1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
|
# Worker Threads
<!--introduced_in=v10.5.0-->
> Stability: 1 - Experimental
The `worker` module provides a way to create multiple environments running
on independent threads, and to create message channels between them. It
can be accessed using the `--experimental-worker` flag and:
```js
const worker = require('worker_threads');
```
Workers are useful for performing CPU-intensive JavaScript operations; do not
use them for I/O, since Node.js’s built-in mechanisms for performing operations
asynchronously already treat it more efficiently than Worker threads can.
Workers, unlike child processes or when using the `cluster` module, can also
share memory efficiently by transferring `ArrayBuffer` instances or sharing
`SharedArrayBuffer` instances between them.
```js
const {
Worker, isMainThread, parentPort, workerData
} = require('worker_threads');
if (isMainThread) {
module.exports = async function parseJSAsync(script) {
return new Promise((resolve, reject) => {
const worker = new Worker(__filename, {
workerData: script
});
worker.on('message', resolve);
worker.on('error', reject);
worker.on('exit', (code) => {
if (code !== 0)
reject(new Error(`Worker stopped with exit code ${code}`));
});
});
};
} else {
const { parse } = require('some-js-parsing-library');
const script = workerData;
parentPort.postMessage(parse(script));
}
```
Note that this example spawns a Worker thread for each `parse` call.
In practice, it is strongly recommended to use a pool of Workers for these
kinds of tasks, since the overhead of creating Workers would likely exceed the
benefit of handing the work off to it.
## worker.isMainThread
<!-- YAML
added: v10.5.0
-->
* {boolean}
Is `true` if this code is not running inside of a [`Worker`][] thread.
## worker.parentPort
<!-- YAML
added: v10.5.0
-->
* {null|MessagePort}
If this thread was spawned as a [`Worker`][], this will be a [`MessagePort`][]
allowing communication with the parent thread. Messages sent using
`parentPort.postMessage()` will be available in the parent thread
using `worker.on('message')`, and messages sent from the parent thread
using `worker.postMessage()` will be available in this thread using
`parentPort.on('message')`.
## worker.threadId
<!-- YAML
added: v10.5.0
-->
* {integer}
An integer identifier for the current thread. On the corresponding worker object
(if there is any), it is available as [`worker.threadId`][].
## worker.workerData
<!-- YAML
added: v10.5.0
-->
An arbitrary JavaScript value that contains a clone of the data passed
to this thread’s `Worker` constructor.
## Class: MessageChannel
<!-- YAML
added: v10.5.0
-->
Instances of the `worker.MessageChannel` class represent an asynchronous,
two-way communications channel.
The `MessageChannel` has no methods of its own. `new MessageChannel()`
yields an object with `port1` and `port2` properties, which refer to linked
[`MessagePort`][] instances.
```js
const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log('received', message));
port2.postMessage({ foo: 'bar' });
// Prints: received { foo: 'bar' } from the `port1.on('message')` listener
```
## Class: MessagePort
<!-- YAML
added: v10.5.0
-->
* Extends: {EventEmitter}
Instances of the `worker.MessagePort` class represent one end of an
asynchronous, two-way communications channel. It can be used to transfer
structured data, memory regions and other `MessagePort`s between different
[`Worker`][]s.
With the exception of `MessagePort`s being [`EventEmitter`][]s rather
than [`EventTarget`][]s, this implementation matches [browser `MessagePort`][]s.
### Event: 'close'
<!-- YAML
added: v10.5.0
-->
The `'close'` event is emitted once either side of the channel has been
disconnected.
### Event: 'message'
<!-- YAML
added: v10.5.0
-->
* `value` {any} The transmitted value
The `'message'` event is emitted for any incoming message, containing the cloned
input of [`port.postMessage()`][].
Listeners on this event will receive a clone of the `value` parameter as passed
to `postMessage()` and no further arguments.
