Google App Engine is a "Platform as a Service" that abstracts away the infrastructure worries and lets you focus only on code. It provides an automatically scalable platform that scales based on the amount of traffic it receives. You just need to upload your code, and it will automatically manage the availability of your app. Google App Engine is an easy and fast way to add additional processing power or trusted execution to your Firebase application.

If you have an App Engine application, you can use the built-in App Engine APIs to share data between Firebase and App Engine, since the Firebase SDKs for Cloud Storage use the Google App Engine default bucket. This is useful for performing computation intensive background processing or image operations, such as creating a thumbnail of uploaded image.

The Google App Engine standard environment provides an environment where your application runs in a sandbox, using the runtime environment of a supported language viz, Python 2.7, Java 8, Java 7, PHP 5.5, and Go 1.8, 1.6. If you have application code that needs some other version of these languages or needs some another language, you can use the Google App Engine flexible environment where your application runs on docker containers, which run on Google Cloud Virtual machines.

There are many differences between these two environments, which can be explored in the Google Cloud documentation at https://cloud.google.com/appengine/docs/the-appengine-environments.

If you want to import an existing Google Cloud Platform project into Firebase and want to make available any existing App Engine objects, you'll need to set the default access control on your objects to allow Firebase to access them by running the following command using gsutil:

gsutil -m acl ch -r -u firebase-storage@system.gserviceaccount.com:O gs://<your-cloud-storage-bucket>