A practical guide to architecting a scalable, secure multi-tenant SaaS application using Next.js App Router, PostgreSQL, and modern auth patterns.
## Multi-Tenancy Fundamentals
Multi-tenant architecture allows multiple customers to share the same application instance while keeping their data completely isolated.
### Tenant Isolation Strategies
**Shared Database, Shared Schema:**
Every row has a `tenant_id`. Simplest to implement, most cost-efficient. Requires careful query scoping.
**Shared Database, Separate Schema:**
Each tenant gets their own PostgreSQL schema. Good balance of isolation and cost.
**Separate Databases:**
Maximum isolation. Highest cost. Reserved for enterprise clients with compliance requirements.
### Next.js App Router for SaaS
```typescript
// middleware.ts
export function middleware(request: NextRequest) {
const hostname = request.headers.get('host')
const tenant = extractTenant(hostname)
const requestHeaders = new Headers(request.headers)
requestHeaders.set('x-tenant-id', tenant)
return NextResponse.next({ request: { headers: requestHeaders } })
}
```
### Authentication
Use JWT tokens with tenant claims. Never let a user from Tenant A access Tenant B's data — validate at every database query layer.
### Billing Integration
Stripe is the gold standard. Map each tenant to a Stripe Customer. Use webhooks to sync subscription status to your database.
### Database Schema
Always add Row-Level Security (RLS) policies in PostgreSQL for an additional safety layer beyond application-level scoping.