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As Node.js runs in a single thread mode but it uses an event-driven paradigm to handle concurrency. It also facilitates creation of child processes to leverage parallel processing on multi-core cpu based systems.

Child processes always have three streams child.stdin, child.stdout, and child.stderr which may be shared with the stdio streams of the parent process.

Node provides child_process module which has following three major ways to create child process.

exec - child_process.exec method runs a command in a shell/console and buffers the output.
spawn - child_process.spawn launches a new process with a given command
fork - The child_process.fork method is a special case of the spawn() to create child processes.

The exec() method

child_process.exec method runs a command in a shell and buffers the output. It has the following signature:

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

Parameters

Here is the description of the parameters used:

command String The command to run, with space-separated arguments
options Object may comprise one or more of the following options:
- cwd String Current working directory of the child process
- env Object Environment key-value pairs
- encoding String (Default: 'utf8')
- shell String Shell to execute the command with (Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows, The shell should understand the -c switch on UNIX or /s /c on Windows. On Windows, command line parsing should be compatible with cmd.exe.)
- timeout Number (Default: 0)
- maxBuffer Number (Default: 200*1024)
- killSignal String (Default: 'SIGTERM')
- uid Number Sets the user identity of the process.
- gid Number Sets the group identity of the process.
callback Function gets three arguments error, stdout and stderr which is called with the following output when process terminates

The exec() method returns a buffer with a max size and waits for the process to end and tries to return all the buffered data at once.

Example

Let us create two js file named support.js and master.js:

File: support.js

console.log("Child Process " + process.argv[2] + " executed." );

File: master.js

const fs = require('fs');
const child_process = require('child_process');

for(var i=0; i<3; i++) {
   var workerProcess = child_process.exec('node support.js '+i,
      function (error, stdout, stderr) {
         if (error) {
            console.log(error.stack);
            console.log('Error code: '+error.code);
            console.log('Signal received: '+error.signal);
         }
         console.log('stdout: ' + stdout);
         console.log('stderr: ' + stderr);
      });

      workerProcess.on('exit', function (code) {
      console.log('Child process exited with exit code '+code);
   });
}

Now run the master.js to see the result:

$ node master.js

Verify the Output. Server has started

Child process exited with exit code 0
stdout: Child Process 1 executed.

stderr:
Child process exited with exit code 0
stdout: Child Process 0 executed.

stderr:
Child process exited with exit code 0
stdout: Child Process 2 executed.

The spawn() method

child_process.spawn method launches a new process with a given command. It has the following signature:

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

Parameters

Here is the description of the parameters used:

command String The command to run
args Array List of string arguments
options Object may comprise one or more of the following options:
- cwd String Current working directory of the child process
- env Object Environment key-value pairs
- stdio Array|String Child's stdio configuration
- customFds Array Deprecated File descriptors for the child to use for stdio
- detached Boolean The child will be a process group leader
- uid Number Sets the user identity of the process.
- gid Number Sets the group identity of the process.

The spawn() method returns streams (stdout & stderr) and it should be used when the process returns large amount of data. spawn() starts receiving the response as soon as the process starts executing.

Example

Create two js file named support.js and master.js:

File: support.js

console.log("Child Process " + process.argv[2] + " executed." );

File: master.js

const fs = require('fs');
const child_process = require('child_process');
 
for(var i=0; i<3; i++) {
   var workerProcess = child_process.spawn('node', ['support.js', i]);

   workerProcess.stdout.on('data', function (data) {
      console.log('stdout: ' + data);
   });

   workerProcess.stderr.on('data', function (data) {
      console.log('stderr: ' + data);
   });

   workerProcess.on('close', function (code) {
      console.log('child process exited with code ' + code);
   });
}

Now run the master.js to see the result:

$ node master.js

Verify the Output. Server has started

stdout: Child Process 0 executed.

child process exited with code 0
stdout: Child Process 1 executed.

stdout: Child Process 2 executed.

child process exited with code 0
child process exited with code 0

The fork method

child_process.fork method is a special case of the spawn() to create Node processes. It has the following signature

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

Parameters

Here is the description of the parameters used:

modulePath String The module to run in the child
args Array List of string arguments
options Object may comprise one or more of the following options:
- cwd String Current working directory of the child process
- env Object Environment key-value pairs
- execPath String Executable used to create the child process
- execArgv Array List of string arguments passed to the executable (Default: process.execArgv)
- silent Boolean If true, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the "pipe" and "inherit" options for spawn()'s stdio for more details (default is false)
- uid Number Sets the user identity of the process.
- gid Number Sets the group identity of the process.

The fork method returns object with a built-in communication channel in addition to having all the methods in a normal ChildProcess instance.

Example

Create two js file named support.js and master.js:

File: support.js

console.log("Child Process " + process.argv[2] + " executed." );

File: master.js

const fs = require('fs');
const child_process = require('child_process');
 
for(var i=0; i<3; i++) {
   var worker_process = child_process.fork("support.js", [i]);	

   worker_process.on('close', function (code) {
      console.log('child process exited with code ' + code);
   });
}

Now run the master.js to see the result:

$ node master.js

Verify the Output. Server has started

Child Process 0 executed.
Child Process 1 executed.
Child Process 2 executed.
child process exited with code 0
child process exited with code 0
child process exited with code 0

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