Why Your Legacy Software is Hard to Maintain – and What to Do About It
Legacy software is like a classic car – it may have been top-of-the-line when it was first released, but it becomes increasingly unreliable, expensive to maintain, and impractical as it ages. And just like that vintage vehicle, legacy systems cannot run forever without intervention.
As a senior developer and software consultant, I‘ve seen first-hand how legacy applications become a growing liability for organizations over time. The code becomes brittle, the integrations break, and the knowledge of how it works gradually walks out the door. But with the right approach, legacy doesn‘t have to be a four-letter word.
In this article, I‘ll dive deep into the technical and business challenges that make legacy software so painful to maintain based on my experience and industry research. I‘ll share real-world examples, key statistics, and practical strategies for overcoming these challenges to keep your legacy apps running smoothly. Finally, I‘ll provide a actionable plan for application modernization to help your organization innovate faster at lower cost and risk.
The High Costs of Legacy Software
The costs of legacy software are far-reaching and often underestimated by business and IT leaders alike. Consider these telling statistics on the impact of outdated systems:
- Prevalence – The average organization relies on software that is over 20 years old, and 31% of their applications are considered legacy^1. In the public sector, some of the oldest legacy systems are over 50 years old^2.
- Maintenance – Organizations spend an average of 60-80% of their IT budgets on maintaining legacy software^3. That‘s a lot of money spent just to keep the lights on, rather than innovating.
- Outages – High-priority legacy applications have 3 production incidents per week on average^4. Each incident costs $12,913 on average and takes 12.4 hours to resolve^5.
- Productivity – Over 90% of IT leaders say that outdated technology is holding their organization back^1. Legacy constraints slow development velocity, hinder the ability to meet business requirements, and hurt employee morale.
- Opportunity – Outdated legacy systems prevent organizations from adopting new technologies like cloud, AI, and IoT that could deliver significant business value. They also make it harder to comply with new regulations and industry standards.
The message is clear – legacy software is not just a minor inconvenience, but a major liability that gets worse over time. Technical debt compounds like financial debt if it‘s not paid down. Next, let‘s look at the specific factors that make legacy software so difficult to maintain.
Why Legacy Software is Hard to Maintain
Legacy software is hard to maintain for a variety of reasons. I like to group these challenges into two main categories – technical factors and organizational factors. Let‘s start with the technical side.
Technical Factors
On the technical front, legacy applications suffer from a range of issues that make them brittle, error-prone, and difficult to update. These challenges get more pronounced the older the software gets.
Outdated technologies are one of the biggest culprits. A recent survey found that the most problematic legacy technologies for maintenance are:
- Mainframes (45%)
- C/C++ (40%)
- Unix (35%)
- Java (25%)
- COBOL (25%)^1
I‘ve seen many applications still running on technology stacks that are 10+ years old. The older the stack, the harder it is to find developers and support. Even seemingly modern languages like Java have gone through major changes that can make older code incompatible without significant refactoring.
Poor design leads to code that is hard to understand, modify and test. One study found that the average legacy code base has over 1 million lines of code, and 41% of developers say that legacy code is "spaghetti code" that needs to be untangled to understand^6. Without clear separation of concerns and modular design, small changes can have ripple effects that break the application in unexpected ways.
Lack of documentation is another challenge in many legacy apps. The original developers may be long gone, and documentation gets lost or becomes outdated over time. In fact, out-of-date documentation was cited as the #1 challenge of maintaining legacy code by 58% of developers^6. Lack of clear documentation makes it hard to determine the purpose of the code, expected inputs/outputs, and potential side effects of changes.
Complex dependencies arise in legacy software over time as different systems are integrated in non-standard ways. Over 90% of applications have undocumented dependencies^7. Changing legacy code risks breaking integrations, especially when the dependent systems are also legacy. Subtle differences between test and production environments also cause unexpected issues.
Non-functional flaws are common in legacy software, even if the functionality works as expected. The most cited issues are performance/scalability (44%), high technical debt (38%), and security vulnerabilities (32%)^8. As usage grows beyond original forecasts, legacy systems often fail to meet expected response times and throughput. Security flaws may exist in open source dependencies or custom code.
