Question

I want to raise error when a user tries to delete an object when some other users are active in update_object view. I feel some sort of mutex-like locking mechanism is needed for that. Do you have any suggestions?

Answer 1

So, there are a handful of ways to do what you’re asking. But a good number of them are not going to be implementation independent: you could use locks or rlocks, but they will really only work on 100% threaded servers and probably not at all in a fork/pre-fork implementation.

That more or less means the locking implementation will be up to you. Two ideas:

1. .lock file on your file system
2. locked property in your model class

In both cases, you have to manually set the lock object on update and check against it on delete. Try something like:

def safe_update(request,model,id):     obj = model.objects.get(id)     if obj.locked:         raise SimultaneousUpdateError #Define this somewhere     else:         obj.lock()         return update_object(request,model,id)  # In models file class SomeModel(models.Model):     locked = models.BooleanField(default = False)     def lock(self):         self.locked = True         super(models.Model,self).save()     def save(self):         # overriding save because you want to use generic views         # probably not the best idea to rework model code to accomodate view shortcuts         # but I like to give examples.         self.locked = False         # THIS CREATES A DIFFERENT CRITICAL REGION!         super(models.Model,self).save() 

This is indeed a clumsy implementation that you’ll have to clean up. You may not be comfortable with the fact that a different critical region has been created, but I don’t see how you’ll do much better if your using the database as an implementation without making the implementation much more complicated. (One option would be to make the locks entirely separate objects. Then you could update them after the save() method is called. But I don’t feel like coding that up.) If you really want to use a file-based locking system, that would also solve the problem. If you’re database-hit-paranoid, this might be the thing for you. Something like:

class FileLock(object):     def __get__(self,obj):         return os.access(obj.__class__+'_'+obj.id+'.lock',os.F_OK)     def __set__(self,obj,value):         if not isinstance(value,bool):             raise AttributeError         if value:             f = open(obj.__class__+'_'+obj.id+'.lock')             f.close()         else:             os.remove(obj.__class__+'_'+obj.id+'.lock')     def __delete__(self,obj):         raise AttributeError   class SomeModel(models.Model):      locked = FileLock()      def save(self):          super(models.Model,self).save()          self.locked = False 

Anyway, maybe there’s some way to mix and match these suggestions to your taste?