Building SaaS Products with Node.js: A Practical Guide to Architecture, Security, and Scaling

Building a multi-tenant SaaS application with Node.js involves carefully designing the architecture to support multiple customers securely and reliably. Key aspects include a clear separation of concerns among API, authentication, tenant, business logic, and persistence layers. Multi-tenancy must be handled thoughtfully to associate customer data with the correct tenant identifier, ensuring that each tenant's resources are isolated and properly managed.

Authentication plays a crucial role, especially in enforcing subscription limits on the server side. For instance, a free plan that allows three projects must reject a fourth project even if the user bypasses the interface. This ensures that subscription rules are enforced consistently across all customers.

In the provided Node.js example, a simple API is built using Express, demonstrating how to manage tenant-scoped projects, apply plan-based limits, and authenticate requests with API keys. The code includes in-memory data structures to represent tenants, plan limits, and project usage, although these would be replaced with a real database in a production environment.

It also shows how to track usage separately for each tenant and log requests without exposing sensitive information like API keys. This foundational example highlights the core principles of building a scalable, secure, and multi-tenant SaaS application using Node.js.