Introduction to Legacy Code Modernization

A practical guide to understanding and transforming aging software systems

Why this matters (in one stat)

43% of global banking transactions still run on COBOL.

If you're modernizing legacy systems, you're handling revenue-critical code.

What you'll learn

  • How to recognize legacy risk quickly
  • The four main modernization paths
  • How to choose the right approach for your system

What is Legacy Code?

  • Software that's critical to business operations
  • Built with outdated technologies or practices
  • Often difficult to change or extend
  • Typically lacks modern development practices

Why legacy sticks around

Stability, revenue, and risk keep old systems alive.

Why Legacy Systems Persist

Legacy code doesn't exist because developers are lazy.

It persists because:

  1. It works – handles millions in revenue
  2. Deep domain knowledge embedded in code
  3. High risk of replacement failures
  4. Resource constraints – easier to patch than rebuild

What it looks like in the real world

Two examples you might recognize.

Real-World Examples

COBOL Banking Systems

IDENTIFICATION DIVISION.
PROGRAM-ID. CALCULATE-INTEREST.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 ACCOUNT-BALANCE PIC 9(10)V99.
01 INTEREST-RATE   PIC 9V999.

Still processing 43% of banking transactions globally.

Visual FoxPro ERP Systems

SELECT customers
SCAN FOR balance > 10000
  DO invoice_customer
  REPLACE last_contact WITH DATE()
ENDSCAN

Powering manufacturing and distribution companies decades after official EOL.

The hard parts

Where modernization efforts usually fail.

Common Modernization Challenges

  • Lack of documentation – original developers long gone
  • Tight coupling – UI, business logic, data access intertwined
  • Hidden dependencies – undocumented integrations
  • Testing gaps – no automated tests to catch regressions
  • Knowledge loss – specialized expertise hard to find

Your strategy options

Pick the path that matches risk and timeline.

Modernization Strategies

1. Strangler Fig Pattern

Gradually replace functionality while keeping the system running.

2. Replatform

Lift-and-shift to modern infrastructure without code changes.

3. Refactor

Incrementally improve code quality and architecture.

4. Rewrite

Start fresh—highest risk, highest potential reward.

Modernization Strategies (cont.)

3. Refactor

Incrementally improve code quality and architecture.

4. Rewrite

Start fresh—highest risk, highest potential reward.

The Strangler Fig Approach

// New modern API wraps legacy system
export class CustomerService {
  async getCustomer(id: string) {
    // Route to new implementation if migrated
    if (await this.isMigrated(id)) {
      return this.newService.get(id)
    }
    // Fall back to legacy system
    return this.legacyService.get(id)
  }
}

Key idea: Route new traffic to the modern service, keep legacy as fallback.

Key Considerations

Before modernizing, ask:

  • What problem are we solving? (Performance? Maintainability? Skills?)
  • What's the business impact of downtime or bugs?
  • Do we have the right team and tools?
  • Can we measure success objectively?

Modernization is a business decision, not just a technical one.

Next Steps

  1. Assess your legacy landscape
  2. Identify highest-value modernization targets
  3. Build a cross-functional team
  4. Start small with low-risk components
  5. Measure and iterate

The best time to start was five years ago. The second best time is now.

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Introduction to Legacy Code Modernization