Building a Production-Grade NestJS Backend for SaaS: Nexus API

In real-world SaaS systems, the hard part is not writing CRUD endpoints.
The real challenge is getting architecture, security, multi-tenancy, and scalability right from day one.

That’s why I built Nexus API — a production-grade NestJS backend foundation designed for serious SaaS products.

Repository:
👉 https://github.com/MertcanMert/nexus-api

🎯 Purpose of the Project

Nexus API is not a demo backend and not a tutorial project.

It is designed to solve real problems commonly ignored in early-stage backends:

• Multi-tenant architecture
• Secure authentication and authorization
• Audit logging and traceability
• Background jobs and async processing
• Modular, testable codebase
• Deployment-ready infrastructure

🧱 Architecture Overview
Modular, Layered Structure

src/
 ├─ common/          → guards, interceptors, decorators, config
 ├─ infrastructure/ → prisma, mail, storage, background jobs
 ├─ modules/        → auth, user, health
 └─ main.ts

Key principles:

• Clear separation between business logic and infrastructure
• Cross-cutting concerns handled via guards and interceptors
• Feature-based module organization
• Framework kept out of core domain logic as much as possible

🔐 Authentication & Authorization

• JWT + Refresh Token strategy
• Role-Based Access Control (RBAC)
• Policy-based authorization using an ability factory pattern
• Ownership guards for resource-level access control

Goal:
Prevent “logged-in but can access everything” systems.

🧬 Real Multi-Tenancy

Every critical action answers:

• Who did it?
• On which tenant?
• When?
• On which resource?

Audit logging is:

• Interceptor-driven
• Processed asynchronously via background jobs
• Non-blocking for API performance

⚙️ Background Jobs & Async Processing

Handled asynchronously:

• Email delivery
• Audit log processing
• Long-running or non-critical tasks

🧪 Testing Strategy

Testing is not cosmetic.

• Unit tests for services, repositories, guards
• End-to-end tests for authentication and tenancy
• Tests cover real scenarios, not just happy paths

📦 DevOps & Deployment Readiness

Out of the box:

• Docker & docker-compose setup
• Environment-based configuration
• Production build separation
• Prisma migrations fully integrated

This backend is ready to be deployed, not just run locally.

📚 Documentation Matters

The repository includes documentation for:

Architecture decisions (ADR)

• Security hardening
• Performance considerations
• API standards
• Roadmap planning

Because in production systems:

Documentation is part of the product.

Who Is This For?

• Backend engineers building SaaS platforms
• Developers using NestJS in real production systems
• Freelancers needing a solid backend foundation
• Engineers who want to avoid rewriting the same infrastructure again

Final Thoughts

Nexus API is:

• Not a framework showcase
• Not tutorial code
• Not a copy–paste boilerplate

It is a backend foundation that can confidently answer this question:

“Can this system go to production tomorrow?”

Repository:
👉 https://github.com/MertcanMert/nexus-api