Legacy Modernization

Safely upgrade and modernize legacy systems.

When to Use

• Framework migrations

• Language version upgrades

• Monolith decomposition

• Technical debt reduction

• Dependency updates

Migration Strategies

Strangler Fig Pattern

┌─────────────────────────────────────┐ │ Load Balancer │ └──────────────┬──────────────────────┘ │ ┌───────┴───────┐ │ │ ┌──────▼──────┐ ┌──────▼──────┐ │ Legacy │ │ New │ │ System │ │ Service │ └─────────────┘ └─────────────┘

1. Route new features to new service
2. Gradually migrate existing features
3. Eventually retire legacy system

Branch by Abstraction

1. Create abstraction layer

class PaymentProcessor(ABC): @abstractmethod def process(self, amount: float) -> bool: pass

2. Wrap legacy implementation

class LegacyPaymentProcessor(PaymentProcessor): def init(self, legacy_system): self.legacy = legacy_system

def process(self, amount: float) -> bool:
    return self.legacy.old_process_method(amount)

3. Create new implementation

class ModernPaymentProcessor(PaymentProcessor): def process(self, amount: float) -> bool: # New implementation pass

4. Use feature flag to switch

processor = (ModernPaymentProcessor() if feature_flag("new_payment") else LegacyPaymentProcessor(legacy))

Migration Checklist

Before Starting

• Document current behavior

• Add tests for existing functionality

• Set up monitoring and alerts

• Create rollback plan

• Communicate with stakeholders

During Migration

• Make incremental changes

• Test after each change

• Monitor error rates

• Keep legacy running in parallel

• Document breaking changes

After Migration

• Remove feature flags

• Clean up legacy code

• Update documentation

• Archive old codebase

• Post-mortem lessons learned

Common Migrations

jQuery to React

// Phase 1: Embed React in jQuery app const root = document.getElementById("new-component"); ReactDOM.render(<NewComponent />, root);

// Phase 2: Shared state window.appState = { user: null }; // Both jQuery and React read from appState

// Phase 3: Gradual replacement // Replace one component at a time

Python 2 to 3

Use future imports for compatibility

from future import print_function, division, absolute_import

Use six for cross-version compatibility

import six if six.PY2: text_type = unicode else: text_type = str

Run 2to3 tool

python -m lib2to3 --write --nobackups src/

Compatibility Layers

Adapter for API changes

class CompatibilityAdapter: def init(self, new_service): self.new = new_service

# Old API signature
def get_user(self, user_id):
    # Translate to new API
    return self.new.fetch_user(id=user_id)

# Deprecation warning
def old_method(self):
    warnings.warn(
        "old_method is deprecated, use new_method instead",
        DeprecationWarning
    )
    return self.new.new_method()

Examples

Input: "Migrate from Express to Fastify" Action: Create adapter layer, migrate routes incrementally, test each step

Input: "Reduce technical debt in this module" Action: Add tests first, refactor incrementally, maintain compatibility