URL
Shortener

GET /abc1234

1. Check in-process cache (Caffeine): ~100ns
   HIT (60%): return HTTP 302 immediately ✅

2. Check Redis cache: ~0.5ms
   HIT (39%): populate L1 cache, return HTTP 302 ✅

3. Cache MISS (1%): query MySQL read replica: ~5ms
   Populate Redis (TTL 24h), populate L1 (TTL 5min), return HTTP 302 ✅

4. Async (does NOT block redirect):
   Publish click event to Kafka → Analytics consumer updates click_count

private static final String ALPHABET = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
private static final int BASE = 62;

public static String encode(long id) {
    if (id == 0) return "0";
    StringBuilder sb = new StringBuilder();
    while (id > 0) {
        sb.append(ALPHABET.charAt((int)(id % BASE)));
        id /= BASE;
    }
    return sb.reverse().toString();
}

public static long decode(String code) {
    long result = 0;
    for (char c : code.toCharArray()) {
        result = result * BASE + ALPHABET.indexOf(c);
    }
    return result;
}

// encode(0)         → "0"
// encode(62)        → "10"  (like binary: 1×62 + 0)
// encode(123456789) → "8M0kX"
// decode(encode(n)) == n  ✅
// 62^7 = 3,521,614,606,208 ≈ 3.5 trillion unique 7-char codes

public String getLongUrl(String shortCode) {
    // L1: in-process
    String url = l1Cache.getIfPresent(shortCode);
    if (url != null) return url;

    // L2: Redis
    url = redis.get("url:" + shortCode);
    if (url != null) {
        l1Cache.put(shortCode, url);    // populate L1
        return url;
    }

    // DB: only 1% of traffic
    UrlRecord rec = db.findByShortCode(shortCode);
    if (rec == null || rec.isExpired()) return null;
    redis.setex("url:" + shortCode, 86400, rec.longUrl); // 24h TTL
    l1Cache.put(shortCode, rec.longUrl);
    return rec.longUrl;
}

// Invalidation: on URL delete
public void deleteUrl(String shortCode) {
    db.softDelete(shortCode);
    redis.del("url:" + shortCode);
    l1Cache.invalidate(shortCode);  // invalidate all instances via broadcast
}