There are cases where you'd want a number of operations to succeed or fail altogether and nothing in between. This is called a transaction, and App Engine's datastore offers support for transactions. In App Engine, you do this by encapsulating all the steps that you want to fail or succeed together in a function and decorate it with the ndb.transactional decorator.

Let's suppose that it so happens that we add a new property called city to indicate the city that the car is in. Sometimes, two cars are exchanged between two dealers in two difference cities. In that case, we need to update the cities of these two listings and basically swap them. There are multiple datastore operations involved here, given that we have keys for both the listings. First, we read both the listings from datastore, then modify the cities, and finally, we write them back. All of these operations shall either fail altogether, or succeed altogether. This is how we will do this by wrapping the function in...