CDN concept and usage in HLD - Scalability & System Analysis

Scalability Analysis - CDN concept and usage

Growth Table: CDN Usage at Different Scales

Users	Traffic Pattern	Latency	Origin Server Load	Cache Hit Rate	Bandwidth Usage
100 users	Low, local requests	Low, direct origin access	Low	Minimal caching needed	Low
10,000 users	Moderate, regional spread	Improved with CDN edge nodes	Moderate	Cache hit ~30-50%	Moderate
1,000,000 users	High, global distribution	Significantly reduced latency	High without CDN, reduced with CDN	Cache hit ~70-90%	High, but origin bandwidth offloaded
100,000,000 users	Very high, global and peak loads	Low latency worldwide	Origin server offloaded heavily	Cache hit >90%	Very high, mostly edge bandwidth

First Bottleneck: Origin Server Bandwidth and Latency

As user count grows beyond 10,000, the origin server bandwidth and response time become the first bottleneck. Without a CDN, the origin must serve all requests, causing high latency and potential overload. Network congestion and geographic distance increase delays for users far from the origin.

Scaling Solutions Using CDN

• Edge Caching: Store static content (images, scripts, videos) on CDN edge servers close to users to reduce origin load and latency.
• Geographic Distribution: Use multiple CDN PoPs (Points of Presence) worldwide to serve users locally.
• Cache Invalidation: Implement smart cache refresh strategies to keep content fresh without overloading origin.
• Load Balancing: Distribute requests among CDN nodes and origin servers to avoid hotspots.
• Compression and Optimization: Use gzip, Brotli, and image optimization at CDN edges to reduce bandwidth.
• SSL Termination: Offload SSL/TLS processing to CDN to reduce origin CPU usage.

Back-of-Envelope Cost Analysis

Assuming 1 million users each making 1 request per second:

• Requests per second (RPS): 1,000,000
• Origin server without CDN: must handle 1M RPS, likely impossible on a single server.
• With CDN cache hit rate 80%, origin handles 200,000 RPS.
• Bandwidth saved at origin: 80% reduction.
• CDN edge bandwidth: high, but distributed globally.
• Storage at CDN edges: depends on cached content size, typically tens of TBs.
• Network bandwidth: 1 Gbps server handles ~125 MB/s; origin needs multiple servers or CDN to scale.

Interview Tip: Structuring CDN Scalability Discussion

Start by explaining the problem of origin server overload and latency. Then describe how CDN edge caching reduces load and improves user experience. Discuss cache hit rates and geographic distribution. Finally, mention cache invalidation and SSL offloading as optimizations. Use numbers to show impact on requests and bandwidth.

Self Check Question

Your origin server handles 1,000 requests per second. Traffic grows 10x to 10,000 RPS. What do you do first and why?

Answer: Deploy a CDN to cache static content at edge servers. This reduces origin load by serving most requests from cache, lowering bandwidth and latency before scaling origin servers.

Key Result

A CDN reduces origin server load and latency by caching content at edge locations, enabling systems to scale from thousands to hundreds of millions of users efficiently.