Exercise 3: Building Automated Billing Disablement Without the Spend Cap Feature — Possible Solution
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Without using GCP's own preview spend-cap budget type, the team needs
to assemble this automated response themselves out of GCP's own real,
separate building blocks, the same way the chapter describes.

First, they configure their Cloud Billing budget to publish real
Pub/Sub notifications at the 100% threshold, rather than (or in
addition to) sending an email alert. Second, they write and deploy a
Cloud Function (covered in Chapter 7) that subscribes to that Pub/Sub
topic - when a notification arrives indicating the budget has been
reached, that function's own code runs the real, programmatic action
of disabling billing on the project. This mirrors the same real
pattern Azure Fundamentals Chapter 9 already established for Azure
Budgets, where an Action Group has to be explicitly attached to
actually do something in response - GCP's own version chains a
Pub/Sub topic to a Cloud Function instead of one native attachment
point.

ANSWER: They would need to configure their Cloud Billing budget to
publish a Pub/Sub notification at the 100% threshold, then build and
deploy a Cloud Function subscribed to that topic which programmatically
disables billing on the project when triggered - assembling the
automated response themselves from Pub/Sub and Cloud Functions, the
same way Azure's own Budgets need an explicitly attached Action Group.

WHY THIS WORKS AS AN ANSWER
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This correctly identifies the real Pub/Sub-to-Cloud-Function pattern
the chapter describes as GCP's own way of building automated budget
response manually, and draws the correct parallel to Azure's own
Action Group requirement.