Podman Architecture

The Pod concept was introduced by Kubernetes. Podman pods are similar to the Kubernetes definition.

Every Podman pod includes an "infra" container. This container does nothing, but go to sleep. Its purpose is to hold the namespaces associated with the pod and allow podman to connect other containers to the pod. This allows you to start and stop containers within the POD and the pod will stay running, where as if the primary container controlled the pod, this would not be possible. The default infra container is based on the k8s.gcr.io/pause image, Unless you explicitly say otherwise, all pods will have container based on the default image.

Most of the attributes that make up the Pod are actually assigned to the "infra" container. Port bindings, cgroup-parent values, and kernel namespaces are all assigned to the “infra” container. This is critical to understand, because once the pod is created these attributes are assigned to the "infra" container and cannot be changed. For example, if you create a pod and then later decide you want to add a container that binds new ports, Podman will not be able to do this. You would need to recreate the pod with the additional port bindings before adding the new container.

In the above diagram, notice the box above each container, conmon, this is the container monitor. It is a small C Program that’s job is to watch the primary process of the container, and if the container dies, save the exit code. It also holds open the tty of the container, so that it can be attached to later. This is what allows podman to run in detached mode (backgrounded), so podman can exit but conmon continues to run. Each container has its own instance of common.

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