Content delivery with CDN in HLD - Scalability & System Analysis

Scalability Analysis - Content delivery with CDN

Growth Table: Content Delivery with CDN

Scale	Users	Traffic	Origin Server Load	CDN Cache Hit Rate	Network Bandwidth	Latency
Small	100 users	Low (few requests/sec)	Low	Moderate	Low	Low
Medium	10,000 users	Moderate (hundreds req/sec)	Moderate	High (70-80%)	Moderate	Low
Large	1,000,000 users	High (tens of thousands req/sec)	High without CDN	Very High (90%+)	High	Very Low (due to edge caching)
Very Large	100,000,000 users	Very High (millions req/sec)	Origin overwhelmed without CDN	Critical to be >90%	Very High (multi Tbps)	Very Low (edge nodes worldwide)

First Bottleneck

The origin server becomes the first bottleneck as user traffic grows beyond thousands of requests per second. Without a CDN, the origin must serve all content, causing high CPU, memory, and bandwidth usage. This leads to slow responses and potential downtime.

Scaling Solutions

• Use CDN Edge Caching: Cache static content at edge servers close to users to reduce origin load and latency.
• Cache Invalidation: Implement smart cache invalidation to keep content fresh without overloading origin.
• Load Balancing: Distribute origin requests across multiple servers to handle higher load.
• Origin Scaling: Vertically scale origin servers (better CPU, RAM) or horizontally scale with multiple origins.
• Compression & Optimization: Compress content and optimize delivery to reduce bandwidth.
• Multi-CDN Strategy: Use multiple CDN providers for redundancy and global coverage at very large scale.

Back-of-Envelope Cost Analysis

• At 1M users, assuming 1 request per user per minute = ~16,667 requests/sec.
• Origin server can handle ~5,000 req/sec; CDN must serve ~11,667 req/sec to reduce origin load.
• Storage: If average content size is 500 KB, bandwidth = 500 KB * 16,667 req/sec = ~8.33 GB/sec (~66.6 Gbps).
• Network bandwidth at origin must support peak misses; CDN edges handle majority of traffic.
• Cost savings come from reduced origin bandwidth and server load due to CDN caching.

Interview Tip

Start by explaining the role of the CDN in reducing origin load and latency. Discuss how traffic grows and which components become bottlenecks. Then, describe caching strategies and scaling methods. Use numbers to justify your choices and show understanding of trade-offs.

Self Check Question

Your origin server handles 1,000 requests per second. Traffic grows 10x to 10,000 requests per second. What is your first action and why?

Key Result

A CDN reduces origin server load by caching content at edge locations, enabling the system to scale from thousands to millions of users by offloading traffic and lowering latency.