REGION_ID is a code that Google assigns based on
the region you select when you create your app. Including
REGION_ID.r in App Engine URLs is optional for
existing apps and will soon be required for all new apps.
To ensure a smooth transition, we are slowly updating App Engine to use region IDs. If we haven't updated your Google Cloud project yet, you won't see a region ID for your app. Since the ID is optional for existing apps, you don't need to update URLs or make other changes once the region ID is available for your existing apps.
Learn more about region IDs.
You can use various methods to communicate between your App Engine services or with other services, including Google Cloud services and external applications.
The simplest approach for communicating with your App Engine service is to send targeted HTTP requests, where the URL includes the name or ID of a resource. For example, you can include the ID of a service or version that you want to target, in addition to the corresponding Cloud project ID:
For details about targeting your services, including how to define HTTPS URLs, see How Requests are Routed. To authorize the requests between your services and other Google Cloud services, see Setting Up Authentication for Server to Server Production Applications.
To share data across databases and your App Engine app or some other external application, see Understanding Data and File Storage.