Django’s post_save
methods allow execution of custom code on an insert or update of a record after the save
method is called.
The serialized object propagates through the signal handlers as the instance
parameter which is eventually committed to the database only if no exceptions occur in any of the methods that listen to the signal.
Handlers:
can be multiple which are synchronously chained
will executed non-deterministically as per the signal’s receiver registration
will execute prior to a transaction commit
will short-circuit execution on failure
Data inconsistencies are caused where proceeding methods in the post_save
chain aren’t called after the one that throws an unhandled exception. The database record is not updated.
Additionally, the instance
parameter has the most up-to date state which on an update, will revise the stale content that resides in the DB.
To avoid issues:
Use transaction on_commit to guarantee the data consistency
Use the
instance
argument over re-reading it from the DB in handlers
References