You can use various methods to communicate between your App Engine services or with other services, including GCP 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 GCP 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 GCP 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. In the standard environment, you can also pass requests between services and from services to external endpoints using the URL Fetch API.
Additionally, services in the standard environment that reside within the same GCP project can also use one of the App Engine APIs for the following tasks: