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What is a background job in the context of a Node.js web application, and why can't long-running work simply happen inside a request handler? What is the difference between a delayed job and a scheduled (repeatable) job in a Node.js job queue? What is job idempotency, and why is it critical when designing background job processing? What is a cron job, and how would you schedule recurring background tasks in a Node.js application using node-cron or BullMQ's repeatable jobs? What is job concurrency in a worker process, and how do you configure it safely in BullMQ? How would you track and expose the progress of a long-running background job to the client that requested it? What are the trade-offs of using an in-memory job queue (like a simple array-based queue) versus a persistent, Redis-backed queue? What is a 'stalled' job in BullMQ, and what causes it? What is a dead-letter queue (DLQ), and how should a Node.js job-processing system use one? How do you implement job prioritization in a Node.js queue system, and when is it necessary?
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How does BullMQ (built on Redis) implement a job queue in Node.js, and what are its core building blocks?

Intermediate

Answer

BullMQ uses Redis as a durable, shared broker between producers (application code adding jobs) and one or more worker processes (consuming and processing jobs) -- its core building blocks are the Queue (used to add jobs), the Worker (which processes jobs, potentially running in a completely separate process), and optionally a QueueEvents listener for observing job lifecycle events like completion or failure from elsewhere in the system.
const { Queue, Worker } = require('bullmq');
const connection = { host: 'localhost', port: 6379 };

const emailQueue = new Queue('emails', { connection });
await emailQueue.add('welcome-email', { userId: 123 });

// In a separate worker process
new Worker('emails', async (job) => {
  await sendWelcomeEmail(job.data.userId);
}, { connection });
Real-world example An e-commerce platform uses BullMQ to queue order-confirmation emails, with the web servers only responsible for calling queue.add() and separate dedicated worker processes actually sending the emails, so a slow email provider never affects the responsiveness of the main checkout API.

Common follow-ups: How do BullMQ's built-in retry and backoff options compare to implementing retry logic manually?;What happens to jobs already in the queue if the Redis instance backing BullMQ restarts?

Caching with Redis;Email & Notifications
All 15 Background Jobs & Queues questions