Refactoring legacy systems is the process of updating and improving the existing codebase of an older system. It is a common practice in the field of software development, as every system will have some old code that needs to be addressed. Instead of trying to rewrite the entire system, refactoring allows us to gradually improve the codebase and fix any issues in small increments. This approach is often more effective and less risky than starting from scratch, as it allows us to maintain the existing functionality of the system while making it more efficient and easier to maintain. By embracing the old code and working to improve it, we can ensure that our legacy systems continue to serve us well into the future.

1. Utilising approval testing to ensure the refactoring does not create bugs

Approval testing is a software testing technique that can be used to ensure that refactoring a legacy system does not introduce any new bugs. In this approach, a test is first run on the original, unmodified code to obtain a known-good output. This output is then used as a reference, or "approval," against which future outputs can be compared. After the refactoring has been completed, the test is run again to generate a new output. This output is then compared to the original reference to see if the refactoring has introduced any changes. If the new output matches the reference, the refactoring can be considered successful. If not, it may indicate that the refactoring has introduced a bug, and the code will need to be further reviewed and tested to identify and fix the issue. By using approval testing, we can ensure that our refactored code maintains the desired behavior and does not introduce any unintended changes.

2. Declutter the code

Decluttering the code is an important aspect of refactoring a legacy system. As systems age, they can become cluttered with old, unused, or redundant code, which can make them more difficult to understand and maintain. By removing this unnecessary code, we can make the system easier to understand and navigate, which can make it easier to identify and fix any issues. Decluttering can also help to improve the overall performance and efficiency of the system by reducing the amount of unnecessary processing that the system needs to do. To declutter the code during refactoring, we can use a variety of techniques, such as removing unused variables and functions, consolidating similar code, and using more expressive and self-documenting names for variables and functions. By taking the time to declutter the code, we can make the refactoring process more efficient and effective, and ultimately improve the overall quality of the system.

3. SOLID

SOLID principles can be useful when refactoring code to improve its design and maintainability. The "S" in SOLID stands for the Single Responsibility Principle, which states that a class should have only one reason to change. When refactoring code, it's often a good idea to identify areas where a class is doing too much, and extract the logic into separate classes that each have a single, well-defined responsibility. This can make it easier to understand and modify the code in the future. The "O" in SOLID stands for the Open/Closed Principle, which states that a class should be open for extension but closed for modification. This means that you should strive to design your classes in such a way that they can be extended to add new behavior without modifying the existing code. This can be achieved through the use of inheritance and polymorphism. The "L" in SOLID stands for the Liskov Substitution Principle, which states that derived classes should be substitutable for their base classes. This means that if you have a base class with a certain behavior, any derived classes should also have that behavior. This can help ensure that your code is consistent and predictable. The "I" in SOLID stands for the Interface Segregation Principle, which states that clients should not be forced to depend on methods they do not use. This means that when designing your classes, you should strive to create small, focused interfaces that define only the methods that are necessary for a specific purpose. This can help reduce coupling and improve the flexibility of your code. Finally, the "D" in SOLID stands for the Dependency Inversion Principle, which states that high-level modules should not depend on low-level modules. Instead, both should depend on abstractions. This can help decouple your code and make it more modular and reusable.

4. Take incremental steps

When refactoring legacy code, it is important to take things step by step and avoid rushing. Making changes too quickly can result in a "tarzan" mode refactoring, where the code is essentially rewritten in a haphazard and unplanned manner. Instead, it is better to take incremental steps, carefully planning and testing each change before moving on to the next. This can be a time-consuming process, but it ultimately leads to a more stable and maintainable codebase. Additionally, being patient and thorough during the refactoring process can help ensure that the code continues to function as intended, without introducing new bugs or other issues.

5. Have GIT by your side

Using Git or any VCS when refactoring legacy code is a good idea because it allows you to commit small changes to your codebase. This way, you can easily keep track of the changes you've made and roll back to a previous version if something goes wrong.

In conclusion, refactoring legacy systems can be a challenging but rewarding process. By following the tips outlined in this blog post, you can approach the task with confidence and ensure that your refactored system is clean, efficient, and easy to maintain. Whether you're working on a large, complex codebase or a small, simple one, these tips can help you make the most of your refactoring efforts and deliver high-quality results.