Your App ←→ Integration Layer ←→ Third-Party API
            ├── Auth (OAuth, API keys)
            ├── Rate limiting
            ├── Retries
            ├── Error handling
            ├── Data transformation
            └── Webhooks (if supported)

interface APIKeyConfig {
  api_key: string
  api_secret?: string
}

class SimpleAPIClient {
  private apiKey: string

  async request(endpoint: string, options: RequestOptions) {
    return fetch(`https://api.service.com${endpoint}`, {
      ...options,
      headers: {
        Authorization: `Bearer ${this.apiKey}`,
        'Content-Type': 'application/json'
      }
    })
  }
}

// 1. Redirect user to authorize
app.get('/connect/slack', (req, res) => {
  const authUrl = new URL('https://slack.com/oauth/v2/authorize')
  authUrl.searchParams.set('client_id', SLACK_CLIENT_ID)
  authUrl.searchParams.set('redirect_uri', 'https://yourapp.com/auth/slack/callback')
  authUrl.searchParams.set('scope', 'channels:read,chat:write')
  authUrl.searchParams.set('state', generateSecureRandomString()) // CSRF protection

  res.redirect(authUrl.toString())
})

// 2. Handle callback
app.get('/auth/slack/callback', async (req, res) => {
  const { code, state } = req.query

  // Verify state to prevent CSRF
  if (state !== req.session.oauthState) {
    throw new Error('Invalid state')
  }

  // Exchange code for access token
  const tokenResponse = await fetch('https://slack.com/api/oauth.v2.access', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({
      client_id: SLACK_CLIENT_ID,
      client_secret: SLACK_CLIENT_SECRET,
      code: code,
      redirect_uri: 'https://yourapp.com/auth/slack/callback'
    })
  })

  const { access_token, refresh_token, expires_in } = await tokenResponse.json()

  // Store tokens securely (encrypted!)
  await db.storeIntegration({
    user_id: req.user.id,
    service: 'slack',
    access_token: encrypt(access_token),
    refresh_token: encrypt(refresh_token),
    expires_at: Date.now() + expires_in * 1000
  })

  res.redirect('/settings/integrations?success=slack')
})

async function getValidAccessToken(userId: string, service: string) {
  const integration = await db.getIntegration(userId, service)

  // Token still valid?
  if (integration.expires_at > Date.now() + 60000) {
    // 1 min buffer
    return decrypt(integration.access_token)
  }

  // Refresh token
  const refreshResponse = await fetch('https://slack.com/api/oauth.v2.access', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({
      client_id: SLACK_CLIENT_ID,
      client_secret: SLACK_CLIENT_SECRET,
      grant_type: 'refresh_token',
      refresh_token: decrypt(integration.refresh_token)
    })
  })

  const { access_token, expires_in } = await refreshResponse.json()

  // Update stored tokens
  await db.updateIntegration(integration.id, {
    access_token: encrypt(access_token),
    expires_at: Date.now() + expires_in * 1000
  })

  return access_token
}

import { RateLimiter } from 'limiter'

class RateLimitedAPIClient {
  private limiter: RateLimiter

  constructor(tokensPerInterval: number, interval: string) {
    this.limiter = new RateLimiter({
      tokensPerInterval,
      interval
    })
  }

  async request(endpoint: string, options: RequestOptions) {
    // Wait for rate limit token
    await this.limiter.removeTokens(1)

    return fetch(`https://api.service.com${endpoint}`, options)
  }
}

// Example: Slack allows ~1 request per second
const slackClient = new RateLimitedAPIClient(1, 'second')

async function requestWithRetry(url: string, options: RequestOptions, maxRetries = 3) {
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    const response = await fetch(url, options)

    if (response.status === 429) {
      // Check Retry-After header
      const retryAfter = response.headers.get('Retry-After')
      const waitTime = retryAfter ? parseInt(retryAfter) * 1000 : Math.pow(2, attempt) * 1000

      console.log(`Rate limited, waiting ${waitTime}ms`)
      await sleep(waitTime)
      continue
    }

    return response
  }

  throw new Error('Max retries exceeded')
}

class APIError extends Error {
  constructor(
    public statusCode: number,
    public response: any,
    public retryable: boolean
  ) {
    super(`API Error: ${statusCode}`)
  }
}

async function safeAPIRequest(endpoint: string, options: RequestOptions) {
  try {
    const response = await fetch(endpoint, options)

    // Success
    if (response.ok) {
      return await response.json()
    }

    // Client errors (4xx) - usually not retryable
    if (response.status >= 400 && response.status < 500) {
      if (response.status === 401) {
        // Token expired, refresh and retry
        await refreshAccessToken()
        return safeAPIRequest(endpoint, options)
      }

      if (response.status === 429) {
        // Rate limited, retry with backoff
        throw new APIError(429, await response.json(), true)
      }

