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Refactoring Legacy Code - Effective Strategy Step-by-Step

Last updated on
February 23, 2024


Strategy for legacy code refactoring

Your refactoring strategy should start with a well-defined purpose of refactoring - it should support business goals. After assessing the current state, you need build various risk mitigation strategies and create a version control system. Then, make a plan and allocate resources. Now, you’re prepared for gradual modernization.

For more details about these steps and descriptions of the process and post-refactoring, scroll down.


Refactoring Legacy Code - Effective Strategy Step-by-Step

If your system is causing financial losses and accumulating technical debt, it's likely time for a legacy app modernization. Neglecting this crucial step can not only damage your brand's reputation but also lead to increasingly expensive fixes and hinder your scalability. Refactoring is one powerful tool for this transformation.

In this guide, you will find practical strategies and tips on how to refactor legacy code to ensure your business remains competitive and adaptable in the digital landscape.

What does it really mean to refactor?

Your refactoring goal is to make the code simpler, cleaner, more efficient, and easier to maintain.

Refactoring legacy code involves changing its current structure while ensuring its functionality remains unchanged.

Refactoring always comes with a risk

Refactoring always comes with a risk and should always be driven by a clear purpose and a close alignment with business goals. - Michał Szopa, JavaScript Software Engineer

Facing a codebase burdened with technical debt and poor quality can naturally lead to the temptation of starting over. The prospect of discarding the old and rebuilding from the ground up is appealing, yet this approach is filled with potential pitfalls and obstacles.

Refactoring should always be driven by a clear purpose and business goals.

A quote explaining that refactoring should always be prepared strategically.

In most cases, you’ll need to refactor incrementally, step by step.

Then, you’ll be able to experience the benefits listed below, while minimizing the risk and cost associated with rewriting the entire system from scratch.

<span class="colorbox1" fs-test-element="box1"><p>Read also: Learn on the real example how to refactor and cope with technical debt by identifying areas for improvement that support business, not just clean the code: Solving Tech Debt Puzzle. Strategy that Supports Business</p></span>

Pros of legacy code refactoring

The pros of legacy code refactoring are:

  • improving software quality
  • enhancing code maintainability
  • reducing the existing technical debt
  • improving app’s performance
  • reducing the amount of bugs
  • increasing scalability
  • accelerating software development process
  • cutting unnecessary IT expenses related to maintaining legacy code
  • driving innovation and company’s potential (developers no longer need to worry about changing the error-susceptible code)
  • ability to introduce modern best practices, such as CI/CD
  • improving interoperability (by integrating the system with modern third-party tools)
  • improving cybersecurity (by removing vulnerabilities caused by outdated programming practices)
  • gaining competitive advantage
  • becoming more attractive employer (using modern practices and technologies attracts and retains talents).
Advantages and challenges of legacy code refactoring

How to refactor legacy code step by step?

How to plan an effective legacy code refactoring process to experience these benefits? Let's delve into a practical guide that will equip you with the tools to develop your strategy and navigate this transformation with confidence.


Before embarking on a refactoring journey, it's essential to lay solid groundwork to ensure a smooth process. This preparation involves thoroughly understanding the existing codebase, establishing clear goals for what the refactoring aims to achieve, and ensuring robust testing coverage to safeguard against regressions.

Step 0: Start with a well-defined purpose and analysis of how refactoring will provide business support

Refactoring should have a well-defined purpose and always support business. It should always be undertaken with a clear objective in mind and must align with broader business goals to ensure that it adds value beyond just technical improvements.

Step 1: Assess the current state

Code audit

The refactoring process begins with conducting an audit of the system to examine how it is structured, assess its feasibility and pinpoint areas requiring attention. A comprehensive understanding of the codebase and its dependencies serves as the base for formulating a well-informed legacy code refactoring plan.

How to identify areas for improvement?

  • Consider the project's direction - Refactoring opportunities should be identified with the project's roadmap in mind, ensuring that improvements to the code are made in alignment with the changing priorities and needs of the business.
  • Investigate bugs - They might share a common origin in a complex part of the code. If so, this could warrant a broader overhaul to replace that segment of the system.
  • Focus on critical application components - Prioritize refactoring efforts on the parts of the application that will most significantly influence the fulfillment of business goals.

Identify core components

Instead of getting sidetracked by the numerous possibilities for improving or altering the code (opportunities), developers should concentrate on the most critical aspects that need attention (priorities).

