Queue events and monitoring in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output when a job is completed in a NestJS queue?

Consider a NestJS queue with an event listener for the 'completed' event. What will be logged when a job with ID 42 completes successfully?

NestJS

queue.on('completed', (job) => {
  console.log(`Job ${job.id} completed with result: ${job.returnvalue}`);
});

queue.add({ task: 'sendEmail' });

// Assume job with ID 42 completes with returnvalue 'Email sent'.

A Job 42 completed

B Job completed with result: Email sent

C Job 42 completed with result: Email sent

D Job completed

💡 Hint

Look at how the job ID and return value are accessed in the event listener.

The 'completed' event listener receives the job object. Accessing job.id and job.returnvalue gives the job's ID and result. The console.log prints both.

🧠 Conceptual

intermediate

Which event should you listen to for retrying failed jobs in NestJS queues?

You want to retry jobs automatically when they fail in a NestJS queue. Which event is best to listen for to trigger retry logic?

A 'failed'

B 'waiting'

C 'completed'

D 'active'

💡 Hint

Retry logic is triggered when a job does not succeed.

The 'failed' event fires when a job fails. Listening to it allows you to implement retry logic.

❓ Configuration

advanced

How to enable detailed queue monitoring with Bull in NestJS?

Which configuration snippet correctly enables the built-in UI dashboard for Bull queue monitoring in a NestJS app?

NestJS

import { BullModule } from '@nestjs/bull';
import { createBullBoard } from '@bull-board/api';
import { BullAdapter } from '@bull-board/api/bullAdapter';
import { ExpressAdapter } from '@bull-board/express';

const serverAdapter = new ExpressAdapter();
createBullBoard({ queues: [new BullAdapter(myQueue)], serverAdapter });

app.use('/admin/queues', serverAdapter.getRouter());

A Use BullModule.forRoot({ monitor: true }) without additional setup

B Install 'bull-monitor' package and import it in main.ts without further config

C Add 'monitoring: true' in queue options in BullModule.registerQueue

D Use BullModule.registerQueue({ name: 'myQueue' }) and set up Bull Board with ExpressAdapter as shown

💡 Hint

Bull Board requires explicit setup with adapters and routes.

Bull Board is a separate UI dashboard for Bull queues. It requires registering queues and mounting its router in the app.

❓ Troubleshoot

advanced

Why does the 'completed' event not fire in a NestJS Bull queue?

You added a listener for the 'completed' event on a Bull queue in NestJS, but it never triggers even though jobs finish successfully. What is the most likely cause?

A Redis server is down

B The event listener is added after jobs are processed

C The queue processor function does not return a value

D The queue is not registered with BullModule

💡 Hint

Check the timing of when the event listener is added relative to job processing.

Event listeners must be added before jobs are added and processed. If added after jobs complete, the event is missed even if jobs succeed.

✅ Best Practice

expert

What is the best practice for monitoring job failures in production NestJS queues?

In a production NestJS app using Bull queues, what is the best practice to monitor and alert on job failures?

A Use the 'failed' event to send alerts to an external monitoring service like Sentry or Datadog

B Log failures to console and check logs manually

C Ignore failures and rely on retries only

D Restart the queue worker process on any failure

💡 Hint

Automated alerting helps catch issues early.

Listening to 'failed' events and sending alerts to monitoring tools ensures prompt detection and response to job failures.