      // Other 4xx errors - don't retry
      throw new APIError(response.status, await response.json(), false)
    }

    // Server errors (5xx) - retryable
    if (response.status >= 500) {
      throw new APIError(response.status, await response.json(), true)
    }
  } catch (error) {
    if (error instanceof APIError) throw error

    // Network errors - retryable
    throw new APIError(0, { message: error.message }, true)
  }
}

async function retryWithBackoff<T>(
  fn: () => Promise<T>,
  maxRetries = 3,
  baseDelay = 1000
): Promise<T> {
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    try {
      return await fn()
    } catch (error) {
      if (error instanceof APIError && !error.retryable) {
        throw error // Don't retry non-retryable errors
      }

      if (attempt === maxRetries - 1) {
        throw error // Last attempt failed
      }

      // Exponential backoff with jitter
      const delay = baseDelay * Math.pow(2, attempt) * (0.5 + Math.random() * 0.5)
      console.log(`Attempt ${attempt + 1} failed, retrying in ${delay}ms`)
      await sleep(delay)
    }
  }

  throw new Error('Unreachable')
}

// Usage
const data = await retryWithBackoff(() => slackClient.postMessage('#general', 'Hello!'))

interface WebhookPayload {
  event_type: string
  data: any
  timestamp: number
  signature: string
}

app.post('/webhooks/stripe', async (req, res) => {
  // 1. Verify signature (CRITICAL for security)
  const signature = req.headers['stripe-signature']
  let event: Stripe.Event

  try {
    event = stripe.webhooks.constructEvent(req.body, signature, STRIPE_WEBHOOK_SECRET)
  } catch (error) {
    console.error('⚠️ Webhook signature verification failed:', error.message)
    return res.status(400).send('Webhook signature verification failed')
  }

  // 2. Respond immediately (don't make webhook wait)
  res.status(200).send('Received')

  // 3. Process asynchronously
  await queue.add('process-webhook', {
    event_type: event.type,
    event_id: event.id,
    data: event.data
  })
})

// Process webhook in background job
async function processWebhook(job: Job) {
  const { event_type, event_id, data } = job.data

  // Idempotency check (handle duplicate webhooks)
  const existing = await db.webhookEvents.findOne({ event_id })
  if (existing) {
    console.log(`Webhook ${event_id} already processed`)
    return
  }

  // Mark as processing
  await db.webhookEvents.create({ event_id, status: 'processing' })

  try {
    switch (event_type) {
      case 'payment_intent.succeeded':
        await handlePaymentSuccess(data)
        break

      case 'customer.subscription.deleted':
        await handleSubscriptionCancelled(data)
        break

      // ... other event types
    }

    // Mark as completed
    await db.webhookEvents.update(event_id, { status: 'completed' })
  } catch (error) {
    // Mark as failed, will retry
    await db.webhookEvents.update(event_id, {
      status: 'failed',
      error: error.message,
      retry_count: (existing?.retry_count || 0) + 1
    })
    throw error // Let queue retry
  }
}

// HMAC signature verification
function verifyWebhookSignature(payload: string, signature: string, secret: string): boolean {
  const expectedSignature = crypto.createHmac('sha256', secret).update(payload).digest('hex')

  // Constant-time comparison to prevent timing attacks
  return crypto.timingSafeEqual(Buffer.from(signature), Buffer.from(expectedSignature))
}

// Timestamp validation (prevent replay attacks)
function validateWebhookTimestamp(timestamp: number, toleranceSeconds = 300) {
  const now = Math.floor(Date.now() / 1000)
  return Math.abs(now - timestamp) < toleranceSeconds
}

interface SyncStrategy {
  // Full sync: Get all data from API
  fullSync(): Promise<void>

  // Incremental sync: Get only changed data since last sync
  incrementalSync(since: Date): Promise<void>

  // Real-time sync: Use webhooks for instant updates
  realTimeSync(webhookData: any): Promise<void>
}

class GoogleCalendarSync implements SyncStrategy {
  async fullSync() {
    const calendars = await googleCalendar.list()

    for (const calendar of calendars) {
      const events = await googleCalendar.events.list({
        calendarId: calendar.id,
        maxResults: 2500
      })

      await db.events.bulkUpsert(events)
    }
  }

  async incrementalSync(since: Date) {
    const events = await googleCalendar.events.list({
      updatedMin: since.toISOString(),
      showDeleted: true // Important: track deletions
    })

    for (const event of events) {
      if (event.status === 'cancelled') {
        await db.events.delete(event.id)
      } else {
        await db.events.upsert(event)
      }
    }
  }

  async realTimeSync(webhookData: any) {
    // Google sends channel notifications
    const { resourceId, resourceUri } = webhookData