This approach ensures that refactoring efforts are directed toward areas that will have the most significant impact on the codebase's health, maintainability, and functionality. By prioritizing, developers can effectively allocate their time and resources to address the most pressing issues, such as fixing bugs, reducing complexity, or improving performance, rather than being overwhelmed by the vast number of potential improvements. This disciplined approach helps in achieving the primary goals of refactoring more efficiently and effectively.

Step 2: Define goals and identify scope

Understand and articulate why you are refactoring. Goals may include improving code maintainability, reducing technical debt, enhancing performance, or preparing the codebase for new features. Determine which parts of the codebase are to be refactored. Not all code needs refactoring, so prioritize areas that will provide the most benefit.

Step 3: Build risk mitigation strategies

Carefully plan the rollout of modernization changes to minimize disruption to users and business operations. Use feature flags, canary releases, or blue-green deployments to test changes in production environments safely.

Testing plan

  • Evaluate the current test coverage. Identify areas lacking tests, as these are risky to refactor without a safety net. Implement a robust suite of automated tests, including unit, integration, and end-to-end tests, to cover as much of the codebase as possible.
  • If necessary, increase test coverage before starting the refactoring. This ensures that you can refactor with confidence, knowing that tests will catch any regressions.

Code review and pair programming

  • Conduct code reviews for refactoring changes to catch potential issues early and ensure adherence to coding standards and practices.
  • Consider pair programming for complex refactoring tasks. This can help identify potential issues in real-time and facilitate knowledge sharing.

Feature flags and branching

  • Use feature flags to toggle new refactored code paths on or off. This allows you to gradually introduce changes and quickly disable them if issues arise.
  • Utilize branching strategies in your version control system to isolate refactoring work from the main development line. This enables safer experimentation and easier rollback if needed.

Step 4: Build a product strategy

Next, let's shift our focus to business logic. At this stage it's crucial to assess whether the app still aligns with current business requirements. In some cases, simply refactoring the code might not be sufficient, and adjustments to functionalities may be required, especially when the code hasn't been reviewed or refactored for an extended period. Developing and validating a product roadmap can aid in this evaluation process and assess whether the app needs to be refactored, rewritten or even rebuilt.

Step 5: Design new architecture

Before diving into the process, it's essential to have a clear vision of the desired new architecture that meets user needs while ensuring manageability and scalability. This vision will serve as your guide once you start the proper refactoring process. 

Step 6: Create a version control system

Having a version control system is essential for any legacy code refactoring project. It enables the tracking of codebase changes, facilitates collaboration among multiple developers working simultaneously, and streamlines identifying and solving issues.

Step 7: Make a detailed plan and allocate resources

Break down the refactoring tasks into small, manageable steps. Plan to implement these changes incrementally, integrating them into your regular development cycles.

Allocate specific time slots for refactoring within your development sprints or cycles. This ensures that refactoring tasks are treated as first-class citizens in the development process.

Determine the resources (developer time, tools) required for the refactoring tasks and plan accordingly.

Now, you’re prepared for gradual modernization

Time to dive into the refactoring process.


The refactoring process involves a series of strategic steps designed to improve the structure of the code without altering its external behavior.

  • Refactor code: Refactor legacy code to improve its structure and readability without changing its external behavior. Focus on applying modern coding standards and practices.
  • Update dependencies: Gradually update or replace outdated libraries and frameworks with more modern, supported versions. This can help improve the system's security and performance.
  • Adopt new technologies: Where appropriate, introduce new technologies (e.g., cloud services, containerization) to enhance the system's capabilities and efficiency. Start with non-critical components to minimize risk.

Step 8: Refactor the code

In this step, we begin rewriting the code while integrating modern approaches, technologies, and frameworks.

Step 9: Introduce TDD

TDD cycle is often summarized as Red-Green-Refactor. 

  • Red: Write a test for the next bit of functionality you want to add. The test should fail because the functionality doesn't exist yet.
  • Green: Write the minimal amount of code required to make the test pass. This often means implementing just enough for the test to pass, not more.
  • Refactor: Refine the code you just wrote, making it cleaner and eliminating any duplication while ensuring that your tests still pass. This step is crucial for maintaining code quality and simplicity.

Step 10: Modernize the codebase incrementally

Modernizing a codebase incrementally is a strategic approach to update and improve a legacy system without undertaking a full rewrite, which can be risky and resource-intensive. Make small, manageable updates over time, focusing on enhancing the system's maintainability, performance, and functionality while minimizing disruption to existing operations.

