How One Developer Built a Multi-Tenant SaaS Using Next.js, NestJS, and PostgreSQL RLS

A developer rebuilt their portfolio around deep technical case studies rather than generic project showcases, using a B2B inventory and POS SaaS called Äbasto as the centerpiece. The application is structured as a monorepo using pnpm workspaces, combining Next.js 16 on the frontend with NestJS 11 and PostgreSQL on the backend. To prevent data leaks between tenants, the system leverages PostgreSQL Row-Level Security, where a custom NestJS guard injects warehouse context into each database session via SET LOCAL commands. Dynamic subdomains are handled through a server-side proxy in Next.js that reads the Host header and rewrites routes internally, while JWT validation ensures users can only access their authorized subdomain. The platform also implements a subscription model with a three-day grace period to avoid abruptly cutting off client access upon plan expiration.