// Problematic: Thundering Herd
async function getPopularPost(postId: string) {
  const cached = await redis.get(`post:${postId}`);
  if (cached) return JSON.parse(cached);

  // 10,000 requests hit DB simultaneously when TTL expires
  const post = await db.query('SELECT * FROM posts WHERE id = $1', [postId]);
  await redis.setEx(`post:${postId}`, 3600, JSON.stringify(post));
  return post;
}

// Solution: Probabilistic Early Expiration
async function getPopularPostSafe(postId: string) {
  const key = `post:${postId}`;
  const result = await redis.get(key);

  if (result) {
    const { data, expiresAt } = JSON.parse(result);

    // Refresh early if within 10% of expiry (probabilistic)
    const timeLeft = expiresAt - Date.now();
    const ttl = 3600 * 1000;
    if (timeLeft < ttl * 0.1 && Math.random() < 0.1) {
      // Only 10% of requests trigger refresh — one will "win"
      refreshCache(postId);
    }

    return data;
  }

  // Mutex: only one request rebuilds the cache
  const lock = await redis.set(`lock:post:${postId}`, '1', {
    NX: true,  // Only set if not exists
    EX: 30     // Lock expires in 30 seconds
  });

  if (!lock) {
    // Another request is rebuilding — wait a bit and retry
    await sleep(100);
    return getPopularPostSafe(postId);
  }

  try {
    const post = await db.query('SELECT * FROM posts WHERE id = $1', [postId]);
    await redis.setEx(key, 3600, JSON.stringify({
      data: post,
      expiresAt: Date.now() + 3600 * 1000
    }));
    return post;
  } finally {
    await redis.del(`lock:post:${postId}`);
  }
}
Caching — אסטרטגיות Cache

Caching — אסטרטגיות Cache

Caching — אסטרטגיות Cache
