On Tue, May 12, 2009 at 02:25:41AM -0700, Daniel Roseman wrote: > No, [get_or_create is] not atomic. You can see the code in > django.db.models.query - it tries a db lookup, and then creates > a new object if one is not found.
It seems that this creates a potentially-troublesome race condition. Wouldn't the object creation fail if the object is created by some other process between the time of the query and the time of the attempt at creation? Shouldn't any operation that relies upon the results of a database query require that a read lock (i.e., a shared or non-exclusive lock) be placed before that initial query and held until the operation has successfully completed? This is rather disturbing. Are there other instances of Django's ORM doing things that are not safe for concurrent database access? It seems that this would be a serious hindrance to scalability. I see some prior discussion [1] of this issue, but the only rationale for the present behavior I see in that discussion is James Bennett's assertion that in order to avoid deadlock, a framework should not automatically lock a table. There's no discussion of DBMS's that perform row-level locking, or of relying upon the DBMS's ability to detect deadlock and proceed in an appropriate manner. It seems risky to try to handle at the application level these things that a DBMS is specifically designed to do. Earlier in the aforementioned thread, Thomas Steinacher quoted Django documentation [2] as stating: > It works like this: When a request starts, Django starts a > transaction. If the response is produced without problems, > Django commits any pending transactions. If the view function > produces an exception, Django rolls back any pending > transactions. Does this not result in get_or_create operating atomically? References: [1]: <http://groups.google.com/group/django-developers/browse_thread/thread/3c8a3a0d431dc4a6/204f0f7953bda2f2> [2]: <http://docs.djangoproject.com/en/1.0/topics/db/transactions/> -- Phil Mocek --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to django-users+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
