Worker threads

Added in: v10.5.0

源代码: lib/worker_threads.js

The node:worker_threads module enables the use of threads that execute JavaScript in parallel. To access it:

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Workers (threads) are useful for performing CPU-intensive JavaScript operations. They do not help much with I/O-intensive work. The Node.js 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.

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The above example spawns a Worker thread for each parseJSAsync() call. In practice, use a pool of Workers 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. 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.

M worker.getEnvironmentData(key)

• key any Any arbitrary, cloneable JavaScript value that can be used as a Map key.
• Returns: any

Within a worker thread, worker.getEnvironmentData() returns a clone of data passed to the spawning thread's worker.setEnvironmentData(). Every new Worker receives its own copy of the environment data automatically.

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M worker.isMainThread

Added in: v10.5.0

• boolean

Is true if this code is not running inside of a Worker thread.

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M worker.markAsUntransferable(object)

Added in: v14.5.0, v12.19.0

Mark an object as not transferable. If object occurs in the transfer list of a port.postMessage() call, it is ignored.

In particular, this makes sense for objects that can be cloned, rather than transferred, and which are used by other objects on the sending side. For example, Node.js marks the ArrayBuffers it uses for its Buffer pool with this.

This operation cannot be undone.

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There is no equivalent to this API in browsers.

M worker.moveMessagePortToContext(port, contextifiedSandbox)

Added in: v11.13.0

• port MessagePort The message port to transfer.

• contextifiedSandbox Object A contextified object as returned by the vm.createContext() method.

• Returns: MessagePort

Transfer a MessagePort to a different vm Context. The original port object is rendered unusable, and the returned MessagePort instance takes its place.

The returned MessagePort is an object in the target context and inherits from its global Object class. Objects passed to the port.onmessage() listener are also created in the target context and inherit from its global Object class.

However, the created MessagePort no longer inherits from EventTarget, and only port.onmessage() can be used to receive events using it.

M worker.parentPort

Added in: v10.5.0

• null | MessagePort

If this thread is a Worker, this is a MessagePort allowing communication with the parent thread. Messages sent using parentPort.postMessage() are available in the parent thread using worker.on('message'), and messages sent from the parent thread using worker.postMessage() are available in this thread using parentPort.on('message').

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M worker.receiveMessageOnPort(port)

• port MessagePort | BroadcastChannel

• 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.

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When this function is used, no 'message' event is emitted and the onmessage listener is not invoked.

M worker.resourceLimits

Added in: v13.2.0, v12.16.0

• Object
  • maxYoungGenerationSizeMb number
  • maxOldGenerationSizeMb number
  • codeRangeSizeMb number
  • stackSizeMb number

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.

M worker.SHARE_ENV

Added in: v11.14.0

• symbol

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.

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M worker.setEnvironmentData(key[, value])

• key any Any arbitrary, cloneable JavaScript value that can be used as a Map key.
• value any Any arbitrary, cloneable JavaScript value that will be cloned and passed automatically to all new Worker instances. If value is passed as undefined, any previously set value for the key will be deleted.

The worker.setEnvironmentData() API sets the content of worker.getEnvironmentData() in the current thread and all new Worker instances spawned from the current context.

M worker.threadId

Added in: 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. This value is unique for each Worker instance inside a single process.

M worker.workerData

Added in: v10.5.0

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.

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C BroadcastChannel extends EventTarget

Instances of BroadcastChannel allow asynchronous one-to-many communication with all other BroadcastChannel instances bound to the same channel name.

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M new BroadcastChannel(name)

Added in: v15.4.0

• name any The name of the channel to connect to. Any JavaScript value that can be converted to a string using `$name` is permitted.

M broadcastChannel.close()

Added in: v15.4.0

Closes the BroadcastChannel connection.

M broadcastChannel.onmessage

Added in: v15.4.0

• Type: Function Invoked with a single MessageEvent argument when a message is received.

