Real-Time Notification Delivery System

Overview

Users expect instant notifications across channels: a live in-app badge, a mobile push, an email, or an SMS. A real-time notification system turns domain events into per-user messages, respects each user's channel preferences, and delivers reliably with deduplication. It must handle live WebSocket connections for in-app updates while batching and rate-limiting other channels.

Use this for collaboration tools, marketplaces, alerting systems, or any product where timely, multi-channel notification drives engagement.

Components

• WebSocket API: maintains live connections for instant in-app delivery.
• EventBridge: routes domain events to the notification pipeline based on type.
• SQS: buffers notification jobs and decouples generation from delivery.
• Lambda: evaluates preferences, renders templates, and dispatches to channels.
• DynamoDB: stores connection state, user preferences, and delivery records.
• SNS: delivers mobile push and SMS through platform endpoints.
• ElastiCache (Redis): caches preferences and enforces per-user rate limits.

Data Flow

A domain event reaches EventBridge, which routes it to a notification handler. The handler looks up the recipient's preferences in DynamoDB (cached in Redis), renders the message, and decides which channels to use. In-app messages push over the WebSocket API to connected clients; offline users get push or email via SNS. Each delivery is recorded for dedup and read-state tracking.

Scaling and Resilience

The WebSocket API and Lambda scale with connection and event volume. SQS buffers bursts so a spike in events does not overwhelm delivery, and dead-letter queues capture failures. Idempotency keys prevent duplicate notifications on retry. Per-user rate limits in Redis prevent notification storms. Channel failures fall back gracefully (e.g., push fails, queue email).

Security

WebSocket connections are authenticated with short-lived tokens and authorized per user. Lambda functions hold least-privilege roles. User contact details and preferences are encrypted at rest, and message rendering avoids leaking data across tenants. Rate limiting and abuse detection protect against notification spam and enumeration.

Trade-offs and Alternatives

Maintaining live WebSocket state adds complexity; for purely transactional alerts, push and email alone may suffice. Multi-channel delivery means handling per-channel quirks, deliverability, and provider limits. A managed notification service can replace much of this for smaller products. Build the full system when channel breadth, preference logic, and real-time in-app updates are core to the product experience.