How do you refactor legacy code to apply design principles and patterns?

Refactoring should always be determined by value. Refactoring to support new requirements of a platform or security or to maintain dependencies is of known high value and should be part of process. If you have code that is maintained in accordance with above and doesn't currently have negative performance issues or isn't a high value/high contact area for the end user/system, that is a determination made by the goals of an organization and should be done so in a migration-style pattern with a set of rules for all developers. You can use tooling to watch for code complexity, duplication, smells, and general analysis worked into your CI/CD for Maintenance practices. Make the refactoring work small and integrated into work already being done.

I would like to add some more case scenarios where the existing code cannot resolve the requirements. Also if the code is too rigid to be changed. When a change is made, its very difficult to deploy the same.

Finding the Trouble: Before fixing anything, figure out what's wrong. Is your code too hard to understand, too connected, or too inflexible? Look for things like repeating code, long confusing parts, or rules that are broken. Using Special Tools: Just like using magic glasses to spot hidden things, there are tools that help find tricky spots in your code. They show where things need fixing. Talking and Listening: Imagine your code as a story. Sometimes, you ask friends to read and give ideas to make the story better. Similarly, talk to others or listen to what people need from your code. It helps understand what the code should do.

Reengineering legacy code to incorporate design principles and patterns requires a systematic approach: First, thoroughly analyze and document the existing codebase to understand its functionality and dependencies. Next, select appropriate design principles and patterns that address specific issues within the code. Implement changes incrementally, prioritizing modules based on business impact and complexity. Validate modifications through rigorous testing to ensure functionality is maintained and edge cases are handled. Document all changes and share knowledge with the team to facilitate understanding and future maintenance. Continuously monitor and refine the codebase based on feedback and evolving requirements.

This is such a great point because sometimes not all code needs refactoring. When it comes to refactoring another great approach that you can employ is just writing new code in the format you want or wish to have. A simple example is for Android, we now Jetpack Compose, but this does not mean all XML code should be refactored, you can easily start writing new components using the latest pattern or tool. Hence, always identify the problem and the why before you start refactoring. Refactoring is also very time-consuming, and if you don't have bandwidth it can be very costly.

Writing tests is essential when refactoring legacy code. Tests ensure your code still works as expected after changes and detect bugs, errors, or regressions. Use unit testing, integration testing, or regression testing to write tests. Consider using TDD or BDD to write tests before you write code. This helps you catch potential issues early and ensure your code meets the desired functionality

Test Driven Development (TDD) is more of a software development methodology than a testing framework. Write a failing test, implement the code and then refactor . That is what is recommended. Some advantages - Makes a developer analyse the module and write tests before development - "Code to abstractions not concretions" is by default. All external dependencies such as repository need to be abstracted for mocking - Code coverage is high, as tests drive the effort Disadvantages - For legacy modules there is an additional effort to refactor the external dependencies before TDD can be applied - This style does not appeal to many developers due to upfront thought process and effort A good fit for projects with IaC framework implemented.

Before refactoring code, it's very important to have a testing process (preferably automated, but even a manual one will do). When writing tests for code you're about to refactor, start with the user-facing interface. These kinds of tests are often called "functional tests" because they focus on ensuring the application continues to function as the user expects. When refactoring, more granular tests, often called "unit tests", are often less important, especially if the unit(s) you're refactoring will change considerably.

Safety Nets for Code: Imagine making a safety net for your code like the safety nets in a trampoline park. Tests are like checking if the trampoline is safe to jump on. Finding Boo-Boos: Just like how a doctor checks you for any problems, tests check the code for mistakes or things that don’t work right. They help make sure changes to the code won’t make it worse. Different Testing Tools: Similar to having different toys for different games, there are tools to test code in different ways. They help find problems before they cause trouble. Writing Tests First: It's like drawing a map before going on an adventure. Sometimes, you write tests before writing the actual code to make sure everything works right from the start.

This is also very valid, tests are a need in any clean software you write - and this is both UI and Unit. I know there is a lot of debate on UI but I believe both are crucial in ensuring your software is up to standard.

To support the same code in multiple deployment modes design patterns are a must . One example is of logging which would be writing unstructured logs to the filesystem in an on-premise deployment but in the cloud environment it requires writing structured logs to a log aggregator . To handle these scenarios the template pattern can be used. This allows specifying the steps of a process with the provision of overriding certain steps in derived classes based on the use case. In above use case the implementing module will invoke the steps need to log a message i.e. prepare message and write message using template methods . Instantiating the derived object based on deployment mode will format message and log it to the desired destination .

Adapter Pattern: It's like using a special connector to fit different puzzle pieces into the same set without changing their shapes. For code, it helps different classes work together by adapting their interfaces. Strategy Pattern: Think of a chef choosing different recipes for pasta based on what customers want. Similarly, in code, it helps pick the best algorithm to solve a problem at runtime. Observer Pattern: Imagine friends in a treasure hunt, shouting when they find clues. Similarly, in code, one object notifies others when it changes, like in a chat app when a message arrives.

One of the many benefits of using patterns is the ease of reading the applied algorithm. Once the pattern is identified, you already know what to expect from the execution. During refactoring, there are many opportunities to apply patterns and programming best practices, and having a deep understanding of them will help you make better choices in your project.

Apply template pattern for logging The template class will have two abstract methods for Generating Message and Log Message as steps which need to be overridden. The derived class for On Premise override the methods, and format the message using log4j to write to filesystem , similarly the Cloud implementation will use Serilog for structured logging (use of key value pairs of clarity) and send to Splunk forwarder . The implementation class needs to instantiate the correct instance based on deployment mode. So the sequence for logging is the same however algorithm differs based on deployment. In some cases the common code can be moved to the template class if both process have some common flow.

Red, Green, Refactor. Cover your code with tests. The change a little portion the run the tests, if it fails, make it pass. Then refactor the code. It will be slow, but steady. We used this technique to upgrade and migrate a very old codebase.

When iteratively refactoring, remember to run your tests between each iteration. Once you have an automated testing process for your code, it's often preferable to setup a "continuation integration" system to run your tests automatically every time your code changes.

BTW, this article is missing many important topics... anyway here are some steps you MUST follow to refactor legacy code effectively: 1- Understand the existing structure and architecture of project. 2- Break down the project into smaller modules/components. 3- Cleanup junk/temp/log/backup files. 4- Write acceptance and integration tests. 5- Apply design principles. (DRY, YAGNI, SOLID...) 6- Run tests. 7- Refactor incrementally. 8- Repeat.

When you refactor old code to apply design principles and patterns, you must first thoroughly understand the existing code and its functionalities. Then, identify parts of the code that can be improved by applying design principles, such as SOLID, and specific design patterns that solve common problems efficiently, like the Strategy pattern for easily changing algorithms. Make these changes gradually, ensuring that the code continues to function correctly after each modification.