Modernize Legacy Applications with Confidence

What is Application Modernization?

Application modernization is the process of updating legacy applications to modern architectures, technologies, and practices. It encompasses everything from updating frameworks and languages to completely rearchitecting monolithic systems into microservices. The goal is to make applications more maintainable, scalable, and aligned with current business needs.

Modernization isn't just about technology—it's about enabling your business to move faster. Legacy applications often become bottlenecks: changes take too long, scaling is expensive, and technical debt accumulates until maintenance consumes most of engineering capacity. Modernization breaks these constraints.

At Salt, we've helped organizations modernize applications ranging from 15-year-old monoliths to systems built just a few years ago that already accumulated significant technical debt. Our approach balances business pragmatism with technical excellence—we modernize what needs modernizing while preserving what works.

Types of Application Modernization

Application modernization takes several forms:

• Rehost: Move applications to cloud infrastructure with minimal changes (lift and shift)
• Replatform: Make targeted optimizations during migration (containerization, managed databases)
• Refactor: Restructure code to improve maintainability while preserving functionality
• Rearchitect: Fundamentally redesign application architecture (monolith to microservices)
• Rebuild: Rewrite from scratch using modern technologies and patterns
• Replace: Substitute with SaaS or off-the-shelf solutions

Modernization Strategies: Choosing the Right Approach

Not every application needs the same modernization approach. The right strategy depends on business value, technical constraints, risk tolerance, and available resources. Here's how to think about choosing a strategy.

When to Refactor

Refactoring improves code quality without changing external behavior. It's the right choice when the application's architecture is fundamentally sound but implementation has degraded over time. Refactoring is lower risk than rearchitecting and can often be done incrementally alongside feature work.

Good candidates for refactoring: applications with solid architecture but accumulated shortcuts, codebases where developers understand the domain well, and systems where a rewrite would be too disruptive.

When to Rearchitect

Rearchitecting is appropriate when fundamental architecture limits prevent meeting business needs. If you can't scale because everything is coupled, or can't deploy frequently because changes affect everything, rearchitecting may be necessary.

Rearchitecting is expensive and risky. We typically recommend incremental approaches like the strangler fig pattern rather than big-bang rewrites. The value of rearchitecting must clearly outweigh the investment and risk.

When to Rebuild

Rebuilding from scratch is rarely the right answer, but sometimes it is. If the existing codebase is beyond recovery—massive technical debt, no tests, poor domain understanding embedded in code—rebuilding may actually be faster and cheaper than trying to salvage what exists.

Be honest about the reasons for rebuilding. "The code is ugly" isn't sufficient justification. "The domain model is fundamentally wrong and fixing it would require touching every module" might be.

When to Replace

Sometimes the best modernization is replacing custom software with SaaS or packaged solutions. If you've built something that's now a commodity, maintaining it is waste. Evaluate build vs. buy for every application in your modernization portfolio.

Microservices Architecture

Microservices architecture structures an application as a collection of loosely coupled services. Each service is independently deployable, owns its data, and communicates with other services through well-defined APIs. This enables teams to work autonomously and deploy without coordinating with others.

Benefits of Microservices

When done right, microservices deliver significant benefits:

• Independent deployment: Change one service without deploying everything
• Technology flexibility: Use the best tool for each service
• Targeted scaling: Scale only the services that need it
• Fault isolation: One service failure doesn't bring down everything
• Team autonomy: Teams own services end-to-end

Microservices Challenges

Microservices aren't free. They introduce distributed systems complexity: network failures, eventual consistency, and operational overhead. Before adopting microservices, ensure you have the engineering maturity to handle these challenges.

Common pitfalls: too many services too fast (nano-services), shared databases that couple services, synchronous chains that create latency and reliability issues, and inadequate observability that makes debugging impossible.

Domain-Driven Design for Service Boundaries

Domain-driven design (DDD) provides principles for identifying service boundaries. Bounded contexts—areas where a particular model applies—often map well to service boundaries. Strategic DDD patterns help identify how contexts relate and where to draw lines.

Good service boundaries minimize cross-service communication. If two services constantly need to call each other, they might belong together. The goal is high cohesion within services and loose coupling between them.

Containerization & Kubernetes

Containers package applications with their dependencies into portable units that run consistently across environments. Docker popularized containers; Kubernetes orchestrates them at scale. Together, they've become the standard platform for modern applications.

Why Containerize?

Containerization delivers several benefits even before Kubernetes:

• Environment consistency: Works the same in development, staging, and production
• Dependency isolation: Each container has exactly what it needs
• Faster deployments: Container images are immutable and fast to transfer
• Resource efficiency: Containers share the OS kernel, unlike VMs

Kubernetes for Orchestration

Kubernetes manages containerized workloads: scheduling containers on nodes, handling failures, scaling based on demand, and managing networking and storage. It's complex, but provides a consistent platform regardless of where it runs—on-premise, AWS, Azure, or GCP.