Step 11: Use refactoring patterns strategically

Start by identifying "code smells," which are indicators of deeper problems in the code. Examples include duplicate code, long methods, large classes, and global data. Each smell may suggest a specific refactoring pattern as a remedy.

Choose the most appropriate patterns based on the specific issue you're addressing. Consider the impact of the refactoring on readability, maintainability, and performance.

Implement the chosen refactoring patterns meticulously, paying close attention to the details of the pattern application. This careful approach helps avoid introducing new issues.

Step 12: Identify and isolate dependencies

Identifying and isolating dependencies during refactoring is a critical step to ensure that changes to the codebase do not inadvertently affect the system's functionality. Dependencies can include libraries, frameworks, external services, or even tightly coupled components within your application. 

  • Utilize static code analysis tools to identify dependencies within your codebase. These tools can help reveal direct and transitive dependencies, making it easier to understand the relationships between different parts of your code.
  • Wrap external libraries/frameworks: Instead of using external libraries or frameworks directly throughout your code, wrap them in your own classes or interfaces. This creates a buffer layer, so if you need to change the external dependency, you only need to update your wrapper, not the entire codebase.
  • Use facades or adapters: For complex dependencies, consider implementing the Facade or Adapter design patterns. These patterns can simplify the interface to an external system or make it compatible with your application without changing the external system itself.
  • Audit third-party libraries: Regularly audit your third-party libraries and frameworks for updates, security vulnerabilities, and compatibility issues. Keeping dependencies up-to-date can reduce the risk during refactoring.
  • Abstract third-party calls: Where possible, abstract calls to third-party services or libraries, providing a clear interface for interaction. This abstraction allows you to replace or update those services with minimal impact on your application.

Step 13: Use documentation as a refactoring tool

Using documentation as a refactoring tool involves leveraging and updating documentation to support and enhance the refactoring process. Good documentation can guide refactoring efforts, help identify areas needing improvement, and ensure changes are understandable and maintainable. By integrating documentation into the refactoring process, you can create a more structured, informed approach to improving your codebase. 

Step 14: Run a QA and performance tests, implement CD/CI

At this stage, it's crucial to confirm that the functionalities remain intact. Once the code has been refactored, it's time to conduct unit tests to ensure the refactored code is functional and free of bugs. These tests can be done manually or automated. Remember to test the code in various environments, across different operating systems, browsers, and devices.

Set up CI/CD pipelines to automate the testing and deployment of changes. This supports a more agile development process and ensures that incremental updates can be rolled out smoothly and quickly.


After the refactoring phase, it's crucial to ensure that the changes made have improved the codebase without introducing new issues. The post-refactoring phase is about validating the work done, documenting the changes, and learning from the process for future refactoring efforts. 

Step 15: Set up performance monitoring

Refactoring legacy code isn't just about enhancing the code itself. Even after wrapping up the process, it's essential to monitor key metrics to ensure that the application continues to work effectively and that its performance hasn't declined. Obviously, it's also a good idea to track performance to see how it has improved and to evaluate the effects of making specific changes.

Step 16: Gather user feedback

It's also important to collect feedback from end-users and stakeholders to see the results of legacy app refactoring from their point of view. This way, it is possible to spot any issues and gather suggestions for further improvements straight from the people who are actually using the software.

How to measure refactoring success?

To measure the refactoring success, you can use the following metrics:

1. Code quality metrics

  • Maintainability index: This metric assesses how easy it is to maintain the code. An increase in post-refactoring indicates success.
  • Cyclomatic Complexity: Measures the complexity of the code. Refactoring should ideally lower this complexity, making the code simpler and easier to understand.
  • Technical Debt Ratio: The ratio of the cost to fix maintainability issues to the total development cost. A decrease in this ratio after refactoring indicates improvement.

2. Automated test coverage

  • Coverage percentage: Ensure that test coverage remains high or improves after refactoring. High test coverage indicates that the codebase is well-protected against regressions.
  • Test suite execution time: Refactoring might improve the efficiency of the code, potentially reducing the time it takes to run automated tests.

3. Performance metrics

  • Application performance: Monitor key performance indicators (KPIs) such as response times, memory usage, and CPU load. Improvements in these metrics can signal successful refactoring, especially if performance optimization was a goal.
  • User Experience: For UI-related refactoring, improvements in load times, navigation speed, and responsiveness are direct measures of success.

