Hybrid applications in NestJS - Mini Project: Build & Apply

Building a Hybrid Application with NestJS

📖 Scenario: You are creating a simple NestJS application that can run both as a REST API server and as a GraphQL server. This hybrid setup allows your app to serve different types of clients easily.

🎯 Goal: Build a NestJS hybrid application that sets up both REST and GraphQL modules and starts the server with both capabilities.

📋 What You'll Learn

Create a NestJS application instance

Configure a REST controller

Configure a GraphQL module

Use the app.select() method to create a hybrid app

Start the application listening on port 3000

💡 Why This Matters

🌍 Real World

Hybrid applications let you serve different client needs from one backend, like mobile apps using REST and web apps using GraphQL.

💼 Career

Understanding hybrid apps with NestJS is useful for backend developers building flexible APIs that support multiple protocols.

1

Create the NestJS application instance

Write code to create a NestJS application instance called app using NestFactory.create(AppModule).

NestJS

// Your code here to create the NestJS app instance

Hint

Use NestFactory.create(AppModule) to create the app instance.

2

Add a configuration variable for the port

Add a constant variable called port and set it to 3000.

NestJS

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

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  // Define the port number
  const port = 3000;
}

bootstrap();

Hint

Use const port = 3000; inside the bootstrap function.

3

Create the hybrid application with REST and GraphQL modules

Use app.select() to select AppModule and create a hybrid app instance called hybridApp.

NestJS

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

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  const port = 3000;
  // Create hybrid app instance
}

bootstrap();

Hint

Use const hybridApp = app.select(AppModule); to create the hybrid app.

4

Start the hybrid application listening on the port

Call hybridApp.listen(port) to start the server on port 3000.

NestJS

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

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  const port = 3000;
  const hybridApp = app.select(AppModule);
  // Start the server
}

bootstrap();

Hint

Use await hybridApp.listen(port); to start the server.

Practice

1. What is a hybrid application in NestJS?

easy

A. An app that combines HTTP and microservices in one project

B. An app that only uses HTTP controllers

C. An app that only uses microservices

D. An app that runs on multiple servers

Solution

1. Step 1: Understand NestJS application types

   NestJS apps can be HTTP-based, microservice-based, or both combined.

2. Step 2: Define hybrid application

   A hybrid app combines HTTP and microservices in one project to handle both communication types.

3. Final Answer:

   An app that combines HTTP and microservices in one project -> Option A

4. Quick Check:

   Hybrid app = HTTP + microservices [OK]

Hint: Hybrid means mixing HTTP and microservices in one app [OK]

Common Mistakes:

• Thinking hybrid means multiple servers
• Confusing hybrid with only HTTP or only microservices
• Assuming hybrid apps are separate projects

2. Which method is used to add microservice support in a NestJS hybrid application?

easy

A. connectMicroservice()

B. createMicroservice()

C. addMicroservice()

D. useMicroservice()

Solution

1. Step 1: Recall NestJS microservice integration method

   The official NestJS method to add microservices to an app is connectMicroservice().

2. Step 2: Verify method names

   Other options like createMicroservice(), addMicroservice(), or useMicroservice() are not valid NestJS methods.

3. Final Answer:

   connectMicroservice() -> Option A

4. Quick Check:

   Use connectMicroservice() to add microservices [OK]

Hint: Remember: connectMicroservice() connects microservices [OK]

Common Mistakes:

• Using createMicroservice() which does not exist
• Confusing with addMicroservice() or useMicroservice()
• Trying to add microservices without this method

3. Given this NestJS hybrid app code snippet, what will be the output when the app starts?

const app = await NestFactory.create(AppModule);
app.connectMicroservice({ transport: Transport.TCP });
await app.startAllMicroservices();
await app.listen(3000);
console.log('App is running');

medium

A. App listens on port 3000 but microservices do not start

B. App is running (and microservices start listening)

C. Error: connectMicroservice is not a function

D. App crashes due to missing microservice configuration

Solution

1. Step 1: Analyze microservice connection

   The code calls connectMicroservice with TCP transport, which is valid to add microservices.

2. Step 2: Check microservice start and app listen

   startAllMicroservices() starts microservices, then app.listen(3000) starts HTTP server. Console logs 'App is running'.

3. Final Answer:

   App is running (and microservices start listening) -> Option B

4. Quick Check:

   connectMicroservice + startAllMicroservices + listen = running app [OK]

Hint: connectMicroservice + startAllMicroservices means microservices run [OK]

Common Mistakes:

• Thinking microservices start automatically without startAllMicroservices()
• Confusing connectMicroservice with createMicroservice
• Assuming app.listen blocks microservices

4. Identify the error in this NestJS hybrid app setup:

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

medium

A. AppModule cannot be used with microservices

B. connectMicroservice() does not accept transport option

C. startAllMicroservices() must be called before app.listen()

D. Missing await before connectMicroservice()

Solution

1. Step 1: Check method call order

   In NestJS, microservices must be started before the HTTP server listens, so startAllMicroservices() should be before app.listen().

2. Step 2: Validate other options

   connectMicroservice() accepts transport option; AppModule supports microservices; connectMicroservice() is synchronous, no await needed.

3. Final Answer:

   startAllMicroservices() must be called before app.listen() -> Option C

4. Quick Check:

   Start microservices before HTTP listen [OK]

Hint: Call startAllMicroservices() before app.listen() [OK]

Common Mistakes:

• Calling startAllMicroservices() after app.listen()
• Adding await to connectMicroservice()
• Thinking transport option is invalid

5. You want to build a NestJS hybrid app that handles HTTP requests and listens to RabbitMQ messages. Which steps correctly set up the microservice part?

hard

A. Use connectMicroservice({ transport: Transport.TCP }) and call startAllMicroservices() after app.listen()

B. Use createMicroservice({ transport: Transport.RMQ }) and call app.listen() before starting microservices

C. Add RabbitMQ config in AppModule imports and call app.listen() only

D. Use connectMicroservice({ transport: Transport.RMQ, options: { urls: ['amqp://localhost'], queue: 'main_queue' } }), then call startAllMicroservices() before app.listen()

Solution

1. Step 1: Choose correct microservice setup for RabbitMQ

   connectMicroservice() with Transport.RMQ and proper options (urls, queue) is the correct way to add RabbitMQ microservice.

2. Step 2: Call startAllMicroservices() before app.listen()

   Microservices must start before HTTP server listens to ensure both run properly.

3. Final Answer:

   Use connectMicroservice({ transport: Transport.RMQ, options: { urls: ['amqp://localhost'], queue: 'main_queue' } }), then call startAllMicroservices() before app.listen() -> Option D

4. Quick Check:

   connectMicroservice + RMQ options + startAllMicroservices before listen [OK]

Hint: Set RMQ options in connectMicroservice and start microservices first [OK]

Common Mistakes:

• Using createMicroservice() which is not a NestFactory method
• Calling app.listen() before starting microservices
• Using wrong transport type for RabbitMQ
• Not providing RabbitMQ connection options