True). This will make the call non-blocking. If a conflicting lock is already acquired by another transaction, DatabaseError will be raised when the queryset is evaluated.

Of course this will only work if the back-end support the “select for update” feature, which for example sqlite doesn’t. Unfortunately: nowait=True is not supported by MySql, there you have to use: nowait=False, which will only block until the lock is released.

Actually, transactions don’t help you much here … unless you want to have transactions running over multiple HTTP requests (which you most probably don’t want).

What we usually use in those cases is “Optimistic Locking”. The Django ORM doesn’t support that as far as I know. But there has been some discussion about adding this feature.

So you are on your own. Basically, what you should do is add a “version” field to your model and pass it to the user as a hidden field. The normal cycle for an update is :

1. read the data and show it to the user
2. user modify data
3. user post the data
4. the app saves it back in the database.

To implement optimistic locking, when you save the data, you check if the version that you got back from the user is the same as the one in the database, and then update the database and increment the version. If they are not, it means that there has been a change since the data was loaded.

You can do that with a single SQL call with something like :

UPDATE ... WHERE version = 'version_from_user';

This call will update the database only if the version is still the same.

Django 1.11 has three convenient options to handle this situation depending on your business logic requirements:

• Something.objects.select_for_update() will block until the model become free
• Something.objects.select_for_update(nowait=True) and catch DatabaseError if the model is currently locked for update
• Something.objects.select_for_update(skip_locked=True) will not return the objects that are currently locked

In my application, which has both interactive and batch workflows on various models, I found these three options to solve most of my concurrent processing scenarios.

The “waiting” select_for_update is very convenient in sequential batch processes – I want them all to execute, but let them take their time. The nowait is used when an user wants to modify an object that is currently locked for update – I will just tell them it’s being modified at this moment.

The skip_locked is useful for another type of update, when users can trigger a rescan of an object – and I don’t care who triggers it, as long as it’s triggered, so skip_locked allows me to silently skip the duplicated triggers.

For future reference, check out https://github.com/RobCombs/django-locking. It does locking in a way that doesn’t leave everlasting locks, by a mixture of javascript unlocking when the user leaves the page, and lock timeouts (e.g. in case the user’s browser crashes). The documentation is pretty complete.

You should probably use the django transaction middleware at least, even regardless of this problem.

As to your actual problem of having multiple users editing the same data… yes, use locking. OR:

Check what version a user is updating against (do this securely, so users can’t simply hack the system to say they were updating the latest copy!), and only update if that version is current. Otherwise, send the user back a new page with the original version they were editing, their submitted version, and the new version(s) written by others. Ask them to merge the changes into one, completely up-to-date version. You might try to auto-merge these using a toolset like diff+patch, but you’ll need to have the manual merge method working for failure cases anyway, so start with that. Also, you’ll need to preserve version history, and allow admins to revert changes, in case someone unintentionally or intentionally messes up the merge. But you should probably have that anyway.

There’s very likely a django app/library that does most of this for you.

Another thing to look for is the word “atomic”. An atomic operation means that your database change will either happen successfully, or fail obviously. A quick search shows this question asking about atomic operations in Django.

The idea above

updated = Entry.objects.filter(Q(id=e.id) && Q(version=e.version))\
      .update(updated_field=new_value, version=e.version+1)
if not updated:
      raise ConcurrentModificationException()

looks great and should work fine even without serializable transactions.

The problem is how to augment the deafult .save() behavior as to not have to do manual plumbing to call the .update() method.

I looked at the Custom Manager idea.

My plan is to override the Manager _update method that is called by Model.save_base() to perform the update.

This is the current code in Django 1.3

def _update(self, values, **kwargs):
   return self.get_query_set()._update(values, **kwargs)

What needs to be done IMHO is something like:

def _update(self, values, **kwargs):
   #TODO Get version field value
   v = self.get_version_field_value(values[0])
   return self.get_query_set().filter(Q(version=v))._update(values, **kwargs)

Similar thing needs to happen on delete. However delete is a bit more difficult as Django is implementing quite some voodoo in this area through django.db.models.deletion.Collector.

It is weird that modren tool like Django lacks guidance for Optimictic Concurency Control.

I will update this post when I solve the riddle. Hopefully solution will be in a nice pythonic way that does not involve tons of coding, weird views, skipping essential pieces of Django etc.

To be safe the database needs to support transactions.

If the fields is “free-form” e.g. text etc. and you need to allow several users to be able to edit the same fields (you can’t have single user ownership to the data), you could store the original data in a variable.
When the user committs, check if the input data has changed from the original data (if not, you don’t need to bother the DB by rewriting old data),
if the original data compared to the current data in the db is the same you can save, if it has changed you can show the user the difference and ask the user what to do.

If the fields is numbers e.g. account balance, number of items in a store etc., you can handle it more automatically if you calculate the difference between the original value (stored when the user started filling out the form) and the new value you can start a transaction read the current value and add the difference, then end transaction. If you can’t have negative values, you should abort the transaction if the result is negative, and tell the user.

I don’t know django, so I can’t give you teh cod3s.. 😉

From here:
How to prevent overwriting an object someone else has modified

I’m assuming that the timestamp will be held as a hidden field in the form you’re trying to save the details of.

def save(self):
    if(self.id):
        foo = Foo.objects.get(pk=self.id)
        if(foo.timestamp > self.timestamp):
            raise Exception, "trying to save outdated Foo" 
    super(Foo, self).save()