M broadcastChannel.onmessageerror

Added in: v15.4.0

• Type: Function Invoked with a received message cannot be deserialized.

M broadcastChannel.postMessage(message)

Added in: v15.4.0

• message any Any cloneable JavaScript value.

M broadcastChannel.ref()

Added in: v15.4.0

Opposite of unref(). Calling ref() on a previously unref()ed BroadcastChannel does 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 has no effect.

M broadcastChannel.unref()

Added in: v15.4.0

Calling unref() on a BroadcastChannel allows the thread to exit if this is the only active handle in the event system. If the BroadcastChannel is already unref()ed calling unref() again has no effect.

C MessageChannel

Added in: 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.

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C MessagePort

• Extends: EventTarget

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 MessagePorts between different Workers.

This implementation matches browser MessagePorts.

E 'close'

Added in: v10.5.0

The 'close' event is emitted once either side of the channel has been disconnected.

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E 'message'

Added in: 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 receive a clone of the value parameter as passed to postMessage() and no further arguments.

E 'messageerror'

Added in: v14.5.0, v12.19.0

• error Error An Error object

The 'messageerror' event is emitted when deserializing a message failed.

Currently, this event is emitted when there is an error occurring while instantiating the posted JS object on the receiving end. Such situations are rare, but can happen, for instance, when certain Node.js API objects are received in a vm.Context (where Node.js APIs are currently unavailable).

M port.close()

Added in: 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.

The 'close' event is emitted on both MessagePort instances that are part of the channel.

M port.postMessage(value[, transferList])

• value any
• transferList Object[]

Sends a JavaScript value to the receiving side of this channel. value is 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 as RegExps, BigInts, Maps, Sets, etc.
• value may contain typed arrays, both using ArrayBuffers and SharedArrayBuffers.
• value may contain WebAssembly.Module instances.
• value may not contain native (C++-backed) objects other than:
  • CryptoKeys,
  • FileHandles,
  • Histograms,
  • KeyObjects,
  • MessagePorts,
  • net.BlockLists,
  • net.SocketAddresses,
  • X509Certificates.

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transferList may be a list of ArrayBuffer, MessagePort, and FileHandle objects. After transferring, they are not 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 are 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.

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The message object is 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 node:v8 module.

Considerations when transferring TypedArrays and Buffers

All TypedArray and Buffer instances are views over an underlying ArrayBuffer. That is, it is the ArrayBuffer that actually stores the raw data while the TypedArray and Buffer objects provide a way of viewing and manipulating the data. It is possible and common for multiple views to be created over the same ArrayBuffer instance. Great care must be taken when using a transfer list to transfer an ArrayBuffer as doing so causes all TypedArray and Buffer instances that share that same ArrayBuffer to become unusable.

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For Buffer instances, specifically, whether the underlying ArrayBuffer can be transferred or cloned depends entirely on how instances were created, which often cannot be reliably determined.

An ArrayBuffer can be marked with markAsUntransferable() to indicate that it should always be cloned and never transferred.

Depending on how a Buffer instance was created, it may or may not own its underlying ArrayBuffer. An ArrayBuffer must not be transferred unless it is known that the Buffer instance owns it. In particular, for Buffers created from the internal Buffer pool (using, for instance Buffer.from() or Buffer.allocUnsafe()), transferring them is not possible and they are always cloned, which sends a copy of the entire Buffer pool. This behavior may come with unintended higher memory usage and possible security concerns.

See Buffer.allocUnsafe() for more details on Buffer pooling.

The ArrayBuffers for Buffer instances created using Buffer.alloc() or Buffer.allocUnsafeSlow() can always be transferred but doing so renders all other existing views of those ArrayBuffers unusable.

Considerations when cloning objects with prototypes, classes, and accessors

Because object cloning uses the HTML structured clone algorithm, non-enumerable properties, property accessors, and object prototypes are not preserved. In particular, Buffer objects will be read as plain Uint8Arrays on the receiving side, and instances of JavaScript classes will be cloned as plain JavaScript objects.

