13. Child Processes: spawn vs exec vs execFile vs fork • Node.js Backend Architecture: Libuv to High-Scale Microservices • Rishabh Ki Kaksha

Jab Node.js ko OS-level shell commands (FFmpeg video transcoding, Python ML script execution, Git automation) run karni hoti hain — **`child_process` module naya independent OS process spawn karta hai!**

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### 2. The 4 Spawn Methods Compared

#### A. `spawn()`: Real-time Streaming (Big Tech Standard ⚡)
Data streams me aata hai (`stdout.on('data')`); memory buffer limit nahi hoti.
```javascript
const { spawn } = require('child_process');
const ls = spawn('ls', ['-lh', '/var/log']);

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

#### B. `exec()`: Buffered Shell Execution (Dangerous if output > 1MB)
Spawns a shell (`/bin/sh` or `cmd.exe`) and buffers entire output in RAM.
```javascript
const { exec } = require('child_process');
exec('git status', { maxBuffer: 1024 * 1024 }, (err, stdout) => {
    console.log(stdout);
});
```

#### C. `execFile()`: Direct Binary Execution (No Shell Injection!)
Direct executable binary run karta hai bina shell spawn kiye (Faster & Secure).

#### D. `fork()`: Node.js to Node.js IPC (Inter-Process Communication) 👑
Naya VDOM/V8 instance spawn karta hai aur direct **`process.send()`** message channel provide karta hai!

```javascript
// parent.js
const { fork } = require('child_process');
const child = fork('./heavy_computation.js');

child.send({ start: 1, end: 10000000 });
child.on('message', (result) => console.log('Computation Result:', result));

// heavy_computation.js
process.on('message', (msg) => {
    let sum = 0;
    for (let i = msg.start; i <= msg.end; i++) sum += i;
    process.send(sum);
});
```

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### 3. Master Comparison Matrix

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### 4. Interview Ke 3 Sawal Is Topic Se

1. **User input ke sath `exec()` call karna security hazard kyu hai?** -> Command Injection attack! Agar user input me `; rm -rf /` bhej de, toh `exec` shell me us injection ko execute kar dega (`execFile` is safe from injection).
2. **`fork()` aur `Worker Threads` me kya difference hai?** -> `fork()` poora naya independent OS process banata hai jiska apna V8 instance aur separate memory space hota hai; Worker Threads same process ke andar lightweight threads hote hain jo memory share kar sakte hain.
3. **Child process ke `close` aur `exit` events me kya fark hai?** -> `exit` tab fire hota hai jab process terminate hota hai; `close` tab fire hota hai jab process ke saare stdio streams completely close ho jaate hain.

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**Next Topic:** 14. Multi-Core Clustering: Node.js Cluster Module & Zero-Downtime Reloads

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Rishabh Gehlot

PHP 8+ & Laravel 11 Master Track