What is Legacy Code?
Legacy code is any software code that is:
- Written in an outdated or unsupported programming language or technology
- An old or obsolete version of a software application that is still in use
Legacy code is often kept by organizations due to the perceived cost and risk associated with updating or replacing it. It can present major challenges for businesses and development organizations.
Swimm is a leading solution for helping enterprises with legacy code. From our experience, one of the primary characteristics of legacy code is that it’s often poorly understood by current staff. The original developers may have moved on, and their knowledge and expertise went with them. Legacy code also commonly lacks comprehensive documentation or comments, making it even harder to understand and maintain.
We’ll explore some of the reasons organizations maintain legacy code, and present several strategies that can be life-savers for teams grappling with the challenges of legacy codebases.
Why Is It So Difficult to Work With Legacy Code?
Working with legacy code is difficult because teams must work with systems that were built under different assumptions, often without adequate documentation or testing. These systems may still perform critical functions, but understanding and modifying them safely is time-consuming and error-prone.
Key reasons include:
- Lack of automated tests: Without tests, it’s hard to verify if changes break existing behavior. This makes developers hesitant to refactor or optimize the code.
- Missing or outdated documentation: Legacy systems often have poor or no documentation. This forces developers to reverse-engineer the system just to understand its behavior.
- Unfamiliar or deprecated technologies: Legacy code might rely on outdated languages, libraries, or frameworks that few developers are trained in, increasing the learning curve.
- High risk of regression: Because the system’s behavior isn’t well understood, even small changes can cause unexpected side effects, often in unrelated parts of the application.
- Tightly coupled components: Many legacy systems are monolithic, with interdependent modules that make isolation and targeted changes difficult.
- Scarcity of institutional knowledge: The original authors of the code may have left the organization, taking with them valuable context about the system’s design and decisions.
- Difficulty integrating with modern tools: Legacy systems may not support or interoperate easily with current development, deployment, or monitoring tools.
Why Do Organizations Maintain Legacy Code?
Continuing Business Value and Importance
Despite its challenges, legacy code remains essential to many businesses. The original software was written to solve a specific business problem, and in many cases, it continues to do so. It delivers essential functionality that the business relies on to operate efficiently. In fact, in many instances, the business may have been built around the software rather than the other way around.
Even if the software is outdated and inefficient, it’s still delivering value to the business. It may not be ideal, but it’s better than not having the software at all. Furthermore, the cost and effort of replacing the software may be prohibitively high, especially when compared to the cost of simply maintaining the existing code.
Costs and Risks Associated with Full System Replacements
When it comes to replacing legacy code, the costs can be substantial. It’s not just about the financial cost of developing new software; there’s also the time and effort required to implement the new system, train staff, and manage any teething problems that arise.
There’s always a risk that the new software won’t deliver the expected benefits or may even create new problems. This is particularly true if the original legacy code is poorly understood. If the developers don’t fully understand the existing system, they may inadvertently remove essential functionality or introduce new bugs.
Additionally, there’s the risk of business disruption. Implementing new software can be a disruptive process, and if it’s not managed effectively, it can lead to a loss of productivity and even customer dissatisfaction.
The Knowledge and Business Logic Embedded Within Legacy Systems
One of the main reasons why legacy code is so hard to replace is because of the knowledge and business logic embedded within it. The original developers built the software to solve specific business problems, and their knowledge and experience are encoded within the software itself.
This business logic is often complex and nuanced, reflecting the unique needs and challenges of the business at the time. It may not be well-documented or easily understood, but it’s essential to the operation of the business. Replacing this knowledge and business logic is not a simple task, and it’s one of the main reasons why replacing legacy code can be so challenging.
Furthermore, the business logic embedded within legacy code often reflects the history and evolution of the business. It’s a record of past decisions, successes, and failures. Replacing this code means losing this history, which can be a significant loss.
Code Examples: Useful Techniques for Dealing with Legacy Code
Here are a few techniques that can be very useful in addressing the challenges of legacy code.
1. Creating Seams to Break Dependencies
Legacy code often includes hard-to-control dependencies, such as queue connection calls. To test such code, you can introduce a seam — a place where behavior can be changed without modifying the original code.
Example (JavaScript):
// QueueConnector is responsible for establishing a connection to a message queue system (e.g., RabbitMQ, Kafka)
export class QueueConnector {
connect() {
// Actual queue connection logic should be implemented here
// For example: connect to RabbitMQ, Kafka, AWS SQS, etc.
}
}
To avoid triggering a real connection in tests, you can override the behavior:
// FakeQueueConnector simulates a queue connection for testing or development purposes
class FakeQueueConnector extends QueueConnector {
connect() {
// Simulate queue connection behavior
console.log(“Fake queue connection”);
}
}
2. Using Characterization Tests
When you don’t fully understand what a function does, you can write tests that capture its existing behavior. These help ensure nothing breaks when you refactor.
// Function that performs the actual API call
async function call_api() {
const response = await fetch(“https://api.example.com/data”);
return response.response_code;
}
// Test case that verifies the API returns a 200 status code
test(“API returns 200 status code”, async () => {
const response = await call_api();
// Ensure the API responded with HTTP 200
expect(response.status).toBe(200);
});
This lets you refactor call_api with confidence that the output remains consistent.
3. Sprout Technique
If you need to add functionality to untested code but can’t refactor it yet, write new code in a separate method or class and call it from the original location.
