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System Design: Push Notification System

Problem Statement

Design a scalable push notification system that can send notifications to millions of users in real-time across different platforms like iOS, Android, and web. The system should handle retries, prioritization, and guarantee delivery.

Requirements

Functional Requirements:

  1. Send Notifications: Ability to send notifications to users across multiple platforms (iOS, Android, Web).
  2. Targeted Notifications: Notifications can be sent to individual users or groups of users.
  3. Notification Queueing: Notifications should be queued and delivered asynchronously.
  4. Retry Mechanism: Automatically retry failed notifications (e.g., if a device is offline).
  5. Scheduling: Notifications should be scheduled for future delivery.
  6. Delivery Confirmation: Track if the notification has been successfully delivered.
  7. Platform-Specific Payload: Support platform-specific payload structures for iOS (APNS), Android (FCM), and Web (Push API).

Non-Functional Requirements:

  1. Scalability: The system should handle millions of users and notifications.
  2. Low Latency: Notifications should be delivered in real-time or near real-time.
  3. Fault Tolerance: Ensure no notifications are lost during system failures.
  4. Reliability: Ensure delivery through retries and acknowledgments.
  5. Security: Ensure notifications are securely sent to the correct device using encryption and authentication.

Components of the System

  1. Notification Service: The core service that handles receiving, processing, and sending notifications.
  2. Message Queue: A queue that holds notifications for processing. This helps in decoupling the senders and receivers, allowing the system to scale.
  3. Push Gateway (APNS/FCM/Web Push API): Acts as the bridge between the notification service and the platform-specific push services like APNS (Apple Push Notification Service), FCM (Firebase Cloud Messaging), or Web Push API.
  4. Retry and Failure Handling: Manages retrying notifications if the device is offline or the push gateway fails.
  5. Notification Scheduler: Schedules notifications for future delivery.
  6. Notification Database: Stores information about notifications, such as user preferences, delivery status, and logs.

High-Level Design

1. Notification Service:

  • API Gateway: External services can send notifications via an API.
  • Platform-Specific Handling: Notifications are structured according to platform requirements (APNS for iOS, FCM for Android, Web Push for browsers).
  • Message Routing: The service routes the notifications to the appropriate push gateway based on the user's platform.

2. Message Queue:

  • Queueing Notifications: Notifications are placed in a queue when they are ready to be processed.
  • Asynchronous Processing: The system processes notifications asynchronously, allowing for scalability and fault tolerance.
  • Message Prioritization: Higher priority notifications (e.g., critical system alerts) are sent before low-priority messages.

3. Push Gateway:

  • APNS: Push gateway for Apple devices.
  • FCM: Push gateway for Android devices.
  • Web Push: Push gateway for browser notifications.
  • Each gateway has different protocols and payload requirements, which the system must adhere to.

4. Retry and Failure Handling:

  • Retry Mechanism: If a notification fails due to a transient issue (e.g., the device is offline), the system retries sending the notification at a later time.
  • Exponential Backoff: For retries, use exponential backoff to avoid overwhelming the gateways.
  • Failure Logging: If a notification repeatedly fails, log the failure and notify the sender (e.g., via an alert or email).

5. Notification Scheduler:

  • Scheduling API: Allows notifications to be scheduled for a future time (e.g., send at 9 AM).
  • Delayed Processing: Notifications are delayed in the message queue until their scheduled time for delivery.

6. Notification Database:

  • User Preferences: Store user notification preferences (e.g., preferred notification types or time to receive notifications).
  • Delivery Logs: Track the status of each notification (pending, sent, delivered, failed).
  • Auditing: Log all notifications for auditing and debugging purposes.

Data Flow

1. Notification Creation:

  • An external service or user sends a notification request through an API to the Notification Service.
  • The Notification Service validates and prepares the notification payload based on the target platform.

2. Queueing:

  • The notification is placed into a message queue for processing.
  • Notifications may be prioritized or scheduled for future delivery.

3. Notification Processing:

  • The Notification Service dequeues the notification and routes it to the appropriate Push Gateway (APNS, FCM, Web Push).
  • The Push Gateway handles delivering the notification to the user’s device.

4. Delivery Confirmation:

  • The Push Gateway sends a confirmation (acknowledgment) back to the Notification Service indicating whether the notification was successfully delivered or not.
  • If unsuccessful, the notification is retried using a backoff strategy.

Key Challenges

1. Scalability:

  • Millions of notifications need to be processed in near real-time.
  • Solution: Use message queues to decouple the processing and ensure that the system can scale horizontally.

2. Platform-Specific Payloads:

  • Different platforms (APNS, FCM, Web Push) have different payload structures and protocols.
  • Solution: Build platform-specific handlers that generate the appropriate payloads and communicate with the corresponding gateways.

3. Retry Handling:

  • Notifications may fail if the device is offline or if the push service is down.
  • Solution: Implement a retry mechanism with exponential backoff and failure logging.

4. Delivery Guarantees:

  • Ensure that notifications are reliably delivered even in case of failures.
  • Solution: Use acknowledgments from push gateways and retry mechanisms to ensure reliable delivery.

Scaling the System

1. Horizontal Scaling:

  • Scale the Notification Service horizontally by adding more instances behind a load balancer.
  • Message queues (e.g., Kafka, RabbitMQ) can be partitioned to allow parallel processing of notifications.

2. Sharding and Partitioning:

  • Shard the user base across multiple databases to reduce the load on a single database and distribute traffic.

3. Caching:

  • Cache frequently accessed data (e.g., user device tokens) in an in-memory store like Redis to reduce the load on the database.

4. Load Balancing:

  • Use load balancers to distribute traffic evenly across instances of the Notification Service to prevent bottlenecks.

Advanced Features

1. Batch Notifications:

  • Allow sending batch notifications (e.g., promotional messages) to millions of users simultaneously.

2. Analytics and Reporting:

  • Provide detailed analytics on notifications, such as open rates, click-through rates, and engagement metrics.

3. Segmentation and Targeting:

  • Segment users based on attributes (e.g., location, activity) and send targeted notifications.

4. Rate Limiting:

  • Implement rate limiting to prevent overloading external push services like APNS or FCM.

5. Multi-Tenant Support:

  • Support multiple clients or organizations with isolated data and separate notification queues for each tenant.

Conclusion

Designing a push notification system requires handling platform-specific payloads, ensuring reliable delivery, and scaling to handle millions of users. By leveraging message queues, retries, and horizontal scaling, the system can efficiently send notifications to users in real-time, ensuring reliability and fault tolerance.

Further Reading