Fastify to NestJS Migration: Zero Downtime with 1,000+ Live Users

Yes. By migrating module-by-module and running both side-by-side behind the same routes, you can shift traffic incrementally and keep the app live throughout.

NestJS adds structure, dependency injection, and modularity that scale better for larger teams and codebases, while still supporting Fastify as its underlying HTTP adapter.

NestJS can run on the Fastify adapter, so raw HTTP performance stays close to Fastify while you gain architectural benefits.