Organizational Factors
Beyond the technical issues, organizational dynamics present additional challenges for legacy modernization. These factors are less tangible than the technical ones, but I‘ve found they are often the biggest barriers.
Knowledge loss is a key risk as experienced developers and SMEs leave the organization over time. When knowledge is concentrated among a small team (or single individual), their departure can make the software very difficult to maintain. I‘ve even seen cases where nobody can confidently explain how key parts of the legacy system work, which is really scary!
Misaligned incentives between business, development, and operations teams can lead to mounting technical debt. Project managers are often goaled on delivering new features as fast as possible, which can incentivize shortcuts and quick hacks. Without business support for a sustainable software practice, maintenance gets neglected until the system reaches a breaking point.
Sunk cost fallacy can keep organizations investing in legacy systems long after they should rationally move on. Stakeholders may be reluctant to replace something that has historically worked and that they‘ve invested a lot of time and resources into already. There‘s a mentality of "if it ain‘t broke, don‘t fix it." But propping up legacy software is throwing good money after bad.
Competing priorities make it difficult for legacy modernization projects to get funding and support. There are always many projects vying for limited resources, and the benefits of modernization may seem abstract compared to new feature development. But continuing to defer maintenance leads to compounding technical debt and lost opportunities in the long run.
Strategies for Maintaining Legacy Software
Now that we understand the factors that make legacy software difficult to maintain, what can we actually do about it? Based on my experience, there are 4 main approaches for maintaining legacy systems: encapsulation, reengineering, rewriting, and replacement. The right strategy depends on your specific situation.
1. Encapsulation
Encapsulation involves putting a wrapper around legacy code to minimize changes and isolate it from other systems. Common encapsulation techniques include:
- Expose as a service – Create APIs that sit on top of legacy code, allowing modern applications to access legacy data and logic without touching the underlying implementation. This allows you to migrate the front-end independently of the back-end.
- Database abstraction – Add a data access layer to abstract database access from the application code. This provides flexibility to refactor the database without touching application code, and vice versa.
- Anti-corruption layer – Place an anti-corruption layer between the legacy system and new applications. This layer translates data between old and new models and handles differences in protocols and interfaces. It prevents new code from depending directly on legacy code.
These techniques let you modernize legacy systems incrementally by putting a more maintainable facade over the old code. However, they require investment to design, build, and maintain the abstraction layers. And you are still stuck with the limitations of the underlying legacy system. Encapsulation is most viable when the legacy software is relatively stable and sufficient for current needs.
2. Reengineering
Reengineering takes an incremental approach to modernizing legacy code from the inside out. Rather than replacing the system all at once, you gradually improve code quality, modularity, and maintainability over time. Common reengineering techniques include:
- Automated refactoring – Use static analysis tools to automatically identify and fix common code quality issues like duplications, magic numbers, long methods, etc. Many IDEs have built-in refactoring tools.
- Modularization – Organize the code base into loosely coupled modules with clear interfaces. Separate concerns like data access, business logic, and presentation. Use design patterns to make the code more readable and maintainable.
- Test coverage – Add unit tests for key behaviors to catch potential bugs and verify the code works as expected. Use code coverage tools to identify untested code. Tests make it safer to refactor and provide living documentation.
- Instrumentation – Add logging and monitoring to provide insight into system behavior in production. Use tools for centralized log management, error tracking, and performance monitoring. This helps identify and diagnose issues faster.
These reengineering practices help pay down technical debt and make the legacy code base more maintainable over time. They require active investment, but deliver compounding benefits in productivity and agility. In my experience, it‘s better to budget at least 20% of development capacity to maintenance and "sharpening the saw" rather than just shipping new features.
3. Rewriting
Rewriting involves creating a new system from scratch to replace the legacy application. This is typically a last resort for systems that are too far gone to salvage through reengineering. Starting fresh can deliver better performance, scalability, and maintainability than trying to gradually modernize a dying system.
However, rewriting is the riskiest approach in my experience. It‘s essentially a "big bang" release, which has a lot more potential for failure than an iterative approach. Rewrites tend to be much more expensive and time-consuming than originally expected. During the transition, you have to maintain both the legacy and new system in parallel. There‘s also a risk that the rewritten system won‘t actually deliver the expected benefits over the legacy one.
