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Node.js v15.0.0-test20200429329279fc84 Documentation
Table of Contents
Worker Threads#
The worker_threads
module enables the use of threads that execute JavaScript
in parallel. To access it:
const worker = require('worker_threads');
Workers (threads) are useful for performing CPU-intensive JavaScript operations. They will not help much with I/O-intensive work. Node.js’s built-in asynchronous I/O operations are more efficient than Workers can be.
Unlike child_process
or cluster
, worker_threads
can share memory. They do
so by transferring ArrayBuffer
instances or sharing SharedArrayBuffer
instances.
const {
Worker, isMainThread, parentPort, workerData
} = require('worker_threads');
if (isMainThread) {
module.exports = 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));
}
The above example spawns a Worker thread for each parse()
call. In actual
practice, use a pool of Workers instead for these kinds of tasks. Otherwise, the
overhead of creating Workers would likely exceed their benefit.
When implementing a worker pool, use the AsyncResource
API to inform
diagnostic tools (e.g. in order to provide asynchronous stack traces) about the
correlation between tasks and their outcomes. See
"Using AsyncResource
for a Worker
thread pool"
in the async_hooks
documentation for an example implementation.
Worker threads inherit non-process-specific options by default. Refer to
Worker constructor options
to know how to customize worker thread options,
specifically argv
and execArgv
options.
worker.isMainThread
#
Is true
if this code is not running inside of a Worker
thread.
const { Worker, isMainThread } = require('worker_threads');
if (isMainThread) {
// This re-loads the current file inside a Worker instance.
new Worker(__filename);
} else {
console.log('Inside Worker!');
console.log(isMainThread); // Prints 'false'.
}
worker.moveMessagePortToContext(port, contextifiedSandbox)
#
-
port
<MessagePort> The message port which will be transferred. -
contextifiedSandbox
<Object> A contextified object as returned by thevm.createContext()
method. -
Returns: <MessagePort>
Transfer a MessagePort
to a different vm
Context. The original port
object will be rendered unusable, and the returned MessagePort
instance will
take its place.
The returned MessagePort
will be an object in the target context, and will
inherit from its global Object
class. Objects passed to the
port.onmessage()
listener will also be created in the target context
and inherit from its global Object
class.
However, the created MessagePort
will no longer inherit from
EventEmitter
, and only port.onmessage()
can be used to receive
events using it.
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')
.
const { Worker, isMainThread, parentPort } = require('worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
worker.once('message', (message) => {
console.log(message); // Prints 'Hello, world!'.
});
worker.postMessage('Hello, world!');
} else {
// When a message from the parent thread is received, send it back:
parentPort.once('message', (message) => {
parentPort.postMessage(message);
});
}
worker.receiveMessageOnPort(port)
#
-
port
<MessagePort> -
Returns: <Object> | <undefined>
Receive a single message from a given MessagePort
. If no message is available,
undefined
is returned, otherwise an object with a single message
property
that contains the message payload, corresponding to the oldest message in the
MessagePort
’s queue.
const { MessageChannel, receiveMessageOnPort } = require('worker_threads');
const { port1, port2 } = new MessageChannel();
port1.postMessage({ hello: 'world' });
console.log(receiveMessageOnPort(port2));
// Prints: { message: { hello: 'world' } }
console.log(receiveMessageOnPort(port2));
// Prints: undefined
When this function is used, no 'message'
event will be emitted and the
onmessage
listener will not be invoked.
worker.resourceLimits
#
Provides the set of JS engine resource constraints inside this Worker thread.
If the resourceLimits
option was passed to the Worker
constructor,
this matches its values.
If this is used in the main thread, its value is an empty object.
worker.SHARE_ENV
#
A special value that can be passed as the env
option of the Worker
constructor, to indicate that the current thread and the Worker thread should
share read and write access to the same set of environment variables.
const { Worker, SHARE_ENV } = require('worker_threads');
new Worker('process.env.SET_IN_WORKER = "foo"', { eval: true, env: SHARE_ENV })
.on('exit', () => {
console.log(process.env.SET_IN_WORKER); // Prints 'foo'.
});
worker.threadId
#
An integer identifier for the current thread. On the corresponding worker object
(if there is any), it is available as worker.threadId
.
This value is unique for each Worker
instance inside a single process.
worker.workerData
#
An arbitrary JavaScript value that contains a clone of the data passed
to this thread’s Worker
constructor.
The data is cloned as if using postMessage()
,
according to the HTML structured clone algorithm.
const { Worker, isMainThread, workerData } = require('worker_threads');
if (isMainThread) {
const worker = new Worker(__filename, { workerData: 'Hello, world!' });
} else {
console.log(workerData); // Prints 'Hello, world!'.
}
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.
const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();
// Prints:
// foobar
// closed!
port2.on('message', (message) => console.log(message));
port2.on('close', () => console.log('closed!'));
port1.postMessage('foobar');
port1.close();
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
.
The 'close'
event will be emitted on both MessagePort
instances that
are part of the channel.
