Perfect 👍 Below is a FINAL, CLEAN README.md in pure GitHub Markdown format.

👉 You can directly copy–paste this exactly as it is into your GitHub repo 👉 No extra explanation, no chat text, no formatting issues 👉 This is production-ready + interview-ready

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# 🚀 Blue-Green Deployment Using Nginx (Zero Downtime)

This repository demonstrates a **zero-downtime Blue-Green deployment strategy** using **Nginx**, **PM2**, and **Node.js**, where traffic is seamlessly switched between two application versions without stopping the service.

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## 📌 Objective

- Run **two versions of the same API** (Blue & Green) in parallel
- Switch live traffic **without downtime**
- No application restart during deployment
- Traffic switches using **Nginx reload**
- Instant rollback capability

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## 🧠 Architecture Overview

Client | v Nginx (Reverse Proxy - Port 8080) | |----> Green App (Port 5000) | |----> Blue App (Port 5001)

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## 🛠️ Tech Stack

| Component | Purpose |
|--------|--------|
| Node.js | Backend API |
| PM2 | Process manager |
| Nginx | Reverse proxy & traffic switch |
| systemd | Reload hook automation |

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## 📁 Project Structure

blue-green-poc/ ├── apps/ │ ├── app-green.js │ └── app-blue.js ├── ecosystem.config.js └── README.md

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## ⚙️ Prerequisites

- Ubuntu/Linux VM
- sudo access
- Node.js (via NVM recommended)
- PM2
- Nginx

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## 🔧 Setup Instructions

### 1️⃣ Install Node.js (NVM)

```bash
curl -fsSL https://raw.githubusercontent.com/nvm-sh/nvm/v0.39.7/install.sh | bash
source ~/.bashrc
nvm install --lts

Verify:

node -v
npm -v

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2️⃣ Install PM2

npm install -g pm2
pm2 -v

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3️⃣ Install Nginx

sudo apt update
sudo apt install -y nginx

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🧩 Application Setup

GREEN App (Port 5000)

const express = require("express");
const app = express();

app.get("/hello", (req, res) => {
  res.json({ message: "Hello from GREEN (5000)" });
});

app.listen(5000);

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BLUE App (Port 5001)

const express = require("express");
const app = express();

app.get("/hello", (req, res) => {
  res.json({ message: "Hello from BLUE (5001)" });
});

app.listen(5001);

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Install Dependencies

npm init -y
npm install express

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⚙️ PM2 Configuration

module.exports = {
  apps: [
    { name: "hello-green", script: "./apps/app-green.js" },
    { name: "hello-blue", script: "./apps/app-blue.js" }
  ]
};

Start applications:

pm2 start ecosystem.config.js
pm2 list

Test:

curl http://localhost:5000/hello
curl http://localhost:5001/hello

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🌐 Nginx Configuration

Disable Default Site

sudo rm /etc/nginx/sites-enabled/default

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Create State Directory

sudo mkdir -p /etc/nginx/state

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Active Backend File

sudo nano /etc/nginx/state/backend_upstream.conf

server 127.0.0.1:5000;

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Upstream Config

sudo nano /etc/nginx/conf.d/backend.conf

upstream backend_app {
    include /etc/nginx/state/backend_upstream.conf;
}

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Server Config

sudo nano /etc/nginx/sites-available/blue-green.conf

server {
    listen 8080;
    server_name _;

    location / {
        proxy_pass http://backend_app;
        proxy_http_version 1.1;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
    }
}

Enable site:

sudo ln -s /etc/nginx/sites-available/blue-green.conf /etc/nginx/sites-enabled/
sudo nginx -t
sudo systemctl restart nginx

Test:

curl http://localhost:8080/hello

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🔁 Zero-Downtime Deployment Demo

Continuous Traffic Simulation

while true; do curl -s http://localhost:8080/hello; sleep 0.5; done

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Manual Traffic Switch

sudo nano /etc/nginx/state/backend_upstream.conf

Change to:

server 127.0.0.1:5001;

Reload Nginx:

sudo nginx -s reload

✔ Traffic switches with zero downtime

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🤖 Automatic Switch on Nginx Reload (Production Style)

Switch Script

sudo nano /usr/local/bin/nginx-blue-green-switch.sh

#!/bin/bash

STATE="/etc/nginx/state/active_port"
UPSTREAM="/etc/nginx/state/backend_upstream.conf"

CURRENT=$(cat $STATE)

if [ "$CURRENT" = "5000" ]; then
  echo "5001" > $STATE
  echo "server 127.0.0.1:5001;" > $UPSTREAM
else
  echo "5000" > $STATE
  echo "server 127.0.0.1:5000;" > $UPSTREAM
fi

sudo chmod +x /usr/local/bin/nginx-blue-green-switch.sh

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Hook Script into Nginx Reload

sudo systemctl edit nginx

[Service]
ExecReload=
ExecReload=/usr/local/bin/nginx-blue-green-switch.sh
ExecReload=/usr/sbin/nginx -s reload

Apply changes:

sudo systemctl daemon-reexec

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🔥 Final Demo

sudo systemctl reload nginx

✔ Automatic traffic switch ✔ Zero downtime ✔ No failed requests

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🔄 Rollback

Rollback is instant:

sudo systemctl reload nginx

🎯 One-Line Summary

Two application versions run in parallel and Nginx reload switches traffic instantly with zero downtime.