bootc
Transactional, in-place operating system updates using OCI/Docker container images.
STATUS: Experimental, subject to change!
Motivation
The original Docker container model of using "layers" to model applications has been extremely successful. This project aims to apply the same technique for bootable host systems - using standard OCI/Docker containers as a transport and delivery format for base operating system updates.
The container image includes a Linux kernel (in e.g. /usr/lib/modules),
which is used to boot. At runtime on a target system, the base userspace is
not itself running in a container by default. For example, assuming
systemd is in use, systemd acts as pid1 as usual - there's no "outer" process.
ostree
This project currently leverages significant work done in the ostree project.
In the future, there may be non-ostree backends.
Modeling operating system hosts as containers
The bootc project suggests that Linux operating systems and distributions to provide a new kind of "bootable" base image, distinct from "application" base images. See below for available images.
Effectively, these images contain a Linux kernel - and while this kernel
is not used when the image is used via e.g. podman|docker run, it is
used when booted via bootc.
In the current defaults, /etc and /var both act a bit like
mounted, persistent volumes. More on this in the ostree docs.
Status
At the current time, bootc is in active development and is not quite considered ready for production use. The command line interface might change. There is not yet stable RPC API.
However, it heavily relies on a lot of underlying technologies which are tested, and the goal is to stabilize everything sometime in 2023.
Using bootc
Installing
- Fedora, CentOS Stream 9: There is a COPR tracking git main with binary packages.
You can also build this project like any other Rust project, e.g. cargo build --release from a git clone.
Base images
Many users will be more interested in base (container) images.
- Fedora CoreOS can be used as a base image; you will need to enable bootc there.
- There is also an in-development Project Sagano for Fedora/CentOS.
However, bootc itself is not tied to Fedora derivatives; this issue tracks the main blocker for other distributions.
Deriving from and switching to base images
A toplevel goal is that every tool and technique a Linux system administrator knows around how to build, inspect, mirror and manage application containers also applies to bootable host systems.
There are a number of examples in e.g. coreos/layering-examples.
First, build a derived container using any container build tooling.
Using bootc install
The bootc install command will write the current container to a disk, and set it up for booting.
In brief, the idea is that every container image shipping bootc also comes with a simple
installer that can set a system up to boot from it. Crucially, if you create a
derivative container image from a stock OS container image, it also automatically supports bootc install.
For more information, please see docs/install.md.
Switching from an existing ostree-based system
If you have an operating system already using ostree then you can use bootc switch:
$ bootc switch --no-signature-verification quay.io/examplecorp/custom:latest
This will preserve existing state in /etc and /var - for example,
host SSH keys and home directories. There may be some issues with uid/gid
drift in this scenario however.
Upgrading
Once a chosen container image is used as the boot source, further
invocations of bootc upgrade will look for newer versions - again
preserving state.
Relationship with other projects
Relationship with podman
It gets a bit confusing to talk about shipping bootable operating systems in container images. Again, to be clear: we are reusing container images as:
- A build mechanism (including running as a standard OCI container image)
- A transport mechanism
But, actually when a bootc container is booted, podman (or docker, etc.) is not involved.
The storage used for the operating system content is distinct from /var/lib/containers.
podman image prune --all will not delete your operating system.
That said, a toplevel goal of bootc is alignment with the https://github.com/containers ecosystem, which includes podman. But more specifically at a technical level, today bootc uses skopeo and hence indirectly containers/image as a way to fetch container images.
This means that bootc automatically also honors many of the knobs available in /etc/containers - specifically
things like containers-registries.conf.
In other words, if you configure podman to pull images from your local mirror registry, then bootc will automatically honor that as well.
The simple way to say it is: A goal of bootc is to be the bootable-container analogue for podman, which runs application containers. Everywhere one might run podman, one could also consider using bootc.
Relationship with Kubernetes
Just as podman does not depend on a Kubernetes API server, bootc will also not depend on one.
However, there are also plans for bootc to also understand Kubernetes API types. See configmap/secret support for example.
Perhaps in the future we may actually support some kind of Pod analogue for representing the host state. Or we may define a CRD which can be used inside and outside of Kubernetes.
Relationship with rpm-ostree
Today rpm-ostree directly links to ostree-rs-ext, and hence
gains all the same container functionality. This will likely
continue. For example, with rpm-ostree (or, perhaps re-framed as
"dnf image"), it will continue to work to e.g. dnf install
(i.e. rpm-ostree install) on the client side system. However, bootc upgrade would
(should) then error out as it will not understand how to upgrade
the system.
rpm-ostree also has significant other features such as
rpm-ostree kargs etc.
Overall, rpm-ostree is used in several important projects and will continue to be maintained for many years to come.
However, for use cases which want a "pure" image based model,
using bootc will be more appealing. bootc also does not
e.g. drag in dependencies on libdnf and the RPM stack.
bootc also has the benefit of starting as a pure Rust project; and while it doesn't have an IPC mechanism today, the surface of such an API will be significantly smaller.
Further, bootc does aim to include some of the functionality of zincati.
But all this said: It will be supported to use both bootc and rpm-ostree together; they are not exclusive.
For example, bootc status at least will still function even if packages are layered.
Relationship with Fedora CoreOS (and Silverblue, etc.)
Per above, it is a toplevel goal to support a seamless, transactional update from existing OSTree based systems, which includes these Fedora derivatives.
For Fedora CoreOS specifically, see this tracker issue.
See also OstreeNativeContainerStable.