API-First Lending: Building Scalable Loan Origination Systems

// Application Management
POST   /api/v1/applications          # Create new application
GET    /api/v1/applications          # List applications (with filters)
GET    /api/v1/applications/{id}     # Get application details
PUT    /api/v1/applications/{id}     # Update application
POST   /api/v1/applications/{id}/submit  # Submit for processing

// Document Management  
POST   /api/v1/applications/{id}/documents  # Upload document
GET    /api/v1/applications/{id}/documents  # List documents
GET    /api/v1/documents/{id}        # Get document details
POST   /api/v1/documents/{id}/process  # Process document with AI

// Underwriting Workflow
POST   /api/v1/applications/{id}/underwrite  # Start underwriting
GET    /api/v1/applications/{id}/underwriting  # Get underwriting status
POST   /api/v1/applications/{id}/approve   # Approve application
POST   /api/v1/applications/{id}/decline   # Decline application

// Example implementation - processing uploaded documents
router.post('/documents/:id/process', async (req, res) => {
  const { id } = req.params;
  const { documentType, applicationId } = req.body;

  // Validate document type
  const validTypes = ['bank-statement', 'tax-return', 'appraisal'];
  if (!validTypes.includes(documentType)) {
    return res.status(400).json({ 
      error: 'Invalid document type' 
    });
  }

  // Process with appropriate AI model
  const extractionResult = await processDocument(id, documentType);
  
  // Emit event for other services
  await emitEvent('DocumentProcessed', {
    documentId: id,
    applicationId,
    documentType,
    extractedData: extractionResult,
    confidence: extractionResult.confidence
  });

  res.json({
    status: 'processed',
    documentId: id,
    extractedFields: extractionResult.fields,
    confidence: extractionResult.confidence
  });
});

// Circuit breaker implementation for credit checks
const options = {
  timeout: 10000, // 10 second timeout
  errorThresholdPercentage: 50, // 50% errors trip breaker
  resetTimeout: 30000 // 30 second reset
};

const creditCheck = async (applicantData) => {
  // Implementation calling credit bureau API
  const response = await axios.post(creditBureauUrl, applicantData);
  return response.data;
};

// Fire with circuit breaker protection
const result = await breaker.fire(applicantData);

// Handle different states
breaker.on('open', () => {
  // Use cached credit data or fallback scoring
});

// Provider abstraction pattern
class DocumentProvider {
  async fetchBankStatements(credentials) {
    throw new Error('Not implemented');
  }
}

class PlaidProvider extends DocumentProvider {
  async fetchBankStatements(credentials) {
    // Plaid-specific implementation
    const response = await plaidClient.getTransactions(credentials);
    return this.normalizeTransactions(response.transactions);
  }
}

// Factory to select provider
class DocumentProviderFactory {
  static createProvider(providerName) {
    switch (providerName) {
      case 'plaid': return new PlaidProvider();
      case 'mx': return new MXProvider();
      default: throw new Error('Unknown provider');
    }
  }
}