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5 Tips for Effective Documentation of Legacy Code

Legacy code is typically inherited from previous developers and is difficult to update unless you want to risk breaking the essential components in your program. Another inherent trait of legacy codes is that they either have low-quality documentation or lack the same altogether, something that can impede your legacy system architecture improvement process. 

This article takes an in-depth look into the documentation of legacy code, why it’s necessary, basic requirements, and an overview of the whole process. Keep reading and get tips on how to write legacy code documentation. 

Why Document Legacy Code?

Why document legacy code, and why are legacy systems still used, yet there are numerous new technologies, probably with higher performance? Legacy code needs to be documented because it’s hard to replace, especially if it drives core business functions. Moreover, legacy systems rely on 1 to 2 developers who originally coded it. If these people leave your organization maintaining the legacy code can be challenging and it might eventually crash. 

Lack of software documentation will most likely result in your team losing focus, underestimating the workload, or building a white elephant. It will prove very challenging, if not nigh impossible to maintain the system when you finally hire a new team. 

“It really doesn’t matter what you build if no one can figure out how to use it.”  

Tim Smith, Chef Product Manager

On the other hand, documenting a legacy codebase gives your current programmers great insights into the building blocks of your system architecture. This comes in handy when upgrading legacy systems with new features or correcting failed components. At the same time, businesses modernize legacy systems to tap into new frameworks and technologies that help you build and deploy digital solutions faster.

Requirements for Legacy Code Documentation 

The process of legacy software documentation starts with understanding what the application does. For instance, which business challenges is it geared to solve? This information should be well described in a written document, as well as an overview of how the system components interact with each other. 

After that, you should map out all the links to the system to understand the sources of your input and the destination of the outputs. Fulfilling this requirement will also help you account for external service providers when preparing documentation of legacy system.

It’s worth noting that you might have a diagram representation in some instances before you start to document the architecture of the application. This is especially true if you want to prepare an architecture document of a complex system. A diagram of the system’s phases will give you a better overview of the legacy code documentation process. 

Overview of Legacy Code Documentation Process

The process of creating documentation of legacy code can be quite complex for the uninitiated, especially if an enterprise-grade system is involved. That said, here is how to document a legacy code project. 

Conduct a system overview 

Evaluate a legacy system to determine the initial perspectives of the system before you start documentation. Critical factors to consider at this stage include:

  • Architecture overview: Create a diagram representation highlighting all the components and how they interlink with each other. 
  • Component list: List all the components on a table alongside their respective name, purpose, service provider, or version number 
  • Functional overview: Describe what the system does briefly and the business problems it intends to solve. 
  • External interfaces: Create another list of all the third-party applications that your system integrates with.

Document other elements 

Other elements you might consider when creating a software documentation depend on whether they are relevant to your project or useful to the development team. At the very list, documenting a project might include:

  • The coding standards inspired by the previous developers.
  • A reference for domain-specific names.
  • Non-functional requirements, such as performance, compatibility, or deployment protocols.
  • Agreements with external platforms or applications.


The next step of documenting a legacy codebase is automation. Automation helps you save time and costs, especially if you’re going to rewrite a code in the future. For instance, you can leverage emerging tools to generate a website from your sum-up legacy application documentation. Similarly, you can also generate diagrams of your system’s parts and automate coding standards in the future. 

Maintain the documentation 

The last step of documentation of legacy systems is ongoing maintenance. While automation will complete most of the work, some maintenance tasks will require manual involvement. Maintenance will make it easy for your team to refactor legacy code. 

5 Tips for Creating an Effective Documentation of Legacy Code

Now that you understand the benefits of legacy code documentation, what are the best practices? Documentation while working with legacy code can be pretty challenging to both junior and senior programmers. Here are 5 tips for efficient documentation with legacy technologies.

Start small and scale gradually 

One of the reasons for legacy code documentation is to minimize mistakes or risks that might impede upgrades in the future. Starting slow ensures that you counter-check every section of the code so that you don’t get overwhelmed by multiple rewrites when refactoring legacy code. 

Document call hierarchy 

Documenting a call hierarchy will help you track down how a specific system part works, especially when working with a legacy time code that is relatively long. This works by highlighting a specific function and listing down the subsequent calls that it makes sequentially. 

Double-check any assumptions 

Making assumptions about what a system’s component does is synonymous with team leaders who love taking the easier route of project management. Inasmuch as this can save you time, it can also culminate in technical debt. Double-check your assumptions to repay this debt before it yields “interest” in untenable risks.  

Testing is paramount 

Testing is obvious as one of the best practices of code documentation but is worth emphasizing. Conducting both unit and function tests will come in handy when cleaning up code, as this practice will reveal bug issues. You can set up logging to achieve the same objective when test cases are consuming too much of your time. 

Incorporate DocOps in your strategy 

You can also implement DocOps practices that extend to automation and integration of software documentation processes across your entire organization — from product and engineering departments, to support and technical writing teams. Do your research and find the best tool for documentation as this approach fosters collaboration and continuous improvement for effective documentation of legacy code. 

Re-engineering of the Large Management Platform

Re-engineering the healthcare reputation management platform to eliminate unnecessary expenses.

Re-engineering of the Large Management Platform

How Can ModLogix Help with Documentation of Legacy Code?

ModLogix is your innovative partner for all-around legacy modernization services that extend to documentation. Our experts know how to document legacy code of any application to help your team get started with new upgrades as soon as possible. We take a custom approach to create a detailed report on your legacy system and map out the priorities for documentation

Based on our 9-years of experience in dealing with outdated software solutions, our dedicated technical expert advises such points you must know about legacy systems documentation:

  1. Leave code comments and a meaningful name for all code elements for your future developers. 
  2. Include a mandatory task for technical description of each feature in your system.
  3. Run through the documentation the same way as code to ensure everything is accurate.
  4. Include and test external components for third-party documentation. 

Contact us to get started. 

Wrapping It Up

Software development has evolved and is no longer just about cracking code — documentation is a critical part of the process as well. A future-centric business will conduct thorough documentation of its legacy system to save time and effort during later upgrades. Leverage this guide and know what to prioritize in legacy code documentation today to avoid costly or nigh impossible improvements in the future.

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