To mitigate these risks, it‘s critical to:
- Ensure strong business alignment on the rationale and expected outcomes of the rewrite
- Prioritize the riskiest technical challenges and validate with a proof-of-concept
- Implement comprehensive testing for functional and non-functional requirements
- Plan for incremental migration rather than an all-at-once cutover
- Perform rigorous data migration testing, validation, and backups
4. Replacement
Finally, there‘s the option to replace the legacy application entirely with a commercial off-the-shelf (COTS) package or SaaS solution. Migrating to a proven solution can deliver faster time-to-value and lower TCO than continuing to maintain an in-house legacy application.
The suitability of "buy" vs. "build" depends on how well the legacy functionality maps to available solutions on the market. For horizontal capabilities like CRM, ERP, and ITSM, there are typically robust solutions available. You‘ll need to assess potential solutions based on functional fit, cost, ease of migration, vendor risk, and other factors.
The main challenges with replacement tend to be around data migration and integration with other in-house systems. Expect to spend significant time and effort on data cleansing, mapping, transformation, and validation. You‘ll also need to rework any integrations between the legacy system and adjacent applications.
Making the Business Case for Modernization
Hopefully you now have a solid understanding of why legacy software is so hard to maintain and what your options are to tackle it. But often the biggest challenge is getting the rest of the organization on board to invest in modernization.
Modernization initiatives live and die based on buy-in from non-technical stakeholders who may not appreciate the urgency. As software leaders, we have to make modernization a business priority, not just a technical one.
My recommended approach is to engage your business partners to understand their goals first. What are the strategic priorities of the organization and how does the business envision technology enabling them? Position legacy modernization in terms of advancing those goals.
For example, if "improve customer experience" is a key business priority, explain how the scalability and reliability issues of legacy applications lead to poor performance, outages, and frustrated customers. Share statistics on the frequency and costs of legacy failures. Provide anecdotes of customers complaining about the legacy app or limitations implementing their requested features. Contrast the legacy experience with that of a modernized system.
Similarly, if "increase speed to market" is a priority, showcase how legacy constraints slow the business down. Share metrics on the long lead times and low throughput of the legacy software delivery process. Give examples of missed opportunities to beat competitors to market. Explain how investing in automation and modularity will help the business ship faster and more frequently.
The key is to build a compelling narrative around the expected business outcomes of modernization. Some outcomes to emphasize:
- Improving system availability and performance
- Reducing the risk of security incidents and compliance issues
- Enabling faster delivery of new features and integrations
- Increasing agility to respond to changing market conditions
- Reducing ongoing maintenance, infrastructure, and licensing costs
- Improving employee satisfaction and productivity
Work with your finance partners to quantify the expected costs and benefits (cost savings, incremental revenue, risk reduction) of modernization. This helps validate the ROI and payback period for the investment. Don‘t expect to get multi-year budget commitments for a big-bang project. Start with a modest seed investment in a pilot project to build credibility.
Conclusion
Legacy software may run your business today, but you can‘t afford to let it hold your business back from innovating for the future. As you‘ve seen, continuing to rely on brittle legacy applications leads to mounting costs, risks, and missed opportunities. It‘s like paying more and more to keep a clunker car running rather than investing in a modern vehicle.
By understanding the factors that make legacy software difficult to maintain, you can choose the right strategies to pay down technical debt and build a more agile foundation. Whether you choose to rewrite, re-engineer, encapsulate, or replace your legacy apps, the keys to success are:
- Secure business alignment on the vision and rationale for modernization
- Embrace an incremental, iterative approach to reduce risk and prove value
- Abstract and isolate changes to legacy systems to enable independent modernization
- Adopt software engineering best practices and invest in automation and testing
- Prioritize knowledge management and documentation to reduce key person risk
Ultimately, preventing new applications from turning into tomorrow‘s legacy is just as important as dealing with today‘s legacy challenges. By building with maintainability in mind from the start, you can keep your software assets from becoming liabilities down the road.
It‘s not easy to teach an old application new tricks, but it‘s one of the most essential challenges that software leaders must take on. Don‘t let legacy be a four-letter word in your organization – turn it into an opportunity to set your business up for agility, growth, and competitive advantage in the digital era.