First impressions of moving from Docker to Podman

It’s been on the horizon for a while but when I decided to port some stuff over to RHEL 8 I was more or less forced to remove my dependency on Docker and use something else instead.

When it comes to the beef between Red Hat and Docker I’ve been on the side of Red Hat. Both for technical and philosophical reasons. Docker is a big fat daemon which I really don’t need to pull a container file from a URL, or to build a container image and save it to disk. Add to that the fact that Docker is very close minded to accepting code changes and that they once thought that just verifying the existence of a checksum as a proper image validation during pull.

But even though I knew I wanted to move away from Docker at some time, I also knew it would come with a bunch of work that I’d much rather spend adding features and fixing bugs.

Anyway, now I am porting stuff to RHEL 8 and this means I need to add support for Podman. So here I will lay out some of my experiences moving from Docker to Podman.

Background

So just to give you a little context on what I do. I develop and package IT systems. The system is installed and configured using Ansible and most services are packaged as containers. While we try to use container images from vendors, we sometimes have to resort to create our own containers. So the main focus here is on adapting our Ansible roles so they start and configure the containers using podman instead of docker.

Here are the services that I decided to port:

  • AWX (upstream for Ansible Tower)
  • Foreman (upstream for Red Hat Satellite)
  • Sonatype Nexus
  • HAProxy
  • NodePKI

Installation

This was the easy part. Podman has to be installed on the target and to do this I just added the following:

package: name=podman state=present

Ansible modules

One of the biggest issue is that there are no Podman equivalents to the Ansible modules docker_network and docker_container. There is a module podman_image though and podman_container was just merged into Ansible core. However, I cannot wait for Ansible 2.9 and need a solution today. These modules are used extensively by us to manage our containers using Ansible and having to resort to the command or shell modules really feels like a step back.

Luckily I actually found a way to make the transition much easier, using systemd services.

Cheating with Systemd services

So before I started the port to podman I decided to adjust all my roles to setup the docker containers so they are managed by systemd. This is quite simple:

Create a sysconfig file:

# {{ ansible_managed }}
C_VOLUMES="{% for x in container_volumes %}--volume {{ x }} {% endfor %}"
C_ENV="{% for k,v in container_env.items() %}--env {{ k }}='{{ v }}' {% endfor %}"
C_PORTS="{% for x in container_ports %}--publish {{ x }} {% endfor %}"
C_IMAGE="{{ container_image }}"
C_COMMAND="{{ container_cmd }}"
C_ARGS="{{ container_args }}"

Create a service unit file:

[Unit]
Description=My container
Wants=syslog.service

[Service]
Restart=always
EnvironmentFile=-/etc/sysconfig/my_service
ExecStartPre=-{{ container_mgr }} stop {{ container_name }}
ExecStartPre=-{{ container_mgr }} rm {{ container_name }}
ExecStart={{ container_mgr }} run --rm --name "{{ container_name }}" \
  $C_VOLUMES $C_ENV $C_PORTS $C_ARGS $C_IMAGE $API_COMMAND
ExecStop={{ container_mgr }} stop -t 10 {{ container_name }}

[Install]
WantedBy=multi-user.target

Start the service:

- service: name=my_service state=started

Thanks to the fact that podman is CLI-compatible with the Docker client, moving to podman is now as easy as setting container_manager to /usr/bin/podman instead of /usr/bin/docker.

Creating networks

Unfortunately Podman has no podman create network to create a private network where I can put a set of containers. This is really a shame. Docker networks makes it easy to create a private namespace for containers to communicate. Docker networks allows me to expose ports only to other containers (keeping them unexposed to the host) and name resolution so containers can find each other easily.

One alternative that was suggested to me on the Podman mailing list was to use a pod. But containers in pods share localhost which means that I run the risk of port collision if two containers use the same port. This also adds more complexity as I need to create and start/stop a new entity (the pod) which I never got working using systemd (systemd just killed the pod directly after starting it).

I also cannot use the built in CNI network, or create additional ones, since they don’t provide name resolution and I have no way of knowing the IP for a given container.

My only solution here was to skip networks all together and use host networking. It comes with some issues:

  • I still have the risk of port collision between containers
  • All ports are published and accessible from outside the host (unless blocked by a firewall)

Working on Mac

Another big thing missing from Podman is a client for macOS. While I use RHEL on all the servers (and Fedora at home) my workstation is a Macbook which means I cannot use Podman when I build containers locally, or troubleshoot podman commands locally. Luckily, I have a really streamlined development environment that makes it a breeze to quickly bring up a virtual machine running CentOS where I can play around. I do miss the ability to build containers on my Mac using Podman but since Docker and Podman both are CNI compatible I can build container images using Docker on my laptop and then manage and run them on RHEL using Podman without problems.

InSpec

My InSpec tests uses some docker resources but I decided to use the service resource instead to verify that the systemd services are running properly, and of course I have a bunch of tests that access the actually software that runs inside the containers.

Summary

So after moving to systemd services it was really easy to port from Docker to Podman. My wishlist for Podman would be the following:

  • Podman modules for Ansible to replace the Docker modules
  • Ability to manage CNI networks using podman network ...
  • Name resolution inside Podman networks
  • Support for macOS

Luckily none of these were showstoppers for me and after figuring it all out it took about a day to convert five Ansible roles from Docker to Podman without loss of end user functionality.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s