Notification System Design

Skill — Notification System Design (Multi-Channel Delivery Architecture)

When this skill activates

When designing notification delivery systems, implementing multi-channel messaging, building preference management, or architecting notification infrastructure. Use for any system that needs to send timely, relevant messages to users across push, email, SMS, in-app, or other channels.

Core principle: Respect over reach — every notification should earn its delivery by being timely, relevant, and respectful of user preferences and attention.

Mandatory actions when this skill is active

Channel Selection (Urgency Matrix)

1. Route notifications by urgency and type:

   | Urgency    | Channels                    | Examples                          |
   |------------|-----------------------------|-----------------------------------|
   | Critical   | Push + Email + SMS          | Security alert, payment failure   |
   | Important  | Push + Email                | Order shipped, appointment reminder|
   | Standard   | Push OR Email               | New feature, weekly digest        |
   | Low        | In-app only                 | Social activity, recommendations  |
   

   Rules:

   • Critical notifications bypass frequency caps (but still respect DND)
   • Never send SMS for non-critical notifications (highest user annoyance cost)
   • In-app is the lowest-friction channel — use it generously
   • Email is async — never rely on it for time-sensitive actions
   • User channel preferences override the urgency matrix (except Critical/security)

Frequency Capping

2. Rate limiting per user:

   {
     "caps": {
       "push": {"max_per_day": 5, "max_per_hour": 2},
       "email": {"max_per_day": 3, "max_per_week": 10},
       "sms": {"max_per_day": 1, "max_per_week": 3},
       "in_app": {"max_per_day": 20}
     },
     "aggregation": {
       "combine_similar": true,
       "digest_threshold": 3,
       "digest_window_minutes": 30
     }
   }
   

   Rules:

   • Caps are per-user, per-channel, across ALL notification types
   • When cap is reached: queue for next window OR downgrade to lower-urgency channel
   • Aggregate similar notifications into digests (e.g., "3 new comments" not 3 separate pushes)
   • Track notification fatigue: if open rate drops below 5%, reduce frequency automatically
   • Critical/security notifications are exempt from caps but tracked separately

Preference Management

3. User preference schema:

   {
     "user_id": "uuid",
     "global": {
       "muted": false,
       "do_not_disturb": {"enabled": true, "start": "22:00", "end": "08:00", "timezone": "America/New_York"}
     },
     "channels": {
       "push": {"enabled": true},
       "email": {"enabled": true, "digest_mode": "daily"},
       "sms": {"enabled": false}
     },
     "categories": {
       "marketing": {"push": false, "email": true},
       "social": {"push": true, "email": false},
       "transactional": {"push": true, "email": true},
       "security": {"push": true, "email": true, "sms": true}
     }
   }
   

   Rules:

   • Users can control preferences at: global, channel, and category level
   • Category-level overrides channel-level (fine-grained control)
   • Security/transactional notifications: always delivered (legal requirement), but user chooses channel
   • DND hours: queue notifications and deliver when DND ends (except Critical)
   • Preference changes take effect immediately (no "takes up to 24 hours")

Delivery Architecture

4. System architecture:

   Event Source → Notification Service → Channel Router → Delivery Providers → Tracking
   
   Components:
   1. Event Ingestion: receives trigger events from application services
   2. Notification Service: applies business logic (templates, personalization, dedup)
   3. Preference Engine: checks user preferences and caps
   4. Channel Router: selects delivery channel(s) per urgency matrix + preferences
   5. Queue (per channel): buffers for rate limiting and retry
   6. Delivery Providers: FCM/APNs (push), SendGrid/SES (email), Twilio (SMS)
   7. Tracking: delivery status, opens, clicks, dismissals
   

5. Delivery guarantees:

   • At-least-once delivery with deduplication
   • Idempotency key per notification (event_type + user_id + dedup_window)
   • Exponential retry with jitter (max 3 retries per channel)
   • Dead letter queue for permanently failed deliveries
   • Track delivery status: queued → sent → delivered → opened → clicked/dismissed

6. Deduplication:

   Dedup key = hash(notification_type + user_id + content_hash)
   Dedup window = configurable per type (default: 1 hour)
   
   If same dedup key seen within window:
   - Suppress duplicate
   - Log suppression (for debugging)
   - Update existing notification if content changed
   

Template System

7. Notification templates:

   Template: order_shipped
   Channels: push, email
   Personalization tokens: {{user.first_name}}, {{order.id}}, {{order.tracking_url}}
   
   Push:
     title: "Your order is on its way!"
     body: "{{user.first_name}}, order #{{order.id}} has shipped. Track it here."
     action_url: "{{order.tracking_url}}"
   
   Email:
     subject: "Order #{{order.id}} shipped"
     template_id: "tmpl_order_shipped_v2"
     variables: {first_name, order_id, tracking_url, items}
   

   Rules:

   • Templates support multi-language (locale selected from user preferences)
   • Preview rendering available before send (catch token errors)
   • Version templates (never edit in place — create new version, deprecate old)
   • A/B test notification copy via experiment-design skill

Self-check before task completion

Before marking a task done when this skill was active:

• Did I define an urgency matrix mapping notification types to channels?
• Are frequency caps defined per-user, per-channel with aggregation rules?
• Do user preferences support global, channel, and category-level control?
• Is DND (do not disturb) implemented with timezone awareness?
• Is delivery at-least-once with deduplication via idempotency keys?
• Are retries exponential with a dead letter queue for failures?
• Are templates versioned, multi-language, and previewable?
• Are Critical/security notifications exempt from caps but still respect DND?