Refactoring: Improving the Design of Existing Code

Key Takeaways

“Refactoring: Improving the Design of Existing Code” was my first real introduction to thinking about code as something more than just a means to complete an assignment. Before reading this book, my approach to programming was largely task-focused: write code that works, submit it, and move on. Fowler’s work fundamentally changed how I think about software development by introducing the concept that code is a living document that requires ongoing care and improvement.

The book’s central premise is that refactoring should be a continuous process rather than a large, risky undertaking. Fowler defines refactoring as “a disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior.” This definition itself was revelatory to someone who had only thought of code in terms of “working” or “broken.”

The Refactoring Process

Fowler breaks down refactoring into a systematic process that relies heavily on testing. The basic cycle involves:

1. Ensuring comprehensive test coverage exists
2. Making small, incremental changes
3. Running tests after each change
4. Committing frequently

This approach makes refactoring much less risky than the large-scale rewrites I had previously imagined were necessary when code became messy.

# Before refactoring - a method doing too many things
def process_customer_order(customer_data, order_items):
    # Validate customer
    if not customer_data.get('email'):
        raise ValueError("Customer email required")
    if not customer_data.get('name'):
        raise ValueError("Customer name required")

    # Calculate total
    total = 0
    for item in order_items:
        price = item['price']
        quantity = item['quantity']
        total += price * quantity

    # Apply discount
    if total > 100:
        total *= 0.9

    # Save to database
    db.save_order({
        'customer': customer_data,
        'items': order_items,
        'total': total
    })

    return total

# After refactoring - responsibilities separated
def validate_customer(customer_data):
    """Validate customer data and raise errors if invalid."""
    if not customer_data.get('email'):
        raise ValueError("Customer email required")
    if not customer_data.get('name'):
        raise ValueError("Customer name required")

def calculate_order_total(order_items):
    """Calculate total price for all items in order."""
    return sum(item['price'] * item['quantity'] for item in order_items)

def apply_discount(total):
    """Apply discount for orders over $100."""
    return total * 0.9 if total > 100 else total

def process_customer_order(customer_data, order_items):
    """Process a customer order through validation, calculation, and storage."""
    validate_customer(customer_data)

    subtotal = calculate_order_total(order_items)
    total = apply_discount(subtotal)

    order = {
        'customer': customer_data,
        'items': order_items,
        'total': total
    }

    db.save_order(order)
    return total

Code Smells and When to Refactor

One of the most practical aspects of the book is Fowler’s catalog of “code smells” - indicators that code might benefit from refactoring. These include concepts like:

• Long Method - Methods that try to do too much
• Large Class - Classes with too many responsibilities
• Duplicate Code - The same logic appearing in multiple places
• Long Parameter List - Methods that require too many arguments
• Feature Envy - When a method seems more interested in another class than its own

Learning to recognize these smells has been invaluable in my current work as a DevOps engineer, where I frequently encounter scripts and automation code that exhibits these problems.

Refactoring Techniques

The book provides a lot of refactoring techniques, with step-by-step instructions. Some of the most useful ones I’ve applied include:

Extract Method - Taking a section of code and turning it into its own method. This is particularly useful in deployment scripts where complex operations can be broken down into understandable steps.

Rename Variable - Choosing better names for variables, functions, and classes. This seems simple but has had a huge impact on code readability in our CI/CD pipelines.

Replace Magic Numbers with Named Constants - Converting hard-coded values into named constants that explain their purpose.

# Before refactoring
#!/bin/bash
sleep 30
if [ $(curl -s -o /dev/null -w "%{http_code}" http://app:8080/health) -eq 200 ]; then
    echo "Service is ready"
else
    echo "Service failed to start"
    exit 1
fi

# After refactoring
#!/bin/bash
readonly STARTUP_WAIT_TIME=30
readonly HEALTH_CHECK_URL="http://app:8080/health"
readonly HTTP_OK=200

wait_for_service_startup() {
    sleep $STARTUP_WAIT_TIME
}

check_service_health() {
    local http_code
    http_code=$(curl -s -o /dev/null -w "%{http_code}" "$HEALTH_CHECK_URL")

    if [ "$http_code" -eq $HTTP_OK ]; then
        echo "Service is ready"
        return 0
    else
        echo "Service failed to start (HTTP $http_code)"
        return 1
    fi
}

wait_for_service_startup
if ! check_service_health; then
    exit 1
fi

Applications in DevOps Work

The principles from this book have been particularly valuable in my DevOps role, where I work with Python automation scripts, Bash deployment scripts, and Groovy Jenkins pipelines.

Configuration Management is another area where these principles shine. Instead of having massive configuration files or scripts, we’ve refactored them into smaller, focused modules that each handle a specific aspect of system configuration.

Monitoring and Alerting Scripts have benefited from the systematic approach Fowler advocates. Rather than having one large script that handles all monitoring tasks, we’ve refactored these into smaller, focused scripts that are easier to test and debug when issues arise.

The book’s emphasis on testing before refactoring has been particularly valuable. In a DevOps context, where changes can affect production systems, having comprehensive tests gives confidence that refactoring won’t introduce unexpected behavior.

Critical Assessment

While “Refactoring” was transformative for my understanding of software development, it’s not without limitations. The book is heavily focused on object-oriented programming, particularly Java, which doesn’t always translate directly to the scripting languages and infrastructure-as-code tools commonly used in DevOps.

Some of the refactoring techniques assume a level of IDE support that isn’t always available when working with configuration files or deployment scripts. The automated refactoring tools that Fowler references work well for traditional application code but are less useful for infrastructure code.

The book’s approach to testing, while sound, can be challenging to implement in environments where infrastructure changes are difficult to test in isolation. Setting up comprehensive test coverage for deployment scripts or configuration changes requires significant infrastructure investment that isn’t always practical.

Additionally, some of the code smells that Fowler identifies may actually be acceptable or even preferred in certain contexts. For example, long parameter lists might be problematic in application code but are sometimes necessary in configuration scripts that need to be explicit about their dependencies.

That said, the core principles of making small, incremental changes and maintaining test coverage have proven valuable even in these contexts. The systematic approach to code improvement that Fowler advocates has made me a more thoughtful programmer and helped our team maintain more reliable infrastructure.

Long-term Impact

Reading “Refactoring” marked a turning point in how I approach software development. It shifted my mindset from viewing code as a disposable artifact to seeing it as a craft that deserves ongoing attention and improvement. This perspective has been invaluable as I’ve moved from academic programming to professional software development.

The book’s emphasis on continuous improvement rather than periodic major rewrites has influenced how our team approaches technical debt. Instead of allowing problems to accumulate until they require major overhauls, we now address issues incrementally as part of our regular development process.

The systematic approach to code improvement that Fowler teaches has also made me more confident in making changes to existing code. Understanding that refactoring can be done safely through small, tested changes has eliminated much of the fear that previously surrounded modifying working code.

Additional Resources

• Fowler’s website contains additional articles and updates to refactoring techniques
• Refactoring.com - the book’s companion website with examples and updates