4. Development metrics

  • Bug rate: A decrease in the number of reported bugs or issues after refactoring can indicate an improvement in code quality.
  • Feature development speed: If new features can be added more quickly or with fewer bugs after refactoring, it suggests that the codebase has become more adaptable and easier to work with.
  • Code churn: Monitoring code churn (the amount of code changed, added, or removed) after refactoring can provide insights into the stability and maturity of the codebase.

5. Team feedback and morale

  • Developer satisfaction: Gather feedback from the development team. Improvements in their satisfaction and productivity are qualitative measures of successful refactoring.
  • Ease of maintenance: If developers find it easier to understand, modify, and extend the codebase post-refactoring, it signifies an improvement in code maintainability.

6. Business impact

  • User satisfaction: Monitor user feedback, support tickets, and satisfaction surveys for any changes post-refactoring. Improvements can indicate a successful refactoring from a user perspective.
  • ROI analysis: Conduct a return on investment (ROI) analysis to evaluate the cost of refactoring against the benefits gained, such as reduced maintenance costs or increased revenue from improved performance.

Legacy code refactoring - best practices

Since refactoring can be pretty complicated, we've gathered some best practices and tips on how to refactor code to help make the process a bit smoother:

  1. Keep your user in mind

As mentioned earlier, it's important to remember that refactoring shouldn't be done just for the sake of it; it should always revolve around improving the user experience. Ultimately, it's the feelings and feedback of the users that determine whether they'll continue using the upgraded, refactored app or not.

  1. Separate refactoring from regular development

For safety reasons, it's best to keep these two processes separate. Making too many changes or mixing refactoring with other code modifications could compromise the safety of the process and the quality of the final codebase.

  1. Split the process

When modernizing a legacy app, we strongly suggest prioritizing incremental changes over modifying large sections of code all at once. This approach ensures reduced complexity and safer, more manageable, and bug-free refactoring. In this way, the entire process is easier to control, and potential issues can be identified and resolved more swiftly. Moreover, this method promotes a clearer understanding of the code and facilitates the creation of documentation.

  1. Analyze dependencies

Dependencies on third-party tools, systems and libraries are very important to consider in legacy app refactoring. Proper integration is key to ensuring the software functions correctly, so understanding these dependencies shouldn't be overlooked during the planning stage. These dependencies can be handled by creating interfaces or abstract classes that make the app more modular and easier to test.

  1. Choose the right priorities

While every part of the app may require attention for fixes and refactoring, it's essential to prioritize high-impact areas. These are the sections that have the most significant influence on the app's performance and code quality. By focusing on these areas first, you'll see the biggest improvements. To identify what exactly should be improved, you can use various tools such as bug reports, coverage reports or performance profiling. 

  1. Apply refactoring patterns

When searching for the best approach to refactor legacy code, there are several patterns you can apply. Implementing these patterns can help to standardize the process, make it easier to follow and enhance the overall quality of the code. To name but a few methods, there are Extract Method, Move Method, Rename Method, or others.

How to implement a refactoring strategy?

If you're unsure about how to refactor code and develop the right refactoring strategy, the support of an experienced development team can be incredibly valuable.

When hiring a team, make sure that they have the necessary skills and approaches for refactoring. Look for expertise in legacy app modernization, a track record of delivering product strategy, proficiency in end-to-end project management, and a commitment to taking ownership of the process.

If you're in search of such a team, don't hesitate to reach out to us. With our expertise, we can assist you in efficiently refactoring your code by developing and implementing a tailored refactoring strategy that aligns with your technical and business needs.

Getting a broader picture - additional questions

Below, we’ve gathered answers to 3 frequently-asked questions about refactoring legacy code:

What is legacy code refactoring?

Refactoring legacy code is a process of enhancing the architecture, structure and design of outdated codebases, all while ensuring the app's functionality remains intact. Post-refactoring, the code emerges cleaner, more comprehensible, and notably easier to maintain and manage.

Legacy code refactoring is a highly recommended practice in software development process. Its aim extends beyond tidying up codebases; it delivers tangible business benefits, including enhanced adaptability, scalability, and overall efficiency.

What are the common characteristics of legacy code?

1. Older technologies and languages

Legacy codebases are typically written in older programming languages or versions of languages that are no longer in widespread use. They may also rely on outdated frameworks, libraries, or platforms that are no longer supported.

