System Design Case Studies

The best way to internalise system design principles is to apply them. This lesson walks through four real-world system designs: a URL shortener, a chat system, a news feed, and a notification service. Each follows the framework: requirements, estimation, high-level design, component breakdown, and trade-offs.

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Case Study 1: URL Shortener

Requirements

Functional:

• Given a long URL, return a short URL (e.g. short.ly/abc123)
• When a short URL is accessed, redirect to the original URL
• Custom short links (optional)
• Link expiration

Non-Functional:

• 100M new URLs per day
• 10:1 read-to-write ratio (1B redirects per day)
• Short URLs should be as short as possible
• Low latency (< 50ms for redirection)
• High availability (cannot go down)

Back-of-the-Envelope

Write QPS = 100M / 86,400 ≈ 1,160 writes/sec
Read QPS  = 1B / 86,400   ≈ 11,600 reads/sec

Storage per URL ≈ 500 bytes (short URL + long URL + metadata)
Daily storage   = 100M × 500 = 50 GB/day
5-year storage  = 50 × 365 × 5 ≈ 91 TB

High-Level Architecture