The idea of building a multi-tenant SaaS application has been on my mind because it represents a significant architectural challenge with practical implications for security, scalability, and cost. Many developers approach multi-tenancy with apprehension, often concerned about data isolation and performance. I believe that with the right tools and patterns, these concerns can be addressed effectively. Let’s explore how to build such a system using NestJS, Prisma, and PostgreSQL’s Row-Level Security.

Have you ever considered what prevents one customer from accidentally accessing another’s data in a shared database? This is where Row-Level Security (RLS) becomes essential. It acts as an invisible gatekeeper at the database level, ensuring each query only returns data belonging to the current tenant. This approach provides robust security without sacrificing performance.

Setting up the foundation begins with our database schema. We design our tables to include a tenant_id column on all tenant-specific data. This simple addition becomes the cornerstone of our isolation strategy.

-- Enable RLS on a table
ALTER TABLE users ENABLE ROW LEVEL SECURITY;

-- Create a policy that filters by tenant_id
CREATE POLICY tenant_isolation_policy ON users
    USING (tenant_id = current_setting('app.current_tenant'));

Now, how do we make this work with our application code? The magic happens through Prisma’s middleware and a custom service that manages tenant context. We intercept each request, identify the tenant, and set the database context accordingly.

// Custom Prisma service with tenant context
@Injectable()
export class TenantAwarePrismaService extends PrismaClient {
    async setTenantContext(tenantId: string) {
        await this.$executeRaw`SELECT set_config('app.current_tenant', ${tenantId}, true)`;
    }
}

But where does this tenant identification occur? In a typical SaaS application, tenants are often identified through subdomains or JWT tokens. We implement middleware that extracts this information early in the request lifecycle.

// Tenant identification middleware
@Injectable()
export class TenantMiddleware implements NestMiddleware {
    constructor(private readonly tenantService: TenantService) {}

    async use(req: Request, res: Response, next: NextFunction) {
        const tenantId = await this.tenantService.identifyTenant(req);
        req.tenantId = tenantId;
        next();
    }
}

What about authentication? We need to ensure that users can only access resources within their assigned tenant. This requires careful coordination between our authentication system and tenant context.

// JWT strategy with tenant validation
@Injectable()
export class JwtStrategy extends PassportStrategy(Strategy) {
    constructor(private readonly tenantService: TenantService) {
        super({
            jwtFromRequest: ExtractJwt.fromAuthHeaderAsBearerToken(),
            secretOrKey: process.env.JWT_SECRET,
        });
    }

    async validate(payload: any) {
        const user = await this.tenantService.validateUserWithinTenant(
            payload.sub,
            payload.tenantId
        );
        return user;
    }
}

Performance considerations are crucial in multi-tenant systems. We must ensure that our database indexes properly support our tenant isolation strategy. Without proper indexing, query performance can degrade significantly as data grows.

// Prisma schema with proper indexing
model User {
    id        String   @id @default(cuid())
    email     String
    tenantId  String
    // ... other fields

    @@index([tenantId])
    @@unique([email, tenantId])
}

Testing becomes more complex in a multi-tenant environment. We need to verify that data isolation works correctly and that users cannot cross tenant boundaries. This requires comprehensive test suites that simulate multiple tenant scenarios.

// Example test verifying tenant isolation
it('should not return data from other tenants', async () => {
    await prismaService.setTenantContext('tenant-a');
    const tenantAData = await prismaService.user.findMany();
    
    await prismaService.setTenantContext('tenant-b');
    const tenantBData = await prismaService.user.findMany();
    
    expect(tenantAData).not.toEqual(tenantBData);
});

Deployment and monitoring require special attention. We need to track performance metrics per tenant and ensure that no single tenant can negatively impact others. Proper logging and monitoring help identify issues before they affect customers.

Building a multi-tenant application requires careful planning and execution, but the benefits in terms of operational efficiency and scalability make it worthwhile. The patterns we’ve discussed provide a solid foundation that can be extended to meet specific business requirements.

I hope this exploration of multi-tenant architecture with NestJS and Prisma has been valuable. If you found this helpful, please consider sharing it with others who might benefit. I’d love to hear about your experiences with multi-tenant systems - what challenges have you faced, and how did you overcome them? Feel free to leave your thoughts in the comments below.