Refactoring Legacy Code in Go: A Step-by-Step Guide

Introduction to Refactoring Legacy Go Code

Working with legacy code is an inevitable part of any Go developer's life. Over time, projects accumulate technical debt: monolithic functions, code duplication, lack of tests, and error handling. Refactoring legacy Go code is not just rewriting, but a strategic process of improving architecture without changing the system's external behavior.

In this article, we will break down the main approaches to refactoring Go projects, provide specific code examples, and show how to make an old codebase cleaner, safer, and more performant.

Why Does Legacy Go Code Require a Special Approach?

Go is known for its simplicity and strictness, but even in Go, problems accumulate:

• Lack of interfaces — tight coupling of components;
• Ignoring errors — return without checking err;
• Global variables — implicit dependencies;
• Logic duplication — copy-paste instead of composition;
• Long functions — more than 50 lines with multiple responsibilities.

Refactoring without tests is like surgery without anesthesia. Therefore, the first step is to ensure test coverage for critical areas.

Refactoring Strategy: From Monolith to Modules

1. Extracting Interfaces and Dependency Inversion

Let's start with typical legacy code where business logic is tightly coupled to the storage implementation:

// Before refactoringtype UserService struct {    db *sql.DB}

func (s *UserService) GetUser(id int) (*User, error) {    row := s.db.QueryRow("SELECT id, name FROM users WHERE id = ?", id)    var u User    err := row.Scan(&u.ID, &u.Name)    return &u, err}

The problem: such code cannot be tested without a real database. The solution is to introduce an interface:

// After refactoringtype Storage interface {    GetUser(id int) (*User, error)}

type UserService struct {    storage Storage}

func NewUserService(storage Storage) *UserService {    return &UserService{storage: storage}}

func (s *UserService) GetUser(id int) (*User, error) {    return s.storage.GetUser(id)}

Now we can easily substitute the implementation with a mock in tests.

2. Splitting Large Functions (Single Responsibility)

Typical legacy code — a function doing everything at once:

// Before refactoringfunc processOrder(order Order) error {    // Validation    if order.Amount <= 0 {        return errors.New("invalid amount")    }    // Discount calculation    discount := 0.0    if order.CustomerType == "vip" {        discount = 0.1    }    // Save to DB    _, err := db.Exec("INSERT INTO orders ...")    if err != nil {        return err    }    // Send email    return sendEmail(order.Email, "Order confirmed")}

After refactoring, we break it down into small, testable functions:

// After refactoringtype OrderProcessor struct {    db      Database    notifier Notifier}

func (p *OrderProcessor) Process(order Order) error {    if err := validateOrder(order); err != nil {        return err    }    order = applyDiscount(order)    if err := p.db.SaveOrder(order); err != nil {        return fmt.Errorf("save order: %w", err)    }    return p.notifier.SendConfirmation(order.Email)}

func validateOrder(order Order) error {    if order.Amount <= 0 {        return errors.New("amount must be positive")    }    return nil}

func applyDiscount(order Order) Order {    if order.CustomerType == "vip" {        order.Amount *= 0.9    }    return order}

3. Replacing Global Variables with DI (Dependency Injection)

Global variables are the main enemy of test