Question

what would be the best way to create mechanism that ensures that for example:

Two people won’t buy same item at the same time in auction based on GAE application using BigTable?

If someone could enlighten me with “under the hood” kind of description I would be grateful.

Answer 1

Such a transaction is quite simple to implement with AppEngine. The key is the run_in_transaction method:

class AuctionItem(db.Model):
    sold = db.BooleanProperty()
    purchaser = db.UserProperty()

def buy(self, buyer):
    def buy_txn():
        sale_success = False
        if self.sold == False:
            self.sold = True
            self.purchaser = buyer
            self.put()
            sale_success = True
        return sale_success

    success = db.run_in_transaction(buy_txn, buyer)
    return success