customerio-reliability-patterns

# Customer.io Reliability Patterns

## Overview
Implement reliability patterns for fault-tolerant Customer.io integrations including circuit breakers, retries, and fallbacks.

## Prerequisites
- Customer.io integration working
- Understanding of failure modes
- Queue infrastructure (optional)

## Instructions

### Pattern 1: Circuit Breaker
```typescript
// lib/circuit-breaker.ts
enum CircuitState {
  CLOSED = 'CLOSED',
  OPEN = 'OPEN',
  HALF_OPEN = 'HALF_OPEN'
}

interface CircuitBreakerConfig {
  failureThreshold: number;
  successThreshold: number;
  timeout: number;
}

export class CircuitBreaker {
  private state: CircuitState = CircuitState.CLOSED;
  private failures: number = 0;
  private successes: number = 0;
  private lastFailureTime: number = 0;
  private config: CircuitBreakerConfig;

  constructor(config: Partial<CircuitBreakerConfig> = {}) {
    this.config = {
      failureThreshold: config.failureThreshold || 5,
      successThreshold: config.successThreshold || 3,
      timeout: config.timeout || 30000
    };
  }

  async execute<T>(operation: () => Promise<T>): Promise<T> {
    if (this.state === CircuitState.OPEN) {
      if (Date.now() - this.lastFailureTime >= this.config.timeout) {
        this.state = CircuitState.HALF_OPEN;
      } else {
        throw new Error('Circuit breaker is OPEN');
      }
    }

    try {
      const result = await operation();
      this.onSuccess();
      return result;
    } catch (error) {
      this.onFailure();
      throw error;
    }
  }

  private onSuccess(): void {
    this.failures = 0;

    if (this.state === CircuitState.HALF_OPEN) {
      this.successes++;
      if (this.successes >= this.config.successThreshold) {
        this.state = CircuitState.CLOSED;
        this.successes = 0;
      }
    }
  }

  private onFailure(): void {
    this.failures++;
    this.lastFailureTime = Date.now();
    this.successes = 0;

    if (this.failures >= this.config.failureThreshold) {
      this.state = CircuitStat