Migrating a Legacy Codebase Across 15+ Modules Without Downtime: An Engineering Transformation Story

Legacy code is not a dirty word. Legacy code is code that makes money. It runs in production, serves real users, and generates the revenue that pays engineering salaries. The problem is not that legacy code exists — it is that legacy code eventually accumulates enough technical debt to slow the entire business down. I know this because I have spent the past three years living it. Since September 2022, I have been working as a Full Stack Engineer at VacancySoft, a data intelligence company whose platform processes and serves vacancy data to clients across the recruitment and professional services industries. When I joined, the platform was built on a legacy monolithic architecture that had served the company well for years — but was buckling under the weight of 50,000+ daily API requests, growing data volumes, and a feature velocity that the architecture could no longer support. Over the following months, I led the architectural redesign and incremental migration of 15+ modules from the