TCP transport in NestJS

Introduction

TCP transport lets different parts of your app talk to each other using the TCP protocol. It helps send and receive messages over a network reliably.

When you want to connect microservices inside your app using a fast and reliable network method.

When you need to send data between servers or services that are on different machines.

When you want to build a simple client-server communication using TCP sockets.

When you want to handle streaming data or continuous connections between services.

When you want to avoid HTTP overhead and use a lower-level network protocol.

Syntax

NestJS

import { Transport } from '@nestjs/microservices';

const tcpOptions = {
  transport: Transport.TCP,
  options: {
    host: '127.0.0.1',
    port: 3001
  }
};

The transport property must be set to Transport.TCP to use TCP transport.

You can specify host and port in options to define where the service listens or connects.

Examples

This creates a TCP microservice server listening on port 4000.

NestJS

const tcpServer = app.connectMicroservice({
  transport: Transport.TCP,
  options: { port: 4000 }
});

This creates a TCP client that connects to the server on localhost port 4000.

NestJS

const client = ClientProxyFactory.create({
  transport: Transport.TCP,
  options: { host: 'localhost', port: 4000 }
});

Sample Program

This code creates a simple TCP microservice that listens on port 3001. It waits for messages with the command 'sum' and returns the sum of the numbers sent.

NestJS

import { Controller } from '@nestjs/common';
import { MessagePattern, Payload } from '@nestjs/microservices';
import { NestFactory } from '@nestjs/core';
import { MicroserviceOptions, Transport } from '@nestjs/microservices';
import { Module } from '@nestjs/common';

@Controller()
class AppController {
  @MessagePattern({ cmd: 'sum' })
  sum(@Payload() data: number[]): number {
    return data.reduce((a, b) => a + b, 0);
  }
}

@Module({
  controllers: [AppController]
})
class AppModule {}

async function bootstrap() {
  const app = await NestFactory.createMicroservice<MicroserviceOptions>(AppModule, {
    transport: Transport.TCP,
    options: { port: 3001 }
  });
  await app.listen();
  console.log('TCP microservice is listening on port 3001');
}

bootstrap();

Important Notes

TCP transport is good for fast and reliable communication but does not have built-in HTTP features like headers or REST.

Make sure the port you choose is free and not blocked by firewalls.

You can test TCP microservices using a TCP client or another NestJS client proxy.

Summary

TCP transport in NestJS helps microservices talk over the network using TCP sockets.

You set it up by specifying Transport.TCP and giving a host and port.

It is useful for fast, low-level communication between services.

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