Introduction to Node.js child processes and applications

This article mainly introduces a brief discussion of Node.js sub-processes and application scenarios. The content is quite good. I will share it with you now and give it as a reference.

Background

Since ons (Alibaba Cloud RocketMQ package) is encapsulated based on C++, it does not support the instantiation of multiple producers and consumers in a single process. Or, in order to solve this problem, Node.js child process is used.

Pits encountered during use

Release: After process management closes the main process, the child process becomes an operating system process (pid is 1)

Several solutions

Treat the child process as an independent running process, record the pid, and process management to close the main process and close the child process at the same time when publishing

The main process listens to the shutdown event and actively closes its own child processes

Type of child process

1. spawn: Execute command

2. exec: execute command (new shell)

3. execFile: execute file

4. fork: execute file

Common events for child processes

1. exit

2. close

3. error

4. message

There is a difference between close and exit. close is when the data stream is closed. The event is triggered when the child process exits. exit is the event that is triggered when the child process exits. Because multiple child processes can share the same data stream, the close event may not necessarily be triggered when a child process exits, because there are other child processes using the data stream at this time.

Subprocess data flow

1. stdin

2. stdout

3. stderr

#Because the main process is the starting point, the data flow of the sub-process is opposite to the conventionally understood data flow direction. stdin: write stream, stdout, stderr: Read stream.

spawn

spawn(command[, args][, options])

Execute a command and return various execution results through the data data stream.

Basic usage

const { spawn } = require('child_process');

const child = spawn('find', [ '.', '-type', 'f' ]);
child.stdout.on('data', (data) => {
  console.log(`child stdout:\n${data}`);
});

child.stderr.on('data', (data) => {
  console.error(`child stderr:\n${data}`);
});

child.on('exit', (code, signal) => {
  console.log(`child process exit with: code $[code], signal: ${signal}`);
});

Common parameters

{
  cwd: String,
  env: Object,
  stdio: Array | String,
  detached: Boolean,
  shell: Boolean,
  uid: Number,
  gid: Number
}

Focus on the detached attribute. Setting detached to true is to prepare the child process to run independently. The specific behavior of the child process is related to the operating system. Different systems behave differently. The Windows system child process will have its own console window, and the POSIX system child process will become the new process group and session leader.

At this time, the child process is not completely independent. The running results of the child process will be displayed on the data stream set by the main process, and the exit of the main process will affect the operation of the child process. When stdio is set to ignore and child.unref(); is called, the child process begins to truly run independently, and the main process can exit independently.

#exec

exec(command[, options][, callback])

Execute a command and return the result through the callback parameter. The part will be cached before the command is executed. results into system memory.

const { exec } = require('child_process');

exec('find . -type f | wc -l', (err, stdout, stderr) => {
  if (err) {
    console.error(`exec error: ${err}`);
    return;
  }

  console.log(`Number of files ${stdout}`);
});

The best of both worlds - spawn instead of exec

Since the result of exec is returned once, before returning Cache in memory, so when the output of the executed shell command is too large, using exec to execute the command cannot complete our work as expected. At this time, we can use spawn instead of exec to execute the shell command.

const { spawn } = require('child_process');

const child = spawn('find . -type f | wc -l', {
  stdio: 'inherit',
  shell: true
});

child.stdout.on('data', (data) => {
  console.log(`child stdout:\n${data}`);
});

child.stderr.on('data', (data) => {
  console.error(`child stderr:\n${data}`);
});

child.on('exit', (code, signal) => {
  console.log(`child process exit with: code $[code], signal: ${signal}`);
});

execFile

child_process.execFile(file[, args][, options][, callback])

Execute a file

The function is basically the same as exec. The difference is that it executes a script file with a given path and directly creates a new process instead of creating a shell environment and then running the script, which is relatively lightweight and more efficient. . However, in Windows systems, files such as .cmd and .bat cannot be run directly. This means that execFile will not work. You can use spawn and exec instead.

#fork

child_process.fork(modulePath[, args][, options])

Execute a Node.js file

// parent.js

const { fork } = require('child_process');

const child = fork('child.js');

child.on('message', (msg) => {
  console.log('Message from child', msg);
});

child.send({ hello: 'world' });

// child.js

process.on('message', (msg) => {
  console.log('Message from parent:', msg);
});

let counter = 0;

setInterval(() => {
  process.send({ counter: counter++ });
}, 3000);

fork is actually a special form of spawn, which fixes the spawn Node.js process and establishes a communication channel between the master and child processes to allow the master and child processes to Event-based communication can be done using the process module.

Subprocess usage scenarios

1. Computationally intensive systems

2. Front-end build tools utilize multi-core CPUs Parallel computing, improve construction efficiency

3. Process management tools, such as: some functions in PM2

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