Stop Fighting Legacy Systems: Streamline Enterprise Integration with Modern TypeScript

Your enterprise runs on a maze of legacy systems—COBOL mainframes, SAP ERP, IBM i, and decades-old databases that somehow still power mission-critical operations. You've been tasked with modernizing, but the thought of touching these systems makes you break out in cold sweats. What if there was a way to bridge these worlds without the typical integration nightmare?

The Enterprise Integration Problem You Know Too Well

Enterprise integration isn't just about connecting systems—it's about navigating a minefield of outdated protocols, undocumented APIs, and systems that were never designed to talk to each other. You're dealing with:

Legacy System Constraints: COBOL systems with no REST APIs, mainframes behind VPN gateways, and databases that predate JSON by decades. Every integration feels like archaeological work.

Data Transformation Chaos: Converting EBCDIC to UTF-8, mapping 40-character fixed-width fields to modern JSON schemas, and handling time zones that nobody documented properly.

Security Compliance Gaps: Legacy systems with weak authentication, no OAuth support, and audit trails that compliance teams hate reviewing.

Operational Brittleness: Integration layers that break when legacy systems have maintenance windows, timeout inconsistencies, and error messages that reveal nothing useful.

Knowledge Silos: The one person who understands the AS/400 system retires next month, and their documentation consists of handwritten notes from 1987.

The Modern Integration Approach That Actually Works

These Cursor Rules transform how you approach legacy integration by establishing a composable, API-first architecture that treats legacy systems as external services rather than architectural constraints.

Instead of building monolithic integration layers that become unmaintainable, you create independent connector modules for each legacy system. Each connector is a self-contained NestJS module with its own error handling, circuit breakers, and health checks.

// Before: Monolithic integration nightmare
class LegacyIntegration {
  async processOrder(order: any) {
    // 500+ lines of mixed SAP, mainframe, and business logic
    // Impossible to test, maintain, or understand
  }
}

// After: Composable connector architecture
@Module({
  providers: [SapOrderService, MainframeInventoryService, ModernOrderService],
  exports: [OrderOrchestrationService]
})
export class OrderProcessingModule {
  // Clean separation of concerns
  // Independent testing and deployment
  // Clear error boundaries
}

The rules enforce strict boundaries between legacy and modern code—no direct database access from modern services, no legacy business logic bleeding into your clean TypeScript codebase.

Workflow Transformation: From Weeks to Days

Before: Adding a new legacy system integration meant weeks of reverse engineering, custom connection handling, and brittle error management.

After: Generate a new connector in minutes with standardized patterns, built-in resilience, and comprehensive monitoring.

Legacy System Assessment (≤2 weeks)

Instead of diving into integration code, you start with a structured assessment that maps out data models, change windows, SLAs, and hidden dependencies. This front-loaded analysis prevents the typical "surprise discoveries" that derail integration projects.

Incremental Strangler-Fig Migration

Route new traffic to modern services while proxying existing functionality to legacy systems. No big-bang migrations that keep you up at night wondering what you broke.

// Gradual migration with zero business disruption
@Controller('orders')
export class OrderController {
  @Get(':id')
  async getOrder(@Param('id') id: string) {
    // New orders from modern system
    if (await this.isModernOrder(id)) {
      return this.modernOrderService.getById(id);
    }
    // Legacy orders still work
    return this.legacyOrderService.getById(id);
  }
}

Built-in Resilience Patterns

Every legacy connector includes circuit breakers, exponential backoff, and rate limiting. Legacy systems won't bring down your modern infrastructure when they inevitably have issues.