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This limitation extends to many built-in objects, such as the global URL object:

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M port.hasRef()

Added in: v18.1.0, v16.17.0

• Returns: boolean

If true, the MessagePort object will keep the Node.js event loop active.

M port.ref()

Added in: v10.5.0

Opposite of unref(). Calling ref() on a previously unref()ed port does 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 has no effect.

If listeners are attached or removed using .on('message'), the port is ref()ed and unref()ed automatically depending on whether listeners for the event exist.

M port.start()

Added in: v10.5.0

Starts receiving messages on this MessagePort. When using this port as an event emitter, this is 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 automatically calls .start(), but unsetting it lets messages queue up until a new handler is set or the port is discarded.

M port.unref()

Added in: v10.5.0

Calling unref() on a port allows 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 has no effect.

If listeners are attached or removed using .on('message'), the port is ref()ed and unref()ed automatically depending on whether listeners for the event exist.

C Worker

Added in: 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 streams may be redirected by the parent thread.
• The require('node:worker_threads').isMainThread property is set to false.
• The require('node: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 copy of the parent thread's environment variables, unless otherwise specified. Changes to one copy are not visible in other threads, and are not visible to native add-ons (unless worker.SHARE_ENV is passed as the env option to the Worker constructor).
• process.title cannot be modified.
• Signals are not 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 Workers is possible.

Like Web Workers and the node:cluster module, two-way communication can be achieved through inter-thread message passing. Internally, a Worker has a built-in pair of MessagePorts 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 MessagePorts 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.

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M 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 WHATWG URL object using file: or data: protocol. When using a data: URL, the data is interpreted based on MIME type using the ECMAScript module loader. If options.eval is true, this is a string containing JavaScript code rather than a path.
• options Object
  • argv any[] List of arguments which would be stringified and appended to process.argv in the worker. This is mostly similar to the workerData but the values are available on the global process.argv as if they were passed as CLI options to the script.
  • env Object If set, specifies the initial value of process.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's process.env object affect the other thread as well. Default: process.env.
  • eval boolean If true and the first argument is a string, 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 is provided as process.execArgv inside the worker. By default, options are inherited from the parent thread.
  • stdin boolean If this is set to true, then worker.stdin provides a writable stream whose contents appear as process.stdin inside the Worker. By default, no data is provided.
  • stdout boolean If this is set to true, then worker.stdout is not automatically piped through to process.stdout in the parent.
  • stderr boolean If this is set to true, then worker.stderr is not automatically piped through to process.stderr in the parent.
  • workerData any Any JavaScript value that is cloned and made available as require('node:worker_threads').workerData. The cloning occurs as described in the HTML structured clone algorithm, and an error is thrown if the object cannot be cloned (e.g. because it contains functions).
  • trackUnmanagedFds boolean If this is set to true, then the Worker tracks raw file descriptors managed through fs.open() and fs.close(), and closes them when the Worker exits, similar to other resources like network sockets or file descriptors managed through the FileHandle API. This option is automatically inherited by all nested Workers. Default: true.
  • transferList Object[] If one or more MessagePort-like objects are passed in workerData, a transferList is required for those items or ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST is thrown. See port.postMessage() for more information.
  • resourceLimits Object An optional set of resource limits for the new JS engine instance. Reaching these limits leads to termination of the Worker instance. These limits only affect the JS engine, and no external data, including no ArrayBuffers. Even if these limits are set, the process may still abort if it encounters a global out-of-memory situation.
    • maxOldGenerationSizeMb number The maximum size of the main heap in MB. If the command-line argument --max-old-space-size is set, it overrides this setting.
    • maxYoungGenerationSizeMb number The maximum size of a heap space for recently created objects. If the command-line argument --max-semi-space-size is set, it overrides this setting.
    • codeRangeSizeMb number The size of a pre-allocated memory range used for generated code.
    • stackSizeMb number The default maximum stack size for the thread. Small values may lead to unusable Worker instances. Default: 4.

