Cluster

自 v0.10.0 版本开始新增

源代码： lib/cluster.js

Clusters of Node.js processes can be used to run multiple instances of Node.js that can distribute workloads among their application threads. When process isolation is not needed, use the worker_threads module instead, which allows running multiple application threads within a single Node.js instance.

The cluster module allows easy creation of child processes that all share server ports.

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Running Node.js will now share port 8000 between the workers:

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On Windows, it is not yet possible to set up a named pipe server in a worker.

How it works

The worker processes are spawned using the child_process.fork() method, so that they can communicate with the parent via IPC and pass server handles back and forth.

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows) is the round-robin approach, where the primary process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the primary process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.

Because server.listen() hands off most of the work to the primary process, there are three cases where the behavior between a normal Node.js process and a cluster worker differs:

1. server.listen({fd: 7}) Because the message is passed to the primary, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.
2. server.listen(handle) Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the primary process.
3. server.listen(0) Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they do listen(0). In essence, the port is random the first time, but predictable thereafter. To listen on a unique port, generate a port number based on the cluster worker ID.

Node.js does not provide routing logic. It is therefore important to design an application such that it does not rely too heavily on in-memory data objects for things like sessions and login.

Because workers are all separate processes, they can be killed or re-spawned depending on a program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers, however. It is the application's responsibility to manage the worker pool based on its own needs.

Although a primary use case for the node:cluster module is networking, it can also be used for other use cases requiring worker processes.

C Worker

自 v0.7.0 版本开始新增

• Extends: EventEmitter

A Worker object contains all public information and method about a worker. In the primary it can be obtained using cluster.workers. In a worker it can be obtained using cluster.worker.

E 'disconnect'

自 v0.7.7 版本开始新增

Similar to the cluster.on('disconnect') event, but specific to this worker.

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E 'error'

自 v0.7.3 版本开始新增

This event is the same as the one provided by child_process.fork().

Within a worker, process.on('error') may also be used.

E 'exit'

自 v0.11.2 版本开始新增

• code number The exit code, if it exited normally.
• signal string The name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

Similar to the cluster.on('exit') event, but specific to this worker.

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E 'listening'

自 v0.7.0 版本开始新增

• address Object

Similar to the cluster.on('listening') event, but specific to this worker.

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It is not emitted in the worker.

E 'message'

自 v0.7.0 版本开始新增

• message Object
• handle undefined | Object

Similar to the 'message' event of cluster, but specific to this worker.

Within a worker, process.on('message') may also be used.

See process event: 'message'.

Here is an example using the message system. It keeps a count in the primary process of the number of HTTP requests received by the workers:

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E 'online'

自 v0.7.0 版本开始新增

Similar to the cluster.on('online') event, but specific to this worker.

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It is not emitted in the worker.

M worker.disconnect()

• Returns: cluster.Worker A reference to worker.

In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel.

In the primary, an internal message is sent to the worker causing it to call .disconnect() on itself.

Causes .exitedAfterDisconnect to be set.

After a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see server.close(), the IPC channel to the worker will close allowing it to die gracefully.

The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.

In a worker, process.disconnect exists, but it is not this function; it is disconnect().

Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the 'disconnect' event has not been emitted after some time.

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M worker.exitedAfterDisconnect

自 v6.0.0 版本开始新增

• boolean

This property is true if the worker exited due to .disconnect(). If the worker exited any other way, it is false. If the worker has not exited, it is undefined.

The boolean worker.exitedAfterDisconnect allows distinguishing between voluntary and accidental exit, the primary may choose not to respawn a worker based on this value.

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M worker.id

自 v0.8.0 版本开始新增

• integer

Each new worker is given its own unique id, this id is stored in the id.

While a worker is alive, this is the key that indexes it in cluster.workers.

M worker.isConnected()

自 v0.11.14 版本开始新增

This function returns true if the worker is connected to its primary via its IPC channel, false otherwise. A worker is connected to its primary after it has been created. It is disconnected after the 'disconnect' event is emitted.

M worker.isDead()

自 v0.11.14 版本开始新增

This function returns true if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns false.

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M worker.kill([signal])

自 v0.9.12 版本开始新增

• signal string Name of the kill signal to send to the worker process. Default: 'SIGTERM'

This function will kill the worker. In the primary worker, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by killing the process with signal.

The kill() function kills the worker process without waiting for a graceful disconnect, it has the same behavior as worker.process.kill().

This method is aliased as worker.destroy() for backwards compatibility.

In a worker, process.kill() exists, but it is not this function; it is kill().

M worker.process

自 v0.7.0 版本开始新增

• ChildProcess

All workers are created using child_process.fork(), the returned object from this function is stored as .process. In a worker, the global process is stored.

See: Child Process module.

Workers will call process.exit(0) if the 'disconnect' event occurs on process and .exitedAfterDisconnect is not true. This protects against accidental disconnection.

M worker.send(message[, sendHandle[, options]][, callback])

• message Object
• sendHandle Handle
• options Object The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties:
  • keepOpen boolean A value that can be used when passing instances of net.Socket. When true, the socket is kept open in the sending process. Default: false.
• callback Function
• Returns: boolean

Send a message to a worker or primary, optionally with a handle.

In the primary, this sends a message to a specific worker. It is identical to ChildProcess.send().

In a worker, this sends a message to the primary. It is identical to process.send().

