Design: URL Shortener

String shortCode = Base62.encode(MD5(longUrl).substring(0, 8));
// Problem: collisions possible when truncating

long id = idGenerator.nextId(); // e.g., Snowflake ID or DB auto-increment
String shortCode = base62Encode(id);
// 62^7 = 3.5 trillion codes — sufficient

String BASE62 = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
String base62Encode(long id) {
    StringBuilder sb = new StringBuilder();
    while (id > 0) { sb.insert(0, BASE62.charAt((int)(id % 62))); id /= 62; }
    return sb.toString();
}

CREATE TABLE urls (
    id          BIGINT PRIMARY KEY AUTO_INCREMENT,
    short_code  VARCHAR(10) UNIQUE NOT NULL,
    long_url    VARCHAR(2048) NOT NULL,
    user_id     BIGINT,
    created_at  TIMESTAMP DEFAULT NOW(),
    expires_at  TIMESTAMP,
    click_count BIGINT DEFAULT 0
);

CREATE INDEX idx_short_code ON urls(short_code); -- critical for redirects

Client
  ↓
CDN (cache popular short codes)
  ↓
Load Balancer
  ↓
[Redirect Service]  [Shorten Service]
  ↓                      ↓
[Redis Cache]      [ID Generator (Snowflake)]
  ↓                      ↓
[MySQL/DynamoDB] ←  [MySQL Write Master]
(read replicas)         ↓
                  [Analytics Queue → ClickHouse]

GET /abc123
  1. Check Redis cache: key="url:abc123"
  2. Cache hit → return 301/302 redirect
  3. Cache miss → query DB → cache result → return redirect
  4. Async: publish click event to message queue