BullMQ supports parent - child relationships between jobs. The basic idea is that a parent job will not be moved to the wait status, i.e. can be picked up by a worker, until all its children jobs have been processed successfully. Apart from that, a parent or a child job are no different from regular jobs.

This functionality enables the creation of flows where jobs are the node of trees of arbitrary depth.

In order to create "flows" you must use the FlowProducer class. The method "add" accepts an object with the following interface:

interface FlowJob {
  name: string;
  queueName: string;
  data?: any;
  prefix?: string;
  opts?: Omit<JobsOptions, 'parent'>;
  children?: FlowJob[];
}

So we can add a flow like this one:

import { FlowProducer } from 'bullmq';
​
// A FlowProducer constructor takes an optional "connection"
// object otherwise it connects to a local redis instance.
const flowProducer = new FlowProducer();
​
const flow = await flowProducer.add({
  name: 'renovate-interior',
  queueName: 'renovate',
  children: [
    { name: 'paint', data: { place: 'ceiling' }, queueName: 'steps' },
    { name: 'paint', data: { place: 'walls' }, queueName: 'steps' },
    { name: 'fix', data: { place: 'floor' }, queueName: 'steps' },
  ],
});

The above code will add atomically 4 jobs, one to the 'renovate' queue and 3 to the 'steps' queue. When the 3 jobs in the 'activities" queue are completed, the parent job in the 'renovate' queue will be processed as a regular job.

The above call will return instances for all the jobs added to the queue.

When the parent job is processed it is possible to access the results generated by its child jobs. For example lets assume the following worker for the child jobs:

import { Worker } from "bullmq"
​
const stepsQueue = new Worker('steps', async (job) => {
  await performStep(job.data);
​
  if(job.name === 'paint') {
    return 2500;
  } else if( job.name === 'fix' ) {
    return 1750;
  }
});

we can implement a parent worker that sums the costs of the children's jobs using the "getChildrenValues" method. This method returns an object with job keys as keys and the result of that given job as a value:

import { Worker } from "bullmq"
​
const stepsQueue = new Worker('renovate', async job => {
  const childrenValues = await job.getChildrenValues();
​
  const totalCosts = Object(childrenValues)
    .values()
    .reduce((prev, cur) => prev + cur, 0);
​
  await sendInvoice(totalCosts);
});

It is possible to add as deep job hierarchies as needed, see the following example where jobs are depending on each other, this allows serial execution of jobs:

import { FlowProducer } from 'bullmq';
const flowProducer = new FlowProducer();
​
const queueName = 'assembly-line';
const chain = await flowProducer.add({
  name: 'car',
  data: { step: 'engine' },
  queueName,
  children: [
    {
      name: 'car',
      data: { step: 'wheels' },
      queueName,
      children: [{ name: 'car', data: { step: 'chassis' }, queueName }],
    },
  ],
});

In this case one job will be processed after the previous one has been completed.

Getters

There are some special getters that can be used in order to get jobs related to a flow. First we have a method in the Job class to get all the dependencies for a given job:

const dependencies = await job.getDependencies();

it will return all the direct dependencies, i.e. the children of a given job.

The Job class also provides a another method that we presented above to get all the values produced by the children of a given job:

const values = await job.getChildrenValues();

Also a new property is available in the Job class, parentKey, with a fully qualified key for the job parent.

Finally, there is also a new state where a job can be in, "waiting-children", for parent jobs that have not yet had their children completed:

const state = await job.getState();
// state will be "waiting-children"

Jobs removal

BullMQ also provides seamless removal functionality for jobs that are part of a flow.

When removing a job that is part of the flow there are several important considerations:

1. If a parent job is removed, all its children will also be removed.

2. If a child job is removed, its parent dependency to said child is also removed, and if the child was the last child in the dependencies list, the parent job will be completed.

3. Since a job can be both a parent and a child in a large flow, both 1 and 2 will occur if removing such a job.

4. If any of the jobs that would be removed happen to be locked, none of the jobs will be removed, and an exception will be thrown.

Apart from the considerations above, removing a job can simply be done by either using the Job or the Queue class:

await job.remove();
// or
await queue.remove(job.id);

Read more:

• 💡 FlowProducer API Reference​

• 💡 Job API Reference​