Storing data in a scalable web application can be tricky. A user could be interacting with any of dozens of web servers at a given time, and the user's next request could go to a different web server than the one that handled the previous request. All web servers need to be interacting with data that is also spread out across dozens of machines, possibly in different locations around the world.
Thanks to Google App Engine, you don't have to worry about any of that. App Engine's infrastructure takes care of all of the distribution, replication and load balancing of data behind a simple API—and you get a powerful query engine and transactions as well.
The App Engine datastore has a sophisticated query engine for data models. Because the App Engine datastore is not a traditional relational database, queries are not specified using SQL. Instead, you can prepare queries using a SQL-like query language we call GQL. GQL provides access to the App Engine datastore query engine's features using a familiar syntax.
转载于:https://www.cnblogs.com/perfectdesign/archive/2008/06/29/1232065.html