Legacy Code Modernization Course

Master the art of safely modernizing legacy systems without breaking production. Learn proven strategies, design patterns, and testing techniques used by industry experts.

5 modules
21 lessons
0 min
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Part 1

Foundations of Legacy Code

Understand what legacy code is, why it exists, and how to approach modernization systematically.

  1. Understanding Legacy Systems

    Learn what defines legacy code and why it's critical to your business despite its challenges.

  2. Introduction to Legacy Code Modernization

    An overview of legacy systems, why they persist, and proven strategies for modernizing them safely.

  3. The Cost of Technical Debt

    Quantify the real impact of technical debt on velocity, morale, and business outcomes.

  4. Risk Management in Modernization

    Learn strategies to minimize risk when making changes to mission-critical legacy systems.

Part 2

Modernization Strategies

Master proven patterns for safely migrating legacy systems without breaking production.

  1. Strangler Fig Pattern

    Learn how to incrementally replace legacy systems by gradually strangling the old system with new functionality.

  2. Branch by Abstraction

    Safely swap dependencies in production without downtime using abstraction layers.

  3. Parallel Run Pattern

    Run old and new implementations side-by-side to validate correctness before switching over.

  4. Feature Flags for Safe Deployments

    Deploy safely and release confidently using feature flags in legacy systems.

Part 3

Testing & Safety

Learn how to safely add tests to untested legacy code and build confidence for refactoring.

  1. Testing Legacy Code Without Tests

    Proven techniques to safely add tests to untested legacy code and enable confident refactoring.

  2. Understanding Seams

    Learn Michael Feathers' concept of seams for testing legacy code without breaking production.

  3. Characterization Tests

    Document existing behavior with tests before making changes to legacy systems.

  4. Approval Testing

    Use approval testing to quickly capture and verify complex outputs in legacy code.

Part 4

Database & Data Migration

Master zero-downtime database migrations and data modernization strategies.

  1. Database Migration Strategies

    Master zero-downtime database migrations using expand-contract, dual writes, and backward-compatible schema changes.

  2. Expand-Contract Pattern

    Learn the expand-contract pattern for making backward-compatible database schema changes.

  3. Dual Writes & Data Synchronization

    Implement dual writes to keep old and new systems in sync during migration.

  4. Data Migration Verification

    Ensure data integrity and correctness when migrating between systems.

Part 5

Design Patterns for Legacy Code

Apply essential design patterns to improve legacy code structure and maintainability.

  1. Decorator Pattern

    Add behavior to objects dynamically without inheritance using the Decorator pattern.

  2. Observer Pattern

    Build event-driven systems and reactive applications with the Observer pattern.

  3. Singleton Pattern

    Understand when to use (and when NOT to use) the Singleton pattern in legacy code.

  4. Factory Pattern

    Create objects without tight coupling to concrete classes using the Factory pattern.

  5. Adapter Pattern

    Integrate incompatible interfaces in legacy systems using the Adapter pattern.