In this lesson, we will cover topics including Datastore Query and Transaction. The Query is one of the most challenging parts on learning App Engine technologies. You will learn what are rules and practices of the Query, and why they are important to enable the high scalability of the service. Then we will look at some workarounds to cope with the restrictions of the Query. In the latter part of the lesson, we will cover Transaction of Datastore. Especially, understanding the Eventual Consistency of the Query is very important to designing consistent and reliable App Engine application. We will examine how Entity Group and Ancestor Query will ensure Strong Consistency of your application.