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name: notification-system description: >- Designs and implements multi-channel notification systems with email, push, and in-app delivery. Use when you need to build a notification service, set up user notification preferences, create delivery pipelines, implement retry logic for failed notifications, or route messages across channels. Trigger words: notification system, push notifications, email notifications, in-app notifications, notification preferences, delivery tracking, unsubscribe, notification routing. license: Apache-2.0 compatibility: "Node.js 18+, Python 3.9+. Requires a message queue (Redis/BullMQ, RabbitMQ, or SQS) for production use." metadata: author: terminal-skills version: "1.0.0" category: development tags: ["notifications", "messaging", "email"]

Notification System

Overview

This skill enables AI agents to architect, implement, and configure multi-channel notification systems. It covers channel routing, user preference management, template rendering across formats, delivery tracking, retry logic, and compliance requirements like CAN-SPAM unsubscribe.

Instructions

1. Notification Architecture

Every notification system needs these components:

Event Source → Notification Router → Channel Adapters → Delivery
                    ↓                      ↓
            Preference Store         Retry Queue
                                         ↓
                                   Dead Letter Queue

Notification Router: Receives a typed event, resolves the user's channel preferences, and dispatches to the appropriate channel adapters.

Channel Adapters: Pluggable handlers for each delivery channel. Must implement a common interface:

interface ChannelAdapter {
  channel: 'email' | 'push' | 'in-app';
  send(notification: FormattedNotification): Promise<DeliveryResult>;
  validateRecipient(userId: string): Promise<boolean>;
}

Preference Store: Database-backed user preferences with per-notification-type, per-channel toggles. Some notifications (transactional/security) must be non-disableable.

2. Notification Type Classification

Always classify notifications into categories that determine default behavior:

Category	Examples	Can Disable?	Default Channels
Security	Password reset, 2FA	No	Email
Transactional	Order confirm, receipt	No	Email + In-app
Activity	Comments, mentions	Yes	Push + In-app
Social	New follower, like	Yes	In-app
Marketing	Digest, announcements	Yes	Email

3. Channel-Specific Constraints

Email:

• Must include unsubscribe link (CAN-SPAM / GDPR)
• Use List-Unsubscribe header for one-click unsubscribe
• HTML + plain text versions
• Sender reputation — batch marketing emails separately from transactional

Push Notifications:

• Title: max 65 characters, Body: max 178 characters (iOS truncation)
• Include data payload for deep linking
• Handle token expiration and refresh
• Respect OS-level notification settings

In-App:

• Store in database with read/unread status
• Deliver in real-time via WebSocket if user is online
• Group related notifications (e.g., "3 people commented on your post")
• Paginate the notification feed

4. Retry and Failure Handling

Implement exponential backoff per channel:

• Email: 3 retries at 1min, 5min, 30min (provider outages are temporary)
• Push: 2 retries at 30s, 2min (invalid tokens should fail fast)
• In-app: 0 retries (direct DB write, either succeeds or doesn't)

After max retries, move to dead letter queue with full context for debugging.

5. Template Strategy

Use a single data context per notification type that renders differently per channel:

interface NotificationContext {
  type: 'new-comment';
  actor: { name: string; avatarUrl: string };
  target: { title: string; url: string };
  content: { preview: string; full: string };
}
// → Email: Full HTML with actor avatar, comment preview, and action button
// → Push: "Alex commented: 'Great analysis of the…'"
// → In-app: "Alex commented on Your Post Title" with link

6. Delivery Tracking

Track these states for every notification:

queued → sent → delivered → read → failed

Store in a notification_deliveries table with: notification_id, user_id, channel, status, attempted_at, delivered_at, read_at, error_message.

Examples

Example 1: Express + BullMQ notification router

Prompt: "Build a notification service for my Express app that sends order confirmations via email and in-app."

Output: The agent creates a BullMQ-backed router that accepts { type: 'order-confirmed', userId, data: { orderId, total, items } }, checks user preferences, and dispatches to the email adapter (SendGrid) and in-app adapter (PostgreSQL insert + Socket.io emit).

Example 2: Preference management API

Prompt: "Create an API for users to manage their notification preferences with a React settings page."

Output: The agent generates REST endpoints for CRUD on notification preferences, a migration for the notification_preferences table with a composite unique constraint on (user_id, notification_type, channel), and a React component rendering a matrix of toggles with notification types as rows and channels as columns.

Guidelines

• Never allow users to disable security notifications (password reset, 2FA codes)
• Always send email notifications from a queue, never synchronously in the request handler
• Batch notification grouping (e.g., "5 new comments") to avoid notification fatigue
• Test with notification-heavy scenarios: a user mentioned in a thread with 50 replies
• Include rate limiting on notifications per user per hour to prevent notification storms
• Log delivery metrics for monitoring: sent/delivered/failed rates by channel