TCP transport in NestJS - Step-by-Step Execution

Concept Flow - TCP transport

Start NestJS App

↓

Create TCP Server

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Listen for TCP Connections

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Receive TCP Message

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Process Message in Handler

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Send Response Back

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Wait for Next Message or Connection

NestJS app starts and creates a TCP server that listens for messages, processes them, and sends responses.

Execution Sample

NestJS

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

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

A NestJS controller listens for TCP messages with command 'sum' and returns the sum of numbers.

Execution Table

Step	Action	Input Message	Handler Called	Handler Output	Response Sent
1	TCP Server starts listening	N/A	N/A	N/A	N/A
2	Client sends message	{ cmd: 'sum', data: [1,2,3] }	sum	6	6
3	Client sends message	{ cmd: 'sum', data: [10,20] }	sum	30	30
4	Client sends message	{ cmd: 'sum', data: [] }	sum	0	0
5	No more messages	N/A	N/A	N/A	N/A

💡 No more messages received, TCP server continues listening.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
data	N/A	[1,2,3]	[10,20]	[]	[]
handlerOutput	N/A	6	30	0	0

Key Moments - 2 Insights

Why does the handler method 'sum' get called only when the message has cmd 'sum'?

Because the @MessagePattern decorator filters messages by the 'cmd' property, so only messages with cmd 'sum' trigger the sum method (see execution_table rows 2-4).

What happens if the client sends a message with a different command?

The TCP server ignores it or does not call the sum handler, so no response is sent for unknown commands (not shown in execution_table but implied).

Visual Quiz - 3 Questions

Look at the execution table, what is the handler output at step 3?

A 6

B 0

C 30

D N/A

💡 Hint

Check the 'Handler Output' column at step 3 in the execution_table.

At which step does the handler receive an empty array as input?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look at the 'Input Message' column in execution_table for an empty data array.

If the client sends a message with cmd 'multiply', what happens?

A No handler is called

B An error is thrown

C sum handler is called with data

D Response is always 0

💡 Hint

Recall that @MessagePattern filters by cmd 'sum' only, so other commands do not trigger handlers.

Concept Snapshot

NestJS TCP Transport Quick Guide:
- Use @MessagePattern({ cmd: 'name' }) to listen for TCP messages.
- TCP server listens continuously for messages.
- Handler methods process messages matching their cmd.
- Responses are sent back over TCP automatically.
- Unknown commands do not trigger handlers.

Full Transcript

This visual execution trace shows how NestJS uses TCP transport. The app starts and creates a TCP server that listens for messages. When a client sends a message with a command 'sum' and data array, the sum handler method is called. It calculates the sum of the numbers and sends the result back. This repeats for each message. If the message command does not match, no handler is called. Variables like data and handlerOutput change with each message. This helps beginners see how TCP messages flow through NestJS microservices.