In today’s rapidly evolving technology landscape, enterprises are under growing pressure to innovate, stay competitive, and reduce operational overhead. At the heart of these demands lies the daunting challenge of modernising legacy applications. From high costs and vendor lock-ins to fragile architectures and limited test coverage, modernisation is a multi-faceted journey fraught with complexity. Before delving further, let’s take a moment to define what we mean by a legacy application.
What Is a Legacy Application?
According to Google, a legacy application is “a software system that is outdated but still in use, often built with older technologies and potentially lacking the support and maintenance that newer systems receive.”
These applications often remain critical to business operations, despite their aging architecture and limitations. Understanding their nature is key to grasping the challenges involved in modernising them.
This article explores the real-world challenges engineers and businesses face and why, in some cases, a full rewrite may be the smartest path forward.
1. Regression Risk: The Silent Killer of Confidence
Modernising code is risky. One of the biggest unknowns is whether the migrated application behaves as expected. Legacy applications, especially those built over decades, often have low or non-existent test coverage or rely heavily on manual testing. When modernising such systems, ensuring no regression is introduced becomes a serious challenge.
The lack of comprehensive, robust automated tests and absence of baseline performance benchmarks makes it hard to:
- Validate critical business logic post-migration
- Ensure that changes in infrastructure or environment don’t break core functionality
- Refactor or modernize code with confidence
Without a solid safety net of tests and regression coverage, teams find it difficult to ensure that the refactored or modernised version doesn’t introduce regressions or functional discrepancies. For instance, even minor changes — like moving to a new deployment model or database — can cause silent failures that may surface in production. This slows down delivery and erodes confidence in the new system.
What You Can Do:
- Invest early in test automation before or during migration
- Design and implement benchmarking processes before migration
- Use shadow deployments or canary releases to mitigate risk
- Prioritise end-to-end tests for critical workflows
2. The Knowledge Gap: Bridging Generations of Tech
A major and often overlooked challenge is legacy knowledge loss. Many legacy systems are written in outdated languages like Perl, COBOL, earlier versions of Java, and usually rely on obsolete tools like SVN, ClearCase, WebSphere, etc. The original developers are often no longer around, and documentation is often minimal, outdated, or missing. This results in:
- High onboarding time for new engineers
- Increased risk when making changes
- Dependence on a shrinking pool of specialists
Bridging this gap requires both upskilling and often reverse engineering, and without robust clear documentation or source control history, teams may spend months just understanding what the legacy system actually does before they can begin migration which slows down projects and adds risk.
3. Legacy Databases Dilemma: Old, Expensive, and Rigid
One of the thorniest parts of legacy modernisation is dealing with enterprise databases like Oracle, DB2, MS SQL, or Sybase. These systems not only lock businesses into costly licenses but also limit agility. These databases were once the gold standard, but they now act as heavy anchors, because:
- Their licensing fees are prohibitive
- Integration options are limited
- Migration paths are complex
- Support and upgrade cycles are expensive
- Running on virtualised / cloud environment is restrictive
Moving to modern alternatives like Amazon Aurora or open-source RDBMS like PostgreSQL, MySQL or MariaDB can yield significant cost savings by up to 50% and improve performance, automation compatibility, agility, and scalability. Tools such as AWS Database Migration Service and Google Database Migration Service simplify the transition, making legacy-to-cloud migrations more feasible than ever.
4. The Security Paradox: Legacy Closed Environments vs. Cloud Realities
Legacy systems often run in highly restricted internal networks with tightly controlled environments often with bespoke security models. While this may seem secure, it creates friction. I recently worked on a modernisation project involving code refactoring, where development was restricted to a virtual machine environment, requiring use of legacy tools like ClearCase which is only accessibly within the VM. Furthermore, permission to install essential development tools within the VM was limited, making the workflow highly restrictive. Some of the other challenges include:
- Manual certificate handling often posed with incompatible Transport Layer Security (TLS) Versions, No centralised PKI,
- Limited TLS support, recurrent issues with hardcoded and expired certificates
- Difficulty in implementing modern authentication / authorisation mechanisms (e.g., OAuth2, OpenID Connect)
- Incompatibility with modern DevSecOps practices
Modernising entails adjusting to new security paradigms:
- Zero trust architecture
- Using ACME-based automation to automate certificate renewal
- Centralising secret and certificate management using tools like HashiCorp Vault, Azure Key Vault, or AWS Secrets Manager
- Offloading TLS to modern proxies like NGINX, Envoy, or AWS ALB
- IAM roles instead of static users / passwords
- Exposure to internet-based risks, requiring strong encryption and logging
Convincing security and compliance teams to shift from their tried and trusted on-prem policies can be an uphill battle. Cloud-native platforms provide built-in security features, compliance controls, and seamless integration with identity providers, reducing the security management overhead while improving visibility and threat detection.
5. The Cost Factor: Breaking Free from Licensing Lock-ins
Application built on proprietary software (think Oracle databases, WebLogic servers, or legacy middleware) can carry cost overheads. One of the most immediate and painful revelations during legacy modernisation is the inflated cost of licensing. Oracle databases, for instance, are licensed based on cores or users and can easily run into six-figure bills annually. By contrast, modern alternatives like PostgreSQL, or managed services like AWS RDS offer:
- Pay-as-you-go pricing
- No upfront licensing fees
- Auto-scaling capabilities
Moving away from legacy enterprise databases like Oracle where possible to alternatives can offer up to 50% cost savings, if not more, with better scalability and native cloud integration 1, 2, 3.
6. Refactor vs Rewrite: When It’s Better to Start Fresh
In many cases, trying to incrementally modernise a monolith is like putting a new engine in a rusted car. While refactoring and replatforming can reduce migration risk, there are cases where it’s more efficient to rewrite the application as cloud-native from scratch. Benefits include:
- Clean, modular architecture using microservices or serverless
- Modern CI/CD and DevOps integration
- Easily testable
- Cost-effective and scalable cloud infrastructure
- Modern dev stack (easier to hire talents for)
- Easier onboarding and documentation
Rewriting the application allows organisations to align the system with current business processes, rather than bending processes to fit legacy constraints. Yes, rewrites come with their own risks — scope creep, delayed delivery, knowledge gaps — but for systems with outdated tech stacks, poor code quality, and minimal test coverage, it may be the faster path to long-term agility.
Final Thoughts
Modernising legacy applications is not just about code. It’s about cost, people, process, and risk. From overpriced licenses and fragile test coverage to security hurdles, knowledge gaps and outdated databases, the challenges are real and non-trivial. But with a strategic mindset, a solid roadmap, and the courage to consider a full rewrite when necessary, organisations can unlock agility, cost savings, and long-term sustainability.
If you’re planning a cloud migration or stuck in the middle of one, take a step back and evaluate: Are you just moving old problems to a new place, or solving them for good? The journey isn’t easy. But staying stuck in the past is far riskier than taking the bold step forward.
- DBX Labs $46 Million Cost Savings ↩
- Open Source is the New Data Center Standard ↩
- PostgreSQL vs. Oracle: Difference in Costs… ↩
The Hard Truths of Modernising Legacy Applications: Costs, Complexities, and Cloud-Native Benefits was originally published by Capgemini at Capgemini Software Engineering on June 06, 2025.
