Large-Scale Refactoring

Refactoring is the art of improving code structure without changing its behavior. It’s where technical debt meets its reckoning, where legacy systems get a second life, and where Claude Code truly shines. This lesson explores how to leverage AI assistance for large-scale refactoring that would take months manually.

The Refactoring Challenge

Scenario: You’ve inherited a 5-year-old e-commerce platform. 200,000 lines of code. jQuery spaghetti in the frontend. PHP 5.6 in the backend. No tests. Inconsistent patterns. The business wants to add real-time features, but touching anything risks breaking everything. Sound familiar?

Month 1: Analysis and planning
- Manual code review
- Document all dependencies
- Create refactoring roadmap

Month 2-3: Write tests for critical paths
- Manually write test cases
- Hope you caught edge cases

Month 4-6: Incremental refactoring
- Update one module at a time
- Fix breaks as they appear
- Pray nothing slips through

Month 7+: Ongoing fixes and regression

Week 1: AI-powered analysis
> Analyze our codebase and identify:
> - Deprecated patterns
> - Inconsistent naming
> - Dead code
> - Circular dependencies
> - Security vulnerabilities

Week 2-3: Automated test generation
> Generate comprehensive tests for all critical paths
> Include edge cases I might miss

Week 4-6: Systematic refactoring
> Refactor module by module with:
> - Automatic pattern updates
> - Dependency management
> - Real-time validation

Week 7: Polish and optimization

Setting Up for Safe Refactoring

Essential Preparations

Create a safety net

> Before we start refactoring:
> 1. Create a comprehensive test suite for current behavior
> 2. Set up git hooks for pre-commit testing
> 3. Configure CI/CD to catch regressions
> 4. Document current behavior

Analyze the current state

> Analyze our codebase and create a refactoring plan:
> - Identify code smells
> - Find duplicate code
> - Map dependencies
> - List deprecated patterns
> - Estimate complexity for each module

Set up refactoring infrastructure Create .claude/commands/refactor-safely.md:

Refactor $ARGUMENTS following these rules:
1. Maintain exact functionality
2. Write tests before changing code
3. Update one pattern at a time
4. Run tests after each change
5. Create descriptive commits
6. Update documentation

Configure incremental approach

{
  "refactoring": {
    "batchSize": 10,
    "testFirst": true,
    "preserveComments": true,
    "updateDocs": true,
    "commitAfterEach": true
  }
}

Refactoring Patterns and Strategies

Pattern 1: Modernizing Legacy JavaScript

From jQuery chaos to modern React/Vue/Angular

> Analyze our jQuery codebase and create a modernization plan.
> We want to migrate to React gradually while keeping the app functional.

Claude’s incremental strategy:

Phase 1: Preparation

// 1. Create abstraction layer
class LegacyBridge {
  static $(selector) {
    // Wrap jQuery for gradual replacement
    return {
      original: $(selector),
      onClick: (handler) => {
        $(selector).on('click', handler);
        // Track for migration
        this.trackUsage('click', selector);
      }
    };
  }
}

// 2. Replace direct jQuery calls
// Before: $('#button').click(handler)
// After: LegacyBridge.$('#button').onClick(handler)

Phase 2: Component Extraction

// Identify UI components in jQuery
> Find all jQuery UI patterns and extract them into components

// Claude identifies patterns like:
$('.user-card').each(function() {
  $(this).find('.name').text(userData.name);
  $(this).find('.email').text(userData.email);
});

// Converts to React component:
const UserCard = ({ user }) => (
  <div className="user-card">
    <div className="name">{user.name}</div>
    <div className="email">{user.email}</div>
  </div>
);

Phase 3: State Management

// From scattered jQuery state
var userLoggedIn = false;
$('#login').click(function() {
  userLoggedIn = true;
  updateUI();
});

// To centralized state
const useAuthStore = create((set) => ({
  isLoggedIn: false,
  login: () => set({ isLoggedIn: true }),
  logout: () => set({ isLoggedIn: false })
}));

Phase 4: Complete Migration

// Final cleanup
> Remove jQuery dependencies
> Update build system
> Remove legacy bridge code
> Optimize bundle size

// Result: Modern React app
// 60% smaller bundle
// 3x faster rendering
// Type-safe with TypeScript

