Message patterns (request-response) in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS microservice handler?

Consider this NestJS microservice handler using a message pattern. What will be the response when the client sends a message with pattern 'sum' and data {a: 3, b: 4}?

NestJS

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

@Controller()
export class MathController {
  @MessagePattern('sum')
  sum(data: { a: number; b: number }) {
    return data.a + data.b;
  }
}

A 7

B undefined

C Throws an error

D NaN

💡 Hint

Think about what the handler returns when it receives the data.

The handler receives an object with properties a and b. It returns their sum, so 3 + 4 = 7.

📝 Syntax

intermediate

Which option correctly defines a message pattern handler in NestJS?

Choose the correct syntax to define a message pattern handler that listens for the pattern 'get_user'.

A

@MessagePattern('get_user')
getUser(data) => {
  return data;
}

B

@MessagePattern get_user
getUser(data) {
  return data;
}

C

@MessagePattern('get_user')
getUser {
  return data;
}

D

@MessagePattern('get_user')
getUser(data) {
  return data;
}

💡 Hint

Remember the decorator syntax requires parentheses and a string argument.

Option D uses the correct decorator syntax with parentheses and a string pattern. Other options have syntax errors.

🔧 Debug

advanced

Why does this NestJS microservice handler not respond to messages?

Given this code, why does the microservice never respond to messages with pattern 'ping'?

NestJS

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

@Controller()
export class PingController {
  @MessagePattern('ping')
  ping() {
    console.log('Ping received');
  }
}

A The pattern string 'ping' is invalid and does not match any messages.

B The handler does not return any value, so the client waits indefinitely.

C The @Controller decorator is missing required parameters.

D The method name 'ping' conflicts with reserved keywords.

💡 Hint

Think about what happens if a handler does not return a response.

The handler logs the message but does not return anything. In request-response, the client expects a response, so it waits forever.

❓ state_output

advanced

What is the value of 'count' after these messages are processed?

Consider this NestJS microservice controller that counts how many times it received a 'count' message. What is the value of count after processing three messages with pattern 'count'?

NestJS

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

@Controller()
export class CounterController {
  private count = 0;

  @MessagePattern('count')
  increment() {
    this.count++;
    return this.count;
  }
}

A 3

B 0

C 1

D Undefined

💡 Hint

Each message triggers the increment method once.

The count starts at 0 and increments by 1 for each message. After 3 messages, it becomes 3.

🧠 Conceptual

expert

Which statement about NestJS message patterns and request-response is true?

Choose the correct statement about how NestJS handles request-response message patterns in microservices.

A Message patterns only support fire-and-forget messages and cannot return responses.

B NestJS automatically sends a default response if the handler method returns void or undefined.

C The handler method must always return a value or a Promise to send a response back to the client.

D The client must manually listen for responses; the handler does not send them automatically.

💡 Hint

Think about how request-response communication works in microservices.

In NestJS, to send a response back to the client, the handler must return a value or a Promise. Otherwise, the client waits indefinitely.