ADR 1 — Base image

Status: accepted

Context#

Corium needs an immutable base in the CoreOS/OSTree lineage, and exposes cloud-init as its configuration surface.

Fedora CoreOS was the obvious starting point: it is the mature member of the lineage, with excellent install tooling (coreos-installer, live ISO, PXE). Three things were verified before ruling on it.

• Image-provided Ignition fragments in /usr/lib/ignition/base.d/ work, and with the right precedence. Ignition merges the user’s config over the image’s, so baked-in plumbing applies wherever the operator is silent and never overrides them. Confirmed in internal/exec/engine.go in coreos/ignition, not from documentation.
• FCOS boots with no Ignition config at all. Maintainers declined to make its absence fatal: “Today, we happily boot and sit there if the user doesn’t provide Ignition” (fedora-coreos-tracker#279). A cloud-init-only node is therefore possible.
• Zincati, FCOS’s auto-update agent, fails on OCI-native deployments and must be masked.

So the design was buildable. The question was whether it should be built.

Decision#

Base on quay.io/fedora/fedora-bootc.

Consequences#

FCOS is Ignition-native. Running cloud-init as the primary surface there means masking Zincati, neutralising Afterburn’s SSH key injection — it overlaps cloud-init’s metadata role, and running both gives two agents racing to provision the same user — and arbitrating between NetworkManager and cloud-init’s network stage. That is a meaningful amount of work whose endpoint is the place fedora-bootc already occupies.

The ecosystem has drawn this line already. ucore, the most developed FCOS derivative, stays on Ignition. k0snode-bootc, which runs k0s on bootc with cloud-init, starts from a generic bootc base rather than FCOS. No project ships FCOS with cloud-init as its primary surface.

Finally, FCOS is itself converging on bootc (fedora-coreos-tracker#2214, targeting Fedora 46). Starting from fedora-bootc is starting where FCOS is going.

What is given up: FCOS’s install tooling and its battle-tested server base. bootc-image-builder covers the first, producing an installable artefact directly from the image rather than FCOS’s two-step of installing a stock image and rebasing onto ours.