Design: URL Shortener

Started learning system design and architecture seriously. I’ll be posting daily notes on whatever i learn about the subject going forward. Here’s the first one–enjoy!

1. The Core: Base62 Encoding

To keep URLs short, we use Base62 encoding ([a-z, A-Z, 0-9]).

• A 7-character string gives us 3.5 Trillion combinations.
• Why not Base64? Base64 includes + and /, which are not URL-safe and can break paths.

2. The Coordination Problem: Key Generation Service (KGS)

In a distributed system, we must prevent “collisions” where two servers generate the same ID.

• The “Bucket” Strategy: We use Apache ZooKeeper to hand out “ranges” of IDs (e.g., Server A gets 1-1000).
• RAM over Disk: These IDs are kept in memory. Even if a server crashes and we lose a few IDs, it’s a “safe waste” that allows the system to remain lightning fast.

3. High Availability vs. Consistency (CAP Theorem)

• Read-Heavy Ratio: 100:1 (Read:Write).
• The Fix: NoSQL (Cassandra/DynamoDB) for scaling and Redis for caching “Hot URLs.”
• Quorum Math ($W+R>N$): We ensure consistency by writing to a majority and reading from a majority.

Thoughts on replicas and waste of resources

In System Design, we often intentionally waste resources (like losing a few IDs during a crash) if it removes a bottleneck and makes the entire global system 100x faster.