Table of Contents

Description #

Solving a sad server - There are three Docker containers defined in the docker-compose.yml file: an HAProxy accepting connections on port :5000 of the host, and two nginx containers, not exposed to the host.

The person who tried to set this up wanted to have HAProxy in front of the (backend or upstream) nginx containers load-balancing them but something is not working.

Test #

Running curl localhost:5000 several times returns both hello there from nginx_0 and hello there from nginx_1 but only nginx_0 is being served:

admin@i-0c6403c79fb500a19:~$ for i in `seq 1 6`; do curl localhost:5000; done
hello there from nginx_0
hello there from nginx_0
hello there from nginx_0
hello there from nginx_0
hello there from nginx_0
hello there from nginx_0

All containers seems to be up and running:

admin@i-0c6403c79fb500a19:~$ docker ps 
CONTAINER ID   IMAGE           COMMAND                  CREATED        STATUS          PORTS                                       NAMES
c79c9eb25143   haproxy:2.8.4   "docker-entrypoint.s…"   8 months ago   Up 56 seconds   0.0.0.0:5000->5000/tcp, :::5000->5000/tcp   haproxy
4052b12402a3   nginx:1.25.3    "/docker-entrypoint.…"   8 months ago   Up 56 seconds   80/tcp                                      nginx_1
2624cd20f3c4   nginx:1.25.3    "/docker-entrypoint.…"   8 months ago   Up 56 seconds   80/tcp                                      nginx_0
admin@i-0c6403c79fb500a19:~$

The haproxy config looks fine:

admin@i-0c6403c79fb500a19:~$ cat haproxy.cfg 
global
    daemon
    maxconn 256

defaults
    mode http
    default-server init-addr last,libc,none
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

frontend http-in
    bind *:5000
    default_backend nginx_backends

backend nginx_backends
    balance roundrobin
    server nginx_0 nginx_0:80 check
    server nginx_1 nginx_1:80 check

The configuration files of both nginx servers looks good but nginx_1 was listening on 81 instead of 80:

admin@i-0c6403c79fb500a19:~$ cat custom-nginx_0.conf 
server {
    listen 80;

    server_name localhost;

    location / {
        root   /usr/share/nginx/html;
        index  index.html;
    }
}

admin@i-0c6403c79fb500a19:~$ cat custom-nginx_1.conf 
server {
    listen 81;

    server_name localhost;

    location / {
        root   /usr/share/nginx/html;
        index  index.html;
    }
}

Fix it by replacing 81 with 80.

Looking at the compose file, we can see that nginx_0 is part of the frontend_network, nginx_1 is part of the backend_network, and haproxy is only part of the frontend_network:

admin@i-0c6403c79fb500a19:~$ cat docker-compose.yml 
version: '3'

services:
  nginx_0:
    image: nginx:1.25.3
    container_name: nginx_0
    restart: always
    volumes:
      - ./custom_index/nginx_0:/usr/share/nginx/html
      - ./custom-nginx_0.conf:/etc/nginx/conf.d/default.conf:ro
    networks:
      - frontend_network

  nginx_1:
    image: nginx:1.25.3
    container_name: nginx_1
    restart: always
    volumes:
      - ./custom_index/nginx_1:/usr/share/nginx/html
      - ./custom-nginx_1.conf:/etc/nginx/conf.d/default.conf:ro
    networks:
      - backend_network

  haproxy:
    image: haproxy:2.8.4
    container_name: haproxy
    restart: always
    ports:
      - "5000:5000"
    depends_on:
      - nginx_0
      - nginx_1
    volumes:
      - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg:ro
    networks:
      - frontend_network

networks:
  frontend_network:
    driver: bridge
  backend_network:
    driver: bridge
admin@i-0c6403c79fb500a19:~$

Fix the compose file by adding the haproxy container to both frontend_network and backend_network. Also, change the nginx_0 container to be part of backend_network instead of frontend_network, even though it would work without this modification. Finally, restart all the containers to ensure they join the updated networks:

admin@i-0c6403c79fb500a19:~$ docker compose down
[+] Running 5/5
 ✔ Container haproxy               Removed                                                          0.2s 
 ✔ Container nginx_0               Removed                                                          0.3s 
 ✔ Container nginx_1               Removed                                                          0.3s 
 ✔ Network admin_frontend_network  Removed                                                          0.2s 
 ✔ Network admin_backend_network   Removed                                                          0.1s 


admin@i-0c6403c79fb500a19:~$ docker compose up -d
[+] Running 5/5
 ✔ Network admin_frontend_network  Created                                                          0.1s 
 ✔ Network admin_backend_network   Created                                                          0.1s 
 ✔ Container nginx_0               Started                                                          0.1s 
 ✔ Container nginx_1               Started                                                          0.1s 
 ✔ Container haproxy               Started                                                          0.0s

Now haproxy is able to reach both nginx containers:

admin@i-0c6403c79fb500a19:~$ for i in `seq 1 6`; do curl localhost:5000; done
hello there from nginx_0
hello there from nginx_1
hello there from nginx_0
hello there from nginx_1
hello there from nginx_0
hello there from nginx_1

I love being married. It’s so great to find that one special person you want to annoy for the rest of your life.
– Rita Rudner.