dev-ops-challenges

Write a Docker Compose File

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:

1. 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.

2. Volumes (volumes): Defines persistent storage resources that can be shared between multiple services or mounted directly to directories on the host system (bind mounts).

3. 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 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

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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.

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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

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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.

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Step 2: Create the Target Directory

Ensure the target directory exists for the Docker Compose configuration:

sudo mkdir -p /opt/docker

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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).

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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.

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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

Expected Output:

Nautilus DevOps Team - Challenge 44

Log out of the Application Server:

exit