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Node.js v10.15.2-rc.0 Documentation
Table of Contents
Worker Threads#
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:
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.
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#
Is true
if this code is not running inside of a Worker
thread.
worker.parentPort#
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#
An integer identifier for the current thread. On the corresponding worker object
(if there is any), it is available as worker.threadId
.
worker.workerData#
An arbitrary JavaScript value that contains a clone of the data passed
to this thread’s Worker
constructor.
Class: MessageChannel#
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.
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#
- 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'#
The 'close'
event is emitted once either side of the channel has been
disconnected.
Event: 'message'#
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()#
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])#
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.
port.ref()#
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()#
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()#
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#
- 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
andprocess.stderr
may be redirected by the parent thread. - The
require('worker_threads').isMainThread
property is set tofalse
. - The
require('worker_threads').parentPort
message port is available, process.exit()
does not stop the whole program, just the single thread, andprocess.abort()
is not available.process.chdir()
andprocess
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('...')
. - Execution may stop at any point as a result of
worker.terminate()
being invoked. - IPC channels from parent processes are not accessible.
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.
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../
. Ifoptions.eval
istrue
, this is a string containing JavaScript code rather than a path.-
options
<Object>eval
<boolean> Iftrue
, 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 asrequire('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 containsfunction
s).- stdin <boolean> If this is set to
true
, thenworker.stdin
will provide a writable stream whose contents will appear asprocess.stdin
inside the Worker. By default, no data is provided. - stdout <boolean> If this is set to
true
, thenworker.stdout
will not automatically be piped through toprocess.stdout
in the parent. - stderr <boolean> If this is set to
true
, thenworker.stderr
will not automatically be piped through toprocess.stderr
in the parent.
Event: 'error'#
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'#
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'#
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'#
The 'online'
event is emitted when the worker thread has started executing
JavaScript code.
worker.postMessage(value[, transferList])#
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()#
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#
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#
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#
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])#
-
callback
<Function>
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#
An integer identifier for the referenced thread. Inside the worker thread,
it is available as require('worker_threads').threadId
.
worker.unref()#
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.