🔥 1️⃣ What is HAProxy?

HAProxy = High Availability Proxy

It works as:

• Layer 4 (TCP) load balancer
• Layer 7 (HTTP) load balancer
• Reverse proxy
• SSL terminator

Used in:

• SaaS platforms
• Fintech systems
• Large-scale web apps

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🔥 2️⃣ Install HAProxy (Local)

🟢 Windows (Docker Recommended)

docker run -d -p 80:80 haproxy

🟢 Ubuntu

sudo apt update
sudo apt install haproxy

Config file location:

/etc/haproxy/haproxy.cfg

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🔥 3️⃣ Basic HAProxy Config Structure

Main config file:

haproxy.cfg

Structure:

global
defaults
frontend
backend
listen

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🔥 4️⃣ Simple Reverse Proxy Example

Goal:

Client → HAProxy → Backend (port 3000)

haproxy.cfg

global
    daemon
    maxconn 256

defaults
    mode http
    timeout connect 5s
    timeout client 50s
    timeout server 50s

frontend http_front
    bind *:80
    default_backend app_servers

backend app_servers
    server app1 127.0.0.1:3000

Restart:

sudo systemctl restart haproxy

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🔥 5️⃣ Load Balancing (Multiple Servers)

Client → HAProxy → app1
                     app2

backend app_servers
    balance roundrobin
    server app1 127.0.0.1:3000 check
    server app2 127.0.0.1:3001 check

Algorithms:

• roundrobin
• leastconn
• source
• uri

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🔥 6️⃣ Health Checks

Notice:

check

HAProxy checks if backend is alive.

If one fails:

• Traffic automatically stops to that server.

You can customize:

server app1 127.0.0.1:3000 check inter 5s fall 3 rise 2

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🔥 7️⃣ SSL Termination

First generate certificate:

cat cert.pem key.pem > haproxy.pem

Config:

frontend https_front
    bind *:443 ssl crt /etc/ssl/haproxy.pem
    default_backend app_servers

Now HAProxy handles HTTPS.

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🔥 8️⃣ Sticky Sessions (Important for Stateful Apps)

Used when:

• Sessions stored in memory
• Need same user → same backend

backend app_servers
    balance roundrobin
    cookie SERVERID insert indirect nocache
    server app1 127.0.0.1:3000 check cookie s1
    server app2 127.0.0.1:3001 check cookie s2

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🔥 9️⃣ HAProxy Stats Dashboard

Add:

listen stats
    bind *:8404
    stats enable
    stats uri /
    stats refresh 10s

Access:

http://localhost:8404

Shows live backend status.

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🔥 10️⃣ HAProxy With Docker Compose

docker-compose.yml

services:
  backend1:
    image: nginx
    expose:
      - "80"

  backend2:
    image: nginx
    expose:
      - "80"

  haproxy:
    image: haproxy:latest
    ports:
      - "80:80"
      - "8404:8404"
    volumes:
      - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg

haproxy.cfg

global
    daemon

defaults
    mode http

frontend http_front
    bind *:80
    default_backend web

backend web
    balance roundrobin
    server web1 backend1:80 check
    server web2 backend2:80 check

listen stats
    bind *:8404
    stats enable
    stats uri /

Run:

docker compose up

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🔥 11️⃣ Layer 4 (TCP Mode)

For database load balancing:

frontend tcp_front
    bind *:3306
    default_backend mysql_servers

backend mysql_servers
    mode tcp
    balance leastconn
    server db1 127.0.0.1:3306 check

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🔥 12️⃣ Advanced Routing (Path-Based)

frontend http_front
    bind *:80
    acl is_api path_beg /api
    use_backend api_servers if is_api
    default_backend web_servers

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🔥 13️⃣ Production Best Practices

• Enable logging
• Set maxconn properly
• Use timeouts
• Enable health checks
• Use SSL offloading
• Monitor stats endpoint
• Use keep-alive tuning

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🔥 14️⃣ Common Errors

502 → Backend down 503 → No backend available Connection refused → Wrong IP/Port Timeout → Backend slow

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🔥 15️⃣ HAProxy vs NGINX

HAProxy	NGINX
Strong L4	Strong static serving
Enterprise LB	Web server + LB
Very fast	More flexible

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🧠 Real DevOps Architecture Example

Internet
   ↓
HAProxy (SSL termination)
   ↓
App Servers (Docker/K8s)
   ↓
Database

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