# Event-Driven Architecture Migration Guide

**Date**: 2025-01-27
**Purpose**: Guide for migrating projects to event-driven architecture
**Status**: Complete

---

## Overview

This guide provides instructions for migrating projects to use the shared event bus (NATS) for event-driven communication.

---

## Prerequisites

- NATS event bus deployed
- Access to event bus
- Understanding of event-driven patterns

---

## Migration Steps

### Step 1: Install NATS Client

```bash
pnpm add nats
```

### Step 2: Create Event Publisher

```typescript
import { connect, NatsConnection } from 'nats';

class EventPublisher {
  private nc: NatsConnection | null = null;

  async connect() {
    this.nc = await connect({
      servers: process.env.NATS_URL || 'nats://nats:4222',
    });
  }

  async publish(subject: string, data: unknown) {
    if (!this.nc) {
      await this.connect();
    }
    await this.nc!.publish(subject, JSON.stringify(data));
  }

  async close() {
    await this.nc?.close();
  }
}
```

### Step 3: Create Event Subscriber

```typescript
import { connect, NatsConnection } from 'nats';

class EventSubscriber {
  private nc: NatsConnection | null = null;

  async connect() {
    this.nc = await connect({
      servers: process.env.NATS_URL || 'nats://nats:4222',
    });
  }

  async subscribe(subject: string, handler: (data: unknown) => void) {
    if (!this.nc) {
      await this.connect();
    }
    const sub = this.nc!.subscribe(subject);
    for await (const msg of sub) {
      const data = JSON.parse(msg.data.toString());
      handler(data);
    }
  }

  async close() {
    await this.nc?.close();
  }
}
```

### Step 4: Define Event Schemas

```typescript
// events/user-events.ts
export interface UserCreatedEvent {
  type: 'user.created';
  userId: string;
  email: string;
  timestamp: Date;
}

export interface UserUpdatedEvent {
  type: 'user.updated';
  userId: string;
  changes: Record<string, unknown>;
  timestamp: Date;
}
```

### Step 5: Publish Events

```typescript
import { EventPublisher } from './event-publisher';
import { UserCreatedEvent } from './events/user-events';

const publisher = new EventPublisher();

async function createUser(userData: UserData) {
  // Create user logic
  const user = await createUserInDB(userData);

  // Publish event
  const event: UserCreatedEvent = {
    type: 'user.created',
    userId: user.id,
    email: user.email,
    timestamp: new Date(),
  };
  await publisher.publish('events.user.created', event);
}
```

### Step 6: Subscribe to Events

```typescript
import { EventSubscriber } from './event-subscriber';
import { UserCreatedEvent } from './events/user-events';

const subscriber = new EventSubscriber();

async function setupEventHandlers() {
  await subscriber.subscribe('events.user.created', async (data: UserCreatedEvent) => {
    // Handle user created event
    await sendWelcomeEmail(data.email);
    await createUserProfile(data.userId);
  });
}
```

---

## Best Practices

### Event Naming
- Use consistent naming: `events.{domain}.{action}`
- Examples: `events.user.created`, `events.order.placed`

### Event Schema
- Define schemas using TypeScript interfaces
- Include type, timestamp, and relevant data
- Version events for compatibility

### Error Handling
- Implement retry logic
- Use dead letter queues
- Log all events

### Monitoring
- Track event rates
- Monitor latency
- Set up alerts

---

## Testing

### Unit Tests

```typescript
import { describe, it, expect } from 'vitest';
import { EventPublisher } from './event-publisher';

describe('EventPublisher', () => {
  it('should publish events', async () => {
    const publisher = new EventPublisher();
    await publisher.publish('test.event', { data: 'test' });
    // Verify event was published
  });
});
```

### Integration Tests

```typescript
import { describe, it, expect } from 'vitest';
import { EventPublisher, EventSubscriber } from './events';

describe('Event Integration', () => {
  it('should publish and receive events', async () => {
    const publisher = new EventPublisher();
    const subscriber = new EventSubscriber();

    const received: unknown[] = [];
    await subscriber.subscribe('test.event', (data) => {
      received.push(data);
    });

    await publisher.publish('test.event', { data: 'test' });
    await new Promise(resolve => setTimeout(resolve, 100));

    expect(received).toHaveLength(1);
  });
});
```

---

## Migration Checklist

- [ ] Install NATS client
- [ ] Create event publisher
- [ ] Create event subscriber
- [ ] Define event schemas
- [ ] Update code to publish events
- [ ] Update code to subscribe to events
- [ ] Test event publishing
- [ ] Test event subscription
- [ ] Set up monitoring
- [ ] Update documentation

---

**Last Updated**: 2025-01-27