Design URL Shortener (TinyURL) - Complete System Design Interview Guide

// Create short URL
POST /api/v1/shorten
Request:
{
  "long_url": "https://example.com/very-long-path/with/many/segments",
  "custom_alias": "my-link",  // optional
  "expiry_date": "2025-12-31" // optional
}
Response: 
{
  "short_url": "https://tiny.url/abc123",
  "expires_at": "2025-12-31T00:00:00Z",
  "created_at": "2025-01-12T10:30:00Z"
}
// Redirect (most important API)
GET /{short_code}
Response: HTTP 301/302 Redirect to original URL
Headers: Location: https://example.com/very-long-path
// Get Analytics
GET /api/v1/stats/{short_code}
Response:
{
  "total_clicks": 15000,
  "unique_visitors": 8500,
  "clicks_by_country": {"IN": 5000, "US": 3000, ...},
  "clicks_by_day": [{"date": "2025-01-01", "count": 500}, ...]
}
// Delete URL (for user-owned URLs)
DELETE /api/v1/url/{short_code}

CREATE TABLE urls (
  id BIGINT PRIMARY KEY AUTO_INCREMENT,
  short_code VARCHAR(7) UNIQUE NOT NULL,
  long_url TEXT NOT NULL,
  user_id BIGINT,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  expires_at TIMESTAMP NULL,
  click_count BIGINT DEFAULT 0,
  is_active BOOLEAN DEFAULT TRUE,
  
  INDEX idx_short_code (short_code),
  INDEX idx_user_id (user_id),
  INDEX idx_expires (expires_at)
);
-- For analytics (separate table for performance)
CREATE TABLE url_analytics (
  id BIGINT PRIMARY KEY AUTO_INCREMENT,
  short_code VARCHAR(7),
  clicked_at TIMESTAMP,
  user_agent TEXT,
  ip_address VARCHAR(45),
  country_code VARCHAR(2),
  referrer TEXT,
  
  INDEX idx_short_code_time (short_code, clicked_at)
);

public class Base62Encoder {
    private static final String ALPHABET = 
        "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    
    public String encode(long id) {
        StringBuilder sb = new StringBuilder();
        while (id > 0) {
            sb.append(ALPHABET.charAt((int)(id % 62)));
            id /= 62;
        }
        // Pad to minimum length
        while (sb.length() < 6) {
            sb.append('0');
        }
        return sb.reverse().toString();
    }
    
    public long decode(String shortCode) {
        long id = 0;
        for (char c : shortCode.toCharArray()) {
            id = id * 62 + ALPHABET.indexOf(c);
        }
        return id;
    }
}

public String generateShortCode(String longUrl) {
    String hash = DigestUtils.md5Hex(longUrl);
    return hash.substring(0, 7);
}

public class RedirectService {
    @Autowired private RedisTemplate<String, String> redis;
    @Autowired private UrlRepository urlRepo;
    @Autowired private AnalyticsPublisher analytics;
    
    public String getLongUrl(String shortCode) {
        // Step 1: Check Redis cache
        String longUrl = redis.opsForValue().get("url:" + shortCode);
        
        if (longUrl != null) {
            // Cache hit - publish analytics async
            analytics.publishAsync(shortCode, request);
            return longUrl;
        }
        
        // Step 2: Cache miss - query database
        Url url = urlRepo.findByShortCode(shortCode);
        
        if (url == null || !url.isActive()) {
            throw new UrlNotFoundException();
        }
        
        // Step 3: Check expiration
        if (url.getExpiresAt() != null && 
            url.getExpiresAt().isBefore(Instant.now())) {
            throw new UrlExpiredException();
        }
        
        // Step 4: Update cache
        redis.opsForValue().set(
            "url:" + shortCode, 
            url.getLongUrl(),
            Duration.ofHours(24)
        );
        
        // Step 5: Publish analytics asynchronously
        analytics.publishAsync(shortCode, request);
        
        return url.getLongUrl();
    }
}

public int getShard(String shortCode) {
    return Math.abs(shortCode.hashCode()) % NUM_SHARDS;
}