How to Design a URL Shortener Like Bitly or TinyURL

Security and Best Practices

Don’t let your shortener become a hacker’s playground.

Authentication and Rate Limiting

• API keys for users.
• Limit requests per IP/key to prevent DDoS.
• Scan for malicious URLs.

Common Pitfalls to Avoid

• Random Slugs: Always customize for readability (Short.io warns against this).
• Dependency Risks: Own your domain to avoid third-party failures.
• SEO Impact: Use 301 redirects for link juice.
• Privacy Issues: Anonymize analytics data.

Expert quote: “URL shorteners can threaten user privacy if not handled carefully,” per UK government communications guidelines.

Analytics and Monitoring

Turn data into insights.

• Track via 302 redirects.
• Use Kafka for async processing.
• Metrics: Clicks, geolocation, devices.

Bitly’s dashboard exemplifies this, showing trends that drive business decisions.

Common Interview Questions on URL Shortener Design

Since this is a staple system design interview question at companies like Google and Amazon, here are 30 real questions often asked, with in-depth answers based on actual experiences from LeetCode, GeeksforGeeks, and interview prep sites. These draw from FAANG-style sessions where candidates must think aloud.

1. What are the core functional requirements for a URL shortener? Answer: Generate unique short aliases, redirect to originals, support customs/expirations, and provide analytics. Focus on uniqueness to avoid conflicts.
2. How would you handle non-functional requirements like scalability? Answer: Use sharded NoSQL, caching, and load balancers. Estimate: 200 writes/sec scales with horizontal additions.
3. Explain Base62 encoding for short keys. Answer: Maps numbers to 62 chars. Code: Divide by 62, append remainders. 7 chars handle trillions.
4. How do you resolve hash collisions in MD5-based shortening? Answer: Append a sequence or retry with salt. Check DB atomically.
5. Why choose NoSQL over SQL for the database? Answer: Better for key-value, scales easily. SQL for ACID if needed, but rare here.
6. Describe the role of caching. Answer: Redis for hot keys, LRU eviction. Reduces DB hits by 80%.
7. How to implement load balancing? Answer: Round Robin at multiple layers. Intelligent for load-aware routing.
8. What’s a Key Generation Service (KGS)? Answer: Pre-generates keys offline, stores in DB. Prevents runtime duplicates.
9. How to handle URL expiration? Answer: Lazy deletion on access; periodic cleanup job.
10. Design the API endpoints. Answer: POST /shorten {url}, returns short. GET /{alias} redirects.
11. Calculate storage needs for 100M URLs/month. Answer: ~500 bytes/entry → 50GB/month, 3TB/5 years.
12. How to track analytics without slowing redirects? Answer: Async via Kafka; store in separate DB.
13. What HTTP code for redirects? Why 302 over 301? Answer: 302 for temporary, allows click tracking each time.
14. Prevent abuse like spam links. Answer: Rate limits, blacklists, CAPTCHA for high-volume users.
15. Handle custom aliases. Answer: Separate DB check; limit length to 16 chars.
16. What if the KGS fails? Answer: Standby replica; sync via Zookeeper.
17. Pros/cons of counter vs. hash encoding. Answer: Counter guarantees uniqueness but needs coordination; hash is simple but collides.
18. Implement data partitioning. Answer: Hash short key for shards; consistent hashing for rebalancing.
19. Cache eviction policy? Answer: LRU for recency; size based on 80/20 rule.
20. How to make keys non-predictable? Answer: Random generation or hash salting.
21. Support for private URLs? Answer: Add permission table; check user ID on redirect.
22. Estimate memory for caching. Answer: 170GB for 20% of daily traffic at 500M/month.
23. Handle high read/write ratios. Answer: Cache reads; shard writes.
24. What about link reuse after expiration? Answer: Return to key pool after cleanup.
25. Database schema details. Answer: ShortKey (PK), LongURL, Expiration, Clicks, UserID.
26. How many characters for short URLs? Answer: 7 in Base62 = 3.5T possibilities; adjust for needs.
27. Integrate security like API keys. Answer: Authenticate requests; quota per key.
28. Follow-up: Distributed counter with Zookeeper. Answer: Zookeeper assigns ranges; servers increment locally.
29. Replication strategy. Answer: Master-slave; eventual consistency OK.
30. Edge case: Same URL shortened multiple times. Answer: Deduplicate with hashing; return same short.

These questions test depth—practice explaining trade-offs.

Conclusion

Designing a URL shortener like Bitly or TinyURL is more than code; it’s about building reliable, scalable systems that solve real problems. From encoding tricks to security safeguards, you’ve now got a solid foundation. Ready to build your own? Start small, test rigorously, and scale up. If this sparked your interest, explore our courses for more hands-on learning. What’s your next project—share in the comments!

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