Step 4: Scale Refinement

Celebrity Detection & Routing:

class CelebrityOptimizer {
// Celebrity threshold configuration
private readonly CELEBRITY_THRESHOLD = 100000;
private readonly MEGA_CELEBRITY_THRESHOLD = 10000000;

async handlePost(userId: string, post: Post) {
  const user = await this.userService.getUser(userId);

  if (user.followerCount >= this.MEGA_CELEBRITY_THRESHOLD) {
    return this.handleMegaCelebrityPost(user, post);
  } else if (user.followerCount >= this.CELEBRITY_THRESHOLD) {
    return this.handleCelebrityPost(user, post);
  } else {
    return this.handleRegularPost(user, post);
  }
}

async handleMegaCelebrityPost(user: User, post: Post) {
  // 1. Store post (no fanout)
  await this.postService.create(post);

  // 2. Cache aggressively
  await Promise.all([
    this.cacheGlobally(post, 86400),     // 24 hour TTL
    this.indexForSearch(post),
    this.addToTrendingPosts(post)
  ]);

  // 3. Notify only VIP followers (< 1000)
  const vipFollowers = await this.getVipFollowers(user.id);
  await this.pushToFeeds(vipFollowers, post);

  // 4. Batch notify regular followers
  await this.queueBatchNotifications(user.id, post.id);
}

async handleCelebrityPost(user: User, post: Post) {
  // Hybrid approach
  const activeFollowers = await this.getActiveFollowers(user.id, 24); // Active in last 24h

  // Push to active followers only (typically 10-20% of total)
  await this.pushToFeeds(activeFollowers, post);

  // Cache for pull model
  await this.cacheRegionally(post, 3600); // 1 hour TTL
}

private async cacheGlobally(post: Post, ttl: number) {
  const regions = ['us-east', 'us-west', 'eu', 'asia', 'sa', 'africa'];

  await Promise.all(regions.map(region =>
    this.regionalCache[region].setex(
      `celebrity:post:${post.id}`,
      ttl,
      post
    )
  ));
}
}