E 'error'

Added in: v10.5.0

• err Error

The 'error' event is emitted if the worker thread throws an uncaught exception. In that case, the worker is terminated.

E 'exit'

Added in: 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 is the passed exit code. If the worker was terminated, the exitCode parameter is 1.

This is the final event emitted by any Worker instance.

E 'message'

Added in: v10.5.0

• value any The transmitted value

The 'message' event is emitted when the worker thread has invoked require('node:worker_threads').parentPort.postMessage(). See the port.on('message') event for more details.

All messages sent from the worker thread are emitted before the 'exit' event is emitted on the Worker object.

E 'messageerror'

Added in: v14.5.0, v12.19.0

• error Error An Error object

The 'messageerror' event is emitted when deserializing a message failed.

E 'online'

Added in: v10.5.0

The 'online' event is emitted when the worker thread has started executing JavaScript code.

M worker.getHeapSnapshot([options])

• options Object
  • exposeInternals boolean If true, expose internals in the heap snapshot. Default: false.
  • exposeNumericValues boolean If true, expose numeric values in artificial fields. Default: false.
• 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 is rejected immediately with an ERR_WORKER_NOT_RUNNING error.

M worker.performance

Added in: v15.1.0, v14.17.0, v12.22.0

An object that can be used to query performance information from a worker instance. Similar to perf_hooks.performance.

M performance.eventLoopUtilization([utilization1[, utilization2]])

Added in: v15.1.0, v14.17.0, v12.22.0

• utilization1 Object The result of a previous call to eventLoopUtilization().
• utilization2 Object The result of a previous call to eventLoopUtilization() prior to utilization1.
• Returns Object
  • idle number
  • active number
  • utilization number

The same call as perf_hooks eventLoopUtilization(), except the values of the worker instance are returned.

One difference is that, unlike the main thread, bootstrapping within a worker is done within the event loop. So the event loop utilization is immediately available once the worker's script begins execution.

An idle time that does not increase does not indicate that the worker is stuck in bootstrap. The following examples shows how the worker's entire lifetime never accumulates any idle time, but is still be able to process messages.

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The event loop utilization of a worker is available only after the 'online' event emitted, and if called before this, or after the 'exit' event, then all properties have the value of 0.

M worker.postMessage(value[, transferList])

Added in: v10.5.0

• value any
• transferList Object[]

Send a message to the worker that is received via require('node:worker_threads').parentPort.on('message'). See port.postMessage() for more details.

M worker.ref()

Added in: v10.5.0

Opposite of unref(), calling ref() on a previously unref()ed worker does 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 has no effect.

M worker.resourceLimits

Added in: v13.2.0, v12.16.0

• Object
  • maxYoungGenerationSizeMb number
  • maxOldGenerationSizeMb number
  • codeRangeSizeMb number
  • stackSizeMb number

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.

M worker.stderr

Added in: 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 is piped to the parent thread's process.stderr stream.

M worker.stdin

Added in: 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.

M worker.stdout

Added in: 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 is piped to the parent thread's process.stdout stream.

M 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.

M worker.threadId

Added in: v10.5.0

• integer

An integer identifier for the referenced thread. Inside the worker thread, it is available as require('node:worker_threads').threadId. This value is unique for each Worker instance inside a single process.

M worker.unref()

Added in: v10.5.0

Calling unref() on a worker allows 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 has no effect.

Notes

Synchronous blocking of stdio

Workers utilize message passing via MessagePort to implement interactions with stdio. This means that stdio output originating from a Worker can get blocked by synchronous code on the receiving end that is blocking the Node.js event loop.

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Launching worker threads from preload scripts

Take care when launching worker threads from preload scripts (scripts loaded and run using the -r command line flag). Unless the execArgv option is explicitly set, new Worker threads automatically inherit the command line flags from the running process and will preload the same preload scripts as the main thread. If the preload script unconditionally launches a worker thread, every thread spawned will spawn another until the application crashes.