Summary

Added functionality to override FCM root URL via Env Variable so that before constructing the FirebaseApp object we can set the Fcm root url in the environment variable. This will help in changing the base URL to localhost while writing FTs(Functional Tests)/Integration Tests.

Global Variable: GOOGLE_FCM_HOST

Open Issue: #802

How setting the base url to localhost help with your functional tests?
While writing functional tests/Integration tests, we do mock all the external services and create mock servers which behaves the same as external service. So here in case of sending push notifications, we are using Firebase admin SDK and to write tests for this use-case, we would need to mock the FCM server with a mock server created in tests.

For example
image

Here, Suppose we are writing tests for Service 1. It exposes a GET API to get some data. The data contains multiple attributes few attributes from DB, a few from the cache, and a few from Service 2(HTTP API).

While setting up the tests we will create a DB, cache server, and a mock HTTP server. Once the server is up, we want Service 1 to point to these servers instead of actual hosts. Since the DB, cache, and mock HTTP server are running in the same machine on different ports. We would want to set the Host to localhost with respective ports.

Now in the case of Firebase admin SDK, the host is hardcoded in the library. Hence, we are not able to override the Host to localhost.