Blue-Green Deployments Explained

What is Blue-Green Deployment?

You maintain two identical production environments:

• Blue — currently serving live traffic
• Green — idle, ready for the next deployment

         ┌──── Blue (v1.0) ← Live traffic
Router ──┤
         └──── Green (v1.1) ← Deploy here, test, then switch

How It Works

1. Deploy new version to Green (Blue still serves traffic)
2. Run smoke tests against Green
3. Switch the router to point to Green
4. Green is now live, Blue is idle
5. If anything goes wrong, switch back to Blue (instant rollback)

Implementation with Nginx

# /etc/nginx/conf.d/upstream.conf
upstream app_blue {
    server 127.0.0.1:3001;
}

upstream app_green {
    server 127.0.0.1:3002;
}

# Active environment (change this to switch)
upstream app_active {
    server 127.0.0.1:3001;  # Blue is active
}

server {
    listen 443 ssl;
    server_name example.com;

    location / {
        proxy_pass http://app_active;
    }
}

Switching

# Switch to Green
sed -i 's/3001/3002/' /etc/nginx/conf.d/upstream.conf
sudo nginx -t && sudo nginx -s reload

# Rollback to Blue
sed -i 's/3002/3001/' /etc/nginx/conf.d/upstream.conf
sudo nginx -t && sudo nginx -s reload

Deployment Script

#!/bin/bash
# deploy.sh

ACTIVE=$(grep -oP 'server 127.0.0.1:\K\d+' /etc/nginx/conf.d/upstream.conf)
if [ "$ACTIVE" = "3001" ]; then
    DEPLOY_PORT=3002
    DEPLOY_ENV="green"
else
    DEPLOY_PORT=3001
    DEPLOY_ENV="blue"
fi

echo "Deploying to $DEPLOY_ENV (port $DEPLOY_PORT)..."

# Deploy new version
cd /var/www/$DEPLOY_ENV
git pull
npm ci --production
npm run build
pm2 restart $DEPLOY_ENV

# Wait for health check
sleep 5
HTTP_CODE=$(curl -s -o /dev/null -w "%{http_code}" http://127.0.0.1:$DEPLOY_PORT/health)
if [ "$HTTP_CODE" != "200" ]; then
    echo "Health check failed! Aborting."
    exit 1
fi

# Switch traffic
sed -i "s/server 127.0.0.1:$ACTIVE/server 127.0.0.1:$DEPLOY_PORT/" /etc/nginx/conf.d/upstream.conf
sudo nginx -t && sudo nginx -s reload

echo "Switched to $DEPLOY_ENV. Old environment available for rollback."

Blue-Green vs Other Strategies

Tip Blue-green deployments are the simplest zero-downtime strategy. The trade-off is running two environments, but on a VPS this just means two process instances on different ports.