Design a Notification System

1// ============================================
2// Notification System — Architecture
3// ============================================
4
5class NotificationService {
6  constructor(queue, db, templateEngine) {
7    this.queue = queue;
8    this.db = db;
9    this.templates = templateEngine;
10    this.channels = {
11      push: new PushWorker(),
12      email: new EmailWorker(),
13      sms: new SMSWorker(),
14      inApp: new InAppWorker(),
15    };
16  }
17
18  async processEvent(event) {
19    // 1. Get user preferences
20    const prefs = await this.db.getUserNotificationPrefs(event.userId);
21
22    // 2. Check deduplication
23    const dedupKey = `notif:\${event.userId}:\${event.type}:\${event.id}`;
24    const alreadySent = await this.db.cache.get(dedupKey);
25    if (alreadySent) return;
26
27    // 3. Route to enabled channels
28    const enabledChannels = this.getEnabledChannels(event.type, prefs);
29
30    for (const channel of enabledChannels) {
31      // 4. Render template for each channel
32      const content = this.templates.render(event.type, channel, event.data);
33
34      // 5. Queue with priority
35      await this.queue.publish(`notifications.\${channel}`, {
36        userId: event.userId,
37        channel,
38        content,
39        priority: this.getPriority(event.type),
40        dedupKey,
41      });
42    }
43
44    // 6. Mark as sent (dedup)
45    await this.db.cache.set(dedupKey, '1', 'EX', 86400);
46  }
47
48  getEnabledChannels(eventType, prefs) {
49    const defaults = {
50      'security.alert': ['push', 'email', 'sms'],
51      'order.shipped': ['push', 'email'],
52      'social.like': ['push', 'inApp'],
53      'marketing.promo': ['email'],
54    };
55    const defaultChannels = defaults[eventType] || ['inApp'];
56    return defaultChannels.filter(ch => prefs[ch] !== false);
57  }
58
59  getPriority(eventType) {
60    if (eventType.startsWith('security')) return 0;  // Critical
61    if (eventType.startsWith('order')) return 1;     // High
62    if (eventType.startsWith('social')) return 2;    // Medium
63    return 3;                                        // Low
64  }
65}
66
67class PushWorker {
68  async send(notification) {
69    console.log(`[Push] → User \${notification.userId}: \${notification.content.title}`);
70    // Call FCM/APNs API
71  }
72}
73class EmailWorker {
74  async send(notification) {
75    console.log(`[Email] → User \${notification.userId}: \${notification.content.subject}`);
76  }
77}
78class SMSWorker {
79  async send(notification) {
80    console.log(`[SMS] → User \${notification.userId}: \${notification.content.body}`);
81  }
82}
83class InAppWorker {
84  async send(notification) {
85    console.log(`[In-App] → User \${notification.userId}: \${notification.content.message}`);
86  }
87}
88
89console.log("Notification system architecture demonstrated.");