Hybrid applications in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when running a NestJS hybrid app with both HTTP and microservice listeners?

Consider a NestJS hybrid application that starts both an HTTP server and a microservice listener. What will happen when you send an HTTP request to the app's endpoint?

NestJS

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { Transport } from '@nestjs/microservices';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.connectMicroservice({ transport: Transport.TCP });
  await app.startAllMicroservices();
  await app.listen(3000);
}
bootstrap();

A The app listens on port 3000 for HTTP requests and also listens for TCP microservice messages simultaneously.

B The app only listens for TCP microservice messages and ignores HTTP requests.

C The app listens only on port 3000 for HTTP requests and does not start the microservice listener.

D The app throws an error because it cannot start both HTTP and microservice listeners at the same time.

💡 Hint

Think about how NestJS handles multiple listeners in a hybrid app.

In a NestJS hybrid app, you can start both HTTP and microservice listeners. The HTTP server listens on the specified port, and the microservice listens on its transport layer (TCP here). Both run simultaneously.

❓ lifecycle

intermediate

What happens if you call app.listen() before app.startAllMicroservices() in a NestJS hybrid app?

In a NestJS hybrid app, you have two ways to start servers: app.listen() for HTTP and app.startAllMicroservices() for microservices. What is the effect of calling app.listen() before app.startAllMicroservices()?

NestJS

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.connectMicroservice({ transport: Transport.REDIS });
  await app.listen(3000);
  await app.startAllMicroservices();
}
bootstrap();

A Only the HTTP server runs; the microservice listener is ignored silently.

B The microservice listener never starts because app.listen() blocks the event loop.

C The app throws an error because startAllMicroservices must be called before listen.

D The HTTP server starts first, then the microservice listener starts after; both run correctly.

💡 Hint

Consider how async/await works and how NestJS manages multiple listeners.

Calling app.listen() starts the HTTP server and returns a promise that resolves when the server is ready. Then app.startAllMicroservices() starts the microservice listeners. Both run correctly regardless of order.

📝 Syntax

advanced

Which option correctly creates a NestJS hybrid app with HTTP and RabbitMQ microservice?

Choose the code snippet that correctly sets up a NestJS hybrid application with an HTTP server and a RabbitMQ microservice listener.

A

const app = await NestFactory.create(AppModule);
app.connectMicroservice({ transport: Transport.RMQ, options: { urls: 'amqp://localhost', queue: 'main_queue' } });
await app.startAllMicroservices();
await app.listen(3000);

B

const app = await NestFactory.create(AppModule);
app.connectMicroservice({ transport: Transport.RMQ, options: { urls: ['amqp://localhost'], queue: 'main_queue' } });
await app.startAllMicroservices();
await app.listen(3000);

C

const app = await NestFactory.create(AppModule);
app.connectMicroservice({ transport: Transport.RMQ, options: { url: 'amqp://localhost', queue: 'main_queue' } });
await app.startAllMicroservices();
await app.listen(3000);

D

const app = await NestFactory.create(AppModule);
app.connectMicroservice({ transport: Transport.RMQ, options: { url: ['amqp://localhost'], queue: 'main_queue' } });
await app.startAllMicroservices();
await app.listen(3000);

💡 Hint

Check the property name for RabbitMQ connection URLs in NestJS microservice options.

The correct property for RabbitMQ connection URLs is urls as an array of strings. Option B uses this correctly. Other options use incorrect property names or types.

🔧 Debug

advanced

Why does this NestJS hybrid app fail to receive microservice messages?

Given the following code, why does the microservice listener not receive any messages?

NestJS

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.connectMicroservice({ transport: Transport.TCP });
  await app.listen(3000);
  // Missing await app.startAllMicroservices();
}
bootstrap();

A Because the transport type TCP is not supported in hybrid apps.

B Because <code>app.listen()</code> blocks microservice messages from being processed.

C Because <code>app.startAllMicroservices()</code> was not called, so the microservice listener never started.

D Because the microservice listener must be started before <code>app.listen()</code> or it fails silently.

💡 Hint

Check if the microservice listener is actually started.

Calling app.connectMicroservice() only configures the microservice. You must call app.startAllMicroservices() to start listening. Without it, microservice messages are not received.

🧠 Conceptual

expert

What is the main advantage of using a NestJS hybrid application?

Why would a developer choose to build a NestJS hybrid application instead of separate HTTP and microservice apps?

A It allows sharing the same codebase and dependency injection container for both HTTP and microservice servers, reducing duplication and improving maintainability.

B It automatically scales microservices independently without any additional configuration.

C It forces the app to use only one transport protocol, simplifying network setup.

D It disables HTTP server features to optimize microservice performance.

💡 Hint

Think about code reuse and architecture benefits.

A NestJS hybrid app runs both HTTP and microservice servers in the same process and shares the same dependency injection container. This reduces code duplication and makes maintenance easier.