How to design a URL shortener like bit.ly

Requirements

URL shortener: classic interview question. Deceptively simple. The depth is in ID generation and redirect latency optimization.

Core Flows

• Write: POST /shorten → generate short ID → store (shortId, longUrl, createdAt) → return short URL
• Read: GET /abc123 → lookup longUrl → HTTP 301/302 redirect

ID Generation: Base62 Encoding

import string

CHARS = string.ascii_letters + string.digits  # 62 chars

def encode(num):
    result = []
    while num:
        result.append(CHARS[num % 62])
        num //= 62
    return ''.join(reversed(result))

# encode(125000000) -> '5lnM' (7-char IDs support 62^7 ~ 3.5 trillion URLs)

Database Schema

CREATE TABLE urls (
  id BIGINT PRIMARY KEY AUTO_INCREMENT,
  short_id VARCHAR(10) UNIQUE,
  long_url TEXT NOT NULL,
  user_id BIGINT,
  created_at TIMESTAMP,
  expires_at TIMESTAMP,
  click_count BIGINT DEFAULT 0
);

Read Optimization

Reads massively outnumber writes. Cache short_id → long_url in Redis with TTL 24h. Cache hit: return redirect in <1ms. Cache miss: DB lookup, populate cache, redirect. Use HTTP 301 (permanent) for analytics bypass, HTTP 302 (temporary) if you want to track every click server-side.