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, the significant differences to JSON
are:
value
may contain circular references.value
may contain instances of builtin JS types such asRegExp
s,BigInt
s,Map
s,Set
s, etc.value
may contain typed arrays, both usingArrayBuffer
s andSharedArrayBuffer
s.value
may containWebAssembly.Module
instances.value
may not contain native (C++-backed) objects other thanMessagePort
s.
const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log(message));
const circularData = {};
circularData.foo = circularData;
// Prints: { foo: [Circular] }
port2.postMessage(circularData);
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.
const { MessageChannel } = require('worker_threads');
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log(message));
const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]);
// This posts a copy of `uint8Array`:
port2.postMessage(uint8Array);
// This does not copy data, but renders `uint8Array` unusable:
port2.postMessage(uint8Array, [ uint8Array.buffer ]);
// The memory for the `sharedUint8Array` will be accessible from both the
// original and the copy received by `.on('message')`:
const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
port2.postMessage(sharedUint8Array);
// This transfers a freshly created message port to the receiver.
// This can be used, for example, to create communication channels between
// multiple `Worker` threads that are children of the same parent thread.
const otherChannel = new MessageChannel();
port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]);
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.
This method exists for parity with the Web MessagePort
API. In Node.js,
it is only useful for ignoring messages when no event listener is present.
Node.js also diverges in its handling of .onmessage
. Setting it will
automatically call .start()
, but unsetting it will let messages queue up
until a new handler is set or the port is discarded.
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 copy of the parent thread's environment variables, unless otherwise specified. Changes to one copy will not be visible in other threads, and will not be visible to native add-ons (unlessworker.SHARE_ENV
has been passed as theenv
option to theWorker
constructor).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.
- The
trace_events
module is not supported. - Native add-ons can only be loaded from multiple threads if they fulfill certain conditions.
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> | <URL> The path to the Worker’s main script or module. Must be either an absolute path or a relative path (i.e. relative to the current working directory) starting with./
or../
, or a WHATWGURL
object usingfile:
protocol. Ifoptions.eval
istrue
, this is a string containing JavaScript code rather than a path.-
options
<Object>argv
<any[]> List of arguments which would be stringified and appended toprocess.argv
in the worker. This is mostly similar to theworkerData
but the values will be available on the globalprocess.argv
as if they were passed as CLI options to the script.env
<Object> If set, specifies the initial value ofprocess.env
inside the Worker thread. As a special value,worker.SHARE_ENV
may be used to specify that the parent thread and the child thread should share their environment variables; in that case, changes to one thread’sprocess.env
object will affect the other thread as well. Default:process.env
.eval
<boolean> Iftrue
and the first argument is astring
, interpret the first argument to the constructor as a script that is executed once the worker is online.execArgv
<string[]> List of node CLI options passed to the worker. V8 options (such as--max-old-space-size
) and options that affect the process (such as--title
) are not supported. If set, this will be provided asprocess.execArgv
inside the worker. By default, options will be inherited from the parent thread.stdin
<boolean> If this is set totrue
, 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 totrue
, thenworker.stdout
will not automatically be piped through toprocess.stdout
in the parent.stderr
<boolean> If this is set totrue
, thenworker.stderr
will not automatically be piped through toprocess.stderr
in the parent.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).transferList
<Object[]> If one or moreMessagePort
-like objects are passed inworkerData
, atransferList
is required for those items orERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST
will be thrown. Seeport.postMessage()
for more information.-
resourceLimits
<Object> An optional set of resource limits for the new JS engine instance. Reaching these limits will lead to termination of theWorker
instance. These limits only affect the JS engine, and no external data, including noArrayBuffer
s. Even if these limits are set, the process may still abort if it encounters a global out-of-memory situation.
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
.
This is the final event emitted by any Worker
instance.
Event: 'message'
#
value
<any> The transmitted value
The 'message'
event is emitted when the worker thread has invoked
require('worker_threads').parentPort.postMessage()
.
See the port.on('message')
event for more details.
All messages sent from the worker thread will be emitted before the
'exit'
event is emitted on the Worker
object.
Event: 'online'
#
The 'online'
event is emitted when the worker thread has started executing
JavaScript code.
worker.getHeapSnapshot()
#
- Returns: <Promise> A promise for a Readable Stream containing a V8 heap snapshot
Returns a readable stream for a V8 snapshot of the current state of the Worker.
See v8.getHeapSnapshot()
for more details.
If the Worker thread is no longer running, which may occur before the
'exit'
event is emitted, the returned Promise
will be rejected
immediately with an ERR_WORKER_NOT_RUNNING
error.
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.resourceLimits
#
Provides the set of JS engine resource constraints for this Worker thread.
If the resourceLimits
option was passed to the Worker
constructor,
this matches its values.
If the worker has stopped, the return value is an empty object.
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()
#
- Returns: <Promise>
Stop all JavaScript execution in the worker thread as soon as possible.
Returns a Promise for the exit code that is fulfilled when the
'exit'
event is emitted.
worker.threadId
#
An integer identifier for the referenced thread. Inside the worker thread,
it is available as require('worker_threads').threadId
.
This value is unique for each Worker
instance inside a single process.
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.