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KNOWLEDGE BASE

Migrating Legacy Systems to Modern Web-Based Enterprise Software: Process and Strategies

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Table of Contents — English

Why Is the Legacy System Still Running?

Legacy systems have managed critical business processes for decades, have been customized in the process, had layers added on top, and become embedded in the company’s DNA. You cannot simply "switch these systems off"; that would mean stopping the company. That is why legacy modernization must be a gradual process, not an abrupt one.

Strangler Fig Pattern: The Risk-Minimizing Approach

The Strangler Fig is a migration pattern inspired by a vine that wraps around an existing tree and gradually replaces it. While the existing system keeps running, new components are built in parallel and gradually replace the old ones.

Implementation Steps

  • Map all the capabilities of the legacy system — which module does what?
  • Start with the least risky and least dependent module
  • Develop and test the new module on the modern stack
  • Gradually shift traffic routing to the new system (5% → 20% → 50% → 100%)
  • Decommission the legacy module and migrate the data
  • Move on to the next module

Managing Migration Risks

Data Migration: The Most Critical Step

Legacy databases often hold un-normalized, inconsistent data in old formats. Data migration should be handled as a project independent of the technical development.

  • Data profiling: Analysis of current data quality and inconsistency
  • ETL pipeline: The Extract → Transform → Load process
  • Parallel run: The two systems running in sync until migration is complete
  • Data validation: Record count, checksum, and critical-field matching for each batch
  • Rollback capacity: The ability to revert at every stage

AI Perspective: 2026–2030

AI-powered code-analysis tools can automatically analyze legacy code bases and produce a dependency map. These tools are increasingly used to optimize the migration order and discover hidden dependencies.

FREQUENTLY ASKED QUESTIONS

This situation is not exceptional; it is common in banking, insurance, and the public sector. With the API Wrapping approach, the COBOL system can be placed behind an API, and modern frontend and middleware layers can be built on top.

With an appropriate migration strategy (Strangler Fig), operational downtime is close to zero. With a Big Bang approach, a controlled maintenance window may be required.

It varies from 6 months to 3 years depending on the system’s complexity. In the Strangler Fig approach, where each module is migrated independently, each stage is managed in 2–4 month iterations.

No. Legacy migration is also an opportunity to reassess business processes. Unused or value-less features may not be migrated.

A reliable estimate is not possible before code analysis and process mapping are complete. The PROJX Discover phase produces this estimate.

Key Takeaways

  • Legacy modernization is not abrupt; it is a gradual, risk-managed process.
  • The Strangler Fig Pattern enables a gradual transition without operational downtime.
  • Data migration is a sub-project independent of the technical development and of equal importance.
  • A migration plan made without comprehensive code and process analysis is unreliable.
  • Legacy migration is an opportunity to optimize business processes and eliminate value-less features.
Content Owner: Projx Digital
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