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    thread-pool (BETA)

    thread-pool is a simple thread pool manager for Node.js. (Currently in Beta)

    It allows you to create a waiting pool of independent OS processes (threads), each running Node.js, to which you can assign tasks and use promises to handle the completion of those tasks.


    Creating a Thread Pool

    Import the Pool class and create a new pool, optionally passing in a pool size. If you don't specify a size, the pool will default to giving you 5 threads.

    import { Pool } from 'thread-pool';
    const pool = new Pool(5);

    You now have 5 asynchronous Node.js processes ready to handle your heavy tasks.

    When you are ready to pass a task off to one of these threads, use pool.assign:

      task: (handler) => {
        setTimeout(() => handler.finish('I processed ' + handler.init), 3000);
      init: 'foo',
      timeout: 10000
    .then(data => console.log('Success!', data))
    .catch(data => console.log('Failure!', data));

    The assign method should be handed an object with three keys – task, init, and timeout. The task key determines the actual task to be accomplished. The init key determines the data that should be processed within that task. The timeout key, which is optional, gives your task a certain amount of time to finish. If it doesn't finish in that time, it is automatically killed.

    In this example, we've passed an actual function in for the task key. It takes a single argument denoting an instance of a Handler class that allows you to "handle" things on the thread. Here's what you get with the handler:

    1. handler.init - The data that was passed in via the init key in pool.assign.
    2. handler.finish - A function to call when the thread has finished executing the task you assigned to it. It takes a single argument containing data to send back to the main thread.
    3. handler.err - A function to be called when you want to manually shut down the thread from the inside.
    4. handler.send - A function to be called when you want to send a message to the main thread. It takes a single argument containing data to send back to the main thread.

    As shown in the example, assign returns a Promise object which resolves upon successful completion of the task, and rejects whenever there was a problem.

    When you call the assign method, thread-pool picks an idle thread and assigns the task to that thread. If no threads are currently idle, your task will be queued up to be executed as soon as one of the threads finishes its current task. All of this is logged to the console so that you can determine whether or not your pool size is correct.

    Of course, it's less likely that you'll want to pass an actual function to your pool and more likely that you'll want to tell it to execute a file. In that case, simply pass in the path to your file instead of a function:

      task: './example-task',
      init: 'foo'
    .then(data => console.log('Success!', data))
    .catch(data => console.log('Failure!', data));

    Because thread-pool gives you Node.js processes, your task files should be JS files. They will be required into your thread upon execution and will have access to your whole project directory as normal. The only stipulation is that your task file needs to export a single function taking the handler, parameter. This will allow you to properly interface with thread-pool. Like so:

    // example-task.js
    import { whatever } from 'wherever';
    export default function (handler) {
      handler.finish(`You sent me the payload ${handler.init}.`);

    Behind the scenes, thread-pool will cache these tasks so that they will not have to be read in every time they are used.


    From the main application thread, you'll always treat your pool as a cohesive unit. In other words, you won't be able to target a specific thread and tell it to take any particular action once a task has been assigned to it.


    Tasks being executed within the pool can pass messages back to the main thread and, when that happens, the main thread can reply to those messages. Let's start by looking at an example task file:

    // example-task.js
    export default function (handler) {
      handler.send('Hello from one of the threads in your pool.')
             .onReply(data => {
               console.log('Parent replied with:', data);

    Whenever we want to send a message to the main thread, we'll use handler.send and pass in whatever serializable data we want. If we're expecting a reply back from the main thread, we can trap it with a subsequent call to onReply.

    Now let's take a look at the main thread:

    import { Pool } from 'thread-pool';
    const pool = new Pool();
    pool.onMessage((data, reply) => {
      console.log('Pool sent me:', data);
      reply('Hello to you too.');
    pool.assign({ task: './example-task' });

    We can trap all incoming messages from the pool by calling pool.onMessage. The message handler we pass to this method accepts two arguments denoting (first) the data that came through and (second) a reply function. By calling the reply function, we can pass data back as a response to this specific message.

    A nice convention might be to pass along an object containing both a message type as well as any other necessary data, then set up an onMessage handler that routes messages to appropriate processes. For example...

    Inside a task file...

      type: 'FOO',
      data: { ... }
      type: 'BAR',
      data: { ... }

    Inside the main application...

    pool.onMessage((data, reply) => {
      switch (data.type) {
        case 'FOO' : return reply(something(;
        case 'BAR' : return reply(somethingElse(;
        default    : throw new Error('Bad message type.');

    What happens when there's an error inside a thread?

    Certain kinds of errors will result in a terminated thread. Also, if you call the err function anywhere within your task, the thread will be terminated. If that happens, obviously your task won't be completed. This is just JavaScript, after all. However, if a thread dies, a new thread will be generated in its place automatically. It won't be able to pick up where the old thread left off, but it will be able to grab any queued up tasks and start running with them.

    Don't forget that you have Promise#catch that you can use to catch any error that might result in your thread dying.

    Shutting Down a Pool

    If you are done with your pool, you don't have to leave threads alive with no purpose forever. Just call pool.killAll(). By calling this function, every thread will be terminated immediately, whether or not their tasks have been completed.

    Re-Initializing a Pool

    If a pool has been shut down, you can bring it back to life by calling pool.up(). Calling this function while a pool is still alive will not hurt anything. It will simply make sure that there are live threads generated to fill the size of your thread pool. So if no threads are alive, all of them will be generated. If, somehow, one thread had permanently died, only one thread would be generated. If all threads are alive, nothing happens.

    Considerations for ES6/7, TypeScript, CoffeeScript, etc.

    If you are using a tool like Babel (or whatever) to compile your cutting-edge JS dialect, keep in mind that your compiler can not automatically be applied to threads in the pool and these threads will not have access to in-memory code held by the main thread.

    As such, this should not cause problems as long as your thread tasks are already pre-processed in such a way that all dialect-specific libraries and polyfills have been included in them, and all the files they import are also already pre-processed.

    To illustrate a little more clearly, JS processors often work like this:

    1. Gather all files to be compiled together.
    2. Inject dialect-specific dependency code ONCE, at the top of the output.
    3. Generate code for imports and exports ONCE, at the top of the output.
    4. Process and concatenate files as necessary.
    5. Write the output.

    Because processors try really hard not to be wasteful, you could easily end up with task files that rely upon dialect-specific dependency code that isn't actually written in those files. Then, when an independent process attempts to execute that task, it won't have access to that code. So, to avoid this, simply make sure that your task files are processed independently of the rest of your application.

    Context Considerations

    If you pass in a function rather than a file for your thread task, keep in mind that the function has to be stringified, passed to a new thread, and then re-evaluated. As such, it will not have access to any context from within the main thread, including any reference to this within the main thread, closure variables, etc.


    npm i thread-pool

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    • jgnewman