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worker

worker.js ūüĒß

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A library most strongly suited for facilitating data parallelism within a series of orchestrated tasks that run in worker threads. Works with both node.js and the browser (including w/ browserify) in an abstraction that unifies Worker and Child Process.

Features

  • Enables the use of pseudo-Transferable objects in node.js between threads by using shared memory when available (meaning faster messaging with less overhead when passing large objects).
  • Serializes messages in node.js using the Structured Clone Algorithm, meaning you can pass actual JavaScript objects (not just JSON as is with process.send) between threads.
  • Allows subworkers (i.e., the ability for a worker to spawn its own nested worker) in all enviornments, including a workaround for browsers that do not support it natively.
  • Supports transmitting/receiving streams across threads (including special treatment for File and Blob in the browser).
  • Compatible with Browserify

If running on node.js, requires v8.0.0 or higher (for v8 serializer)

Contents


Example


Let's take a long list of words, reverse the letters in each word, and then sort that list. Pretty straightforward?

serial.js:

const fs = require('fs');
 
// load a few hundred thousand words into an array
let a_words = fs.readFileSync('/usr/share/dict/words', 'utf8').split('\n');
 
// start the timer
console.time('serially');
 
// reverse each word, then sort that list
let a_sorted_words_reversed = a_words
    .map(s => s.split('').reverse().join(''))
    .sort((s_a, s_b) => s_a.localeCompare(s_b));
 
// write to disk
fs.writeFile('out', a_sorted_words_reversed.join('\n'), (e_write) => {
  if(e_write) throw new Error(e_write);
 
    // stop the timer
    console.timeEnd('serially');
});

Parallelize

In this scenario, we can save a bit of time by dividing the task among multiple cores.

First, we define tasks in the-worker.js:

const worker = require('worker');
 
const F_SORT_ALPHABETICAL = (s_a, s_b) => s_a.localeCompare(s_b);
 
worker.dedicated({
    // take a list of words and reverse the letters in each word
    reverse_letters(a_list) {
        return a_list.map(s => s.split('').reverse().join(''));
    },
 
    // take a list of words and sort them alphabetically
    sort(a_list) {
        return a_list.sort(F_SORT_ALPHABETICAL);
    },
 
    // take two sorted lists of words and merge them in sorted order
    merge(a_list_a, a_list_b) {
        return worker.merge_sorted(a_list_a, a_list_b, F_SORT_ALPHABETICAL);
    },
});

Then, we define how to use those tasks in the-master.js:

const fs = require('fs');
const worker = require('worker');
 
// load a few hundred thousand words into an array
let a_words = fs.readFileSync('/usr/share/dict/words', 'utf8').split('\n');
 
// start the timer
console.time('parallel');
 
// create a group of workers (size defaults to os.cpus().length)
let k_group = worker.group('./the-worker.js');
 
// processing pipeline
k_group
    // bind data from our list, dividing array evenly among workers
    .data(a_words)
 
    // send data to workers and push them thru the first transform
    .map('reverse_letters')
 
    // as soon as each worker finishes its previous task, forward each result
    //   to a new task in the same thread (keeping data in the same thread)
    .thru('sort')
 
    // reduce multiple results into a single one
    .reduce('merge').then((a_sorted_words_reversed) => {
        fs.writeFile('out', a_sorted_words_reversed.join('\n'), (e_write) => {
            if(e_write) throw new Error(e_write);
 
            // stop the timer
            console.timeEnd('parallel');
        });
    });

Results

This is just a demonstration to show how to use this library, and the results from my machine here shows the potential benefit of dividing such a task:

serially: 2328.937ms
parallel: 1355.964ms

Intro


Data Parallelism

One form of parallelization is to divide a large set of data across multiple processors by spawning workers that run identical code but are assigned different subsets of the data.

Task Parallelism

Another form of parallelization is to assign a different task to each processor. An effective approach to parallelism can combine both forms together. With worker, you can use Group to emply both forms cooperatively (i.e., cores perform data parallelism while individually advancing to the next serial task), in order to maximize use of all available processing power.

Pseudo-Datatypes:

Throughout this API document, the following datatypes are used to represent expectations imposed on primitive-datatyped parameters to functions, uses of primitives in class methods, and so forth:

  • key - a string used for accessing an arbitrary value in a plain object
  • path - a string that conforms to an expected syntax (e.g., URL, file path, etc.)
  • struct - an interface for a plain object (i.e., one that has expected key names)
  • hash - a plain object whose keys are arbitrary (i.e., defined by you, the user)
  • list - a one-dimensional array containing only elments of the same type/class
  • uint - a non-negative integer
  • any - any object or primitive data type that are serializable via the Structured Clone Algorithm

API Documentation


Factory

The module's main export is the Factory. const worker = require('worker');

Constructors:

