To build an App Engine application, you write code for one or more request handlers, and describe to App Engine which requests go to which handlers, using configuration. The life of a request handler begins when a single request arrives, and ends when the handler has done the necessary work and calculated the response.
App Engine does all the heavy lifting of accepting incoming TCP/IP connections, reading HTTP request data, ensuring that an instance of your app is running on an application server, routing the request to an available instance, calling the appropriate request handler code in your app, and collecting the response from the handler and sending it back over the connection to the client.
The system that manages and routes requests is known generally as the frontend. You can configure the frontend to handle different requests in different ways. For instance, you can tell the frontend to route requests for some URLs to App Engine’s static file servers instead of the application servers, for efficient delivery of your app’s images, ...