Child Processes in Node.js

In Node.js, child processes allow us to execute external programs or scripts from within our Node.js application. This is useful for various tasks, such as:

• Running system commands: Executing shell commands like ls, pwd, or custom scripts.
• Spawning external processes: Creating new processes to handle specific tasks, such as running a web server or a database.
• Forking the current process: Creating a new process that shares the same memory space as the parent process.

Node.js provides three primary methods to create child processes:

1. child_process.spawn()

• Purpose: Spawns a new process, providing fine-grained control over input and output streams.
• Usage:

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

const childProcess = spawn('python', ['script.py']);

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

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

childProcess.on('close', (code) => {
    console.log(`child process exited with code ${code}`);
});

Explanation:

• spawn() takes the command to execute and an array of arguments as input.
• It returns a ChildProcess object, which provides access to the child process’s input, output, and error streams.
• We can listen to events like 'data' and 'close' to handle the child process’s output and termination.

2. child_process.exec()

• Purpose: Executes a command and captures its output as a single string.
• Usage:

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

exec('ls -la', (error, stdout, stderr) => {
    if (error) {
        console.error(`error: ${error.message}`);
        return;
    }
    if (stderr) {
        console.error(`stderr: ${stderr}`);
        return;
    }
    console.log(`stdout: ${stdout}`);
});

Explanation:

• exec() takes the command to execute and a callback function as input.
• The callback function receives three arguments: error, stdout, and stderr.
• This method is simpler to use than spawn() when we only need to capture the output of a command.

3. child_process.fork()

• Purpose: Creates a new Node.js process that shares the same memory space as the parent process.
• Usage:

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

const childProcess = fork('child.js', ['arg1', 'arg2']);

childProcess.on('message', (message) => {
    console.log(`Message from child: ${message}`);
});

childProcess.send({ foo: 'bar' });

Explanation:

• fork() is primarily used for inter-process communication (IPC) between Node.js processes.
• The child process can access the parent process’s modules and variables.
• Both the parent and child processes can use the process.send() and process.on('message') methods to communicate.

Choosing the Right Method:

• spawn(): Use when we need fine-grained control over the child process’s input and output streams.
• exec(): Use when we want to execute a simple command and capture its output.
• fork(): Use for inter-process communication between Node.js processes.

By understanding these methods, we can effectively leverage child processes to enhance the capabilities of our Node.js applications.