This example will echo back all messages from the primary:

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E 'disconnect'

自 v0.7.9 版本开始新增

• worker cluster.Worker

Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with worker.disconnect()).

There may be a delay between the 'disconnect' and 'exit' events. These events can be used to detect if the process is stuck in a cleanup or if there are long-living connections.

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E 'exit'

自 v0.7.9 版本开始新增

• worker cluster.Worker
• code number The exit code, if it exited normally.
• signal string The name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

When any of the workers die the cluster module will emit the 'exit' event.

This can be used to restart the worker by calling .fork() again.

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See child_process event: 'exit'.

E 'fork'

自 v0.7.0 版本开始新增

• worker cluster.Worker

When a new worker is forked the cluster module will emit a 'fork' event. This can be used to log worker activity, and create a custom timeout.

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E 'listening'

自 v0.7.0 版本开始新增

• worker cluster.Worker
• address Object

After calling listen() from a worker, when the 'listening' event is emitted on the server, a 'listening' event will also be emitted on cluster in the primary.

The event handler is executed with two arguments, the worker contains the worker object and the address object contains the following connection properties: address, port, and addressType. This is very useful if the worker is listening on more than one address.

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The addressType is one of:

• 4 (TCPv4)
• 6 (TCPv6)
• -1 (Unix domain socket)
• 'udp4' or 'udp6' (UDPv4 or UDPv6)

E 'message'

• worker cluster.Worker
• message Object
• handle undefined | Object

Emitted when the cluster primary receives a message from any worker.

See child_process event: 'message'.

E 'online'

自 v0.7.0 版本开始新增

• worker cluster.Worker

After forking a new worker, the worker should respond with an online message. When the primary receives an online message it will emit this event. The difference between 'fork' and 'online' is that fork is emitted when the primary forks a worker, and 'online' is emitted when the worker is running.

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E 'setup'

自 v0.7.1 版本开始新增

• settings Object

Emitted every time .setupPrimary() is called.

The settings object is the cluster.settings object at the time .setupPrimary() was called and is advisory only, since multiple calls to .setupPrimary() can be made in a single tick.

If accuracy is important, use cluster.settings.

M cluster.disconnect([callback])

自 v0.7.7 版本开始新增

• callback Function Called when all workers are disconnected and handles are closed.

Calls .disconnect() on each worker in cluster.workers.

When they are disconnected all internal handles will be closed, allowing the primary process to die gracefully if no other event is waiting.

The method takes an optional callback argument which will be called when finished.

This can only be called from the primary process.

M cluster.fork([env])

自 v0.6.0 版本开始新增

• env Object Key/value pairs to add to worker process environment.
• Returns: cluster.Worker

Spawn a new worker process.

This can only be called from the primary process.

M cluster.isMaster

自 v16.0.0 版本开始弃用

Deprecated alias for cluster.isPrimary.

M cluster.isPrimary

自 v16.0.0 版本开始新增

• boolean

True if the process is a primary. This is determined by the process.env.NODE_UNIQUE_ID. If process.env.NODE_UNIQUE_ID is undefined, then isPrimary is true.

M cluster.isWorker

自 v0.6.0 版本开始新增

• boolean

True if the process is not a primary (it is the negation of cluster.isPrimary).

M cluster.schedulingPolicy

自 v0.11.2 版本开始新增

The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once either the first worker is spawned, or .setupPrimary() is called, whichever comes first.

SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.

cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are 'rr' and 'none'.

M cluster.settings

• Object
  • execArgv string[] List of string arguments passed to the Node.js executable. Default: process.execArgv.
  • exec string File path to worker file. Default: process.argv[1].
  • args string[] String arguments passed to worker. Default: process.argv.slice(2).
  • cwd string Current working directory of the worker process. Default:undefined (inherits from parent process).
  • serialization string Specify the kind of serialization used for sending messages between processes. Possible values are 'json' and 'advanced'. See Advanced serialization for child_process for more details. Default: false.
  • silent boolean Whether or not to send output to parent's stdio. Default: false.
  • stdio Array Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an 'ipc' entry. When this option is provided, it overrides silent.
  • uid number Sets the user identity of the process. (See setuid(2).)
  • gid number Sets the group identity of the process. (See setgid(2).)
  • inspectPort number | Function Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the primary's process.debugPort.
  • windowsHide boolean Hide the forked processes console window that would normally be created on Windows systems. Default: false.

After calling .setupPrimary() (or .fork()) this settings object will contain the settings, including the default values.

This object is not intended to be changed or set manually.

M cluster.setupMaster([settings])

Deprecated alias for .setupPrimary().

M cluster.setupPrimary([settings])

自 v16.0.0 版本开始新增

• settings Object See cluster.settings.

setupPrimary is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings.

Any settings changes only affect future calls to .fork() and have no effect on workers that are already running.

The only attribute of a worker that cannot be set via .setupPrimary() is the env passed to .fork().

The defaults above apply to the first call only; the defaults for later calls are the current values at the time of cluster.setupPrimary() is called.

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This can only be called from the primary process.

M cluster.worker

自 v0.7.0 版本开始新增

• Object

A reference to the current worker object. Not available in the primary process.

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M cluster.workers

自 v0.7.0 版本开始新增

• Object

A hash that stores the active worker objects, keyed by id field. This makes it easy to loop through all the workers. It is only available in the primary process.

A worker is removed from cluster.workers after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the cluster.workers list happens before the last 'disconnect' or 'exit' event is emitted.

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