In Chapter 1, Getting Started, we provided the definition of a web application, and one chapter after another, you learned to implement an application using App Engine. At this point, we know enough about the anatomy of such kinds of software to understand the differences between the backend and frontend components of a web application: the first provides logic, definition, and access to data, and the latter provides the user interface.

We did not make a clear distinction between these two components in the previous chapters, and the code we wrote so far provided both the frontend and the backend components without too much separation. In this chapter, we will tear apart the frontend component of our Notes application, implementing a standalone backend server ready to exchange data with different clients, from mobile applications to rich JavaScript clients running in a browser.

Once again, to implement our application, we...