Even in today’s modern world, legacy applications are all around us, powering critical infrastructure we rely on in our daily lives. These applications are in turn driven by legacy code that continues to be actively maintained and developed. In this article, we’ll look at some lessons learned from maintaining legacy applications, while highlighting some of the tools we’ve found useful in this work.

Legacy applications and codebases usually contain technical debt. Practices that were standard when these applications were developed in the past are not necessarily best practices now—or worse, may introduce issues such as security vulnerabilities and performance degradation. For these reasons, it is important to continually minimize technical debt and strive not just to maintain but to improve legacy code. The following are some tools and techniques that can help.

Static analysis tools

Static analysis tools analyze a codebase to identify potential issues, such as

• Code smell. These are indicators of duplicated, unused, or poorly structured code that may benefit from refactoring.
• Outdated libraries. Keeping libraries up to date helps avoid deprecated functions, inefficient code, and compatibility issues.
• Security vulnerabilities. Unsafe coding practices and outdated dependencies can expose the codebase to security risks.

Audits for security compliance

While often performed at the organizational level, security audits can also apply directly to a codebase. Common compliance standards include

• SOC 2 for data security and privacy
• HIPAA for handling protected health information (PHI)
• Cloud Security Alliance (CSA) Certification for cloud-based environments

Prioritizing and categorizing technical debt

Not all technical debt is equal. It can be helpful to group identified issues into categories—such as security, performance, or cosmetic—and prioritize them based on factors like severity, impact, and ease of resolution.

Modern tooling

Many modern development tools help detect and reduce technical debt. Modern integrated development environments (IDEs) like Visual Studio include tools to analyze codebases for outdated libraries and packages and generate compiler warnings when deprecated or obsolete libraries or functions are in use. Some modern tooling can even interface with legacy tools used to maintain legacy applications. For example, Synergy DBL Integration (SDI) for Visual Studio includes interfaces to Synergy repository tools (rpsutl, schema file editing, etc.).

Through our experience modernizing tools and practices, we’ve learned the importance of not alienating developers of legacy applications. A key part of this is preserving existing developer workflows, or improving them when possible. One example is a recent SDI change to tab-only IntelliSense completion so that it now better matches completion behavior for Visual Studio’s built-in languages (C#, F#, etc.). During development, we consulted Synergy DBL developers and identified requirements that are unique to the language. For example, many Synergy developers press the TAB key after typing a variable name to insert a tab between the name and its type—a pattern that differs from other Visual Studio languages, which place the type before the name. To support this workflow, we added logic to ensure Synergy developers are able to get the behavior they want in this situation: tab insertion rather than auto-completion. This avoids frustrating developers who would otherwise find a long-established workflow suddenly disrupted by a change in IDE behavior after an update.

Working on legacy applications also fosters techniques and imparts knowledge that isn’t immediately obvious. By definition, legacy applications have been in production for many years, and examining their code reveals which coding practices really work and are robust enough to be out in the field, as opposed to code that looks simple or clean in tutorials. These applications are also often mission-critical, requiring high uptime and resilient features to meet business needs. As such, they offer insight into past business decisions—something valuable in an era where engineering and business choices are increasingly siloed due to increased departmentalization. Legacy applications also embody practical coding principles such as YAGNI (You Aren’t Gonna Need It), Fail Fast (to avoid cascading failure), and others, providing developers with real-world examples of techniques that can be applied in modern application development.

We hope this article has provided useful insights into the lessons developers can learn from working with legacy applications. Whether maintaining a legacy system or creating new functionality, developers can gain valuable skills and expertise along the way.