dev-ops-challenges

Deploy an App on Docker Containers

Technical Overview

Deploying modern applications on Docker typically involves orchestrating a multi-tier architecture (e.g., a web frontend, an application API, and a database backend). Rather than running each layer in a single monolithic container, the microservices design pattern isolates each component into its own dedicated container.

Core Deployment Concepts

To deploy a multi-container application on Docker, three primary parameters must be configured:

1. Service Dependency and Startup Order: Web applications require their database backends to be active and ready before initialization. In Docker Compose, the depends_on instruction defines the boot order.

2. Container Network Inter-connectivity: Containers on the same user-defined network communicate using automatically registered DNS hostnames. For example, a PHP application container can connect to MySQL by using the container name mysql_host as the DB server address.

3. Data Persistence via Volume Bind Mounts: Containers are ephemeral; any files written inside them are lost when the container terminates. Map directories on the host (e.g., /var/lib/mysql for databases, /var/www/html for website assets) to locations inside the container to persist data.

graph TD
    Client[Client Browser] -->|Port 8089| Web["Web Server (php_host / Port 80)"]
    subgraph Docker Network
        Web -->|Internal Query: port 3306| DB["Database (mysql_host / Port 3306)"]
    end
    subgraph Host Storage
        WebVol["/var/www/html/"] -->|Bind Mount| Web
        DBVol["/var/lib/mysql/"] -->|Bind Mount| DB
    end

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Environment Variables for Database Configuration

When deploying standard database images (like MySQL or MariaDB), environment variables are passed to initialize security credentials and create default databases:

• MYSQL_ROOT_PASSWORD: Sets the password for the superuser root account.
• MYSQL_DATABASE: Automatically creates a database with the specified name upon initialization.
• MYSQL_USER & MYSQL_PASSWORD: Creates a new database user and grants them all privileges on the created database.

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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)
• Configuration File: /opt/docker/docker-compose.yml
• Web Service:
  • Service/Container Name: php_host
  • Base Image: php:apache
  • Port Mapping: Host Port 8089 mapped to Container Port 80
  • Volume Mount: Host /var/www/html mapped to Container /var/www/html
• Database Service:
  • Service/Container Name: mysql_host
  • Base Image: mariadb:latest
  • Port Mapping: Host Port 3306 mapped to Container Port 3306
  • Volume Mount: Host /var/lib/mysql mapped to Container /var/lib/mysql
  • Environment Variables:
    • MYSQL_DATABASE: database_host
    • MYSQL_USER: app_user
    • MYSQL_PASSWORD: password_user
    • MYSQL_ROOT_PASSWORD: R00tP@ss!

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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 Configuration Folder

Ensure that the target directory for the Docker Compose file exists:

sudo mkdir -p /opt/docker

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Step 3: Write the docker-compose.yml File

Create and edit the Docker Compose configuration file:

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

Add the following multi-container configuration:

version: '3.8'
services:
  web:
    image: php:apache
    container_name: php_host
    ports:
      - "8089:80"
    volumes:
      - /var/www/html:/var/www/html
    depends_on:
      - db
    restart: always

  db:
    image: mariadb:latest
    container_name: mysql_host
    ports:
      - "3306:3306"
    volumes:
      - /var/lib/mysql:/var/lib/mysql
    environment:
      MYSQL_DATABASE: database_host
      MYSQL_USER: app_user
      MYSQL_PASSWORD: password_user
      MYSQL_ROOT_PASSWORD: R00tP@ss!
    restart: always

Save and close the file (:wq).

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Step 4: Deploy the Application Stack

Start both containers in detached mode:

cd /opt/docker
sudo docker compose up -d

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Post-Deployment Verification

1. Check Container Health

Ensure both containers are active and in the Up state:

docker compose ps

Expected Output:

NAME         IMAGE            COMMAND                  SERVICE   CREATED         STATUS         PORTS
mysql_host   mariadb:latest   "docker-entrypoint.s…"   db        5 seconds ago   Up 5 seconds   0.0.0.0:3306->3306/tcp
php_host     php:apache       "docker-php-entrypoi…"   web       5 seconds ago   Up 5 seconds   0.0.0.0:8089->80/tcp

2. Verify Database Connection

Inspect the database server configuration and test container network communication:

# Query PHP container logs to verify database handshake
docker logs php_host

# Perform connection check from Jump Host to App Server port
curl -I http://localhost:8089

Log out of the Application Server:

exit