### port.close()
<!-- YAML
added: v10.5.0
-->
Disables further sending of messages on either side of the connection.
This method can be called when no further communication will happen over this
`MessagePort`.
### port.postMessage(value[, transferList])
<!-- YAML
added: v10.5.0
-->
* `value` {any}
* `transferList` {Object[]}
Sends a JavaScript value to the receiving side of this channel.
`value` will be transferred in a way which is compatible with
the [HTML structured clone algorithm][]. In particular, it may contain circular
references and objects like typed arrays that the `JSON` API is not able
to stringify.
`transferList` may be a list of `ArrayBuffer` and `MessagePort` objects.
After transferring, they will not be usable on the sending side of the channel
anymore (even if they are not contained in `value`). Unlike with
[child processes][], transferring handles such as network sockets is currently
not supported.
If `value` contains [`SharedArrayBuffer`][] instances, those will be accessible
from either thread. They cannot be listed in `transferList`.
`value` may still contain `ArrayBuffer` instances that are not in
`transferList`; in that case, the underlying memory is copied rather than moved.
Because the object cloning uses the structured clone algorithm,
non-enumerable properties, property accessors, and object prototypes are
not preserved. In particular, [`Buffer`][] objects will be read as
plain [`Uint8Array`][]s on the receiving side.
The message object will be cloned immediately, and can be modified after
posting without having side effects.
For more information on the serialization and deserialization mechanisms
behind this API, see the [serialization API of the `v8` module][v8.serdes].
### port.ref()
<!-- YAML
added: v10.5.0
-->
Opposite of `unref()`. Calling `ref()` on a previously `unref()`ed port will
*not* let the program exit if it's the only active handle left (the default
behavior). If the port is `ref()`ed, calling `ref()` again will have no effect.
If listeners are attached or removed using `.on('message')`, the port will
be `ref()`ed and `unref()`ed automatically depending on whether
listeners for the event exist.
### port.start()
<!-- YAML
added: v10.5.0
-->
Starts receiving messages on this `MessagePort`. When using this port
as an event emitter, this will be called automatically once `'message'`
listeners are attached.
### port.unref()
<!-- YAML
added: v10.5.0
-->
Calling `unref()` on a port will allow the thread to exit if this is the only
active handle in the event system. If the port is already `unref()`ed calling
`unref()` again will have no effect.
If listeners are attached or removed using `.on('message')`, the port will
be `ref()`ed and `unref()`ed automatically depending on whether
listeners for the event exist.
## Class: Worker
<!-- YAML
added: v10.5.0
-->
* Extends: {EventEmitter}
The `Worker` class represents an independent JavaScript execution thread.
Most Node.js APIs are available inside of it.
Notable differences inside a Worker environment are:
- The [`process.stdin`][], [`process.stdout`][] and [`process.stderr`][]
may be redirected by the parent thread.
- The [`require('worker_threads').isMainThread`][] property is set to `false`.
- The [`require('worker_threads').parentPort`][] message port is available,
- [`process.exit()`][] does not stop the whole program, just the single thread,
and [`process.abort()`][] is not available.
- [`process.chdir()`][] and `process` methods that set group or user ids
are not available.
- [`process.env`][] is a read-only reference to the environment variables.
- [`process.title`][] cannot be modified.
- Signals will not be delivered through [`process.on('...')`][Signals events].
- Execution may stop at any point as a result of [`worker.terminate()`][]
being invoked.
- IPC channels from parent processes are not accessible.
- The [`trace_events`][] module is not supported.
Currently, the following differences also exist until they are addressed:
- The [`inspector`][] module is not available yet.
- Native addons are not supported yet.
Creating `Worker` instances inside of other `Worker`s is possible.
Like [Web Workers][] and the [`cluster` module][], two-way communication can be
achieved through inter-thread message passing. Internally, a `Worker` has a
built-in pair of [`MessagePort`][]s that are already associated with each other
when the `Worker` is created. While the `MessagePort` object on the parent side
is not directly exposed, its functionalities are exposed through
[`worker.postMessage()`][] and the [`worker.on('message')`][] event
on the `Worker` object for the parent thread.
