Automated Billing Circuit Breaker Architecture

This document details the recommended architecture for implementing an absolute hard dollar spend cap in Google Cloud Platform using Cloud Billing Budgets, Pub/Sub, and Cloud Functions / Cloud Run.

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1. The Architectural Problem: Alerts vs. Caps

In GCP, standard Billing Budget alerts only send notifications (emails or Pub/Sub messages). GCP does not automatically stop services when a budget threshold is exceeded.

To enforce an absolute hard cap (e.g., stopping all workloads when monthly spend reaches $100), an automated circuit breaker must be deployed.

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2. Circuit Breaker Architecture Overview

[ GCP Billing Engine ]
         │
         ▼ (Publishes spend JSON every ~15-30 mins)
[ Cloud Pub/Sub Topic: billing-alerts ]
         │
         ▼
[ Cloud Function: billing-circuit-breaker ]
         │
         ├─ Spend >= 100% of Budget?
         │
         ├─ YES ──► Unlink Project from Billing OR Disable Costly APIs
         │          (gcloud beta billing projects unlink <PROJECT_ID>)
         │
         └─ NO  ──► Log current spend & take no action

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3. Deployment Steps

Step 1: Create a Pub/Sub Topic

gcloud pubsub topics create billing-alerts --project=<ADMIN_PROJECT_ID>

Step 2: Attach Billing Budget to Pub/Sub

gcloud billing budgets create \
  --billing-account=<BILLING_ACCOUNT_ID> \
  --display-name="Automated $100 Hard Cap" \
  --budget-amount=100USD \
  --threshold-rule=percent=1.0 \
  --notifications-rule-pubsub-topic=projects/<ADMIN_PROJECT_ID>/topics/billing-alerts

Step 3: Deploy Automated Circuit Breaker Function (Node.js / Python)

import base64
import json
from googleapiclient import discovery

def stop_billing_on_budget_exceeded(event, context):
    pubsub_data = base64.b64decode(event['data']).decode('utf-8')
    data = json.loads(pubsub_data)

    cost_amount = data.get('costAmount')
    budget_amount = data.get('budgetAmount')
    project_id = "<TARGET_PROJECT_ID>"

    if cost_amount is not None and budget_amount is not None:
        if cost_amount >= budget_amount:
            print(f"CRITICAL: Budget exceeded (${cost_amount} >= ${budget_amount}). Disabling billing for {project_id}...")
            billing = discovery.build('cloudbilling', 'v1')
            billing.projects().updateBillingInfo(
                name=f"projects/{project_id}",
                body={'billingAccountName': ''}
            ).execute()
            print(f"Billing successfully unlinked for {project_id}.")

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4. Operational Considerations

• Propagation Delay: GCP Billing telemetry publishes every 15–30 minutes. High-velocity bursts during that window may slightly exceed the threshold before disablement occurs.
• Service Disruption: Unlinking billing will immediately pause paid API calls and Compute Engine VMs. Re-linking billing instantly restores operations once approved.