You want to receive a message on your mobile device each time your Database Administrator logs into a Firebase database, while minimizing effort. What GCP service will best achieve this?

Creating a Cloud Function triggered by Firebase Authentication events is the most efficient solution. Firebase Authentication events, such as a user logging in, can directly trigger a Cloud Function. This function can then use a service like Firebase Cloud Messaging (FCM) to send a notification to your mobile device. This approach is serverless, scales automatically, and requires minimal operational overhead. Implementing mobile notifications using Compute Engine would involve managing a server and manually integrating notification services, which is more effort. Using Kubernetes Engine with Firebase Admin SDK is also overkill, introducing unnecessary complexity and management overhead for a simple notification task. Utilizing App Engine along with Pub/Sub for notifications is possible but less direct than a Cloud Function for this specific trigger. Configuring alerts in Stackdriver Monitoring is primarily for operational metrics and logs, not for real-time, event-driven notifications from Firebase Authentication.