Ansible Playbook --- Build & Run Python App (Docker)

This playbook builds a Docker image for the Python application, removes any running containers that use the configured host port, and starts the container with the PORT environment variable that the app requires.

The playbook uses Ansible modules only (community.docker) and is fully idempotent.

What the playbook does

1. Builds the Docker image using your local Dockerfile.\
2. Finds and removes any container currently using the configured host port.\
3. Removes the previous python-app container if present (idempotent).\
4. Runs the container with:
   • the correct environment variable (PORT)\
   • port mapping (host_port:listen_port)\
   • restart_policy: unless-stopped

Requirements

• Python ≥ 3.8\
• Docker installed and running\
• Ansible installed\
• The community.docker collection installed

Install dependencies:

pip install -r requirements.txt
ansible-galaxy collection install community.docker

Variables

---

Variable Description Default

---

image_name The Docker image name (no tag). cyrixrr/python-app

image_tag The tag for the Docker image. v2

listen_port The internal port that the Python app listens 8000 on.

host_port The port on the host that will be published {{ listen_port }} and checked for conflicts.

You can override them when running the playbook:

ansible-playbook playbook.yml -e image_tag=v3
ansible-playbook playbook.yml -e host_port=8001

How to run

From the project root:

ansible-playbook playbook.yml

Test the application:

curl http://localhost:8000/

Idempotency

This playbook is fully idempotent:

• If no container uses the configured host port, nothing is removed.\
• If the image has not changed, it will not rebuild.\
• If the python-app container is already running with the correct settings, no changes occur.\
• If a different container is occupying the selected host port, it will be removed automatically.

Troubleshooting

Port already in use

This playbook automatically removes Docker containers that use host_port.

If a non-Docker process is using the port:

sudo lsof -i :8000 -nP
sudo kill -9 <PID>

Environment variable must be a string

PORT must be passed as a string:

PORT: "{{ listen_port | string }}"

Summary

This playbook demonstrates: - Use of Ansible variables\

• Docker image building via docker_image\
• Running containers via docker_container\
• Idempotent cleanup logic\
• Environment variable injection\
• Port mapping and collision handling