Key Kubernetes concepts: Pods (groups of containers), Deployments (manage rollouts), Services (network access), ConfigMaps/Secrets (configuration), and Ingress (external access). Helm packages Kubernetes resources into reusable charts.

Managed Kubernetes

Running Kubernetes yourself is significant operational overhead. Managed offerings—EKS (AWS), AKS (Azure), GKE (Google)—handle control plane operations, upgrades, and integration with cloud services. For most organizations, managed Kubernetes is the right choice.

The Strangler Fig Pattern

The strangler fig pattern is our preferred approach for incremental modernization. Named after strangler fig trees that grow around host trees eventually replacing them, this pattern allows you to gradually replace legacy functionality with modern services.

How It Works

The strangler fig pattern follows these steps:

• Facade: Put a facade in front of the legacy system (often an API gateway)
• Extract: Build new functionality as separate services behind the facade
• Redirect: Route traffic for new functionality to modern services
• Retire: Eventually decommission the legacy system as all functionality moves

Benefits of Strangler Fig

Unlike big-bang rewrites, strangler fig delivers value continuously. Each increment is a working system. Risk is contained—if a new service has problems, traffic routes back to legacy. Teams learn and adjust as they go rather than betting everything on upfront design.

Implementation Considerations

Successful strangler fig requires careful attention to data. New services often need access to legacy data during transition. Patterns like event sourcing, data replication, and anti-corruption layers help manage this complexity. Plan data strategy early—it's often the hardest part.

Common Modernization Challenges

Application modernization is complex. Understanding common challenges helps you plan effectively and set realistic expectations.

Data Migration

Data is often the hardest part of modernization. Legacy schemas don't map cleanly to new models. Data quality issues surface during migration. Maintaining consistency during transition requires careful synchronization strategies.

Integration Complexity

Legacy systems often have years of integrations—some documented, many not. Modernized systems need to maintain these integrations during and after transition. API gateways and anti-corruption layers help manage integration complexity.

Organizational Change

Modernization isn't just technical—it changes how teams work. Microservices require different skills than monoliths. Team structures may need to change to own services end-to-end. DevOps practices require cultural shifts.

Scope Creep

Modernization projects easily expand beyond original scope. "While we're here, let's also..." is dangerous. Maintain focus on the specific goals that justified modernization. Additional improvements can come later.

Underestimating Effort

Modernization typically takes longer than expected. Legacy systems have hidden complexity. Dependencies emerge that weren't documented. Plan for contingency and deliver in increments that provide value even if the full scope takes longer than planned.

Modernization Best Practices

Based on many modernization projects, these practices improve outcomes:

Start with Assessment

Understand what you have before deciding what to do. Assess applications across business value, technical health, and modernization effort. Prioritize based on value vs. effort. Not everything needs modernizing—focus resources where they matter.

Define Success Metrics

What does successful modernization look like? Deployment frequency? Lead time for changes? Reliability metrics? Cost reduction? Define measurable goals before starting and track progress throughout.

Invest in Platform First

Build the modernization platform—container infrastructure, CI/CD pipelines, observability—before migrating applications. This investment accelerates all subsequent work and ensures consistency.

Modernize Incrementally

Avoid big-bang rewrites. Use patterns like strangler fig to modernize incrementally. Each increment should deliver working software. This reduces risk and allows course correction based on what you learn.

Automate Testing

Comprehensive automated tests are essential for safe modernization. Without tests, every change risks breaking something. Invest in testing before major refactoring. Consider characterization tests to capture existing behavior.

Document Architecture Decisions

Architecture Decision Records (ADRs) capture why decisions were made. Modernization involves many decisions with long-lasting implications. Future engineers (including future you) will benefit from understanding the reasoning.

Why Salt for Application Modernization?

Salt brings a differentiated approach to application modernization. Here's what sets us apart:

Pragmatic Approach: We modernize what needs modernizing, not everything. We evaluate every decision against business value and recommend the simplest approach that achieves your objectives. Shiny new technology isn't the goal—business outcomes are.

Incremental Delivery: We use patterns like strangler fig to deliver value continuously rather than betting everything on a big-bang rewrite. Each increment is working software. Risk is contained and you can adjust course based on what you learn.

Full-Stack Capability: Our managed pods handle everything—backend services, frontend applications, data migration, infrastructure, and DevOps. No coordination overhead between multiple vendors.

SPARK™ Framework: Our SPARK™ framework brings structure to modernization projects. Clear phases, quality gates, and success metrics ensure predictable delivery. You always know where you are and what's next.

Knowledge Transfer: We don't create dependency—we build capability. Every engagement includes documentation and training so your team can maintain and evolve the modernized system independently.

AI-Augmented Delivery: We use AI tools to accelerate modernization—automated code analysis, refactoring assistance, and test generation. This isn't about replacing engineers; it's about making them more effective.

Ready to start your modernization journey? Schedule a free assessment with our team to discuss your applications and how Salt can help you modernize with confidence.