Strong consistency in GAE Datastore

Question

I'm writing GAE/Python app and need strong consistency. I've read about Model parent/ancestor queries but still doesn't understand.

Let's say I have User model. And also I have something like Item model. User may possess few items. Will it be enough to set every item parent as that User? I mean for sake of strong consistency for each kind of item operation (add/remove/update).

What if user have money and every item addition costs some? I need in strong consistent transaction deduce money from that user and add item to him. So here I need to make Wallet model and attach it to each User as its child? Because as far as I understand update of User field will not be strong consistent where update of Wallet model instance with that User as parent will be, am I right?

How I can check that queries indeed strong consistent on developing machine/GAE?

Faisal Faisal · Accepted Answer · 2013-07-25T14:37:04

You don't need to have the kinds in same group anymore with ndb but I'm not really sure with old db if it ever supported the cross group transactions. But in case it didn't yes you need to have these parent/ancestor in your entities. Then use transactional methods remember you need to do queries from multiple models outside of this method:

For old db:

@db.transactional
def get_item(user, item):
    user.price -= item.price
    item.user = user.key
    user.put()
    item.put()

For ndb and cross group transactions meaning no need to be parent to do transactions:

@ndb.transactional(xg=True)
def get_item(user, item):
    user.price -= item.price
    item.user = user.key
    user.put()
    item.put()

Strong consistency in GAE Datastore

2 Answers