Legacy Code Survival Guide: Tips for Enhancing Maintainability

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ITNEXT

6 min readJun 13, 2023

A pervasive reality in a developer’s routine is interacting with legacy code, essentially code authored by someone else. This task can be daunting, particularly when dealing with a lack of documentation or tests. Although the syntax and specifics might seem elementary, such as a for-loop, data manipulation APIs, or unique library patterns, understanding the entire code structure often poses a significant challenge.

The complexities arise when dealing with a large class full of diverse functions, misaligned function names, or confusing abstractions. Developers often hesitate to modify such code due to unpredictable outcomes. Consequently, this code is either left untouched or developed further following the current pattern, which increases the isolation and antiquation of these legacy modules.

The transient nature of the developer role further compounds this problem, leaving more legacy code with each departure until it reaches a point of being virtually unmaintainable.

I’ve gathered valuable insights throughout my journey of deciphering various legacy codebases — from an advertisement system in Java to a GIS system monitoring network traffic performance or a banking system for data collection. I’d like to categorize these tips into two primary areas: outside and inside the codebase.

Outside the Codebase

Understanding the Business

Establishing communication with the software users is crucial. Reach out to the end-user, collaborate with the business analyst or product owner in the team, or conduct interviews with domain experts.

The aim is to become familiar with the language being used, also referred to as Ubiquitous Language in Domain Driven Design (a term coined by Eric Evans). This understanding is critical to view the problem holistically and appreciate the codebase’s role in resolving domain issues.

Documentation

Despite often being dismissed as outdated or unreliable, documentation can offer valuable insights into a legacy codebase. Acronyms, references to other documents or projects, and high-level architecture diagrams, which are typically less mutable than code, can be found in the documentation.

Creating a new document based on your findings is a practical approach, making sure to include references to your sources.

End-to-End Tests

Though end-to-end tests can exist within the codebase under testing, they primarily assess the system from an external perspective. These tests simulate user interaction with the application, revealing the critical user journeys and important paths.

it('intercepts a request and returns mocked data', () => {
  cy.intercept('GET', 'https://api.openweathermap.org/data/2.5/weather*', {
    fixture: 'melbourne-weather.json'
  }).as('getWeather')

  cy.visit('http://localhost:3000/');

  cy.get('[data-testid="search-input"]').type('Melbourne');
  cy.contains('Search').click();

  cy.get('[data-testid="search-results"] .search-result').first().click();

  cy.get('[data-testid="my-weather-list"]').contains('Melbourne');
  cy.get('.weather-category').contains('clouds');
  cy.get('.temperature').contains('14°');
})

Tools like Cypress or Playwright allow developers to verify whether the application successfully enables the end user to accomplish their tasks.

Cypress running against a Web application

From an end-to-end testing perspective, your application is treated as a black box, simulating an end user's experience. It is crucial to prioritize the inclusion of the most critical user journeys in the end-to-end test suite, considering that these tests can be resource-intensive to run and debug.

Inside the Codebase

As a developer, you can perform numerous tasks within the codebase itself. Before making any modifications, it is always wise to establish a safety net to avoid serious errors.

Integration Tests

End-to-end tests should only cover the most critical user journeys. Integration tests prove valuable for contingencies like network failures or backend service errors. These tests require mock servers (like json-server) and fake email servers to simulate potential failure scenarios.

The objective is to ensure that various components can function collectively in a relatively isolated environment. Tools like mirage.jsor msw can simulate the network layer in the browser, facilitating the verification of interactions between components.

In my book Maintainable React, I have covered a lot of patterns to address the code smells and how to refacotring them with Test-Driven Development approach.

Code Smells

Code smells are usually not bugs; they do not prevent the program from functioning. Instead, they indicate design weaknesses that may slow development or increase the risk of bugs or failures in the future. Common examples of code smell in a legacy codebase include:

Large Classes or Methods: These take time to understand and maintain. A method should do one thing, and a class should have a single responsibility.
Duplicated Code: This usually means an opportunity to abstract or generalize the code.
Dead Code: Code that is no longer in use should be removed to reduce clutter and confusion.
Inappropriate Naming: Code should be self-documenting. Names of variables, functions, and classes should clearly express what they do.
Tight Coupling: High dependency between classes or modules can make the system hard to change and maintain.

Refactoring

Refactoring is the process of changing a software system in such a way that it does not alter the external behaviour of the code, yet improves its internal structure. When dealing with legacy code, the following refactoring strategies can be helpful:

Simplifying Conditional Expressions: This can make your code more readable and easier to understand.
Extracting Methods or Classes: This helps reduce code duplication and complexity. It also increases reusability and maintainability.
Renaming Variables, Methods, or Classes: This can improve the readability and clarity of your code.
Replacing Magic Numbers with Named Constants: This makes the code more readable and reduces potential errors.
Moving Features between Objects: This can help you organize your code better and adhere to the Single Responsibility Principle.

Refactoring should be done incrementally, and each change should be small. If something breaks, you will know exactly what caused it.

Test-Driven Development (TDD)

Test-Driven Development (TDD) is a software development process that relies on repeating a concise development cycle: requirements are turned into particular test cases, and then the code is improved so that the tests pass.

Here’s how you can apply TDD when working with a legacy codebase:

Identify a Small Piece of Functionality to Change or Add: Start small, so verifying whether your changes are correct is easy.
Write a Test for the Desired Functionality: This might be challenging with legacy code. If the code isn’t structured to be testable, you may need to refactor it first.
Run the Test and See it Fail: This verifies that your test works correctly and can catch a failure.
Update the Code to Make the Test Pass: This can involve adding or modifying new code.
Run All Tests to Ensure They All Pass: This ensures that your change didn’t break anything else in the system.
Refactor the Code: Now that you know the code is working (because all tests pass), you can safely clean it up.

Using TDD in a legacy codebase might be challenging, especially if the codebase lacks tests. However, it’s worth the effort. TDD can help you understand the code, prevent bugs, and make the code easier to change and maintain in the future.

Once you have established your initial set of tests, they act as a safety net, guarding against mistakes and allowing you to proceed with increasing speed and confidence.

Summary

In summary, dealing with legacy codebases requires a balance of understanding both the internal structure and the external factors influencing the codebase. Implementing robust testing mechanisms and applying strategies like refactoring and test-driven development will ensure that the legacy codebase remains a valuable asset rather than a liability.

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