Queue consumers (processors) in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a NestJS queue consumer throws an error?

Consider a NestJS queue consumer that processes jobs. What is the default behavior if the consumer function throws an error during job processing?

NestJS

import { Processor, Process } from '@nestjs/bull';
import { Job } from 'bull';

@Processor('email')
export class EmailProcessor {
  @Process('send')
  async handleSendEmail(job: Job) {
    throw new Error('Failed to send email');
  }
}

A The job is marked as failed and retried automatically if retry options are set.

B The job is silently ignored and removed from the queue.

C The entire queue stops processing further jobs until the error is fixed.

D The job is marked as completed despite the error.

💡 Hint

Think about how Bull handles job failures and retries by default.

When a job processor throws an error, Bull marks the job as failed. If retry options are configured, Bull will retry the job automatically. Otherwise, the job stays failed.

📝 Syntax

intermediate

Which option correctly defines a NestJS queue processor for a 'notifications' queue?

Choose the correct code snippet that defines a NestJS queue processor for a queue named 'notifications'.

A

import { Processor, Process } from '@nestjs/bull';

@Processor('notifications')
export class NotificationsProcessor {
  @Process('send')
  async handleSend(job: Job) {
    // process job
  }
}

B

import { Processor, Process } from '@nestjs/bull';

@Processor('notifications')
export class NotificationsProcessor {
  @Process()
  async handle(job: Job) {
    // process job
  }
}

C

import { Processor, Process } from '@nestjs/bull';

@Processor()
export class NotificationsProcessor {
  @Process('notifications')
  async handle(job: Job) {
    // process job
  }
}

D

import { Processor, Process } from '@nestjs/bull';

@Processor('notifications')
export class NotificationsProcessor {
  @Process('notifications')
  async handle(job: Job) {
    // process job
  }
}

💡 Hint

The @Processor decorator takes the queue name. The @Process decorator takes the job name.

The @Processor decorator specifies the queue name. The @Process decorator specifies the job name within that queue. Option A correctly uses @Processor('notifications') and @Process('send').

❓ state_output

advanced

What is the value of job.attemptsMade after 2 failed retries?

Given a NestJS queue job configured with 3 attempts, what will be the value of job.attemptsMade inside the processor when the job is being retried for the third time?

NestJS

import { Processor, Process } from '@nestjs/bull';
import { Job } from 'bull';

@Processor('tasks')
export class TasksProcessor {
  @Process('run')
  async handleRun(job: Job) {
    console.log(job.attemptsMade);
    throw new Error('Fail');
  }
}

// Job options: { attempts: 3 }

A 1

B 3

C 2

D 0

💡 Hint

Remember that attemptsMade counts how many times the job has been tried before the current attempt.

job.attemptsMade is the number of times the job has been attempted before the current run. On the third attempt, attemptsMade is 2.

🔧 Debug

advanced

Why does this NestJS queue consumer never process jobs?

Review the following NestJS queue consumer code. Why does it never process any jobs?

NestJS

import { Processor, Process } from '@nestjs/bull';
import { Job } from 'bull';

@Processor('orders')
export class OrdersProcessor {
  @Process('')
  async handleOrder(job: Job) {
    console.log('Processing order', job.id);
  }
}

A The Job type is imported incorrectly.

B The @Process decorator has an empty string, so it does not match any job name.

C The @Processor decorator is missing the queue name.

D The handleOrder method is not async.

💡 Hint

Check the @Process decorator argument and how job names are matched.

The @Process decorator requires a job name string to match jobs. An empty string means no jobs match, so the processor never runs.

🧠 Conceptual

expert

Which statement about NestJS queue processors and concurrency is true?

In NestJS using Bull queues, which statement about concurrency in queue processors is correct?

A Concurrency is automatically handled by Bull and cannot be configured in NestJS.

B Concurrency is controlled only by the number of queue processors registered, not by any decorator option.

C NestJS queue processors process jobs strictly one at a time, concurrency is not supported.

D Setting concurrency in @Process decorator allows multiple jobs of the same type to be processed in parallel within the same processor instance.

💡 Hint

Think about how Bull and NestJS allow parallel job processing.

The @Process decorator accepts a concurrency option that allows multiple jobs of the same type to be processed in parallel by the same processor instance.