Re: Locking / serializing access to one element in database

Hi Joakim,

I would suggest you use select_for_update() in a transaction.

It's hard to provide a full example without more details about
the kind of calculation required.

Does it need to be run on new_text even if now rows match
the provided id?

Simon

Le mercredi 21 octobre 2015 18:01:50 UTC-4, Joakim Hove a écrit :

Hello;

this arises in the context of a django application  - but it might be a more general Python/Postgres/... problem.

[ The django application is existing all right - but the problem I am describing here is yet only in my head; I am seeking advice on how to proceed. ] Assume I have a model with a large text field:

class TextModel(models.Model):

     text = models.TextField( ... )

     @classmethod

     def update(cls, id , new_text):

           # Fetch existing ID - or alternatively create a new one.

           try: 

               tm = TextModel.objects.get( pk = id )

           except TextModel.DoesNotExist:

               tm = TextModel( )

           # Perform time consuming calculation (strcat just for demonstration) and save again.

           tm.text += new_text   

           tm.save()

Now - obviously the whole update() method is one big screaming race-condition, but it is not clear to ensure that only one thread/process is accessing this DB element at a time. Suggestions on how to solve this race condition - or suggestions of an alternative race free approach would be highly appreciated.

Joakim

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