OpenShift 4 is an operator-focused platform, and the Machine Config operator extends that to the operating system itself, managing updates and configuration changes to essentially everything between the kernel and kubelet.
To repeat for emphasis, this operator manages updates to systemd, cri-o/kubelet, kernel, NetworkManager,
etc. It also offers a new MachineConfig CRD that can write configuration files onto the host.
The approach here is a "fusion" of code from the original CoreOS Tectonic as well as some components of Red Hat Enterprise Linux Atomic Host, as well as some fundamentally new design.
The MCO (for short) interacts closely with both the installer as well as Red Hat CoreOS. See also the machine-api-operator which handles provisioning of new machines - once the machine-api-operator provisions a machine (with a "pristine" base Red Hat CoreOS), the MCO will take care of configuring it.
One way to view the MCO is to treat the operating system itself as "just another
Kubernetes component" that you can inspect and manage with oc.
The MCO uses CoreOS Ignition as a configuration format. Operating system updates use rpm-ostree, with ostree updates encapsulated inside a container image. More information in OSUpgrades.md.
As of release 4.12, you can try out OCP CoreOS Layering which lets you use more familiar "Containerfile" (Dockerfile) syntax to apply configuration to your pools.
This one git repository generates 4 components in a cluster; the machine-config-operator
pod manages the remaining 3 sub-components. Here are links to design docs:
Because the MCO is a cluster-level operator, you can inspect its status just like any other operator that is part of the release image. If it's reporting success, then that means that the operating system is up to date and configured.
oc describe clusteroperator/machine-config
One level down from the operator CRD, the machineconfigpool objects
track updates to a group of nodes. You will often want to run a command
like this:
oc describe machineconfigpool
Particularly note the Updated and Updating columns.
The MCO has "high level" knobs for some components of the cluster state; for
example, SSH keys and kubelet configuration. However, there are obviously a
quite large number of things one may want to configure on a system. For example,
offline environments may want to specify an internal NTP pool. Another example
is static network configuration. By providing a MachineConfig object
containing Ignition configuration,
systemd units can be provided, arbitrary files can be laid down into writable
locations (i.e. /etc and /var).
See the OCP product documentation for more information.
Once you create a MachineConfig fragment like the above, the controller will generate a new "rendered" version that will be used as a target. For more information, see MachineConfig.
In particular, you should look at oc describe machineconfigpool and oc describe clusteroperator/machine-config as noted above.
The model implemented by the MCO is that the cluster controls the operating system. OS updates are just another entry in the release image. For more information, see OSUpgrades.md.
See HACKING.md.
See FAQ.md.
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