Day 44 · Docker

Write a Docker Compose File

By Varun DeshpandePublished 2026-07-12Docker / Linux~1h to complete

Technical Overview

Managing multi-container applications using standard docker run commands can quickly become complex, verbose, and error-prone. Each container requires separate flags for port exposures, environment variables, network attachments, and volume mounts.

Docker Compose is a tool that simplifies this process by allowing developers and system administrators to define and manage multi-container Docker applications declaratively using a single YAML file (typically named docker-compose.yml).

Core Concepts of Docker Compose

A standard Docker Compose file is structured around three key configurations:

Services (services): Defines the distinct containers that make up your application (e.g., a database service, a caching service, and a web server service). You specify the base image, build contexts, environment configurations, and restart policies within each service.
Volumes (volumes): Defines persistent storage resources that can be shared between multiple services or mounted directly to directories on the host system (bind mounts).
Networks (networks): Defines virtual isolated networks. By default, Docker Compose sets up a single default network for your application. Each container for a service joins this network and is both reachable and discoverable by other containers using their service name as the hostname.

graph TD subgraph Host["Host Machine (Port 6100)"] HostDir["Host Directory (/opt/data/)"] end subgraph Docker Compose Environment httpd["Apache Container (httpd)"] httpd -->|Maps Port 80 to Host Port 6100| Host HostDir -->|Volume Bind Mount| WebRoot["Container Web Root (/usr/local/apache2/htdocs/)"] end

Docker Compose Command Reference

Below are the essential commands used to manage environments defined by Docker Compose:

docker compose up: Builds, (re)creates, starts, and attaches to containers for a service.
- Use -d (detached) mode to run containers in the background: docker compose up -d
docker compose down: Stops and removes containers, networks, volumes, and images created by up.
docker compose ps: Lists the status of running containers managed by the current Compose file.
docker compose logs: Streams output logs from all running containers in the stack.
docker compose exec <service> <command>: Executes a command inside the running container of a specific service.

Infrastructure & Configuration Requirements

Target Host: Nautilus App Server 3 (stapp03) (can vary in labs, e.g., stapp01, stapp02, stapp03)
SSH User: banner (associated with stapp03; tony for stapp01, steve for stapp02)
File Location (Host): /opt/docker/docker-compose.yml
Service Name: httpd
Container Name: httpd
Base Image: httpd:latest
Port Mapping: Host Port 6100 mapped to Container Port 80
Volume Mount: Host Directory /opt/data mounted to Container Web Root /usr/local/apache2/htdocs

Step-by-Step Implementation

Step 1: Connect to the Application Server

Establish an SSH connection from the Jump Host to App Server 3:

ssh banner@stapp03

Provide the server password when prompted.

Step 2: Create the Target Directory

Ensure the target directory exists for the Docker Compose configuration:

sudo mkdir -p /opt/docker

Step 3: Create the docker-compose.yml File

Create and edit the YAML file using a text editor (e.g., vi):

sudo vi /opt/docker/docker-compose.yml

Add the following configuration block:

version: '3.8'
services:
  httpd:
    image: httpd:latest
    container_name: httpd
    ports:
      - "6100:80"
    volumes:
      - /opt/data:/usr/local/apache2/htdocs
    restart: always

Save and exit the text editor (:wq).

Step 4: Deploy the Stack using Docker Compose

Navigate to the directory containing the Compose file and start the stack in detached mode:

cd /opt/docker
sudo docker compose up -d

If using older versions of Docker Compose, run sudo docker-compose up -d instead.

Post-Deployment Verification

1. Verify Container Status

Check if the service container is active and running:

docker compose ps
# or
docker ps

Expected Output:

CONTAINER ID   IMAGE          COMMAND              CREATED         STATUS         PORTS                  NAMES
2a3b4c5d6e7f   httpd:latest   "httpd-foreground"   5 seconds ago   Up 5 seconds   0.0.0.0:6100->80/tcp   httpd

2. Verify Volume Mount and Service Response

Create a test file in the host data directory and verify that it is served correctly by the web server:

# Write test file on Host
echo "Nautilus DevOps Team - Challenge 44" | sudo tee /opt/data/index.html

# Query the HTTP service on host port 6100
curl http://localhost:6100