npm package: @stdlib/utils-parallel
Description: Execute scripts in parallel.
Author: The Stdlib Authors
Latest version: 0.3.4

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<details>
  <summary>
    About stdlib...
  </summary>
  <p>We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.</p>
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</details>

# Parallel

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> Execute scripts in parallel.

<section class="installation">

## Installation

```bash
npm install @stdlib/utils-parallel
```

</section>

<section class="usage">

## Usage

```javascript
var parallel = require( '@stdlib/utils-parallel' );
```

#### parallel( files, \[options,] clbk )

Executes scripts in parallel.

<!-- FIXME: revert back to JavaScript once can handle errors -->

<!-- run-disable -->

```text
var files = [
    './a.js',
    './b.js'
];

function done( error ) {
    if ( error ) {
        console.error( 'Exit code: '+error.code );
        console.error( 'Signal: '+error.signal );
        throw error;
    }
    console.log( 'Done!' );
}

parallel( files, done );
```

The function accepts the following `options`:

-   **cmd**: executable file/command. Default: `'node'`.
-   **workers**: number of workers. Default: number of CPUs minus `1`.
-   **concurrency**: number of scripts to execute concurrently. Default: `options.workers`.
-   **ordered**: `boolean` indicating whether to preserve the order of script output. Default: `false`.
-   **maxBuffer**: maximum child process `stdio` buffer size. This option is **only** applied when `options.ordered = true`. Default: `200*1024*1024` bytes.
-   **uid**: child process user identity.
-   **gid**: child process group identity.

By default, the number of workers running scripts is equal to the number of CPUs minus `1` (master process). To adjust the number of workers, set the `workers` option.

<!-- FIXME: revert back to JavaScript once can handle errors -->

<!-- run-disable -->

```text
var files = [
    './a.js',
    './b.js'
];

function done( error ) {
    if ( error ) {
        console.error( 'Exit code: '+error.code );
        console.error( 'Signal: '+error.signal );
        throw error;
    }
    console.log( 'Done!' );
}

var opts = {
    'workers': 8
};

parallel( files, opts, done );
```

By default, the number of scripts running concurrently is equal to the number of workers. To adjust the concurrency, set the `concurrency` option.

<!-- FIXME: revert back to JavaScript once can handle errors -->

<!-- run-disable -->

```text
var files = [
    './a.js',
    './b.js'
];

function done( error ) {
    if ( error ) {
        console.error( 'Exit code: '+error.code );
        console.error( 'Signal: '+error.signal );
        throw error;
    }
    console.log( 'Done!' );
}

var opts = {
    'concurrency': 6
};

parallel( files, opts, done );
```

By specifying a concurrency greater than the number of workers, a worker may be executing more than `1` script at any one time. While not likely to be advantageous for synchronous scripts, setting a higher concurrency may be advantageous for scripts performing asynchronous tasks.

By default, each script is executed as a [Node.js][node-js] script.

```text
$ node <script_path>
```

To run scripts via an alternative executable or none at all, set the `cmd` option.

<!-- FIXME: revert back to JavaScript once can handle errors -->

<!-- run-disable -->

```text
var files = [
    './a.js',
    './b.js'
];

function done( error ) {
    if ( error ) {
        console.error( 'Exit code: '+error.code );
        console.error( 'Signal: '+error.signal );
        throw error;
    }
    console.log( 'Done!' );
}

var opts = {
    'cmd': '' // e.g., if scripts contain a shebang
};

parallel( files, opts, done );
```

By default, the `stdio` output for each script is interleaved; i.e., the `stdio` output from one script **may** be interleaved with the `stdio` output from one or more other scripts. To preserve the `stdio` output order for each script, set the `ordered` option to `true`.

<!-- FIXME: revert back to JavaScript once can handle errors -->

<!-- run-disable -->

```text
var files = [
    './a.js',
    './b.js'
];

function done( error ) {
    if ( error ) {
        console.error( 'Exit code: '+error.code );
        console.error( 'Signal: '+error.signal );
        throw error;
    }
    console.log( 'Done!' );
}

var opts = {
    'ordered': true
};

parallel( files, opts, done );
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   Relative file paths are resolved relative to the current working directory.
-   Ordered script output does **not** imply that scripts are executed in order. To preserve script order, execute the scripts sequentially via some other means.
-   Script concurrency cannot exceed the number of scripts.
-   If the script concurrency is less than the number of workers, the number of workers is reduced to match the specified concurrency.

</section>

<!-- /.notes -->

<section class="examples">

## Examples

<!-- FIXME: re-enable running of code block once able to set `maxBuffer` configuration -->

<!-- run-disable -->

<!-- eslint-disable node/no-sync -->

<!-- eslint no-undef: "error" -->

