You have a system that has been running for years. It's not pretty, it has parts nobody wants to touch, and the technical team does gymnastics every time something new needs to be added. But it keeps working — and there's the dilemma. Migrating it involves risk. Not migrating it involves growing cost in maintenance, security and development speed. This article explains how to make that migration safely, without stopping operations.
Why Legacy Software Is So Hard to Migrate
Legacy software has characteristics that make it difficult to replace. First, it's been in production for years absorbing use cases that are never documented. Second, it has dependencies with other systems added over time. Third, the technical teams that built it are no longer there.
Our software audit and consulting service is designed precisely for these cases — analyzing the real state of the system before making any decisions.
The Right Approach: Incremental Migration, Not Big Bang
The biggest mistake in legacy software migrations is trying to rewrite everything at once — what's known as a big bang rewrite. The right approach is incremental migration: identify the parts of the system with the most strategic value or maintenance cost, migrate them one by one, and keep the old system running in parallel.
Strangler Fig Pattern
The Strangler Fig pattern — popularized by Martin Fowler — is the most used methodology for incremental migrations. Build the new system around the old one, gradually redirecting traffic from old to new feature by feature, until the old system is left unused.
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The Phases of a Well-Executed Legacy Migration
Phase 1: Inventory and Analysis
Before writing a line of new code, understand what the current system has. At MiTSoftware we perform this analysis as part of our software review and consulting service.
Phase 2: Target Architecture Definition
Define the architecture of the new system. The stack choice depends on the case: Python for complex business systems, Node.js for high-performance APIs, or React for modern frontends.
Phase 3: Incremental Migration by Domain
Prioritize modules to migrate by value and risk criteria. Having a dedicated team specifically for the migration, separate from the current system maintenance team, significantly reduces risk.
Phase 4: Cutover and Elimination of Old System
Once all modules are migrated, deactivate the old system in a controlled manner with an observation period before final elimination.
Why MiTSoftware
At MiTSoftware we have executed legacy system migrations for companies in Spain and the US. Our software support, maintenance and consulting service includes legacy system analysis and migration roadmap planning. See also our DevOps services to understand how we structure CI/CD processes during migrations.
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