System Design FAQ: Top Questions

A Notification System delivers asynchronous messages like emails, SMS, or push notifications to users based on rules, preferences, or system events. It must handle retries, user targeting, templates, and vendor integrations.

📋 Functional Requirements

• Support for Email, SMS, and Push
• User preference handling
• Templating, personalization, and rate limits
• Retry failed sends with backoff

📦 Non-Functional Requirements

• High throughput, low latency
• Reliability and observability
• Vendor failover (e.g., fallback SMS provider)

🏗️ Architecture Overview

• Producer: Services publish events (e.g., order placed)
• Message Queue: Kafka/SQS decouples producer/consumer
• Worker: Consumes queue, evaluates preferences, renders templates
• Notifier: Sends via vendor APIs (Twilio, SendGrid, Firebase)

📄 Notification Template Example (Handlebars)

Hi {{name}},

Your order #{{orderId}} has been shipped and will arrive by {{deliveryDate}}.

Thanks,
Team
        

🔁 Retry Logic with Exponential Backoff

async function sendWithRetry(payload, maxRetries = 5) {
  for (let attempt = 1; attempt <= maxRetries; attempt++) {
    try {
      await sendToVendor(payload);
      return;
    } catch (err) {
      const delay = Math.pow(2, attempt) * 100;
      await sleep(delay);
    }
  }
  logError("Notification permanently failed", payload);
}
        

🔌 Vendor Integration Example (SendGrid)

const sgMail = require('@sendgrid/mail');
sgMail.setApiKey(process.env.SENDGRID_API_KEY);

sgMail.send({
  to: 'user@example.com',
  from: 'noreply@company.com',
  subject: 'Welcome!',
  text: 'Thanks for joining our app!'
});
        

🧠 User Preference Management

• DB table per user with channel toggles (email=true, sms=false)
• Default settings for new users
• Consent audit logging

📈 Observability

• Delivery rate, bounce rate
• Failure trends by channel
• Time-to-notify metric (event → delivery)

🔐 Security & Compliance

• Rate limiting per user/org
• Secure token usage in templates (e.g., password reset)
• Support for GDPR unsubscribe mechanisms

🧰 Tools & Infra

• Queue: Kafka, SQS, RabbitMQ
• Vendors: SendGrid, SES, Twilio, Firebase
• Templates: Handlebars, Mustache, MJML
• Infra: Node.js, Golang, Python workers

📌 Final Insight

Decouple message generation from delivery using queues and vendor-agnostic workers. Retry intelligently and monitor for deliverability across all channels. Consider UI for audit logs, preferences, and failure inspection.