Before:
class ImagePreprocessor {
preprocessImages(images) {
const processedImages = [];
for (let image of images) {
const processed = this.preprocess(image);
processedImages.push(processed);
}
imageManager.add(processedImages);
}
After Sprouting:
class ImagePreprocessor {
preprocessImages(images) {
for (let image of images) {
this.applyPreprocessing(image);
}
imageBundle.getImageManager().add(images);
}
applyPreprocessing(image) {
// Simulate preprocessing logic like resizing, normalizing, etc.
console.log(`Preprocessing image: ${image.name}`);
image.processed = true;
image.timestamp = Date.now(); // Example metadata
// More processing can go here
}
}
4. Wrap Technique
When adding behavior before or after a legacy method, you can wrap it with a new one.
Original:
class ImagePreprocessor {
preprocessImages(images) {
// Legacy posting logic
}
}
Wrapped:
class ImagePreprocessor {
preprocessImages(images) {
this.process_images(images)
}
process_images(images) {
// Original logic moved here
}
}
Now preprocess_images() can be mocked in tests.
5. Scratch Refactoring
This is a temporary, throwaway refactor to help you understand the code. You extract functions, rename variables, and restructure — but none of these changes are committed.
const isSupportedType = [“png”, “jpeg”, “gif”].includes(image.type);
const isTallEnough = image.height > 300;
const isWideEnough = image.width > 700;
const isImageValid = image && isSupportedType && isTallEnough && isWideEnough;
After gaining clarity, you revert and write proper tests before making real changes.
These techniques help you safely navigate complex, brittle code by introducing tests incrementally and isolating new behavior. They are essential tools for modernizing legacy systems without breaking them.
Critical Best Practices for Managing Legacy Code
Working with legacy code can be a daunting task. However, with the right strategies in place, development teams can be successful.
1. Understanding the Codebase
The first and most crucial step in dealing with legacy code is understanding the codebase. This involves getting a sense of the overall architecture, the language used, and the techniques implemented.
When dealing with a large and complex codebase, breaking it down into smaller, manageable components can be helpful. This approach allows for a more in-depth exploration of the code, thereby facilitating a thorough understanding of its inner workings.
2. Establish a Version Control System
The next step in effectively working with legacy code is to establish a version control system. Version control allows developers to track and manage changes to the codebase, making it easier to pinpoint when and where issues are introduced, and roll back changes if necessary.
A version control system also makes collaboration easier. With it, multiple developers can work on different parts of the codebase simultaneously without stepping on each other’s toes.
3. Create a Safety Net with Tests
Testing is a vital component of software development, and it becomes even more critical when working with legacy code. The older a system is, the more likely it is to have hidden bugs and issues that can cause problems down the line.
By creating a comprehensive suite of tests, you can catch these issues early on and address them before they become major problems. Additionally, these tests serve as a safety net, giving you the confidence to make changes to the codebase without fear of breaking anything.
4. Incremental Refactoring
Rather than trying to overhaul the entire codebase at once, I’ve found that making small, incremental changes over time can be much more manageable and effective.
Refactoring involves improving the structure of the code without changing its functionality. By doing this incrementally, you can make significant improvements to the codebase without disrupting the system’s operation.
Learn more in our detailed guide to legacy code refactoring
5. Documentation and Knowledge Sharing
When working with legacy code, it’s often the case that the original developers are no longer available to provide insight into the system’s inner workings. In such cases, it’s up to you to create comprehensive documentation.
Documenting the codebase, its architecture, its functionality, and any quirks it may have can be incredibly useful for future developers. Additionally, sharing this knowledge with your team can help ensure that everyone is on the same page and can contribute to the project effectively.
6. Isolate Dependencies
Legacy code often comes with a host of dependencies, which can make it difficult to understand and modify. One effective strategy for dealing with this is to isolate these dependencies.
By isolating dependencies, you can focus on understanding and modifying one component of the system at a time, rather than trying to grapple with the entire codebase at once. This approach can make the process of working with legacy code much more manageable.
7. Employ Continuous Integration
Continuous integration is a software development practice that involves integrating changes to the codebase regularly and automatically testing them to catch bugs early. When dealing with legacy code, this practice can be incredibly beneficial.
By continuously integrating changes, you can ensure that any issues introduced by modifications to the code are caught and addressed promptly.
8. Prioritize Technical Debt
Technical debt refers to the future cost of potential rework caused by choosing a quick and dirty solution now instead of using a better approach that would take longer. Legacy code often comes with a significant amount of technical debt, which can make it difficult to work with.
Prioritizing this technical debt and addressing it systematically can significantly improve the quality of the codebase and make it easier to maintain and modify in the future.
9. Stay Updated on Security
Finally, it’s essential to stay updated on security when working with legacy code. Older systems often have security vulnerabilities that can be exploited, so it’s vital to regularly review the codebase for potential issues and update it as necessary to ensure its security.
In some cases, you will not be able to fully address security issues in legacy systems. Even so, being aware of them can help you find other strategies, such as isolating the legacy system from networks or other critical systems.
Documenting Legacy Code with Swimm
Legacy code is a critical part of many organizations’ software ecosystems, despite the challenges it presents. It continues to deliver business value and embodies essential knowledge and business logic. However, dealing with legacy code requires careful strategies to ensure its maintenance and improvement.
Understanding the codebase is the first step, followed by establishing a version control system for better management. Creating a safety net with tests helps catch hidden issues, while incremental refactoring makes the codebase more manageable. Documentation and knowledge sharing are crucial, especially when original developers are no longer available.
Swimm, with its code-coupled documentation and AI assistance for creating documentation, can be an invaluable tool in this process. It aids developers in understanding and maintaining legacy code, ultimately improving team productivity and code quality, aligning with the product’s key use cases and the needs of senior developers and development teams dealing with legacy code.
