This solution describes a standard, event-driven, and fully automated pattern for scaling Google Cloud resources that do not have a native autoscaling feature. A Cloud Monitoring (formerly Stackdriver) alerting policy can monitor the Cloud Spanner instance's CPU utilization. When the utilization crosses a predefined threshold (either high or low), the policy triggers a notification to a webhook. A Cloud Function is configured with an HTTP trigger to listen on this webhook's endpoint. Upon receiving the alert, the function executes code that uses the Cloud Spanner API to resize the instance by adding or removing nodes, thus achieving automatic scaling.

A. A cron job is a scheduled, time-based trigger. While it can automate scaling for predictable patterns, it is less responsive to real-time metric fluctuations compared to an event-driven alerting policy.

B. This option relies on manual intervention by an on-call SRE to scale the instance. This fails to meet the requirement for an automatic scaling solution.

C. Involving Google Cloud Support for routine operational tasks like scaling is not a standard or appropriate procedure. This process is manual and not the intended use of the support service.

1. Google Cloud Blog - Tutorial: Autoscaling Cloud Spanner: This official tutorial explicitly details the recommended architecture for autoscaling Cloud Spanner. It states

"In this architecture

Cloud Monitoring periodically checks the CPU utilization of your Spanner instance. When the CPU utilization crosses a threshold

Monitoring sends a notification to a webhook. The notification triggers a Cloud Function that adds or removes a node from your Spanner instance." This directly validates option D.

Source: Google Cloud Blog

"Autoscaling Cloud Spanner".

2. Cloud Monitoring Documentation - Notification options: The official documentation lists webhooks as a notification channel. It explains

"You can use webhooks to send a notification to other services... When you create a webhook notification channel

you provide a URL to which Monitoring sends a POST request when a notification is triggered." This confirms the trigger mechanism.

Source: Google Cloud Documentation

Cloud Monitoring

"Notification options"

Section: "Webhooks".

3. Cloud Spanner Documentation - Monitoring with Cloud Monitoring: The documentation highlights high-priority CPU utilization as a key metric for monitoring Spanner performance and determining when to scale. It recommends keeping CPU utilization below 65% for multi-region instances and 75% for regional instances to ensure optimal performance. This validates using CPU metrics as the basis for scaling decisions.

Source: Google Cloud Documentation

Cloud Spanner

"Monitoring with Cloud Monitoring"

Section: "Alerting".

4. Cloud Functions Documentation - HTTP Triggers: This documentation explains how to create functions that can be invoked via HTTP requests

which is precisely how a webhook integration works. "HTTP functions can be invoked from standard HTTP requests. These functions must send a response."

Source: Google Cloud Documentation

Cloud Functions

"Develop"

"HTTP Triggers".