Introduction⌗

In this blog post, I’ll explain the steps that I followed to migrate from Docker to Podman. Whether you are a sysadmin or a regular developer, I’m quite sure that you’ve heard of Docker before. It’s a great piece of software, and it really helped to improve and simplify software delivering. Believe me, I was there before Docker exists, and it was quite chaotic.

Then, why to move away from Docker? Well, if you’re after better integration with the system, like using systemd, or if you’re after more security options, or if you prefer software that doesn’t use daemons, then Podman is definitely worth a try!

Podman is an alternative container management tool developed as open source by Red Hat. It was developed after Docker and that meant Podman’s developers could learn a lot of lessons from it. Moreover, they dared to improve it, specially targeting the weakest point of Docker: security. They were very clever: they realised it would be hard to take market share of a well established software, so they made Podman fully compatible with Docker’s API. To me, this is a key to success because regardless on whether they offer a better piece of software or not, there’s a lot of inertia towards Docker and reverting it would be almost impossible, unless you make an offer that you can’t refuse: use Podman as you were using Docker. So simple, so brilliant.

I’m not going to get into the nitty-gritty or compare them side-by-side, as there are already some articles out there doing that. I’ll just point you to the e-book Podman in Action by Daniel J. Walsh. As of today, it is free to download, and it is really well written. If you’re interested in Podman, you should definitely check it out.

Today’s blog post focuses on the migration from Docker to Podman. We’ll take advantage of the compatible API, and we’ll dare to seamless restart our containers using the same command that we were using with Docker with an single change: replace docker by podman.

Hence, no fancy stuff today, just a straightforward migration. I think it’s best to keep it simple, as we’re about to touch an important part of your system, and we need to feel confident enough before moving forward. What would be better than proving that we can run Podman as we were doing with Docker, let’s get started!

Save Your Containers⌗

You might be reluctant to move because you have several containers running, and you can’t afford losing the content of those. Many images are prepared to place modifiable content in a particular path, this way you could use the option --volume to specify a host folder in which data can outlive the container.

But, what if you forgot to use an external volume? Or, what if your particular image keeps many modifications within the running container? If you’re in this situation, don’t panic! There’s a simple procedure to save all the content, and prepare the containers to be loaded into Podman.

How To Save a Running Container⌗

First, we’ll get the hash identifier or the name of the containers that we aim to migrate. If there are stopped containers, we’ll need docker ps --all.

We’ll use docker commit for the purpose of saving a running container into an image that we can later import into Podman. The process is simple, just use:

$ docker commit <container id or name> <image name>

If the container is running, that command will pause it for a while, so keep that in mind in case you can’t interrupt your underlying service.

Once we’ve made an image of a container, we need to export it this way:

$ docker save <image name> | gzip > <image name>.tar.gz

Repeat that process for each container that you need to migrate, and you’ll be ready to start using Podman.

Tip: If you were running docker as superuser, the images that you’ll export will be owned by root. As we’ll run Podman as a regular user, remember to change the permissions of those images before proceeding with the following steps.

Import Your Saved Images Into Podman⌗

At this stage, we’re ready to start running our containers using Podman instead of Docker. Just a heads-up: you might want to stop your running containers under Docker now, otherwise ports being used by your running containers in Docker would avoid running your containers with Podman using exactly the same configuration.

First, we’ll import all the images that from the previous step using:

$ podman load < <image name>.tar.gz

When you are done, go and check everything went OK using podman image ls. You shall see your images listed. Now, it’s time to fire up a few containers using those images. One of the greatest things of Podman is that its command line API is 100% compatible with Docker’s. So take the command that you used before to run your containers with Docker: docker run [...], and replace docker by podman, that’s it!

Final Steps⌗

There’s not much else to do. We saved all our beloved Docker containers and imported them into Podman. We made a big jump but we had a great safety net. Something didn’t go as expected? Just start your stopped container in Docker and you lost nothing but time. However, I hope everything ran smoothly and you faced no big issues during the process.

Keeping Docker won’t harm you, so don’t be forced to clean it up before you feel comfortable using Podman. But if you reached this point, you are more than ready to free some space and remove Docker from your system.

So far, we made no use of any special feature, as we are using Podman as a 1:1 replacement for Docker. However, Podman offers many interesting features to improve security. I’ll cover some interesting features that Podman includes in following blog posts, so keep an eye on my blog!