App modernisation: When to refactor vs rewrite

The decision to refactor or rewrite a legacy application is a key consideration for technical leaders navigating app modernisation. Both approaches offer pathways to improve performance, scalability, and alignment with modern business needs, but they come with vastly different levels of effort, risk, and impact. How can software architects and engineering leads determine the best course for their organisation?

This guide explores the nuances of app modernisation, breaking down key criteria, real-world examples, and hybrid approaches to help you confidently decide.

What Is app modernisation?

App modernisation refers to updating legacy applications to meet modern requirements such as performance, security, and scalability. Businesses modernise software to remain competitive, leverage new technologies, and optimise workflows.

Types of modernisation

• Refactor: Improves the internal structure of the codebase without redefining its functionality. Ideal for technical debt reduction or enhancing maintainability.

• Replatform: Migrates applications to a more modern platform (e.g., cloud-enabled) with minimal changes to core code.

• Rewrite (Rebuild): Reconstructs the application from scratch, utilising modern frameworks for a future-ready platform. Best for systems with severe limitations.

Why modernise?

• Performance: Improve processing speed, system responsiveness, and user experience.

• Security: Adhere to evolving security standards and address vulnerabilities.

• Scalability: Ensure the system can adapt to growth and increased demand.

Refactor vs rewrite

Understanding the differences between refactoring and rewriting is key. Both approaches aim to improve applications, but they deliver results through distinct methods.

What refactoring entails

Refactoring involves reorganising an application’s existing code for better performance and clarity, without altering external behavior.

Pros:

• Low risk and cost-effective

• Preserves existing structure and user experience

• Quick, incremental improvements

Cons:

• Limited by the existing architecture

• May require ongoing effort and maintenance

Best for:

• Applications with minimal technical debt

• Situations requiring faster time-to-market adjustments

What rewriting entails

Rewriting starts from scratch, building a completely new codebase while retaining the application’s core purposes.

Pros:

• Eliminates all legacy constraints

• Utilises the latest tech stacks and practices

• Addresses deep design flaws and scalability issues

Cons:

• Higher cost and time investment

• Potential risks during implementation

Best for:

• Platforms constrained by outdated frameworks

• Situations demanding architectural overhaul

Effort vs impact spectrum

Refactoring is less resource-intensive but provides incremental benefits, while rewriting requires significant effort with the potential for long-term payoff. Understanding where your project falls on this spectrum is essential for decision-making.

Decision-making criteria

The choice between refactoring and rewriting depends on several factors. Here are six key criteria to guide your decision-making process.

1. Codebase quality

Assess whether the existing code is salvageable. A well-maintained yet outdated codebase is a strong candidate for refactoring. Conversely, spaghetti code riddled with technical debt might require rewriting.

2. Business continuity

Consider how each approach affects ongoing operations. Refactoring is usually less disruptive as it doesn’t overhaul the entire system. Rewriting, however, might involve temporary downtime.

3. Team capability

Gauge your team’s expertise. Refactoring works well if your team understands the current architecture deeply. Rewriting may require hiring external experts or learning new frameworks.

4. Time-to-market

Determine how urgently updates are needed. Refactoring enables faster fixes, while rewriting can extend timelines despite greater long-term benefits.

5. Budget constraints

Evaluate financial resources. Rewriting demands a larger upfront investment, whereas refactoring spreads costs over time.

6. User risk tolerance

Consider user impact. Refactoring retains the familiar user experience, while rewriting often involves retraining users and potential feedback on new features.

Hybrid & incremental approaches

Sometimes, you don’t have to choose an all-or-nothing path. Hybrid approaches allow you to leverage the strengths of both strategies.

Strangler fig pattern

Replace legacy systems gradually by building new applications around the old one. Over time, legacy components are phased out as the new system evolves.

Example: An e-commerce site retains its catalog system while replacing the checkout process with a modern microservice for better scalability.

Feature-by-feature replatforming

Modernise piecemeal by refactoring some modules and rewriting others.

Benefits:

• Minimizes risk

• Allows early-stage user feedback

• Distributes resource investment

Real-world examples

Case study 1: Refactoring for enhanced scalability

Scenario: A global CRM tool faced performance issues but had a secure and scalable core architecture. Refactoring optimised specific components, boosting speed and enabling integrations without overhauling the backend.

Outcome: Rapid improvements with minimal disruption to users.

Case study 2: Rewriting for modernisation

Scenario: A legacy healthcare application could no longer integrate with modern compliance tools. A rewrite created a cloud-enabled, mobile-responsive application with enhanced security and analytics.

Outcome: Higher ROI due to the adoption of cutting-edge technology.

How to decide if you’re ready

Not sure where to start? Use this checklist to decide if it’s time to refactor, rewrite, or take a hybrid approach.

• Does your codebase have high levels of technical debt?

• Are your current systems blocking scalability?

• Are compliance or security issues a growing concern?

• Is your team struggling with development velocity within the existing architecture?

• Do you need to implement entirely new features that can’t be supported by the current system?

If you answered "yes" to most of these, you’re ready for app modernisation.