2. Lack of documentation

One of the hallmarks of legacy code is insufficient or outdated documentation. This makes understanding the system's functionality and making changes more difficult, especially for new developers on the project.

3. Tightly coupled architecture

Legacy systems often exhibit tightly coupled components, meaning that different parts of the application are highly dependent on each other. This coupling makes it challenging to modify or update one part of the system without affecting others.

4. Limited or no automated tests

Automated tests are scarce or entirely absent in many legacy codebases. The lack of tests increases the risk of introducing new bugs when making changes and makes refactoring efforts more risky and time-consuming.

5. Complexity and technical debt

Over time, legacy code often accumulates complexity and technical debt due to quick fixes, workarounds, and the addition of features without adequately refactoring. This accumulation makes the codebase harder to understand, maintain, and extend.

6. Resistance to change

Legacy systems are often critical to an organization's operations, making any changes risky. There can be a resistance to modifying the code due to fears of breaking functionality or the high costs associated with testing and validation.

7. Performance and scalability issues

Given that legacy code was developed under different requirements and technological constraints, it might not perform well under current demands. It may struggle with scalability, unable to handle increased loads or integrate smoothly with modern, scalable platforms.

8. Security vulnerabilities

Older codebases might not adhere to current security standards or might use libraries and dependencies with known vulnerabilities. This exposure makes the system a target for security breaches and compliance issues.

9. Difficult integration with modern systems

Legacy systems often have difficulty integrating with newer technologies and platforms. The differences in technologies, data formats, and communication protocols can create significant integration challenges.

10. Specialized knowledge requirement

Maintenance and development of legacy code often require specialized knowledge of outdated technologies and the specific idiosyncrasies of the codebase. This dependency can create a knowledge bottleneck if few team members understand how to work with the code effectively.

What are the common challenges of refactoring?

1. Identifying what to refactor

  • Recognizing code smells: Developers must be adept at identifying "code smells," indicative of deeper problems in the code, which isn’t always straightforward.
  • Prioritizing issues: Deciding which parts of the codebase to refactor first can be challenging, especially in large or complex systems where technical debt is widespread.

2. Lack of comprehensive testing

  • Insufficient test coverage: A common challenge is the lack of a comprehensive automated test suite. Without adequate tests, there's a higher risk of introducing bugs during refactoring.
  • Testing legacy code: Legacy systems often come with little to no automated tests, making it risky to change existing code without inadvertently breaking functionality.

3. Understanding the existing codebase

  • Complexity and documentation: Legacy or complex systems can be difficult to understand, especially with inadequate documentation. Developers need to spend significant time just understanding the code before even beginning to refactor.
  • Hidden dependencies: Identifying and understanding all dependencies within the code can be challenging, leading to unexpected issues when changes are made.

4. Balancing refactoring with new development

  • Time and resource allocation: Allocating time and resources to refactoring can be difficult, especially when there are pressing features to develop or bugs to fix.
  • Development delays: Refactoring can cause delays in new feature development, which may not be well-received by stakeholders looking for immediate improvements or new functionalities.

5. Technical and logical challenges

  • Changing architecture: Making architectural changes without affecting the external behavior of the application requires careful planning and execution.
  • Maintaining compatibility: Ensuring that refactored code remains compatible with other parts of the system and external systems it interacts with is often a complex task.

6. Team skills and experience

  • Skill variability: The success of refactoring efforts can heavily depend on the team's familiarity with the codebase and their technical expertise, particularly in understanding advanced refactoring techniques.
  • Knowledge sharing: In teams with varied levels of experience, ensuring that everyone understands the rationale behind refactoring decisions and learns from the process can be challenging.

Frequently Asked Questions

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Olga Gierszal
IT Outsourcing Market Analyst & Software Engineering Editor

Software development enthusiast with 7 years of professional experience in the tech industry. Experienced in outsourcing market analysis, with a special focus on nearshoring. In the meantime, our expert in explaining tech, business, and digital topics in an accessible way. Writer and translator after hours.

Olga Gierszal
IT Outsourcing Market Analyst & Software Engineering Editor

Software development enthusiast with 7 years of professional experience in the tech industry. Experienced in outsourcing market analysis, with a special focus on nearshoring. In the meantime, our expert in explaining tech, business, and digital topics in an accessible way. Writer and translator after hours.

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