Problem Statement

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Webhook Notification Service - Problem Statement

Overview

Design a webhook delivery system that can reliably send HTTP notifications to external services with retry logic, failure handling, and delivery guarantees. The system should handle millions of webhook deliveries per day with high reliability and comprehensive monitoring.

Functional Requirements

Core Webhook Features

  • Webhook Registration: Register webhook endpoints with authentication
  • Event Subscription: Subscribe to specific event types
  • Webhook Delivery: Send HTTP POST requests to registered endpoints
  • Retry Logic: Automatic retry with exponential backoff
  • Delivery Confirmation: Track successful and failed deliveries
  • Webhook Management: Update, disable, delete webhooks

Event Types

  • User Events: User registration, login, profile updates
  • Transaction Events: Payments, refunds, order status
  • System Events: Errors, alerts, maintenance notifications
  • Custom Events: Application-specific events
  • Batch Events: Multiple events in single webhook call

Delivery Guarantees

  • At-Least-Once: Guarantee delivery with possible duplicates
  • Ordered Delivery: Maintain event order per webhook
  • Idempotency: Include idempotency keys for duplicate detection
  • Acknowledgment: Require 2xx response for success
  • Timeout Handling: Configurable timeout (default 30s)

Retry Configuration

  • Retry Attempts: Configurable max retries (default 5)
  • Retry Strategy: Exponential backoff with jitter
  • Retry Intervals: 1s, 2s, 4s, 8s, 16s
  • Dead Letter Queue: Failed webhooks after max retries
  • Manual Retry: Ability to manually retry failed webhooks

Security Features

  • Signature Verification: HMAC-SHA256 signatures
  • Secret Management: Secure storage of webhook secrets
  • IP Allowlisting: Restrict webhook sources
  • TLS/SSL: Encrypted webhook delivery
  • Authentication: Support for Bearer tokens, Basic auth

Monitoring and Debugging

  • Delivery Status: Track success, failure, pending
  • Delivery Logs: Complete request/response logs
  • Webhook Analytics: Success rate, latency, error patterns
  • Real-time Alerts: Notify on delivery failures
  • Webhook Testing: Test endpoint before registration

Non-Functional Requirements

Performance Requirements

  • Delivery Latency: <1 second from event to webhook delivery
  • Throughput: 10,000 webhooks per second
  • Concurrent Deliveries: 100,000 concurrent HTTP requests
  • Retry Latency: Follow exponential backoff schedule
  • Query Performance: <100ms to query webhook status

Scalability Requirements

  • Total Webhooks: Support 1 million registered webhooks
  • Events per Day: 100 million events triggering webhooks
  • Concurrent Subscriptions: 10 million active subscriptions
  • Horizontal Scaling: Linear scaling with traffic growth
  • Geographic Distribution: Multi-region deployment

Reliability Requirements

  • System Uptime: 99.99% availability
  • Delivery Success Rate: 99.9% for healthy endpoints
  • No Event Loss: 100% event capture and queuing
  • Fault Tolerance: Survive worker and queue failures
  • Data Durability: No webhook loss on system failure
  • Recovery Time: <30 seconds after failure

Delivery Requirements

  • Timeout: 30 seconds default, configurable up to 5 minutes
  • Max Retries: 5 attempts default, configurable up to 10
  • Backoff: Exponential with jitter
  • Circuit Breaker: Disable failing webhooks automatically
  • Rate Limiting: Limit deliveries per endpoint

Use Cases

1. Payment Notifications

Scenario: Notify merchant of payment status
Events: payment.success, payment.failed, payment.refunded
Delivery: <1 second
Retries: 5 attempts
Guarantee: At-least-once

2. Order Status Updates

Scenario: Notify customer of order status
Events: order.created, order.shipped, order.delivered
Delivery: <5 seconds
Retries: 3 attempts
Guarantee: At-least-once

3. System Alerts

Scenario: Notify ops team of system issues
Events: error.critical, service.down, disk.full
Delivery: <1 second
Retries: 10 attempts
Guarantee: At-least-once with escalation

4. Integration Webhooks

Scenario: Sync data with external systems
Events: user.created, user.updated, user.deleted
Delivery: <10 seconds
Retries: 5 attempts
Guarantee: Exactly-once (with idempotency)

Edge Cases and Constraints

Endpoint Failures

  • Timeout: Endpoint doesn't respond within 30s
  • 5xx Errors: Server errors (retry)
  • 4xx Errors: Client errors (don't retry, except 429)
  • Network Errors: Connection refused, DNS failure
  • SSL Errors: Certificate validation failures

Delivery Challenges

  • Slow Endpoints: Response time >10 seconds
  • Rate Limiting: Endpoint returns 429 Too Many Requests
  • Endpoint Down: Extended outage (hours/days)
  • Payload Too Large: Webhook payload exceeds limit
  • Circular Webhooks: Webhook triggers another webhook

System Challenges

  • Event Burst: 10x normal traffic suddenly
  • Retry Storm: Many webhooks retrying simultaneously
  • Queue Backlog: Millions of pending webhooks
  • Worker Failures: Workers crash during delivery
  • Network Partitions: Split brain scenarios

Success Metrics

Performance Metrics

  • Delivery Latency P50: <500ms
  • Delivery Latency P99: <2 seconds
  • Throughput: 10K+ webhooks/second
  • Concurrent Requests: 100K+
  • Queue Processing Rate: 10K+ events/second

Reliability Metrics

  • Delivery Success Rate: 99.9%
  • First Attempt Success: 95%
  • System Uptime: 99.99%
  • Event Loss Rate: 0%
  • Duplicate Rate: <0.1%

Business Metrics

  • Cost per Webhook: <$0.001 per delivery
  • Infrastructure Cost: <$5K per million webhooks
  • Customer Satisfaction: >4.5/5 rating
  • Support Tickets: <0.1% of deliveries

This problem statement provides the foundation for designing a robust, scalable, and reliable webhook delivery system.