Transactional, in-place operating system updates using OCI/Docker container images.
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.
This project currently leverages significant work done in the ostree project.
In the future, there may be non-ostree backends.
The bootc project suggests that Linux operating systems and distributions to provide a new kind of "bootable" base image, distinct from "application" base images. A reference example available today is Fedora CoreOS.
At the current time, there are no official binary releases; this will
likely change in the future. For now, assuming you've done a cargo build --release
and you have a target/release/bootc binary, you can copy that onto
a target host system that is booted using ostree.
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.
Next, given a disk image (e.g. AMI, qcow2, raw disk image) installed on a host
system and set up using ostree by default, the bootc switch command
can be used to switch the system to use the targeted container image:
$ 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.
Once a chosen container image is used as the boot source, further
invocations of bootc upgrade will look for newer versions - again
preserving state.
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.