    // Fetch the changed resource
    const event = await googleCalendar.events.get(resourceId)
    await db.events.upsert(event)
  }
}

interface SyncJob {
  user_id: string
  service: string
  type: 'full' | 'incremental'
}

// Schedule sync jobs
async function scheduleSyncJobs() {
  // Full sync: Weekly for all active integrations
  cron.schedule('0 0 * * 0', async () => {
    const integrations = await db.integrations.findActive()

    for (const integration of integrations) {
      await queue.add('sync', {
        user_id: integration.user_id,
        service: integration.service,
        type: 'full'
      })
    }
  })

  // Incremental sync: Every 15 minutes
  cron.schedule('*/15 * * * *', async () => {
    const integrations = await db.integrations.findActive()

    for (const integration of integrations) {
      await queue.add('sync', {
        user_id: integration.user_id,
        service: integration.service,
        type: 'incremental'
      })
    }
  })
}

// Process sync job
async function processSyncJob(job: Job<SyncJob>) {
  const { user_id, service, type } = job.data

  const sync = getSyncStrategy(service)

  if (type === 'full') {
    await sync.fullSync()
  } else {
    const lastSync = await db.syncLog.getLastSync(user_id, service)
    await sync.incrementalSync(lastSync.completed_at)
  }

  await db.syncLog.create({
    user_id,
    service,
    type,
    completed_at: new Date()
  })
}

class SlackIntegration {
  async postMessage(channel: string, text: string, attachments?: any[]) {
    const accessToken = await getValidAccessToken(userId, 'slack')

    return await retryWithBackoff(() =>
      fetch('https://slack.com/api/chat.postMessage', {
        method: 'POST',
        headers: {
          Authorization: `Bearer ${accessToken}`,
          'Content-Type': 'application/json'
        },
        body: JSON.stringify({
          channel,
          text,
          attachments
        })
      })
    )
  }

  async getChannels() {
    const accessToken = await getValidAccessToken(userId, 'slack')

    const response = await fetch('https://slack.com/api/conversations.list', {
      headers: { Authorization: `Bearer ${accessToken}` }
    })

    const data = await response.json()

    if (!data.ok) {
      throw new Error(`Slack API error: ${data.error}`)
    }

    return data.channels
  }
}

class StripeIntegration {
  async createSubscription(customerId: string, priceId: string) {
    try {
      const subscription = await stripe.subscriptions.create({
        customer: customerId,
        items: [{ price: priceId }],
        payment_behavior: 'default_incomplete',
        expand: ['latest_invoice.payment_intent'],
        metadata: {
          user_id: userId,
          plan: 'pro'
        }
      })

      return {
        subscription_id: subscription.id,
        client_secret: subscription.latest_invoice.payment_intent.client_secret
      }
    } catch (error) {
      if (error instanceof Stripe.errors.StripeError) {
        // Handle specific Stripe errors
        if (error.type === 'card_error') {
          throw new Error('Card was declined')
        }
      }
      throw error
    }
  }

  async cancelSubscription(subscriptionId: string) {
    // Cancel at period end (don't refund)
    await stripe.subscriptions.update(subscriptionId, {
      cancel_at_period_end: true
    })
  }
}

import { google } from 'googleapis'

class GoogleIntegration {
  async sendEmail(to: string, subject: string, body: string) {
    const auth = await this.getOAuth2Client()
    const gmail = google.gmail({ version: 'v1', auth })

    const message = [`To: ${to}`, `Subject: ${subject}`, '', body].join('\n')

    const encodedMessage = Buffer.from(message)
      .toString('base64')
      .replace(/\+/g, '-')
      .replace(/\//g, '_')
      .replace(/=+$/, '')

    await gmail.users.messages.send({
      userId: 'me',
      requestBody: {
        raw: encodedMessage
      }
    })
  }

  async listCalendarEvents(calendarId: string, timeMin: Date, timeMax: Date) {
    const auth = await this.getOAuth2Client()
    const calendar = google.calendar({ version: 'v3', auth })

    const response = await calendar.events.list({
      calendarId,
      timeMin: timeMin.toISOString(),
      timeMax: timeMax.toISOString(),
      singleEvents: true,
      orderBy: 'startTime'
    })

    return response.data.items
  }
}

// tests/mocks/stripe.ts
export class MockStripe {
  subscriptions = {
    create: jest.fn().mockResolvedValue({
      id: 'sub_123',
      status: 'active'
    }),

    cancel: jest.fn().mockResolvedValue({
      id: 'sub_123',
      status: 'canceled'
    })
  }
}