  • worker.spawn(source: path)
    • returns a new Worker
    • example:
      let k_worker = worker.spawn('./eg.js');
  • worker.pool(source: path[, limit: int]) -- create a pool that will spawn up to limit workers (defaults to number of cores, i.e., navigator.hardwareConcurrency or os.cpus().length). If limit is negative, it indicates how many cores to attempt to reserve (such as os.cpus().length -1), but if there are not enough cores, then the final number of workers in the pool will always end up greater than or equal to 1. Each worker will be spawned from the same source.
    • returns a new Pool
    • example:
      let k_pool = worker.pool('./eg.js');
  • worker.group(source: path[, count: int]) -- create a group (i.e., a cooperative pool) that will spawn up to count workers (defaults to number of cores, i.e., navigator.hardwareConcurrency or os.cpus().length). If count is negative, it indicates how many cores to attempt to reserve (such as os.cpus().length -1), but if there are not enough cores, then the final number of workers in the group will always end up greater than or equal to 1. Each worker will be spawned from the same source.
    • returns a new Group
    • example:
      let k_group = worker.group('./eg.js');
  • worker.manifest(args: Array<any>[, transfers: list<Paths>]) -- create an object that encapsulates the serializable objects in args, optionally declaring a list of those objects that are (a) instances of stream or (b) Transferable (for the browser) or SharedMemory (for node.js). If transfers is omitted or true, then each item in args will be exhaustively searched to find all streams/Transferable/SharedMemory objects. Providing a list of transfers spares the extra computation.
    • returns a new Manifest
    • example:
      let h_album = {
         name: 'pics',
         images: [
             new Uint32Array(800*600),
             new Uint32Array(1024*768),
         ],
      };
      let km_args = worker.manifest([h_album], [
         [0, 'images', 0],  // paths to transferable items
         [0, 'images', 1],
           /* OR */
         [0, 'images'],  // path to container of all transferable items
      ]);
      worker.spawn('./eg.js').run('collage', km_args);

Methods:

  • worker.dedicated(tasks: hash{name =>TaskHandler}) -- declare the current thread as a dedicated worker while passing a hash of tasks that associates a task's name to its corresponding TaskHandler.
    • returns undefined
    • example:
      // i-am-a-worker.js
      worker.dedicated({
         namedTask1(a_subset) {...},
         namedTask2(a_subset) {...},
         ...
      });
  • worker.globals([scope: Object]) -- get and optionally set the identifiers for SharedMemory, such as Uint8ArrayS.
    • returns scope, or a new struct
    • example:
      worker.globals(global);  // or `window`, e.g.
      let at_test = new Uint8ArrayS();
  • worker.merge_sorted(left: list<any>, right: list<any>[, sort: callback]) -- helper function for merging two sorted lists; useful on the worker side.
    • returns a list<any> that is the sorted combination of left and right
    • example:

class Worker

An abstraction of a WebWorker or ChildProcess. Create an instance by using the Factory method worker.spawn().

Methods:

  • .run(taskName: string[, args: Array<any> |Manifest[, events:EventHash]) -- run the task given by taskName on the worker, optionally passing args and events. For understanding whether or not you need to create a Manifest object, see the Manifest documentation.
    • returns a new Promise
    • example:
      async function example() {
         let k_worker = worker.spawn('./eg.js');
         
         // example of a simple call with an arg, using await to get return value
         let x_result_1 = await k_worker.run('factorial', [170]);
         
         // example with event callbacks
         await k_worker.run('examine', [s_latex_doc], {
            paragraph() { ... },
            figure() { ... },
         });
      }
  • .kill([signal='SIGTERM': string | uint]) -- terminate the worker, optionally sending a kill signal to the child process if running on node.js
    • returns a new Promise
    • example:
      k_worker.kill().then(() => {
         // worker is gone
      });

class Pool

A pool of Workers for simple task parallelism. Create an instance by using the Factory method worker.pool().

Methods:

  • .run(taskName: string[, args: Array<any> |Manifest[, events:EventHash]) -- pull a single Worker out of the pool and assign it this task, or queue this task if all workers in the pool are busy.
  • .start() -- starts a new point in the queue from which to wait until all further queued tasks will complete
    • returns undefined
  • .stop() -- wait until all previously queued tasks (starting at the start point and ending here) have completed. This also implicitly calls .start(), resetting the start point to here.
  • .kill([signal='SIGTERM': string | uint]) -- terminate all workers in the pool, optionally sending a kill signal to the child process if running on node.js.

Example:

let k_pool = worker.pool('./eg.js', 3);
 
// start a task on one worker
k_pool.run('render');
 
// a series of tasks is about to be queued
k_pool.start();
 
// start several more tasks as workers become available
a_downloads.forEach(async (p_url) => {
    let d_blob = await k_pool.run('download', [p_url]);
    download_complete(p_url, d_blob);
});
 
// as soon as the last download is done, start uploading
k_pool.stop().then(() => {
    k_pool.run('upload');
});

class Group

A group is a cooperative pool of Workers that are spawned from the same source. Create an instance of this class by using the Factory method worker.group().