Pattern 2: Backend Modernization

From PHP 5.6 spaghetti to modern PHP 8+ or Node.js

> Our backend is PHP 5.6 with:
> - No namespace organization
> - Mixed HTML/PHP files
> - Direct MySQL queries (no ORM)
> - Global variables everywhere
>
> Create a plan to modernize to PHP 8.3 with Laravel

Claude’s systematic approach:

// Before: PHP 5.6 style
function calculateTotal($items, $tax) {
    $total = 0;
    foreach ($items as $item) {
        $total += $item['price'] * $item['quantity'];
    }
    return $total + ($total * $tax);
}

// After: PHP 8.3 with types
declare(strict_types=1);

function calculateTotal(
    array $items,
    float $tax
): float {
    $subtotal = array_reduce(
        $items,
        fn($carry, $item) => $carry + ($item->price * $item->quantity),
        0.0
    );
    return $subtotal * (1 + $tax);
}

// Before: Procedural mess
include 'db.php';
$user_id = $_GET['id'];
$result = mysql_query("SELECT * FROM users WHERE id = $user_id");
$user = mysql_fetch_assoc($result);

// After: Repository pattern
namespace App\Repositories;

class UserRepository
{
    public function __construct(
        private PDO $db
    ) {}

    public function find(int $id): ?User
    {
        $stmt = $this->db->prepare(
            'SELECT * FROM users WHERE id = :id'
        );
        $stmt->execute(['id' => $id]);

        $data = $stmt->fetch(PDO::FETCH_ASSOC);
        return $data ? User::fromArray($data) : null;
    }
}

// Migrate to Laravel structure
> Convert our procedural PHP to Laravel:
> 1. Create models for each table
> 2. Convert queries to Eloquent
> 3. Extract business logic to services
> 4. Create proper routing
> 5. Add middleware for auth

// Result: Clean MVC architecture
Route::middleware(['auth'])->group(function () {
    Route::get('/users/{user}', [UserController::class, 'show']);
    Route::post('/users', [UserController::class, 'store']);
});

class UserController extends Controller
{
    public function __construct(
        private UserService $userService
    ) {}

    public function show(User $user): JsonResponse
    {
        return response()->json(
            new UserResource($user)
        );
    }
}

Pattern 3: Database Schema Evolution

Refactoring database structure without downtime

> We need to refactor our database:
> - users table has 80 columns (denormalized)
> - No foreign keys or constraints
> - Inconsistent naming (userId vs user_id)
> - No indexes except primary keys
>
> Plan a gradual migration to normalized schema

Create migration strategy

-- Phase 1: Add new normalized tables alongside old
CREATE TABLE user_profiles (
  id BIGINT PRIMARY KEY,
  user_id BIGINT NOT NULL,
  first_name VARCHAR(100),
  last_name VARCHAR(100),
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  FOREIGN KEY (user_id) REFERENCES users(id)
);

CREATE TABLE user_addresses (
  id BIGINT PRIMARY KEY,
  user_id BIGINT NOT NULL,
  type ENUM('billing', 'shipping'),
  street VARCHAR(255),
  city VARCHAR(100),
  -- ... more fields
  FOREIGN KEY (user_id) REFERENCES users(id),
  INDEX idx_user_type (user_id, type)
);

Implement dual-write pattern

class UserService {
  public function updateProfile($userId, $data) {
    // Write to both old and new structure
    DB::transaction(function() use ($userId, $data) {
      // Legacy write
      DB::table('users')->where('id', $userId)->update([
        'fname' => $data['first_name'],
        'lname' => $data['last_name']
      ]);

      // New structure write
      UserProfile::updateOrCreate(
        ['user_id' => $userId],
        [
          'first_name' => $data['first_name'],
          'last_name' => $data['last_name']
        ]
      );
    });
  }
}

Migrate data progressively

> Create a background job to migrate user data:
> - Process in batches of 1000
> - Track progress
> - Verify data integrity
> - Handle failures gracefully

Switch reads to new schema

// Feature flag controlled
if (Feature::enabled('use_normalized_schema')) {
  return User::with(['profile', 'addresses'])->find($id);
} else {
  return DB::table('users')->where('id', $id)->first();
}

Complete the migration

-- After verification, drop old columns
ALTER TABLE users
DROP COLUMN fname,
DROP COLUMN lname,
DROP COLUMN address1,
-- ... etc

