Push notification strategies

p 3: Build Async Routing & Retry Engine Synchronous delivery blocks event loops and cascades timeouts. Use a message queue with exponential backoff, idempotency keys, and token lifecycle management.

import { Redis } from 'ioredis';

export class DeliveryEngine {
  private redis: Redis;
  private providers: Map<string, PushProvider>;
  private readonly IDEMPOTENCY_TTL = 3600; // 1 hour

  constructor(redis: Redis, providers: PushProvider[]) {
    this.redis = redis;
    this.providers = new Map(providers.map(p => [p.id, p]));
  }

  async deliver(eventId: string, payload: PushPayload): Promise<DeliveryResult> {
    const idempotencyKey = `push:idem:${eventId}`;
    const cached = await this.redis.get(idempotencyKey);
    if (cached) return JSON.parse(cached) as DeliveryResult;

    const provider = this.selectProvider(payload.platform);
    if (!provider.validate(payload)) {
      throw new Error('Payload validation failed');
    }

    const result = await this.executeWithRetry(provider, payload, 3);
    await this.redis.set(idempotencyKey, JSON.stringify(result), 'EX', this.IDEMPOTENCY_TTL);
    return result;
  }

  private async executeWithRetry(
    provider: PushProvider,
    payload: PushPayload,
    attempts: number
  ): Promise<DeliveryResult> {
    for (let i = 0; i < attempts; i++) {
      const start = Date.now();
      const result = await provider.send(payload);
      result.latencyMs = Date.now() - start;

      if (result.status === 'sent') return result;
      if (result.status === 'throttled') {
        await this.backoff(i);
        continue;
      }
      if (result.status === 'invalid_token') {
        await this.purgeToken(payload.tokens[0]);
        return result;
      }
    }
    throw new Error('Delivery failed after max retries');
  }

  private async backoff(attempt: number): Promise<void> {
    const delay = Math.min(1000 * Math.pow(2, attempt), 30000);
    await new Promise(res => setTimeout(res, delay));
  }
}

Architecture Decisions & Rationale

• Facade Pattern: Decouples business logic from provider SDKs. Enables runtime provider switching and A/B testing of delivery channels.
• Async Queue Processing: Prevents thread blocking, absorbs traffic spikes, and enables batch optimization.
• Idempotency via Redis: Guarantees exactly-once delivery semantics for critical events (e.g., transaction alerts).
• Conservative Payload Limits: 3800 bytes accounts for routing headers, platform metadata, and UTF-8 encoding overhead. Prevents silent truncation.
• Token Lifecycle Management: Invalid tokens are purged immediately to reduce API waste and improve delivery accuracy.

Pitfall Guide

1. Ignoring Platform Payload Limits APNs and FCM both cap payloads at ~4KB, but developers frequently exceed usable space by embedding large custom objects. Providers silently drop oversized notifications or truncate keys. Always serialize, measure byte length, and strip non-essential fields before routing.

2. Treating Delivery as Synchronous Blocking request threads on provider APIs causes timeout cascades during peak hours. Push delivery must be async. Use a queue (Redis, Kafka, or SQS) to decouple ingestion from execution.

3. Missing Idempotency & Deduplication Network retries, SDK reinitializations, and user actions generate duplicate events. Without idempotency keys, users receive identical notifications. Implement request hashing or event ID tracking with TTL-based cache.

4. Over-Relying on Provider Retry Defaults FCM and APNs implement their own retry logic, but it is optimized for volume, not latency. Blindly trusting provider retries leads to exponential backoff misalignment and delayed delivery. Implement client-side retry with jitter and circuit breakers.

5. Skipping Quiet Hours & Timezone Awareness Sending notifications at 2 AM local time triggers immediate opt-outs. Always attach timezone metadata to tokens and enforce quiet-hour windows at the routing layer, not the client.

6. No Delivery Analytics or Failure Segregation Logging 200 OK as success is insufficient. Track invalid_token, throttled, payload_truncated, and delivery_timeout separately. Without granular metrics, optimization is guesswork.

7. Hardcoding Provider Credentials in Runtime Embedding API keys in environment variables without rotation or secret management leads to credential drift and quota exhaustion. Use a centralized secret store with automatic rotation and quota monitoring.

Best Practices from Production:

• Centralize notification config in a versioned registry.
• Implement A/B testing for payload structure and send windows.
• Gracefully degrade to in-app messages or email when push fails consistently.
• Monitor provider health endpoints and implement fallback routing.
• Enforce strict schema validation at ingestion, not at delivery.

Production Bundle

Action Checklist

• Abstract push providers behind a unified interface to prevent vendor lock-in
• Implement byte-level payload validation before API routing
• Route all delivery through an async queue with idempotency tracking
• Add timezone-aware quiet hours and token lifecycle management
• Instrument delivery with granular status codes (sent, throttled, invalid_token, truncated)
• Configure circuit breakers and client-side retry with exponential backoff
• Establish fallback channels (in-app, email) for persistent delivery failures
• Rotate provider credentials via secret management with quota alerts

Decision Matrix

Configuration Template

push:
  providers:
    fcm:
      enabled: true
      endpoint: https://fcm.googleapis.com/v1/projects/${PROJECT_ID}/messages:send
      max_payload_bytes: 3800
      retry:
        max_attempts: 3
        base_delay_ms: 500
        max_delay_ms: 30000
        jitter: true
    apns:
      enabled: true
      endpoint: https://api.push.apple.com/3/device/${TOKEN}
      max_payload_bytes: 3800
      retry:
        max_attempts: 2
        base_delay_ms: 1000
        max_delay_ms: 15000
        jitter: true
  routing:
    strategy: behavioral
    quiet_hours:
      enabled: true
      start: "22:00"
      end: "07:00"
      timezone: user_local
    idempotency:
      ttl_seconds: 3600
      store: redis
  analytics:
    track: [sent, throttled, invalid_token, payload_truncated, delivery_timeout]
    retention_days: 90

Quick Start Guide

1. Install dependencies: npm install ioredis axios zod
2. Initialize provider clients: Configure FCM service account and APNs AuthKey in your environment. Instantiate PushProvider implementations with the configuration template.
3. Spin up the delivery queue: Deploy a Redis instance or managed queue. Configure the DeliveryEngine with idempotency TTL and retry parameters.
4. Integrate ingestion endpoint: Expose a POST route that validates incoming events, generates idempotency keys, and pushes payloads to the queue.
5. Monitor delivery health: Instrument the analytics pipeline with provider status codes. Set alerts for invalid_token spikes and throttled rates exceeding 5%.