Methods:

  • .data(items: Array[, strategy: DivisionStrategy]) -- divide items into multiple subsets using the EqualDivisionStrategy by default, or by specifying a different strategy.
  • .use(subsets: list<any>) -- assign each item in subsets to a worker. The length of subsets must be less than or equal to the number of workers in this group.
  • .wait -- declare an event listener, or a list of events that should be triggered consequently, for the given event(s)
    • ...(lock|s: string|list<string>, unlocked: callback) -- calls unlocked once the event named lock, or each and every event listed in locks, is triggered.
    • ...(lock|s: string|list<string>, dependency|ies: string|list<string>) -- triggers the event named dependency, or each and every event listed in dependencies, once the event named lock, or each and every event listed in locks, is triggered.
    • returns this
  • .unlock -- triggers any callbacks that are currently waiting for the given event(s), or as soon as they attach a listener. In other words, event binding via .wait() can occur before or after .unlock() and the results will be the same.
    • ...(lock: string) -- triggers the event named lock.
    • ...(locks: list<string>) -- triggers each event listed in locks.
    • returns this

class ArmedGroup

A Group that has data attached but has not yet been assigned any tasks.

Methods:

  • .map(taskName: string[, args: Array<any> |Manifest[, events:EventHash]]) -- dispatch workers to run the task given by taskName, on the currently binded data, in parallel. The first argument to each task's call will be its corresponding subset, followed by args. For understanding whether or not you need to create a Manifest object, see the Manifest documentation.
    • returns a new ActiveGroup
    • example:
      let a_sequence = [1, 2, 3, 4];
      worker.group('./eg.js').data(a_sequence)
         .map('multiply', [2])
         .reduce('sum', (x_actual) => {
             let x_expect = a_sequence.map(x => x*2).reduce((c, x) => c + x, 0);
             assert.equal(x_actual, x_expect);
         });

class ActiveGroup

A Group that has data attached and has been assigned at least one task.

Methods:

  • .thru(taskName: string[, args: Array<any> |Manifest[, events:EventHash]]) -- rather than passing the result back to the master thread, keep each worker's result data in their own thread and simply forward it to another task. For understanding whether or not you need to create a Manifest object, see the Manifest documentation.
  • .each(each:TaskResultCallback[, then: callback(error=null)]) -- handle each task result as soon as it completes, calling each for each subset of data whether or not they happen to be ready in order. If an error is thrown by one of the workers, or once all tasks complete, then will be called.
  • .series(each:TaskResultCallback[, then: callback(error=nul)]) -- handle each task result in order, calling each for each subset of data once it has been processed by the preceeding task. If an error is thrown by one of the workers, or once all tasks complete, then will be called.
  • .end([then: callback(error)]) -- once all the previous tasks end, call then. This essentially ignores the results returned by the workers.
  • .reduce(taskName: string[, args: Array<any> |Manifest[, events:EventHash]]) -- merge adjacent task results until there is one single result remaining. For understanding whether or not you need to create a Manifest object, see the Manifest documentation.

callback TaskResultCallback

A function to implement on the 'master' side that gets called when a task successfully completes along with the result it returned. As indicated by the signature, this can optionally be an async function which returns a Promise.

Signature: [async] function(result: any, subsetIndex: uint)

Parameters: 0. result: any -- what the worker returned

  1. subsetIndex: uint -- the index (from 0 to # workers - 1) of the subset that this result derives from

Returning one of the following will carry the result downstream:

  • An instanceof a Promise that resolves with something that is not an instanceof Error and is not undefined.
  • Anything that is not undefined

Returning one of the following will take whatever action is defined by the task stream for handling errors:

  • An instanceof an Error
  • An instanceof a Promise that rejects with some reason.
  • undefined

interface EventHash

A hash that binds callback listener functions to arbitrary event names, which can be emitted by the worker.

Signature: [eventName] => callback(...arguments: any)

Example:

worker.spawn('./eg.js').run('scan', [fs.createReadStream('./input.txt')], {
    line() {
        // newline encountered
    },
});

class Manifest

An array of objects to transmit/receive between master and worker threads such that special objects are handled properly, i.e., such that streams can be transmitted across threads, Transferable objects can be handled by the browser, or that SharedMemory objects can be exchanged over IPC. Create an instance by using Factory's worker.manifest().

For convenience, any method that accepts a Manifest argument can also accept an Array<any>. In this context, the Array must not contain any special objects below one traversable depth. In other words, so long as you are passing special objects at the top level of the Array, or no special objects at all, then you do not need to create a Manifest.