Real-World Refactoring Examples

Example 1: Microservices Extraction

Challenge: Extract services from a monolithic Rails application

> Our Rails monolith handles:
> - User authentication
> - Product catalog
> - Order processing
> - Email notifications
> - Reporting
>
> Plan extraction of order processing into microservice

Claude’s extraction strategy:

# Claude analyzes and finds:
> Analyzing order processing boundaries...

Found order-related code in:
- app/models/order.rb (and 12 related models)
- app/controllers/orders_controller.rb (and 4 related)
- app/services/order_service.rb (and 8 related)
- lib/payment_gateway.rb
- app/jobs/order_notification_job.rb

Shared dependencies:
- User model (for customer info)
- Product model (for catalog)
- Notification service

# Create abstraction layer
module OrderServiceInterface
  class Client
    def initialize(internal: true)
      @internal = internal
      @client = internal ? InternalOrderService : HTTPOrderService
    end

    def create_order(params)
      @client.create_order(params)
    end

    def get_order(id)
      @client.get_order(id)
    end
  end
end

# This allows gradual migration

# New microservice structure
> Create order microservice with:
> - Separate database
> - REST API matching our interface
> - Message queue integration
> - Own deployment pipeline

# Generated service includes:
class OrdersController < ApplicationController
  def create
    order = OrderService.create(order_params)

    # Publish event for other services
    EventBus.publish('order.created', order)

    render json: OrderSerializer.new(order)
  end
end

# Feature flag controlled routing
class OrdersController < ApplicationController
  def create
    if Feature.enabled?(:use_order_microservice, current_user)
      # Route to microservice
      response = OrderServiceClient.create_order(params)
      render json: response
    else
      # Use monolith code
      @order = Order.create!(order_params)
      render json: @order
    end
  end
end

Example 2: Async Architecture Refactoring

Challenge: Convert synchronous operations to event-driven architecture

> Our system does everything synchronously:
> - Send email after order
> - Update inventory immediately
> - Calculate analytics in-request
> - Generate PDFs during API call
>
> Refactor to async event-driven pattern

Claude implements event-driven refactoring:

// Before: Synchronous coupling
async function createOrder(orderData: OrderData) {
  const order = await db.orders.create(orderData);

  // All of these block the response
  await sendOrderConfirmationEmail(order);
  await updateInventory(order.items);
  await calculateAnalytics(order);
  await generateInvoicePDF(order);

  return order;
}

// After: Event-driven
async function createOrder(orderData: OrderData) {
  const order = await db.orders.create(orderData);

  // Publish event and return immediately
  await eventBus.publish('order.created', {
    orderId: order.id,
    customerId: order.customerId,
    items: order.items,
    total: order.total
  });

  return order;
}

// Separate handlers process async
eventBus.subscribe('order.created', async (event) => {
  await Promise.all([
    emailService.sendOrderConfirmation(event),
    inventoryService.updateStock(event.items),
    analyticsService.recordOrder(event),
    pdfService.generateInvoice(event.orderId)
  ]);
});

Advanced Refactoring Techniques

Automated Pattern Detection

> Scan our codebase for anti-patterns and create fixes:
> - God objects (classes doing too much)
> - Shotgun surgery (changes require many edits)
> - Feature envy (methods using other class data)
> - Data clumps (same parameters everywhere)

Claude’s pattern detection and fixes:

God Object Fix

// Detected: UserService with 47 methods
class UserService {
  // Authentication methods
  login() {}
  logout() {}
  resetPassword() {}

  // Profile methods
  updateProfile() {}
  uploadAvatar() {}

  // Notification methods
  sendEmail() {}
  sendSMS() {}

  // ... 40 more methods
}

// Refactored into focused services
class AuthService {
  login() {}
  logout() {}
  resetPassword() {}
}

class ProfileService {
  updateProfile() {}
  uploadAvatar() {}
}

class NotificationService {
  sendEmail() {}
  sendSMS() {}
}

Data Clump Fix

// Detected: Same 4 parameters in 15 methods
function createInvoice(
  street: string,
  city: string,
  state: string,
  zip: string,
  // ... other params
) {}

function shipOrder(
  street: string,
  city: string,
  state: string,
  zip: string,
  // ... other params
) {}

