TCP transport in NestJS - Mini Project: Build & Apply

Basic TCP Transport with NestJS

📖 Scenario: You are building a simple NestJS microservice that communicates over TCP. This microservice will receive a message and respond with a greeting.

🎯 Goal: Create a NestJS microservice using TCP transport that listens for a message with the pattern { cmd: 'greet' } and responds with a greeting message.

📋 What You'll Learn

Create a NestJS microservice using TCP transport

Define a message pattern { cmd: 'greet' }

Implement a handler that returns a greeting string

Start the microservice to listen for TCP messages

💡 Why This Matters

🌍 Real World

TCP transport is used in microservices to enable fast and efficient communication between services in distributed systems.

💼 Career

Understanding TCP transport in NestJS is valuable for backend developers working with microservices architecture and building scalable server applications.

1

Create the microservice bootstrap function

Create a function called bootstrap that creates a NestJS microservice using TcpTransport from @nestjs/microservices. Use await to create the microservice instance and assign it to a variable called app.

NestJS

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

async function bootstrap() {
  // Your code here
}

Hint

Use NestFactory.createMicroservice with AppModule and set transport to TcpTransport.

2

Add microservice listen configuration

Inside the bootstrap function, add a line to start the microservice listening by calling await app.listen().

NestJS

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

async function bootstrap() {
  const app = await NestFactory.createMicroservice(AppModule, {
    transport: TcpTransport,
  });
  // Start listening
  // Your code here
}

Hint

Call await app.listen() to start the microservice.

3

Create a message handler for the 'greet' command

In AppController, create a method called handleGreet decorated with @MessagePattern({ cmd: 'greet' }). This method should return the string 'Hello from TCP microservice!'.

NestJS

import { Controller } from '@nestjs/common';
import { MessagePattern } from '@nestjs/microservices';

@Controller()
export class AppController {
  // Add a message handler for { cmd: 'greet' }
  // Your code here
}

Hint

Use @MessagePattern decorator with the pattern { cmd: 'greet' } on the method.

4

Add AppController to AppModule providers

In AppModule, add AppController to the controllers array to register it.

NestJS

import { Module } from '@nestjs/common';
import { AppController } from './app.controller';

@Module({
  // Add AppController to controllers
  // Your code here
})
export class AppModule {}

Hint

Include AppController in the controllers array of @Module.

Practice

1. What is the main purpose of using Transport.TCP in a NestJS microservice?

easy

A. To connect to a database using TCP.

B. To serve static files over HTTP.

C. To enable communication between microservices using TCP sockets.

D. To handle WebSocket connections.

Solution

1. Step 1: Understand the role of Transport.TCP

   Transport.TCP is used in NestJS to set up microservices communication over TCP sockets.

2. Step 2: Compare with other options

   Serving static files, database connections, and WebSocket handling are different concerns not related to Transport.TCP.

3. Final Answer:

   To enable communication between microservices using TCP sockets. -> Option C

4. Quick Check:

   Transport.TCP = Microservice TCP communication [OK]

Hint: Remember TCP transport is for microservice socket communication [OK]

Common Mistakes:

• Confusing TCP transport with HTTP or WebSocket protocols
• Thinking it handles database connections
• Assuming it serves static files

2. Which of the following is the correct way to create a TCP microservice in NestJS?

easy

A. const app = await NestFactory.createMicroservice(AppModule, { protocol: Transport.TCP, port: 3000 });

B. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 3000 } });

C. const app = await NestFactory.createMicroservice(AppModule, { transport: 'tcp', port: 3000 });

D. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.HTTP, options: { port: 3000 } });

Solution

1. Step 1: Check NestJS microservice creation syntax

   The correct syntax uses transport: Transport.TCP and an options object with port.

2. Step 2: Identify incorrect options

   The other options use wrong properties like protocol instead of transport, pass port directly without an options object, use string 'tcp' instead of the Transport.TCP enum, or use the wrong transport type like Transport.HTTP.

3. Final Answer:

   const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 3000 } }); -> Option B

4. Quick Check:

   Use transport: Transport.TCP with options object [OK]

Hint: Use Transport.TCP enum and options object for port [OK]

Common Mistakes:

• Using 'protocol' instead of 'transport'
• Passing port directly without options object
• Using string 'tcp' instead of Transport.TCP enum
• Confusing TCP with HTTP transport

3. Given this NestJS TCP microservice setup:

const app = await NestFactory.createMicroservice(AppModule, {
  transport: Transport.TCP,
  options: { host: '127.0.0.1', port: 4000 }
});
await app.listen();
console.log('Microservice is listening');

What will be the output when the microservice starts successfully?

medium

A. Microservice failed to start

B. Error: Port 4000 is already in use

C. Listening on HTTP port 4000

D. Microservice is listening

Solution

1. Step 1: Understand the code flow

   The microservice is created with TCP transport on host 127.0.0.1 and port 4000, then listens successfully.

2. Step 2: Identify the console output

   After await app.listen(), the console logs 'Microservice is listening'. No errors are shown.

3. Final Answer:

   Microservice is listening -> Option D

4. Quick Check:

   Successful listen logs message [OK]

Hint: Successful listen logs 'Microservice is listening' [OK]

Common Mistakes:

• Assuming automatic HTTP server start
• Expecting error without port conflict
• Confusing TCP with HTTP output

4. Identify the error in this NestJS TCP microservice code:

const app = await NestFactory.createMicroservice(AppModule, {
  transport: Transport.TCP,
  options: { port: '3000' }
});
await app.listen();

medium

A. Port should be a number, not a string.

B. Missing host option causes failure.

C. Transport.TCP is invalid here.

D. app.listen() must be called with a callback.

Solution

1. Step 1: Check the options object

   The port value is given as a string '3000', but it must be a number.

2. Step 2: Validate other parts

   Host is optional, Transport.TCP is valid, and app.listen() can be awaited without callback.

3. Final Answer:

   Port should be a number, not a string. -> Option A

4. Quick Check:

   Port must be numeric type [OK]

Hint: Port option must be a number, not quoted string [OK]

Common Mistakes:

• Passing port as string instead of number
• Thinking host is mandatory
• Expecting app.listen to require callback

5. You want to create a NestJS TCP microservice that listens on all network interfaces on port 5000 and logs a message when a client connects. Which code snippet correctly implements this?

hard

A. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { host: '0.0.0.0', port: 5000 } });
await app.listen();
app.on('connection', () => console.log('Client connected'));

B. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { host: '0.0.0.0', port: 5000 } });
app.listen(() => console.log('Client connected'));

C. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { host: 'localhost', port: 5000 } });
app.listen(() => console.log('Client connected'));

D. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 5000 } });
app.listen(() => console.log('Client connected'));

Solution

1. Step 1: Set host and port correctly

   To listen on all interfaces, host must be '0.0.0.0' and port 5000 is correct.

2. Step 2: Attach client connection event properly

   TCP microservice emits 'connection' event; attaching listener after await app.listen() is correct.

3. Step 3: Evaluate distractors

   Distractors either use the wrong host like 'localhost', omit the host, fail to await app.listen(), or attach the logger to the listen() callback instead of the 'connection' event.

4. Final Answer:

   const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { host: '0.0.0.0', port: 5000 } }); await app.listen(); app.on('connection', () => console.log('Client connected')); -> Option A

5. Quick Check:

   Host 0.0.0.0 + event listener after listen [OK]

Hint: Use host '0.0.0.0' and listen before event binding [OK]

Common Mistakes:

• Using 'localhost' instead of '0.0.0.0' to listen on all interfaces
• Logging inside listen callback instead of connection event
• Not awaiting app.listen() before event binding