// tests/integration.test.ts
describe('Stripe Integration', () => {
  let stripeIntegration: StripeIntegration

  beforeEach(() => {
    stripe = new MockStripe()
    stripeIntegration = new StripeIntegration(stripe)
  })

  it('creates a subscription', async () => {
    const result = await stripeIntegration.createSubscription('cus_123', 'price_123')

    expect(result.subscription_id).toBe('sub_123')
    expect(stripe.subscriptions.create).toHaveBeenCalledWith({
      customer: 'cus_123',
      items: [{ price: 'price_123' }]
      // ...
    })
  })

  it('handles card errors gracefully', async () => {
    stripe.subscriptions.create.mockRejectedValue(
      new Stripe.errors.StripeCardError('Card declined')
    )

    await expect(stripeIntegration.createSubscription('cus_123', 'price_123')).rejects.toThrow(
      'Card was declined'
    )
  })
})

// Generate valid webhook signatures for testing
function generateTestWebhook(payload: any, secret: string) {
  const timestamp = Math.floor(Date.now() / 1000)
  const signature = crypto
    .createHmac('sha256', secret)
    .update(`${timestamp}.${JSON.stringify(payload)}`)
    .digest('hex')

  return {
    payload,
    headers: {
      'stripe-signature': `t=${timestamp},v1=${signature}`
    }
  }
}

describe('Webhook Handler', () => {
  it('processes valid webhook', async () => {
    const webhook = generateTestWebhook(
      {
        type: 'payment_intent.succeeded',
        data: {
          /* ... */
        }
      },
      STRIPE_WEBHOOK_SECRET
    )

    const response = await request(app)
      .post('/webhooks/stripe')
      .set(webhook.headers)
      .send(webhook.payload)

    expect(response.status).toBe(200)
    // Verify processing happened
  })

  it('rejects invalid signature', async () => {
    const response = await request(app)
      .post('/webhooks/stripe')
      .set({ 'stripe-signature': 'invalid' })
      .send({ type: 'payment_intent.succeeded' })

    expect(response.status).toBe(400)
  })
})

interface IntegrationHealth {
  service: string
  status: 'healthy' | 'degraded' | 'down'
  last_success: Date
  last_failure?: Date
  error_rate_24h: number
}

async function checkIntegrationHealth(service: string): Promise<IntegrationHealth> {
  const logs = await db.integrationLogs.findRecent(service, '24h')

  const total = logs.length
  const failures = logs.filter(l => l.status === 'failed').length
  const errorRate = failures / total

  let status: 'healthy' | 'degraded' | 'down'
  if (errorRate < 0.01) status = 'healthy'
  else if (errorRate < 0.1) status = 'degraded'
  else status = 'down'

  return {
    service,
    status,
    last_success: logs.find(l => l.status === 'success')?.timestamp,
    last_failure: logs.find(l => l.status === 'failed')?.timestamp,
    error_rate_24h: errorRate
  }
}

// Log all API requests
async function logAPIRequest(
  service: string,
  endpoint: string,
  statusCode: number,
  duration: number,
  error?: Error
) {
  await db.apiLogs.create({
    service,
    endpoint,
    status_code: statusCode,
    duration_ms: duration,
    error: error?.message,
    timestamp: new Date()
  })

  // Alert on high error rates
  if (statusCode >= 500) {
    const recentErrors = await db.apiLogs.count({
      service,
      status_code: { $gte: 500 },
      timestamp: { $gte: new Date(Date.now() - 5 * 60 * 1000) } // Last 5 min
    })

    if (recentErrors > 10) {
      await sendAlert({
        title: `High error rate for ${service} integration`,
        message: `${recentErrors} errors in last 5 minutes`
      })
    }
  }
}

interface IntegrationStatus {
  connected: boolean
  last_sync: Date
  sync_in_progress: boolean
  error?: string
}

// Show integration status to user
<Card>
  <IntegrationIcon service="slack" />
  <div>
    <h3>Slack</h3>
    {status.connected ? (
      <>
        <Badge color="green">Connected</Badge>
        <p>Last synced {formatRelative(status.last_sync)}</p>
        {status.sync_in_progress && <Spinner />}
      </>
    ) : (
      <>
        <Badge color="red">Disconnected</Badge>
        {status.error && <p className="error">{status.error}</p>}
        <Button onClick={reconnect}>Reconnect</Button>
      </>
    )}
  </div>
  <Button variant="secondary" onClick={disconnect}>
    Disconnect
  </Button>
</Card>