Next.js + NestJS: Useful Architecture or Unnecessary Complexity?

Next.js and NestJS are often discussed as if you must choose between the two, but this comparison is somewhat misleading. While Next.js is a React framework that can handle server-side logic, API endpoints, authentication and database access, NestJS is a backend framework focused on modules, dependency injection, controllers, services, queues, WebSockets, microservices and structured application architecture. The more relevant question is when adding NestJS to a Next.js application is actually necessary.

For many applications, using Next.js alone is sufficient. For example, a small SaaS application with a React frontend, authentication, a database, API endpoints, payments and server-side business logic can be built effectively using Next.js. The entire application can be deployed, monitored and maintained as a single unit. In this case, adding NestJS would only create an additional network boundary without providing any clear benefits.

NestJS becomes more interesting when the backend becomes complex and independent from the frontend. Consider an application that:

1. Serves requests from a Next.js frontend and provides services for multiple clients like mobile apps, internal admin tools or public APIs. In this case, keeping the backend inside Next.js might be reasonable, but introducing a separate NestJS API would allow all clients to access the same backend, making the architecture clearer.

2. Requires background jobs such as generating invoices, processing videos, sending emails or synchronizing external systems. These tasks often need queues, retries, workers, scheduling and monitoring. A dedicated NestJS backend with built-in support for these features can make managing such jobs more efficient.

3. Develops complex business logic that goes beyond simple CRUD endpoints. For example, an endpoint may need to validate inventory, calculate discounts, reserve stock, create payments, send notifications, update analytics and publish events. NestJS's structured approach with modules, services and dependency injection can help organize such complex domain logic.

4. Needs to scale independently. If your Next.js application handles normal web traffic, but your API performs CPU-intensive or high-volume workloads, deploying them as separate applications allows each part to scale according to its own workload. NestJS can provide the necessary scalability and performance.

However, adding NestJS comes with additional costs such as another application, deployment process, set of environment variables, network communication, authentication between systems, duplicated types, additional logging and monitoring, and the potential for failures in another part of the system. These costs should only be incurred when the backend actually requires the additional complexity and benefits that NestJS provides.

In summary, use Next.js alone for applications where the backend mainly supports the Next.js application. Consider using Next.js + NestJS when the backend becomes an independent system, such as when dealing with multiple clients, complex domain logic, background jobs, or the need for independent scaling. The key is to evaluate whether the architectural boundary created by NestJS provides genuine value in your specific case.