NestJS Case Studies & Practical Application 5

Question 1

A NestJS app must support both REST and GraphQL endpoints for the same business logic. What is the best way to share service logic?

Accepted Answer

Extract business logic into service classes injected into both REST controllers and GraphQL resolvers

Answer

NestJS services are protocol-agnostic; injecting the same service into both a controller and a resolver eliminates duplication and keeps business logic in one place.

Question 2

During a load test, a NestJS app with Scope.REQUEST providers on hot paths shows poor throughput. What is the root cause?

Accepted Answer

REQUEST-scoped providers create a new DI subtree for every request, adding instantiation overhead at high RPS

Answer

Every request triggers full instantiation of the provider and all its dependencies; at high throughput this overhead is significant compared to singleton reuse.

Question 3

A NestJS event-driven system uses EventEmitter2 internally. A bug causes some events to be lost silently. What is the most likely cause and fix?

Accepted Answer

Event listeners are registered after the event is emitted; move listener registration to onModuleInit

Answer

If a module's listener is registered lazily after app bootstrap, events emitted during startup may fire before the listener exists.

Question 4

You are migrating a NestJS monolith to microservices. The first extracted service needs to communicate synchronously with the monolith. Which transport requires the least infrastructure change?

Accepted Answer

TCP transport via @nestjs/microservices, since it requires no broker and works over the local network

Answer

TCP transport needs no external message broker — both sides open a socket, making it the fastest path to microservice extraction with minimal infrastructure.

Question 5

A NestJS app must rate-limit API endpoints per user, not per IP. Which implementation is correct?

Accepted Answer

Use @nestjs/throttler with a custom ThrottlerGuard that overrides getTracker to return req.user.id

Answer

Overriding getTracker in a custom ThrottlerGuard supplies req.user.id as the rate-limit key instead of the default IP address.

NestJS Practice Test

NestJS Practice Test

NestJS Case Studies & Practical Application 5