NestJS Case Studies & Practical Application 4

Question 1

A NestJS app needs feature flags that can be toggled without redeployment. Which design best achieves this?

Accepted Answer

Store flags in a Redis/database store, inject a FeatureFlagService, and guard feature branches with it

Answer

A database or cache-backed FeatureFlagService allows runtime toggling without redeployment, while remaining injectable across the NestJS DI graph.

Question 2

Your NestJS service must call three independent external APIs and aggregate results. What is the most efficient approach?

Accepted Answer

Use Promise.all([apiA(), apiB(), apiC()]) to call them concurrently

Answer

Promise.all fires all three requests simultaneously, reducing total latency to the slowest individual call rather than the sum of all three.

Question 3

A NestJS unit test for a service that depends on a repository always hits the real database. What is wrong?

Accepted Answer

The test module imports TypeOrmModule.forRoot() instead of mocking the repository with { provide: getRepositoryToken(Entity), useValue: mockRepo }

Answer

Providing a mock via getRepositoryToken(Entity) replaces the real repository in the DI container, ensuring the service never touches a database.

Question 4

A NestJS app is deployed on Kubernetes with 5 replicas. Session data stored in memory on each instance causes inconsistent behavior. What is the fix?

Accepted Answer

Move session storage to a shared Redis cluster using connect-redis with the NestJS session middleware

Answer

A shared Redis store makes session data consistent across all replicas; sticky sessions are a workaround that fails when pods restart.

Question 5

You need to add OpenTelemetry tracing to a NestJS app so every HTTP request gets a trace ID propagated to downstream services. Which integration point is correct?

Accepted Answer

Initialize the OTEL SDK before NestFactory.create() in main.ts and use the @opentelemetry/instrumentation-http package

Answer

The OTEL SDK must initialize before any modules load so auto-instrumentation can patch Node.js's http module and establish context propagation from the start.

NestJS Practice Test

NestJS Practice Test

NestJS Case Studies & Practical Application 4