SaaS Product Architecture with Next.js in 2025

When building a SaaS product, architectural decisions determine the product's scalability. We cover the key layers of a SaaS built with Next.js App Router in 2025.

Multi-tenant structure: Each customer (tenant) can access their own data but not others'. Row Level Security (RLS) with tenant_id-based isolation in PostgreSQL is the most secure and performant approach. Supabase provides an ideal foundation for this.

Auth layer: Organization-based auth with Clerk or Auth.js. User sessions automatically protect API routes and Server Actions. tenant_id is stored in the JWT.

Payment: Subscription management with Stripe Billing. Webhooks arrive at Next.js API routes and update the database. Free/Pro/Enterprise tier transitions are processed automatically.

AI integration: OpenAI or Anthropic API calls are made directly from Server Actions. For streaming responses, the response body is streamed with the `ai` SDK. Rate limiting is applied per tenant.

Deployment: The Vercel + PlanetScale/Neon combination creates a production-ready stack. Geographic routing and A/B testing are possible with Edge middleware.