Design Notification System - Push, SMS, Email at Scale (Interview Guide)

POST /api/v1/notifications
{
  "user_id": "user123",
  "template_id": "order_confirmed",
  "channels": ["push", "email"],
  "data": {
    "order_id": "ORD456",
    "amount": "₹1,500"
  },
  "priority": "high",
  "schedule_at": null  // null = immediate
}

-- Notification Templates
CREATE TABLE templates (
  id VARCHAR(50) PRIMARY KEY,
  channel VARCHAR(20),
  subject TEXT,
  body TEXT,
  variables JSON
);
-- User Preferences
CREATE TABLE user_preferences (
  user_id BIGINT,
  channel VARCHAR(20),
  enabled BOOLEAN,
  PRIMARY KEY (user_id, channel)
);
-- Notification Log
CREATE TABLE notifications (
  id BIGINT PRIMARY KEY,
  user_id BIGINT,
  channel VARCHAR(20),
  status VARCHAR(20),
  sent_at TIMESTAMP,
  error TEXT
);

public class NotificationWorker {
    public void process(NotificationMessage msg) {
        // 1. Fetch user preferences
        UserPrefs prefs = prefsService.get(msg.getUserId());
        
        // 2. Check if channel enabled
        if (!prefs.isEnabled(msg.getChannel())) {
            return; // Skip
        }
        
        // 3. Rate limit check
        if (rateLimiter.isLimited(msg.getUserId())) {
            requeue(msg); // Try later
            return;
        }
        
        // 4. Render template
        String content = templateEngine.render(
            msg.getTemplateId(), 
            msg.getData()
        );
        
        // 5. Send via provider
        ProviderResponse resp = provider.send(msg.getChannel(), content);
        
        // 6. Log result
        logService.save(msg, resp);
    }
}