Podman

Introduction

Podman is an alternative to Docker for running containers on Linux. It is similar to Docker in many respects, but differs in some key areas:

• Doesn't require root privileges by default
• Containers can continue running when Podman is stopped
• Upgrading Podman doesn't stop all containers (unlike upgrading Docker)

Naming changes

• Podman refers to layers as blobs

Traps for the unwary

• Installing the podman package on Ubuntu doesn't include compose - you have to install podman-compose as well
• Short image names are not supported by default - you have to convert them (e.g. mariadb:10.11 becomes docker.io/mariadb:10.11)
• Rootless by default means you cannot forward ports from the container to a privileged port (below 1024) on the host (you can run software within the container on privileged ports)

Container orchestrators

A container orchestrator performs orchestration, i.e. the management of containers across nodes (which may be across different machines). For example, if you have two web nodes (with a load balancer) and a database node, your orchestrator will ensure that all the nodes are configured and can talk to each other as needed.

Orchestrators include:

• Kubernetes
• Apache Mesos
• Docker Swarm

Orchestrators are generally not compatible with each other(?)

Container engines

A container engine configures applications to run on nodes. These are what you will typically interact with during development.

Engines include:

• Docker
• Podman

Container runtimes

A container runtime is the interface between the kernel and the containerised application. A container engine will usually support multiple container runtimes.