HAProxy Load Balancer: Installation, Health Checks, and Sticky Sessions

# Ubuntu / Debian
sudo apt update && sudo apt install -y haproxy

# CentOS / RHEL
sudo yum install -y haproxy

# Check version
haproxy -v

# Start the service
sudo systemctl enable --now haproxy

/etc/haproxy/haproxy.cfg

global
    log /dev/log local0
    maxconn 50000
    user haproxy
    group haproxy
    daemon

defaults
    log     global
    mode    http
    option  httplog
    option  dontlognull
    timeout connect 5s
    timeout client  30s
    timeout server  30s
    option  forwardfor        # Add X-Forwarded-For header
    option  http-server-close

# Frontend (accept client connections)
frontend web_front
    bind *:80
    bind *:443 ssl crt /etc/ssl/mysite.pem
    redirect scheme https if !{ ssl_fc }   # HTTP → HTTPS redirect
    default_backend web_back

# Backend (server pool)
backend web_back
    balance roundrobin            # Algorithm: roundrobin, leastconn, source
    option  httpchk GET /health   # Health check path

    server web1 192.168.1.10:3000 check weight 1
    server web2 192.168.1.11:3000 check weight 1
    server web3 192.168.1.12:3000 check weight 2  # 2x weight

backend api_back
    option httpchk GET /api/health HTTP/1.1\r\nHost:\ api.example.com
    http-check expect status 200

    server api1 10.0.0.1:8080 check inter 2s rise 2 fall 3
    server api2 10.0.0.2:8080 check inter 2s rise 2 fall 3
    # inter: check interval, rise: successes to mark up, fall: failures to mark down

    # Backup server (used when all servers are down)
    server backup 10.0.0.99:8080 check backup

backend app_back
    balance leastconn
    cookie SERVERID insert indirect nocache   # Cookie-based sticky sessions

    server app1 10.0.0.1:3000 check cookie app1
    server app2 10.0.0.2:3000 check cookie app2

frontend stats
    bind *:8404
    stats enable
    stats uri /stats
    stats refresh 10s
    stats auth admin:StrongPass123   # Basic authentication
    stats show-legends

# Check configuration syntax
sudo haproxy -c -f /etc/haproxy/haproxy.cfg

# Zero-downtime reload (keeps existing connections)
sudo systemctl reload haproxy

# Check status
sudo systemctl status haproxy