Migrating a Monolith to a Modular Next.js Architecture

A growing SaaS product was built as a React SPA with a Node.js Express backend. As the team scaled from 3 to 12 engineers, the monolithic architecture became a bottleneck — slow builds, merge conflicts, and difficulty onboarding new developers.

I led the migration to a Next.js App Router architecture with clear module boundaries. The approach was incremental — we migrated one feature at a time while keeping the existing app running. Phase 1: Established the new Next.js project with shared component library and design system.

Phase 2: Migrated read-heavy public pages first, using Server Components for instant performance gains.

Phase 3: Moved authenticated features, replacing the Express API with Next.js API routes and Supabase.

Phase 4: Deprecated the old SPA once all features were migrated and traffic was shifted.

• Maintaining feature parity during migration without freezing development
• Training team members on Server Components and the App Router paradigm
• Handling authentication state across old and new systems during the transition
• Ensuring SEO improvements didn't cause ranking disruptions

• Built a feature flag system to gradually shift traffic between old and new implementations
• Created comprehensive internal documentation and ran weekly migration workshops
• Implemented a shared authentication layer that worked across both systems
• Set up proper redirects and canonical URLs to preserve SEO equity

Built a feature flag system to gradually shift traffic between old and new implementations Created comprehensive internal documentation and ran weekly migration workshops Implemented a shared authentication layer that worked across both systems Set up proper redirects and canonical URLs to preserve SEO equity