How would you design a URL shortener?

Clarify the requirements first. Assume billions of links, a read-to-write ratio around 100:1, redirects with p99 under 50 milliseconds, links that may expire, and optional click analytics. Out of scope: full content hosting or a social feed.

API and data model

• POST /urls with a long URL returns a short code and the full short URL
• GET /{code} returns a 301 or 302 redirect to the long URL
• The primary table is code to long URL, owner, created time and expiry
• Analytics are written asynchronously so the redirect path stays fast

function encodeBase62(id) {
  const alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
  let value = id;
  let out = "";
  while (value > 0) {
    out = alphabet[value % 62] + out;
    value = Math.floor(value / 62);
  }
  return out;
}

For key generation, a distributed counter or a time-ordered id such as Snowflake avoids collisions without a random read. A random key needs a uniqueness check and a retry loop. Base62 encoding of a numeric id is compact and predictable, but sequential ids leak volume and enable enumeration, so add a random salt or use a non-sequential id when that matters.

The mapping store is a partitioned key-value database keyed by code, with replication for availability. Cache the hot codes in Redis and at the CDN edge, but do not cache a 404 for long because a key may be created later. Use 302 when you need to count every click or change the destination, and 301 when permanent caching is acceptable.