Can you design the architecture of a TinyURL service?

What the interviewer wants to learn

  • Whether you can break a large problem into smaller components.
  • Your understanding of scalability, latency, data consistency, and fault‑tolerance.
  • How you choose storage, caching, and load‑balancing strategies.
  • Your ability to justify trade‑offs (e.g., read‑heavy vs. write‑heavy, latency vs. cost).

How to structure your answer

1. **Clarify requirements** – functional (shorten URL, redirect, analytics) and non‑functional (high availability, low latency, scalability).

2. **Outline high‑level components** – API gateway, URL‑shortening service, database, cache, redirect service, monitoring.

3. **Dive into each component** – data model, sharding/partitioning, read/write path, cache invalidation.

4. **Discuss scaling** – horizontal scaling, load balancers, CDN for redirects.

5. **Address reliability** – replication, backups, graceful degradation.

6. **Mention security & ops** – rate limiting, abuse detection, logging, health checks.

7. **Wrap up with trade‑offs** – consistency vs. latency, cost of storage vs. speed.

Example answer

“Sure. First, let’s clarify the core requirements: we need an API that accepts a long URL and returns a short alias, and a redirect endpoint that takes the alias and forwards the user. The service must handle billions of URLs, support very fast redirects (sub‑10 ms), and stay available even if a data center goes down.

At a high level we have three layers: a front‑end load balancer, a URL‑shortening service, and a storage layer. The client hits our load balancer, which routes to stateless shortener instances. When creating a short URL we generate a unique key—either a sequential ID encoded in base‑62 or a hash with collision handling—and write a mapping {key → long‑URL} to a sharded NoSQL store like DynamoDB. The write is also pushed to a write‑through cache (Redis) so that subsequent reads hit the cache.

For redirects, the request again goes through the load balancer to a lightweight redirect service. It first checks Redis; a cache hit returns the long URL instantly, otherwise it reads from DynamoDB, updates the cache, and issues a 301 redirect. To keep latency low globally we place edge CDN nodes that cache the key‑to‑URL mapping.

We ensure high availability by replicating data across three AZs, using automatic failover for the cache, and deploying the services in multiple regions with DNS‑based traffic routing. Monitoring, rate limiting, and abuse detection are added to protect against spam and DDoS attacks. The main trade‑off is that we favor eventual consistency for writes to keep the redirect path fast, which is acceptable because a short URL being temporarily unavailable is rare and can be retried.”

Mistakes to avoid

  • **Skipping requirement clarification** – jumping straight into architecture can miss critical constraints.
  • **Over‑engineering** – proposing overly complex components (e.g., message queues) when a simple design suffices.
  • **Ignoring scaling details** – no discussion of sharding, load balancing, or CDN for the redirect path.
  • **Neglecting reliability** – forgetting replication, backups, or graceful degradation.
  • **Being vague about trade‑offs** – not explaining why you chose eventual consistency, base‑62 encoding, etc.
  • **Using too much code** – for system design, a short pseudo‑code snippet is fine, but the focus should stay on architecture, not implementation details.

More system design questions