Example:

// without explicitly creating a Manifest:
worker.spawn('./eg.js').run('parse', [
    // OK: just some serializable objects
    'hello', 42, /structured_clone(_algorithm)?/i,
 
    // OK: stream is at top level in array
    fs.createReadStream('./package.json'),
    
    // OK: shareable object is at top level in array
    new Uint8ArrayS(1024),
    
    // NOT OK: shareable object is nested; need to use Manifest instead
    {
        type: 'img',
        data: new Uint8ArrayS(1024),
    },
]);
 
// with Manifest, we can nest special objects
worker.spawn('./eg.js').run('parse', worker.manifest([
    {
        type: 'img',
        data: new Uint8ArrayS(1024),
    },
], [
    // optional: tell Manifest where to find special objects
    [0, 'data'],
]))...

Streams

When working in node.js, instances of ReadableStream and WritableStream can be passed between threads thru worker's standard messaging interface. Each event and method call on such streams are transmitted across threads, so keep in mind the additional overhead costs this may incur when streaming data. See Manifest for an example.

When deploying in the browser using browserify, the same rules apply to streams as with node.js since a reliable stream module can be used by both parties.

function TaskHandler

A function to implement on the 'worker' side that accepts an input subset as its first argument and any user-defined values for the rest of its arguments.

Signature: [async] function(subset: Array[, ...args])

Must return one of the following:

Example:

worker.dedicated({
    // simple synchronous task returning serializable object
    add(a_nums, x_value) {
        return a_nums.map(x => x + x_value);
    },
    // synchronous task returning transferable data
    encode(a_strings) {
        let a_encoded = a_strings.map(s_string => (new TextEncoder()).encode(s_string));
        return worker.response(a_encoded, true);  // auto-find transferable objects
    },
    // asynchronous task returning promise that resolves to serializable object
    async info(s_wikipedia_page) {
        let s_html = await fetch('http://en.wikipedia.org/wiki/'+x);
        return extract_infobox(s_html);
    },
});

Each time a task handler function is called, its this will have the following fields:

  • Properties:

    • .events: hash{name => 1} -- a 'simple set' of events that the user requests to be notified about. Useful for determining if a certain event even needs to be emitted.
  • Methods:

    • .put(key: string, data: any)

      • stores data under the given key to a hash store that will be available for future tasks in the current pipeline running on the same thread.
      • Note: this hash store is safe to use even if the current thread is reassigned to repeat the same task on a different subset; i.e., it protects against task-level collisions.

    • .get(key: string)

      • retrieves data from the previous task(s) in the current thread pipeline.
      • returns data set by the user during a call to .put
    • .emit(eventName: string[, ...args])

      • emits the event given by eventName along with the given args by sending a message to the current worker's master thread.

      Note: each of the args must be serializable by the Structured Clone Algorithm.

      • example:
        worker.dedicated({
            parse(at_code) {
                ds_stream.on('data', (s_data) => {
                    if(s_data.startsWith('warn:')) this.emit('warn', s_data);
                });
            },
        });

SharedMemory


In node.js, this library spawns a new process for each worker. Communication between processes (IPC) is normally done via pipes, which is a copy-on-write operation requiring outgoing data to be duplicated in memory. To alleviate the overhead when transferring larger objects between processes, you can allocate ArrayBuffers and TypedArrays in shared memory space before filling them with data.

The following classes are provided to ease the process of creating shared memory. They behave the same as their corresponding TypedArray constructors:

ArrayBufferS
Int8ArrayS
Uint8ArrayS
Uint8ClampedArrayS
Int16ArrayS
Uint16ArrayS
Int32ArrayS
Uint32ArrayS
Float32ArrayS
Float64ArrayS

In the browser, each of these classes will invoke the SharedArrayBuffer constructor to create shared memory.

In node.js, each of these classes will attempt to create shared memory on the operating system (right now only support for systems that support POSIX shm and mmap).

Alternatively, you may wish to transfer objects in the browser rather than creating shared memory. In node.js however, IPC still requires shared memory. For this scenario, you can use the transfer-if-able TypedArray constructors:

ArrayBufferT
Int8ArrayT
Uint8ArrayT
Uint8ClampedArrayT
Int16ArrayT
Uint16ArrayT
Int32ArrayT
Uint32ArrayT
Float32ArrayT
Float64ArrayT

DivisionStrategies


A division strategy is an algorithm that divides a list of elements into multiple subsets, so as to share the load among workers in a data parallelism paradigm.

EqualDivisionStrategy

A default division strategy that will attempt to divide the input data into equal subsets over the number of workers in the current Group. Inputs that are not evenly divisible by the number of workers in their Group will result in slightly larger subsets nearer the end, increasing the liklihood that the workers assigned the first subsets finish before those assigned the last subsets; an outcome intended to favor responses that process results based on their order within the original input data.