Master Docker Compose: From Installation to HAProxy Load Balancing

1 docker-compose

1.1 Introduction to Compose

Compose

is a tool for defining and running multi‑container Docker applications. By using a YML file you can configure all services required by an application, then start them with a single command.

The three steps to use Compose are:

Define the application environment with a Dockerfile.

Describe the services in a docker-compose.yml file.

Run docker-compose up to start the whole application.

1.2 Installing Docker‑Compose

Install Docker‑Compose on a CentOS system:

# Install pip
yum install -y python2-pip
# Install docker‑compose
pip install docker-compose

Enable a faster pip mirror in China:

mkdir ~/.pip/
cat > ~/.pip/pip.conf <<'EOF'
[global]
index-url = https://mirrors.aliyun.com/pypi/simple/
[install]
trusted-host=mirrors.aliyun.com
EOF

1.3 Orchestrating and Starting Images

Create a project directory:

mkdir /opt/my_wordpress/
cd /opt/my_wordpress/

Write a docker-compose.yml that defines a MySQL database and a WordPress service:

version: '3'
services:
  db:
    image: mysql:5.7
    volumes:
      - /data/db_data:/var/lib/mysql
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: somewordpress
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wordpress
      MYSQL_PASSWORD: wordpress
  wordpress:
    depends_on:
      - db
    image: wordpress:latest
    volumes:
      - /data/web_data:/var/www/html
    ports:
      - "8000:80"
    restart: always
    environment:
      WORDPRESS_DB_HOST: db:3306
      WORDPRESS_DB_USER: wordpress
      WORDPRESS_DB_PASSWORD: wordpress

Start the stack:

docker-compose up          # foreground
docker-compose up -d       # background

Visit http://10.0.0.100:8000 to complete the WordPress installation.

1.4 HAProxy Proxy for Backend Docker Containers

Modify the compose file to expose port 80, then scale WordPress to two instances:

docker-compose scale wordpress=2
# Warning: the scale command is deprecated; use "docker-compose up --scale wordpress=2"

Install HAProxy:

yum install haproxy -y

Backup and edit /etc/haproxy/haproxy.cfg (full configuration omitted for brevity). After editing, start HAProxy:

systemctl start haproxy
systemctl enable haproxy

Access http://10.0.0.100:8000 through HAProxy to see load‑balancing, and view statistics at http://10.0.0.100:8888/haproxy-status:

1.5 Installing socat to Control HAProxy via Socket

yum install socat.x86_64 -y

Disable a backend node:

echo "disable server backend_www_example_com/web-node2" | socat stdio /var/lib/haproxy/stats

Enable a backend node:

echo "enable server backend_www_example_com/web-node3" | socat stdio /var/lib/haproxy/stats

1.6 Compose YML Configuration Reference

1.6.1 Simple Commands

version

: specify the Compose file version. cap_add, cap_drop: add or drop kernel capabilities. cgroup_parent: set the parent cgroup for the container. command: override the default command. container_name: give a custom name to the container. devices: list device mappings. dns, dns_search: custom DNS servers and search domains. entrypoint: override the default entrypoint. env_file: load environment variables from a file. environment: set environment variables (use quotes for booleans). expose: expose a port without publishing it. extra_hosts: add host‑name mappings. healthcheck: define a health‑check for the container. image: specify the image to use (e.g., redis, ubuntu:14.04, etc.). restart: set restart policy ( no, always, on-failure, unless-stopped). security_opt: set security options such as SELinux labels. stop_grace_period: time to wait after SIGTERM before SIGKILL. stop_signal: alternative signal to stop the container. sysctls: kernel parameters inside the container. tmpfs: mount temporary file systems. ulimits: override default ulimit settings. volumes: bind host paths or named volumes into the container.

1.6.2 build

The build section defines how to build an image: context: path to the build context. dockerfile: name of the Dockerfile. args: build‑time variables. labels: metadata labels for the image. target: specify a build stage in a multi‑stage Dockerfile.

1.6.3 depends_on

Controls service start order. docker-compose up starts dependencies first; docker-compose stop stops in reverse order.

1.6.4 deploy

Deployment configuration used only in Swarm mode. Options include mode (replicated or global), replicas, resources (cpu/memory limits), restart_policy, rollback_config, and update_config.

1.6.5 logging

Configure log driver (default json-file) and options such as max-size and max-file. For syslog driver, you can set syslog-address.

1.6.6 network_mode

Set the network mode (e.g., bridge, host, none, service:[name], container:[id]) and define networks with aliases.

1.6.7 secrets

Store sensitive data like passwords. Example:

version: "3.1"
services:
  mysql:
    image: mysql
    environment:
      MYSQL_ROOT_PASSWORD_FILE: /run/secrets/my_secret
    secrets:
      - my_secret
secrets:
  my_secret:
    file: ./my_secret.txt

For the full reference, see the original source link.