Reproducible Computational Environments Using Containers: Introduction to Podman: Key Points

Introducing Containers

Almost all software depends on other software components to function, but these components have independent evolutionary paths.
Small environments that contain only the software that is needed for a given task are easier to replicate and maintain.
Critical systems that cannot be upgraded, due to cost, difficulty, etc. need to be reproduced on newer systems in a maintainable and self-documented way.
Virtualization allows multiple environments to run on a single computer.
Containerization improves upon the virtualization of whole computers by allowing efficient management of the host computer’s memory and storage resources.
Containers are built from ‘recipes’ that define the required set of software components and the instructions necessary to build/install them within a container image.
Podman is just one software platform that can create containers and the resources they use.

You will typically interact with Podman using the command line.
To learn how to run a certain Podman command, we can type the command followed by the --help flag.

The podman image pull command downloads container images from the internet.
The podman image ls command lists Podman container images that are (now) on your computer.
The podman container run command creates running containers from container images and can run commands inside them.
When using the podman container run command, a container can run a default action (if it has one), a user specified action, or a shell to be used interactively.

podman container has subcommands used to interact and manage containers.
podman image has subcommands used to interact and manage container images.
podman container ls or podman ps can provide information on currently running containers.

A container registry is an online repository of container images.
“Docker Hub is the most widely used container registry.”
Many Docker Hub container images are public, and may be officially endorsed.
Each Docker Hub page about a container image provides structured information and subheadings
Most Docker Hub pages about container images contain sections that provide examples of how to use those container images.
Many Docker Hub container images have multiple versions, indicated by tags.
The naming convention for Docker container images when using Podman is: docker.io/OWNER/CONTAINER_IMAGE_NAME:TAG

Dockerfiles specify what is within container images.
The podman image build command is used to build a container image from a Dockerfile.
You can share your container images through the Docker Hub so that others can create containers from your container images.

Podman allows containers to read and write files from the Podman host.
You can include files from your Podman host into your container images by using the COPY instruction in your Dockerfile.

There are many ways you might use Docker and existing container images in your research project.

Container images allow us to encapsulate the computation (and data) we have used in our research.
Using a service such as Docker Hub allows us to easily share computational work we have done.
Using container images along with a DOI service such as Zenodo allows us to capture our work and enables reproducibility.