System Design Use Cases
URL Shortener

URL Shortener

You are asked to design a URL shortening service like TinyURL or Bitly. A user submits a long URL such as https://example.com/very/long/path?with=query&params=here and receives a short link like https://sho.rt/aB3xZ9. When anyone visits the short link, they are redirected to the original URL.

The service looks trivial — “just store a key and a value” — but the interesting part is scale: a popular shortener serves billions of redirects per day with a read:write ratio in the thousands, must keep redirect latency in the low tens of milliseconds globally, and must generate short, collision-free keys across a fleet of servers without coordinating on every request.

Functional Requirements

  1. Shorten: Given a long URL, return a unique short URL. The short key should be as short as practical (7 characters).
  2. Redirect: Given a short URL, redirect the client to the original long URL.
  3. Custom alias: Optionally let the user pick a custom key (e.g. sho.rt/my-brand).
  4. Expiration: Optionally attach a TTL; expired links stop resolving.
  5. Analytics (basic): Count clicks per short URL (approximate is acceptable).

Out of Scope

  • User accounts, authentication, and billing (assume an upstream API-key gateway handles identity).
  • Rich analytics dashboards, geographic/referrer breakdowns beyond a click counter.
  • Malware/phishing URL scanning (would be a separate pipeline).
  • Link editing after creation (short keys are immutable).

Non-Functional Requirements

  • Scale: 100M new URLs/month; 10B redirects/day. Read:write ≈ 3000:1 (extremely read-heavy).
  • Latency: Redirect p99 < 50 ms end-to-end; shorten p99 < 200 ms.
  • Availability: 99.9% for redirects (a dead redirect breaks every embedded link). Creation can tolerate slightly lower availability.
  • Consistency: Redirects may be eventually consistent — a newly created link resolving a few hundred ms later is fine. Key uniqueness must be strongly guaranteed (never hand out the same key twice).
  • Durability: Links are effectively permanent (default 5-year retention); losing the mapping is unacceptable.
  • Scalability of keyspace: 7 base62 characters = 62⁷ ≈ 3.5 trillion keys, comfortably above 100M/month × many years.