Design a Chat System (WhatsApp/Slack)

1// ============================================
2// Chat System — Core Architecture
3// ============================================
4
5class ChatServer {
6  constructor(db, cache, messageQueue) {
7    this.db = db;
8    this.cache = cache;
9    this.queue = messageQueue;
10    this.connections = new Map(); // userId → WebSocket
11  }
12
13  // Handle new WebSocket connection
14  onConnect(userId, ws) {
15    this.connections.set(userId, ws);
16    this.setOnline(userId);
17    this.deliverQueuedMessages(userId);
18  }
19
20  onDisconnect(userId) {
21    this.connections.delete(userId);
22    this.setOffline(userId);
23  }
24
25  async sendMessage(senderId, conversationId, content) {
26    // 1. Generate sequence number (per conversation)
27    const seqNum = await this.cache.incr(`seq:\${conversationId}`);
28
29    // 2. Create message
30    const message = {
31      id: this.generateId(),
32      conversationId,
33      senderId,
34      content,
35      sequenceNum: seqNum,
36      createdAt: Date.now(),
37      status: 'sent',
38    };
39
40    // 3. Persist to database
41    await this.db.insert('messages', message);
42
43    // 4. Deliver to recipients
44    const members = await this.getConversationMembers(conversationId);
45    for (const memberId of members) {
46      if (memberId === senderId) continue;
47
48      const ws = this.connections.get(memberId);
49      if (ws) {
50        // Online: push via WebSocket
51        ws.send(JSON.stringify({ type: 'new_message', message }));
52      } else {
53        // Offline: queue for later delivery
54        await this.cache.rpush(`offline:\${memberId}`, JSON.stringify(message));
55      }
56    }
57
58    return message;
59  }
60
61  async deliverQueuedMessages(userId) {
62    const messages = await this.cache.lrange(`offline:\${userId}`, 0, -1);
63    if (messages.length > 0) {
64      const ws = this.connections.get(userId);
65      ws.send(JSON.stringify({ type: 'queued_messages', messages: messages.map(JSON.parse) }));
66      await this.cache.del(`offline:\${userId}`);
67    }
68  }
69
70  async setOnline(userId) { await this.cache.set(`online:\${userId}`, '1', 'EX', 300); }
71  async setOffline(userId) { await this.cache.del(`online:\${userId}`); }
72
73  async getConversationMembers(convId) {
74    return this.db.query('SELECT member_id FROM conversation_members WHERE conversation_id = $1', [convId]);
75  }
76
77  generateId() { return Date.now().toString(36) + Math.random().toString(36).slice(2); }
78}
79
80console.log("Chat system architecture demonstrated.");