```javascript
var fs = require( 'fs' );
var path = require( 'path' );
var writeFileSync = require( '@stdlib/fs-write-file' ).sync;
var unlinkSync = require( '@stdlib/fs-unlink' ).sync;
var parallel = require( '@stdlib/utils-parallel' );

function template( id ) {
    var file = '';

    file += '\'use strict\';';

    file += 'var id;';
    file += 'var N;';
    file += 'var i;';

    file += 'id = '+id+';';
    file += 'N = 1e5;';
    file += 'console.log( \'File: %s. id: %s. N: %d.\', __filename, id, N );';

    file += 'for ( i = 0; i < N; i++ ) {';
    file += 'console.log( \'id: %s. v: %d.\', id, i );';
    file += '}';

    return file;
}

function createDir() {
    var dir = path.join( __dirname, 'examples', 'tmp' );
    fs.mkdirSync( dir );
    return dir;
}

function createScripts( dir, nFiles ) {
    var content;
    var fpath;
    var files;
    var i;

    files = [];
    for ( i = 0; i < nFiles; i++ ) {
        content = template( i.toString() );
        fpath = path.join( dir, i+'.js' );
        writeFileSync( fpath, content, {
            'encoding': 'utf8'
        });
        files.push( fpath );
    }
    return files;
}

var nFiles = 10;

// Make a temporary directory to store files:
var dir = createDir();

// Create temporary files:
var files = createScripts( dir, nFiles );

// Set the runner options:
var opts = {
    'concurrency': 3,
    'workers': 3,
    'ordered': false
};

// Run all temporary scripts:
parallel( files, opts, done );

function done( error ) {
    if ( error ) {
        console.log( error.message );
    }
    cleanup();
    console.log( 'Done!' );
}

function cleanup() {
    var i;

    // Delete the temporary files...
    for ( i = 0; i < files.length; i++ ) {
        unlinkSync( files[ i ] );
    }
    // Remove temporary directory:
    fs.rmdirSync( dir );
}
```

</section>

<!-- /.examples -->



<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

## See Also

-   <span class="package-name">[`@stdlib/utils-parallel-cli`][@stdlib/utils-parallel-cli]</span><span class="delimiter">: </span><span class="description">CLI package for use as a command-line utility.</span>


</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->


<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

<!-- /.stdlib -->

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<section class="links">

[@stdlib/utils-parallel-cli]: https://www.npmjs.com/package/@stdlib/utils-parallel-cli

[npm-image]: http://img.shields.io/npm/v/@stdlib/utils-parallel.svg
[npm-url]: https://npmjs.org/package/@stdlib/utils-parallel

[test-image]: https://github.com/stdlib-js/utils-parallel/actions/workflows/test.yml/badge.svg?branch=v0.3.4
[test-url]: https://github.com/stdlib-js/utils-parallel/actions/workflows/test.yml?query=branch:v0.3.4

[coverage-image]: https://img.shields.io/codecov/c/github/stdlib-js/utils-parallel/main.svg
[coverage-url]: https://codecov.io/github/stdlib-js/utils-parallel?branch=main

<!--

[dependencies-image]: https://img.shields.io/david/stdlib-js/utils-parallel.svg
[dependencies-url]: https://david-dm.org/stdlib-js/utils-parallel/main

-->

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[chat-url]: https://stdlib.zulipchat.com

[stdlib]: https://github.com/stdlib-js/stdlib

[stdlib-authors]: https://github.com/stdlib-js/stdlib/graphs/contributors

[cli-section]: https://github.com/stdlib-js/utils-parallel#cli
[cli-url]: https://github.com/stdlib-js/utils-parallel/tree/cli
[@stdlib/utils-parallel]: https://github.com/stdlib-js/utils-parallel/tree/main

[umd]: https://github.com/umdjs/umd
[es-module]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

[deno-url]: https://github.com/stdlib-js/utils-parallel/tree/deno
[deno-readme]: https://github.com/stdlib-js/utils-parallel/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/utils-parallel/tree/umd
[umd-readme]: https://github.com/stdlib-js/utils-parallel/blob/umd/README.md
[esm-url]: https://github.com/stdlib-js/utils-parallel/tree/esm
[esm-readme]: https://github.com/stdlib-js/utils-parallel/blob/esm/README.md
[branches-url]: https://github.com/stdlib-js/utils-parallel/blob/main/branches.md

[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/utils-parallel/main/LICENSE

[node-js]: http://nodejs.org/

</section>

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