To create custom messaging channels (which is encouraged over using the default
global channel because it facilitates separation of concerns), users can create
a `MessageChannel` object on either thread and pass one of the
`MessagePort`s on that `MessageChannel` to the other thread through a
pre-existing channel, such as the global one.
See [`port.postMessage()`][] for more information on how messages are passed,
and what kind of JavaScript values can be successfully transported through
the thread barrier.
```js
const assert = require('assert');
const {
Worker, MessageChannel, MessagePort, isMainThread, parentPort
} = require('worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
const subChannel = new MessageChannel();
worker.postMessage({ hereIsYourPort: subChannel.port1 }, [subChannel.port1]);
subChannel.port2.on('message', (value) => {
console.log('received:', value);
});
} else {
parentPort.once('message', (value) => {
assert(value.hereIsYourPort instanceof MessagePort);
value.hereIsYourPort.postMessage('the worker is sending this');
value.hereIsYourPort.close();
});
}
```
### new Worker(filename[, options])
* `filename` {string} The path to the Worker’s main script. Must be
either an absolute path or a relative path (i.e. relative to the
current working directory) starting with `./` or `../`.
If `options.eval` is `true`, this is a string containing JavaScript code
rather than a path.
* `options` {Object}
* `eval` {boolean} If `true`, interpret the first argument to the constructor
as a script that is executed once the worker is online.
* `workerData` {any} Any JavaScript value that will be cloned and made
available as [`require('worker_threads').workerData`][]. The cloning will
occur as described in the [HTML structured clone algorithm][], and an error
will be thrown if the object cannot be cloned (e.g. because it contains
`function`s).
* stdin {boolean} If this is set to `true`, then `worker.stdin` will
provide a writable stream whose contents will appear as `process.stdin`
inside the Worker. By default, no data is provided.
* stdout {boolean} If this is set to `true`, then `worker.stdout` will
not automatically be piped through to `process.stdout` in the parent.
* stderr {boolean} If this is set to `true`, then `worker.stderr` will
not automatically be piped through to `process.stderr` in the parent.
### Event: 'error'
<!-- YAML
added: v10.5.0
-->
* `err` {Error}
The `'error'` event is emitted if the worker thread throws an uncaught
exception. In that case, the worker will be terminated.
### Event: 'exit'
<!-- YAML
added: v10.5.0
-->
* `exitCode` {integer}
The `'exit'` event is emitted once the worker has stopped. If the worker
exited by calling [`process.exit()`][], the `exitCode` parameter will be the
passed exit code. If the worker was terminated, the `exitCode` parameter will
be `1`.
### Event: 'message'
<!-- YAML
added: v10.5.0
-->
* `value` {any} The transmitted value
The `'message'` event is emitted when the worker thread has invoked
[`require('worker_threads').postMessage()`][]. See the [`port.on('message')`][]
event for more details.
### Event: 'online'
<!-- YAML
added: v10.5.0
-->
The `'online'` event is emitted when the worker thread has started executing
JavaScript code.
### worker.postMessage(value[, transferList])
<!-- YAML
added: v10.5.0
-->
* `value` {any}
* `transferList` {Object[]}
Send a message to the worker that will be received via
[`require('worker_threads').parentPort.on('message')`][].
See [`port.postMessage()`][] for more details.
### worker.ref()
<!-- YAML
added: v10.5.0
-->
Opposite of `unref()`, calling `ref()` on a previously `unref()`ed worker will
*not* let the program exit if it's the only active handle left (the default
behavior). If the worker is `ref()`ed, calling `ref()` again will have
no effect.