Real Developer Workflows: Daily Integration Tasks

Adding a New Legacy System Connection

# Generate connector boilerplate
nest generate legacy-connector --name=sap-hr

# Implement required interfaces
export class SapHrConnector implements LegacyConnector<SapEmployee> {
  async fetchEmployees(): Promise<SapEmployee[]> {
    // Connector-specific logic with built-in error handling
  }
}

# Register health check
@Module({
  providers: [SapHrConnector, SapHrHealthIndicator]
})

Handling Data Transformation

Clean separation between legacy data formats and modern DTOs with dedicated mapper classes:

@Injectable()
export class SapEmployeeMapper {
  toModernFormat(sapData: SapEmployeeRecord): Employee {
    return {
      id: sapData.PERNR.trim(),
      name: this.convertFromCp1252(sapData.VORNA + ' ' + sapData.NACHN),
      hireDate: this.convertSapDate(sapData.BEGDA)
    };
  }
}

Debugging Integration Issues

Comprehensive observability with OpenTelemetry spans, structured logging, and error classification:

// Every legacy call is traced
@Trace('sap.fetch-employees')
async fetchEmployees(): Promise<Employee[]> {
  try {
    const employees = await this.sapClient.getEmployees();
    this.logger.info('SAP employee fetch successful', { count: employees.length });
    return employees;
  } catch (error) {
    if (error instanceof SapTimeoutError) {
      this.circuitBreaker.recordFailure();
      throw new LegacyTimeoutError();
    }
    throw error;
  }
}

Implementation: Getting Started in 30 Minutes

1. Initialize Your Integration Project

npm install @nestjs/core @nestjs/common class-validator opossum

2. Configure TypeScript for Enterprise Integration

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "exactOptionalPropertyTypes": true,
    "useUnknownInCatchVariables": true
  }
}

3. Create Your First Legacy Connector

// src/legacy/mainframe/mainframe.module.ts
@Module({
  providers: [MainframeService, MainframeHealthIndicator],
  exports: [MainframeService]
})
export class MainframeModule {}

// src/legacy/mainframe/mainframe.service.ts
@Injectable()
export class MainframeService {
  private readonly circuitBreaker = new CircuitBreaker(
    this.callMainframe.bind(this),
    { timeout: 5000, errorThresholdPercentage: 50 }
  );

  async getCustomer(id: string): Promise<Customer> {
    return this.circuitBreaker.fire(id);
  }
}

4. Add Health Monitoring

@Injectable()
export class MainframeHealthIndicator extends HealthIndicator {
  async isHealthy(key: string): Promise<HealthIndicatorResult> {
    try {
      await this.mainframeService.ping();
      return this.getStatus(key, true);
    } catch (error) {
      return this.getStatus(key, false, { error: error.message });
    }
  }
}

5. Configure Security and Compliance

// OAuth 2.1 with PKCE for legacy system access
@Injectable()
export class LegacyAuthService {
  async getAccessToken(): Promise<string> {
    // Implement OAuth 2.1 client credentials flow
    // Cache tokens with appropriate TTL
    // Handle token refresh automatically
  }
}

Results: Measurable Integration Success

Development Velocity

New connector setup: 3 hours → 30 minutes
Integration testing: 2 days → 4 hours with container-based simulators
Documentation generation: Manual weeks → Automatic with OpenAPI

Operational Reliability

Legacy system failures: Isolated to specific connectors, no cascade failures
Error debugging: Structured logs with correlation IDs across system boundaries
Monitoring coverage: 100% health check coverage for all legacy dependencies

Compliance and Security

Audit trail completeness: Every legacy interaction logged with user context
Data encryption: Automatic TLS 1.3 for all legacy connections
PII protection: Automatic log sanitization and data masking

Team Productivity

Knowledge transfer: Standardized connector patterns reduce tribal knowledge
Code review efficiency: Consistent error handling and testing patterns
Onboarding time: New developers productive with legacy integrations in days, not weeks

The Integration Architecture You've Been Looking For

Stop treating legacy integration as a necessary evil. With these Cursor Rules, you build integration layers that are as maintainable, testable, and reliable as your modern microservices. Your legacy systems become composable building blocks rather than architectural constraints.

The result? Enterprise integration that actually works—incremental migration paths, built-in resilience, and developer workflows that don't make you dread touching legacy systems.

Ready to transform how your team approaches legacy integration? These rules provide the foundation for integration architecture that scales with your business, not against it.

Legacy-Modern Integration Rulebook