// Refactored with value object
class Address {
  constructor(
    public street: string,
    public city: string,
    public state: string,
    public zip: string
  ) {}
}

function createInvoice(
  address: Address,
  // ... other params
) {}

function shipOrder(
  address: Address,
  // ... other params
) {}

Incremental Strangler Fig Pattern

For replacing legacy systems gradually:

> Implement strangler fig pattern for our legacy order system:
> - Current: Monolithic order processing
> - Target: Modern microservices architecture
> - Requirement: Zero downtime, gradual migration

Create facade

class OrderFacade {
  async createOrder(data: OrderData) {
    if (await this.shouldUseNewSystem(data)) {
      return this.newOrderService.create(data);
    }
    return this.legacyOrderService.create(data);
  }

  private async shouldUseNewSystem(data: OrderData) {
    // Gradual rollout logic
    if (this.featureFlags.isEnabled('new_order_system', data.customerId)) {
      return true;
    }
    if (data.total < 100 && Math.random() < 0.1) { // 10% of small orders
      return true;
    }
    return false;
  }
}

Implement comparison mode

// Run both systems and compare
class OrderComparison {
  async createAndCompare(data: OrderData) {
    const [legacy, modern] = await Promise.all([
      this.legacySystem.create(data),
      this.modernSystem.create(data)
    ]);

    const differences = this.compareResults(legacy, modern);
    if (differences.length > 0) {
      await this.logDifferences(differences);
    }

    // Return legacy result but log modern
    return legacy;
  }
}

Gradual cutover

Week 1: 1% traffic to new system
Week 2: 10% traffic
Week 3: 50% traffic
Week 4: 90% traffic
Week 5: 100% traffic
Week 6: Remove legacy code

Refactoring Metrics and Validation

Track Progress

> Create a refactoring dashboard that tracks:
> - Code coverage improvement
> - Cyclomatic complexity reduction
> - Performance improvements
> - Bug reduction
> - Technical debt score

Claude generates tracking code:

class RefactoringMetrics {
  async analyze() {
    const metrics = {
      coverage: await this.getTestCoverage(),
      complexity: await this.getCyclomaticComplexity(),
      performance: await this.getPerformanceMetrics(),
      codeQuality: await this.getCodeQualityScore(),
      technicalDebt: await this.calculateTechnicalDebt()
    };

    return {
      ...metrics,
      improvements: this.compareWithBaseline(metrics),
      roi: this.calculateROI(metrics)
    };
  }

  async getTestCoverage() {
    const coverage = await exec('npm run coverage -- --json');
    return {
      statements: coverage.total.statements.pct,
      branches: coverage.total.branches.pct,
      functions: coverage.total.functions.pct,
      lines: coverage.total.lines.pct
    };
  }
}

Validate Behavior Preservation

> Generate comprehensive tests to ensure refactoring doesn't change behavior:
> - Capture current behavior
> - Create golden master tests
> - Property-based testing
> - Regression test suite

Best Practices for Large-Scale Refactoring

1. Always Work in Branches

> For each refactoring:
> 1. Create feature branch
> 2. Make incremental commits
> 3. Run tests continuously
> 4. Use pull requests for review

2. Document Everything

> While refactoring, also:
> - Update code comments
> - Document new patterns
> - Create migration guides
> - Record decision rationale

3. Maintain Backwards Compatibility

> When changing APIs:
> - Add new endpoints alongside old
> - Deprecate gradually
> - Provide migration tools
> - Support both versions temporarily

4. Use Feature Flags Extensively

if (Feature.enabled('new_payment_system')) {
  return this.newPaymentProcessor.charge(amount);
} else {
  return this.legacyPaymentGateway.process(amount);
}

Testing Strategies Ensure refactoring doesn't break functionality

Architecture Patterns Design patterns for modern system architecture

Performance Optimization Improve performance during refactoring

Next Steps

You’ve learned how to leverage Claude Code for large-scale refactoring that would be impossibly time-consuming manually. The key is thinking systematically: analyze comprehensively, refactor incrementally, validate continuously.

Remember: Refactoring is not about perfection - it’s about continuous improvement. Use Claude Code to handle the mechanical transformations while you focus on architectural decisions and business value. With AI assistance, you can tackle technical debt that’s been accumulating for years and transform legacy systems into modern, maintainable codebases.