Queue consumers (processors) in NestJS - Step-by-Step Execution

Concept Flow - Queue consumers (processors)

Job added to Queue

↓

Queue stores job

↓

Consumer listens to Queue

↓

Consumer receives job

↓

Process job logic

↓

Job completed or failed

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Consumer waits for next job

Jobs are added to a queue, consumers listen and pick jobs one by one, process them, then wait for more.

Execution Sample

NestJS

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

@Processor('email')
export class EmailProcessor {
  @Process('send')
  async handleSendEmail(job: Job) {
    console.log(`Sending email to ${job.data.email}`);
  }
}

This code defines a consumer that listens to the 'email' queue and processes 'send' jobs by logging the email address.

Execution Table

Step	Event	Queue State	Consumer Action	Output
1	Job added to 'email' queue with data {email: '[e-mail]'}	Queue: [{send, {email: '[e-mail]'}}]	Consumer idle, waiting	No output
2	Consumer detects job in queue	Queue: [{send, {email: '[e-mail]'}}]	Consumer picks job	No output
3	Consumer processes job	Queue: []	Runs handleSendEmail with job data	Logs: Sending email to [e-mail]
4	Job completed	Queue: []	Consumer waits for next job	No output
5	Job added to 'email' queue with data {email: '[e-mail]'}	Queue: [{send, {email: '[e-mail]'}}]	Consumer idle, waiting	No output
6	Consumer detects job in queue	Queue: [{send, {email: '[e-mail]'}}]	Consumer picks job	No output
7	Consumer processes job	Queue: []	Runs handleSendEmail with job data	Logs: Sending email to [e-mail]
8	Job completed	Queue: []	Consumer waits for next job	No output
9	No more jobs	Queue: []	Consumer idle	No output

💡 No more jobs in queue, consumer waits idle

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	After Step 7	Final
Queue	[]	[{send, {email: '[e-mail]'}}]	[]	[{send, {email: '[e-mail]'}}]	[]	[]
Consumer State	Idle	Idle	Processing job	Idle	Processing job	Idle
Current Job Data	null	{email: '[e-mail]'}	null	{email: '[e-mail]'}	null	null

Key Moments - 3 Insights

Why does the consumer wait idle after processing a job?

Because the consumer listens continuously but only acts when a job is in the queue, as shown in steps 4 and 9 where the queue is empty and consumer state is idle.

How does the consumer know which job to process next?

The consumer picks the next job from the queue in order, as seen in steps 2 and 6 where it detects and picks the first job in the queue.

What happens if multiple jobs are added quickly?

Jobs queue up and the consumer processes them one by one in order, as the queue state shows jobs added and removed sequentially in the execution table.

Visual Quiz - 3 Questions

Look at the execution table, what is the consumer state after step 3?

A Processing job

B Idle

C Waiting for job

D Disconnected

💡 Hint

Check the 'Consumer State' variable after step 3 in variable_tracker.

At which step does the queue become empty after processing a job?

A Step 2

B Step 3

C Step 5

D Step 7

💡 Hint

Look at the 'Queue' variable state in variable_tracker after each step.

If a new job is added at step 9, what will the consumer do next?

A Ignore the job

B Crash due to no handler

C Process the job immediately

D Wait until manually restarted

💡 Hint

Refer to the flow where consumer detects and processes jobs as soon as they appear in the queue.

Concept Snapshot

Queue consumers listen to a queue for jobs.
When a job arrives, the consumer picks it up.
It runs the processing function with job data.
After processing, it waits for the next job.
Jobs are processed one at a time in order.
This keeps work organized and scalable.

Full Transcript

In NestJS, queue consumers called processors listen to queues for jobs. When a job is added, the consumer detects it and picks it from the queue. It runs the processing function with the job's data, for example sending an email. After finishing, the consumer waits idle until the next job arrives. This process repeats continuously. The queue holds jobs in order, and the consumer processes them one by one. This helps handle background tasks efficiently and reliably.