### worker.stderr
<!-- YAML
added: v10.5.0
-->
* {stream.Readable}
This is a readable stream which contains data written to [`process.stderr`][]
inside the worker thread. If `stderr: true` was not passed to the
[`Worker`][] constructor, then data will be piped to the parent thread's
[`process.stderr`][] stream.
### worker.stdin
<!-- YAML
added: v10.5.0
-->
* {null|stream.Writable}
If `stdin: true` was passed to the [`Worker`][] constructor, this is a
writable stream. The data written to this stream will be made available in
the worker thread as [`process.stdin`][].
### worker.stdout
<!-- YAML
added: v10.5.0
-->
* {stream.Readable}
This is a readable stream which contains data written to [`process.stdout`][]
inside the worker thread. If `stdout: true` was not passed to the
[`Worker`][] constructor, then data will be piped to the parent thread's
[`process.stdout`][] stream.
### worker.terminate([callback])
<!-- YAML
added: v10.5.0
-->
* `callback` {Function}
* `err` {Error}
* `exitCode` {integer}
Stop all JavaScript execution in the worker thread as soon as possible.
`callback` is an optional function that is invoked once this operation is known
to have completed.
**Warning**: Currently, not all code in the internals of Node.js is prepared to
expect termination at arbitrary points in time and may crash if it encounters
that condition. Consequently, only call `.terminate()` if it is known that the
Worker thread is not accessing Node.js core modules other than what is exposed
in the `worker` module.
### worker.threadId
<!-- YAML
added: v10.5.0
-->
* {integer}
An integer identifier for the referenced thread. Inside the worker thread,
it is available as [`require('worker_threads').threadId`][].
### worker.unref()
<!-- YAML
added: v10.5.0
-->
Calling `unref()` on a worker will allow the thread to exit if this is the only
active handle in the event system. If the worker is already `unref()`ed calling
`unref()` again will have no effect.
[`Buffer`]: buffer.html
[`EventEmitter`]: events.html
[`EventTarget`]: https://developer.mozilla.org/en-US/docs/Web/API/EventTarget
[`MessagePort`]: #worker_threads_class_messageport
[`SharedArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer
[`Uint8Array`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array
[`Worker`]: #worker_threads_class_worker
[`cluster` module]: cluster.html
[`inspector`]: inspector.html
[`port.on('message')`]: #worker_threads_event_message
[`port.postMessage()`]: #worker_threads_port_postmessage_value_transferlist
[`process.abort()`]: process.html#process_process_abort
[`process.chdir()`]: process.html#process_process_chdir_directory
[`process.env`]: process.html#process_process_env
[`process.exit()`]: process.html#process_process_exit_code
[`process.stderr`]: process.html#process_process_stderr
[`process.stdin`]: process.html#process_process_stdin
[`process.stdout`]: process.html#process_process_stdout
[`process.title`]: process.html#process_process_title
[`require('worker_threads').isMainThread`]: #worker_threads_worker_ismainthread
[`require('worker_threads').parentPort.on('message')`]: #worker_threads_event_message
[`require('worker_threads').parentPort`]: #worker_threads_worker_parentport
[`require('worker_threads').postMessage()`]: #worker_threads_worker_postmessage_value_transferlist
[`require('worker_threads').threadId`]: #worker_threads_worker_threadid
[`require('worker_threads').workerData`]: #worker_threads_worker_workerdata
[`trace_events`]: tracing.html
[`worker.on('message')`]: #worker_threads_event_message_1
[`worker.postMessage()`]: #worker_threads_worker_postmessage_value_transferlist
[`worker.terminate()`]: #worker_threads_worker_terminate_callback
[`worker.threadId`]: #worker_threads_worker_threadid_1
[HTML structured clone algorithm]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm
[Signals events]: process.html#process_signal_events
[Web Workers]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API
[browser `MessagePort`]: https://developer.mozilla.org/en-US/docs/Web/API/MessagePort
[child processes]: child_process.html
[v8.serdes]: v8.html#v8_serialization_api
|