Thread pools

Pool basics

A Pool allows you to create a set of workers and queue worker calls. The queued tasks are pulled from the queue and executed as previous tasks are finished.

Use it if you have a lot of work to offload to workers and don’t want to drown them in a pile of work at once, but run those tasks in a controlled way with limited concurrency.

import { spawn, Pool, Worker } from "threads"

const pool = Pool(() => spawn(new Worker("./workers/multiplier")), 8 /* optional size */)

pool.queue(async multiplier => {
  const multiplied = await multiplier(2, 3)
  console.log(`2 * 3 = ${multiplied}`)

await pool.completed()
await pool.terminate()

Note that pool.queue() will schedule a task to be run in a deferred way. It might execute straight away or it might take a while until a new worker thread becomes available.

Pool creation

interface PoolOptions {
  concurrency?: number
  maxQueuedJobs?: number
  name?: string
  size?: number

function Pool(threadFactory: () => Thread, size?: number): Pool
function Pool(threadFactory: () => Thread, options?: PoolOptions): Pool

The first argument passed to the Pool() factory must be a function that spawns a worker thread of your choice. The pool will use this function to create its workers.

The second argument is optional and can either be the number of workers to spawn as a number or an options object (see PoolOptions):

  • options.concurrency: number of tasks to run simultaneously per worker, defaults to one
  • options.maxQueuedJobs: maximum number of tasks to queue before throwing on .queue(), defaults to unlimited
  • give the pool a custom name to use in the debug log, so you can tell multiple pools apart when debugging
  • options.size: number of workers to spawn, defaults to the number of CPU cores

Scheduling tasks

let pool: Pool<ThreadType>
type TaskFunction<ThreadType, T> = (thread: ThreadType) => Promise<T> | T

pool.queue<T>(task: TaskFunction<ThreadType, T>): Promise<T>

The promise returned by pool.queue() resolves or rejects when the queued task function has been run and resolved / rejected. That means you should usually not await that promise straight away when calling pool.queue(), since the code after this line will then not be run until the task has been run and completed.

Whenever a pool worker finishes a job, the next pool job is de-queued (that is the function you passed to pool.queue()). It is called with the worker as the first argument. The job function is supposed to return a promise - when this promise resolves, the job is considered done and the next job is de-queued and dispatched to the worker.

The promise returned by pool.completed() will resolve once the scheduled callbacks have been executed and completed. A failing job will make the promise reject. Use pool.settled() if you need a promise that resolves without an error even if a task has failed.

Cancelling a queued task

You can cancel queued tasks, too. If the pool has already started to execute the task, you cannot cancel it anymore, though.

const task = pool.queue(multiplierWorker => multiplierWorker(2, 3))

Pool termination

// Terminate gracefully

// Force-terminate pool workers

By default the pool will wait until all scheduled tasks have completed before terminating the workers. Pass true to force-terminate the pool immediately.

Waiting for tasks to complete

The pool comes with two methods that allow await-ing the completion of all tasks.

The first one is pool.completed(). It returns a promise that resolves once all tasks have been executed and there are no more tasks left to run. If a task fails, the promise will be rejected.

The second one is pool.settled(). It also returns a promise that resolves when all tasks have been executed, but it will also resolve instead of reject if a task fails. The returned promise resolves to an array of errors.

As outlined before, pool tasks provide a Promise-like .then() method. You can use it to await the completion of a subset of a pool’s queued tasks only.

// (Created a pool and queued other pool tasks before…)

const myTasks: QueuedTask[] = []

for (let input = 0; input < 5; input++) {
  const task = pool.queue(worker =>

await Promise.all(myTasks)
console.log("All tasks have completed. Other pool tasks might still be running.")