System requirements

Functional:

• Create short URL from long URL
• Redirect short URL to original URL
• Support optional expiration (default: 1 year)
• Optional custom alias (nice to have)

Non-Functional:

• High read throughput (read-heavy system)
• Low latency redirects (<50ms)
• High availability (no single point of failure)
• Horizontally scalable
• Globally available (multi-region)

Estimations:

• 100K users × 10 URLs/day = 1M writes/day (~12 writes/sec)
• Reads ≈ 1000/sec
• Keyspace: Base62 → 62⁶ ≈ 56.8 billion combinations (no need for recycling)

API Design

POST /create Flow:

1. Receive long_url
2. Generate unique ID (Snowflake)
3. Convert ID → Base62 → url_key
4. Store mapping in DB
5. Cache in Redis
6. Return short URL

GET /url_key Flow:

1. Check cache
2. If hit → redirect to long_url
3. If miss → query DB
4. Cache result
5. Check expiration
6. Redirect to long_url or return error if expired

High-Level Architecture

Client → GeoDNS → Nearest Region

Each region contains:

• Load Balancer
• Stateless App Servers
• Redis (cache)
• Sharded Database (primary + replicas)
• Distributed ID Generator (Snowflake)

Detailed component design

Key Generation (Core Decision)

• Use Snowflake ID generator for globally unique IDs
• Convert ID to short key using Base62 encoding

Why:

• No collisions
• No race conditions
• No need for KGS or coordinator
• Works across multiple regions

Database Design

Table: url_mapping

• url_key (Primary Key)
• long_url
• created_at
• expiration_date
• user_id (optional)

Sharding:

• Partition by hash(url_key)

Replication:

• Primary + read replicas per region
• Async replication across regions

Caching Strategy

• Cache hot URLs in Redis
• Key → url_key, Value → long_url
• Use LRU eviction
• Store millions of entries to reduce DB load

Multi-Region Design

• GeoDNS routes user to nearest region
• Each region handles reads + writes independently
• IDs generated locally using Snowflake

Replication:

• Asynchronous between regions

Consistency:

• Eventual consistency (acceptable for URL shortener)

Expiration & Cleanup

• Store expiration_date in DB
• On read: check expiry → reject if expired
• Background job deletes expired entries
• No need to recycle keys (huge keyspace)

Scaling & Optimizations

• Add DB shards for write scaling
• Use read replicas for read-heavy traffic
• Use CDN like Cloudflare to cache redirects at edge
• Increase cache size to reduce DB hits

Tradeoffs

• Sequential Base62 keys → predictable URLs
• → Can add randomness if needed
• Eventual consistency across regions
• → Slight delay in propagation but acceptable

Final Summary

• Use Snowflake + Base62 for key generation (no KGS)
• Use Redis cache for fast reads
• Use sharded DB + replicas for scalability
• Use GeoDNS + multi-region deployment
• Use async replication for global availability