URL Shortener

Design a public service that creates unique short links and redirects visitors to the original URL. Links expire after a configurable lifetime. Analytics must never slow the redirect path.

The dominant workload is redirect lookup, while creation establishes durable identity. Separate whether a link exists, whether it is still active and whether a click was counted; these outcomes have different correctness and latency requirements.

Begin with a unique mapping table and one API. Direct database lookup is understandable, but viral links concentrate repeated reads. Add caching without relaxing expiry checks, and move click processing outside the redirect dependency chain.

1. Create a short link from an HTTP(S) URL, with optional expiry.
2. Redirect active links; return 404 for unknown links and 410 for expired links.
3. Keep a durable mapping from short code to destination.
4. Collect click events asynchronously and handle hot links.

Workload: 10,000 reads/s, 100 writes/s, 500 B records, 100 ms read p99 target. These are exercise assumptions; the Solution includes capacity formulas.

The unique code index is the final allocation authority: checking availability before insert still races. Cache entries are derived copies, so both cached and database records must pass the same expiry check. Keep analytics admission bounded because a queue outage must not consume all redirect workers.

Suppose code Ab12 expires at 12:00. A request at 11:59:59 can redirect, but a cached copy at 12:00 returns 410. If analytics is unavailable, the redirect still succeeds and a dropped-event counter increases. Retrying a colliding creation allocates a different code without overwriting the first destination.