DevOps Day 44: Declarative Container Management with Docker Compose¶

Today's task was a major step up in how I manage containers. I moved from using long, imperative docker run commands to defining my application's configuration declaratively using a Docker Compose file. This is the standard, professional way to manage containerized applications, making them version-controlled, reproducible, and easy to understand.

My objective was to deploy an httpd (Apache) web server, but instead of typing out all the port and volume mapping flags, I codified them into a simple, readable docker-compose.yml file. This was a fantastic exercise in "Infrastructure as Code" at the container level.

The Task¶

My objective was to deploy a containerized httpd server on App Server 1 using a Docker Compose file. The specific requirements were: - The configuration must be in a file named exactly /opt/docker/docker-compose.yml. - The container must use the httpd:latest image. - The container itself must be named httpd. - Host port 8083 had to be mapped to container port 80. - The host directory /opt/data had to be mapped as a volume to the container's document root, /usr/local/apache2/htdocs.

My Step-by-Step Solution¶

Prepare the Host: I first connected to App Server 1 (ssh tony@stapp01). I then created the necessary directories on the host system.
```
sudo mkdir -p /opt/docker /opt/data
```

Create the docker-compose.yml file: This was the core of the task. I created the file (sudo vi /opt/docker/docker-compose.yml) and wrote the following YAML configuration, paying close attention to the indentation.

version: '3.8'

services:
  apache_service:
    image: httpd:latest
    container_name: httpd
    ports:
      - "8083:80"
    volumes:
      - /opt/data:/usr/local/apache2/htdocs

Launch the Application: With the configuration file in place, launching the container was incredibly simple. I first navigated to the correct directory.
```
cd /opt/docker
```
Then, I launched the entire stack with a single command.
```
sudo docker compose up -d
```
Verification: The final step was to confirm everything was running as defined in my file. I ran sudo docker ps and the output showed a single container running with all the correct attributes: the name httpd, the httpd:latest image, and the 0.0.0.0:8083->80/tcp port mapping. This was the definitive proof of success.

Key Concepts (The "What & Why")¶

Docker Compose: This is a tool for defining and running multi-container Docker applications. Even for a single container like in this task, it's a best practice because it makes your configuration declarative and easy to manage.
Declarative vs. Imperative: This is the key difference.
- docker run ...: This is imperative. I am giving the computer a long, detailed command telling it how to run a container. It's a one-time action.
- docker-compose.yml: This is declarative. I am creating a file that describes what my final application should look like (one container, with these ports, and these volumes). I then let Docker Compose figure out the "how."
Benefits of Docker Compose:
1. Readability and Simplicity: The YAML file is much easier for humans to read and understand than a long docker run command with many flags.
2. Version Control: I can check my docker-compose.yml file into Git. This means my entire application's configuration is version-controlled, and I can track every change.
3. Reproducibility: Anyone on my team can clone the repository, run docker compose up, and get the exact same environment running instantly.

Commands I Used¶

sudo mkdir -p /opt/docker /opt/data: Makes directories. The -p flag creates parent directories if they don't exist, which is a good habit.
sudo vi /opt/docker/docker-compose.yml: Creates and edits the YAML configuration file with root privileges.
cd /opt/docker: A critical step to change directory into the location of the compose file before running it.
sudo docker compose up -d: The hero command.
- docker compose: The main command for interacting with the Docker Compose tool.
- up: The subcommand to create and start the application as defined in the .yml file.
- -d: Runs the containers in detached (background) mode.
sudo docker ps: The standard Docker command to list running containers. I used this to verify that my container was created with the correct name, image, and port/volume mappings as defined in my compose file.