Troubleshooting HAProxy Access Issue in Docker-Compose Setup

Your setup looks like this:

       Internet
           |
          [81]
       HAProxy
           |
          [80]
        Web App
           |
          [82]

• Requests come in on port 81 (HAProxy), which should then forward them to the web application listening on port 80 inside its container.
• The web app container maps port 80 internally to port 82 externally.

Try the following docker-compose.yml file for a proper networking and communication between the webapp and lb services:

version: '3'
services:
  webapp:
    image: httpd
    ports:
      - 82:80
    networks:
      - webnet

  lb:
    image: 'dockercloud/haproxy:latest'
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
    ports:
      - 81:80
    depends_on:
      - webapp
    networks:
      - webnet

networks:
  webnet:

As suggested, the links: option is obsolete and not necessary for modern Docker networking. Containers in the same network can discover and communicate with each other by service names.

This defines a custom network webnet and assigned both services to it. That makes sure they can communicate using the service name as the hostname.
Make sure your HAProxy configuration within the dockercloud/haproxy container is correctly set up to route traffic to the webapp service. HAProxy needs to know the correct service name and port to forward requests appropriately.

docker exec -it [container_id_or_name] /bin/sh
cat /etc/haproxy/haproxy.cfg
...
# you should see a backend section like:
backend http_back
    balance roundrobin
    server webapp webapp:80 check

And check that HAProxy is configured to listen on 0.0.0.0:80 or *:80 to make sure it accepts connections from all IP addresses, at least for testing.

Still not working, but when I checked haproxy.cfg backend section, it looks like this:

backend app 
balance roundrobin 
server app1 127.0.0.1:5001 check 
server app2 127.0.0.1:5002 check 
server app3 127.0.0.1:5003 
check server app4 127.0.0.1:5004 check 

and no other containers are currently active

I tried to use port 5001 instead of 81, but still no luck

If auto-discovery is failing and you have specific routing needs, consider manually specifying your HAProxy configuration. You can mount a custom haproxy.cfg file into a standard HAProxy container, replacing dockercloud/haproxy for debugging purposes:

version: '3'
services:
  webapp:
    image: httpd
    networks:
      - webnet

  haproxy:
    image: haproxy:latest
    volumes:
      - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg:ro
    ports:
      - "81:80"
    networks:
      - webnet

networks:
  webnet:

In haproxy.cfg, explicitly define the backend to point to your webapp service, making sure it matches the service name and port within the Docker network:

backend webapp_backend
    balance roundrobin
    server webapp webapp:80 check