Design a Social Media Feed (Twitter/X)

1// ============================================
2// Social Media Feed — Fan-Out Architecture
3// ============================================
4
5class FeedService {
6  constructor(db, cache, queue) {
7    this.db = db;
8    this.cache = cache;
9    this.queue = queue;
10    this.CELEBRITY_THRESHOLD = 10000;
11  }
12
13  async postTweet(userId, content) {
14    // Save tweet
15    const tweet = await this.db.insert('tweets', {
16      id: this.generateId(), userId, content, createdAt: Date.now(),
17    });
18
19    const followerCount = await this.db.getFollowerCount(userId);
20
21    if (followerCount < this.CELEBRITY_THRESHOLD) {
22      // Regular user: Fan-out on write (push to followers)
23      await this.queue.publish('fanout', {
24        type: 'push',
25        tweetId: tweet.id,
26        userId,
27        content,
28        createdAt: tweet.createdAt,
29      });
30    }
31    // Celebrity: Don't fan out — their tweets are pulled on read
32
33    return tweet;
34  }
35
36  // Fan-out worker: pushes tweet to each follower's feed
37  async processFanout(event) {
38    const followers = await this.db.getFollowers(event.userId);
39    for (const followerId of followers) {
40      // Add to follower's cached feed (Redis sorted set)
41      await this.cache.zadd(
42        `feed:\${followerId}`,
43        event.createdAt, // Score = timestamp for sorting
44        JSON.stringify({ tweetId: event.tweetId, userId: event.userId, content: event.content })
45      );
46      // Trim to latest 800 tweets per feed
47      await this.cache.zremrangebyrank(`feed:\${followerId}`, 0, -801);
48    }
49  }
50
51  // Get user's home feed
52  async getFeed(userId, page = 0, pageSize = 20) {
53    const start = page * pageSize;
54    const end = start + pageSize - 1;
55
56    // Get pre-computed feed from cache
57    let feedItems = await this.cache.zrevrange(
58      `feed:\${userId}`, start, end
59    );
60
61    // Merge celebrity tweets (fan-out on read)
62    const celebrities = await this.getCelebrityFollowees(userId);
63    if (celebrities.length > 0) {
64      const celebrityTweets = await Promise.all(
65        celebrities.map(celeb => this.getRecentTweets(celeb, pageSize))
66      );
67      // Merge and sort all tweets by timestamp
68      feedItems = this.mergeAndSort(
69        feedItems.map(item => JSON.parse(item)),
70        celebrityTweets.flat()
71      ).slice(0, pageSize);
72    }
73
74    return feedItems;
75  }
76
77  async getCelebrityFollowees(userId) {
78    // Get followed users with > 10K followers
79    return this.db.query(
80      'SELECT followee_id FROM follows WHERE follower_id = $1 AND followee_followers > $2',
81      [userId, this.CELEBRITY_THRESHOLD]
82    );
83  }
84
85  async getRecentTweets(userId, limit) {
86    const cached = await this.cache.get(`tweets:\${userId}`);
87    if (cached) return JSON.parse(cached);
88    return this.db.query(
89      'SELECT * FROM tweets WHERE user_id = $1 ORDER BY created_at DESC LIMIT $2',
90      [userId, limit]
91    );
92  }
93
94  mergeAndSort(feed1, feed2) {
95    return [...feed1, ...feed2].sort((a, b) => b.createdAt - a.createdAt);
96  }
97
98  generateId() { return Date.now().toString(36) + Math.random().toString(36).slice(2); }
99}
100
101console.log("Feed system architecture demonstrated.");