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Cloud CDN: ACE Exam Study Guide (2026)

Cloud CDN

Image source: Google Cloud Documentation

1. Cloud CDN Overview

Cloud CDN (Content Delivery Network) uses Google’s global edge network to serve content closer to users, which reduces latency and lowers serving costs.

Key Characteristics

  • Edge Caching: Content is stored in Google’s Edge Points of Presence (PoPs) globally.
  • Integration: Works exclusively with the Global External Application Load Balancer (HTTP/S).
  • Origin Servers: Backends can be Instance Groups (VMs), Cloud Storage buckets, or external origins.
  • Media CDN: For large-scale streaming, use Media CDN, which is built on the same infrastructure as YouTube.

2. Core Features

  • Anycast IP: Uses a single, global IP address for the load balancer.
  • Cache Keys: Defines what makes a request unique for caching purposes (e.g., URL, query parameters, headers).
  • Signed URLs/Cookies: Used to serve private content only to authorized users (e.g., premium video or paid downloads).
  • Cache Invalidation: Allows you to manually remove content from the cache before its TTL (Time To Live) expires.
  • Dynamic Compression: Automatically compresses text-based responses (Gzip/Brotli) to save bandwidth.

3. Caching Behavior

  • TTL (Time To Live): Defines how long content stays in the cache.
    • Default TTL: 3,600 seconds (1 hour).
    • Maximum TTL: 31,536,000 seconds (1 year).
  • Cache Modes:
    • USE_ORIGIN_HEADERS: Respects Cache-Control headers from the backend.
    • CACHE_ALL_STATIC: Caches all static content regardless of headers.
    • FORCE_CACHE_ALL: Unconditionally caches all responses (use with caution).

4. Administrative Operations

  • Enabling CDN: You enable Cloud CDN on a per-backend-service basis within a Global External Application Load Balancer.
  • Invalidation: gcloud compute url-maps invalidate-cdn-cache [MAP_NAME] --path "/*"
    • Exam Tip: Cache invalidations are global but can take several minutes to propagate.

5. Security

  • SSL/TLS: Handled at the Load Balancer level (SSL termination).
  • Cloud Armor integration: You can use Cloud Armor security policies (WAF, Geo-blocking) in front of your CDN-enabled backends.

6. Essential gcloud Commands

  • Enable CDN on existing backend: gcloud compute backend-services update [BACKEND_NAME] --enable-cdn --global
  • Invalidate a specific path: gcloud compute url-maps invalidate-cdn-cache [MAP_NAME] --path "/images/*"
  • Describe backend CDN settings: gcloud compute backend-services describe [BACKEND_NAME] --global

7. Exam Tips

  • Load Balancer Requirement: It requires a Global External Application Load Balancer (no other Load balancer can use Cloud CDN).
  • Static vs. Dynamic: CDN is primarily for static content (images, CSS, JS). While it can cache dynamic content, it is less common.
  • Cost Savings: Cloud CDN reduces “egress” costs because traffic from cache to user is cheaper than traffic from origin to user.
  • Cloud Storage: When using Cloud Storage as a backend, ensure the bucket or files have public access (unless using Signed URLs).