How Do Pull vs Push CDN Models and Configuration Propagation Trade Offs Impact Production Systems?

Pull CDN Architecture
In pull-based CDN architecture, edge nodes fetch content from origin on demand. When a user requests content not in cache (cache miss), the edge node retrieves it from origin, caches it, and serves the response. This is also called lazy loading or on-demand caching. Pull CDNs require zero proactive origin configuration. The tradeoff: first requests experience origin latency (100-300ms additional), but subsequent requests serve from cache (10-50ms). This model works well for unpredictable access patterns where preloading would waste bandwidth.
Push CDN Architecture
Push-based CDNs proactively distribute content to edge nodes before users request it. Origins push updates via APIs or event-driven mechanisms. When content changes, the origin publishes to a message queue (a system that buffers messages between producers and consumers, enabling asynchronous communication). Edge nodes subscribe to relevant content channels and pull updates. This eliminates cache miss latency entirely. The tradeoff: requires infrastructure to track what content exists at which edges, consumes bandwidth for content that may never be requested, and adds complexity for invalidation. Push works best for predictable, high-traffic content like video streaming catalogs or news homepages.
Hybrid Strategies
Production systems combine both approaches. Tiered caching uses push for top-tier PoPs (Points of Presence, CDN server locations) in major metros and pull for smaller edge locations. Predictive prefetching analyzes traffic patterns to push content likely to be requested: if 80% of users requesting video A also request video B, push B when A is requested. Origin shield (a mid-tier cache layer between origin and edges) uses push from origin to shield and pull from shield to edges, reducing origin load by 90%+ while maintaining cache freshness.
Decision Framework
Choose pull when: Traffic is unpredictable, content catalog is large (millions of items), cold start latency is acceptable. Choose push when: Content set is bounded, updates are infrequent but must propagate immediately, zero cache miss latency is critical. Choose hybrid when: You have a long-tail distribution where 10% of content gets 90% of traffic. Push the popular 10%, pull the rest. Monitor cache hit ratios: below 85% suggests either TTL misconfiguration or need for more aggressive prefetching.
Key Trade-off: Pull optimizes for storage efficiency (only cache what is requested). Push optimizes for latency (zero cache misses). Hybrid balances both by pushing popular content while pulling long-tail items on demand.

💡 Key Takeaways

✓Pull CDN fetches on cache miss adding 100-300ms first-request latency; works for unpredictable traffic

✓Push CDN proactively distributes content eliminating cache miss latency; suits predictable high-traffic content

✓Hybrid strategies use tiered caching: push to major PoPs, pull to smaller edges, predictive prefetch

✓Decision framework: pull for long-tail, push for bounded catalog, hybrid for 10/90 traffic distribution

📌 Interview Tips

1Explain the first-request latency penalty of pull CDN (100-300ms) vs subsequent cached requests (10-50ms)

2Describe origin shield as hybrid approach: push origin-to-shield, pull shield-to-edge, reduces origin load 90%+

3Mention cache hit ratio target of 85%+ as indicator of CDN health and configuration correctness

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