The second phase of the layering effort involved creating a BuildController, whose job is to start and manage builds of OS images. While it should be able to perform those functions on its own, getting the built OS image onto each of the cluster nodes involves modifying other parts of the MCO to be layering-aware. To that end, there are three pieces involve, some of which will require modification:

Render Controller

Right now, the render controller listens for incoming MachineConfig changes. It generates the rendered config which is comprised of all of the MachineConfigs for a given MachineConfigPool. Once rendered, the Render Controller updates the MachineConfigPool to point to the new config. This portion of the MCO will not likely need any modification that I'm aware of at the moment.

Node Controller

The Node Controller listens for MachineConfigPool config changes. Whenever it identifies that a change has occurred, it applies the machineconfiguration.openshift.io/desiredConfig annotation to all the nodes in the targeted MachineConfigPool which causes the Machine Config Daemon (MCD) to apply the new configs. With this new layering mechanism, we'll need to add the additional annotation of machineconfiguration.openshift.io/desiredOSimage which will contain the fully-qualified pullspec for the new OS image (referenced by the image SHA256 sum). To be clear, we will not be replacing the desiredConfig annotation with the desiredOSimage annotation; both will still be used. This will allow Config Drift Monitor to continue to function the way it does with no modification required.

Machine Config Daemon

Right now, the MCD listens to Node objects for changes to the machineconfiguration.openshift.io/desiredConfig annotation. With the new desiredOSimage annotation being present, the MCD will need to skip the parts of the update loop which write files and systemd units to disk. Instead, it will skip directly to the rpm-ostree application phase (after making sure the correct pull secrets are in place, etc.).

Done When:

• The above modifications are made.
• Each modification has been done with appropriate unit tests where feasible.