Question

Let’s say I want to create 2 new objects.
I want to do it in a transaction because I want to be sure that either both exist or none.

But I also need a big calculation so my question is where is best fitted for the calculation to be done. Inside the transaction scope? Or outside? (The key of a1 is irrelevant to the calculation so it does not need to be saved first)

@db.transactional()
def _register(self):
 a1.put()
 result = big_calculation_like_hashing(a1)
 a2.prop = result
 a2.put()

Answer 1

As always it depends. Since the calculation relies on a2 being complete it is probably best to be performed after the a2 object is created, outside of the transaction. Unless of course these are all shared objects in some fashion, if that is the case you will need to do this within the context of the transaction to prevent data corruption.