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MachineConfigDaemon.md

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MachineConfigDaemon

Goals

  1. Apply new machine configuration during update.

  2. Validate and verify machine's state to the requested machine configuration.

Non Goals

  1. MachineConfigDaemon does not execute scripts on the machines.

Overview

MachineConfigDaemon is scheduled on the machines in a cluster as a DaemonSet. This daemon is responsible for performing machine updates in OpenShift 4. The update will include tasks related to the systemd units, files on disk, operating system upgrades etc. The MachineConfigDaemon updates a machine to configuration defined by MachineConfig as instructed by the MachineConfigController.

The MachineConfigDaemon is also responsible for annotating a node with machineconfiguration.openshift.io/ssh=accessed when it detects an SSH access to the machine.

Supported vs Unsupported Ignition config changes

The MachineConfigDaemon receives machine configuration in the form of a "rendered" or merged MachineConfig which is generated from applicable fragments by the controller.

If the updated Ignition config contains changes compatible with the current config, the node will be updated in place. Otherwise, it will enter a "degraded" state; the idea is that a human or automation tooling can then re-provision degraded machines.

Not all Ignition config sections are supported; see the following table:

Ignition spec 2 sections Supported
Files YES
systemd Units YES
Networkd NO
Users NO *
Directories NO
FileSystems NO
Links NO
Disks NO
RAID NO
Ignition spec 3 sections Supported
Files YES
systemd Units YES
Users NO *
Groups NO
Directories NO
FileSystems NO
Links NO
Disks NO
RAID NO

* At this time only updates to sshAuthorizedKeys for user core are permitted. Please see Update-SSHKeys for details.

Coordinating updates

The MachineConfigDaemon uses annotations defined on the Node object to coordinate updates with MachineConfigController for the machine.

MachineConfigDaemon update flow

States

  1. Done when daemon sets currentConfig = desiredConfig

  2. Working when daemon starts updating the machine.

  3. Degraded when daemon cannot continue to apply the update.

OS updates

In addition to handling Ignition configs, the MachineConfigDaemon also takes care of updating the base operating system.

Updates are provided via the OSImageURL component of a MachineConfig object. This should generally be controlled by the cluster-version-operator, and its current existence in MachineConfig objects should be though of as an implementation detail.

MachineConfigDaemon only supports updating Red Hat CoreOS, which uses rpm-ostree. The OSImageURL refers to a container image that carries inside it an OSTree payload. When the OSImageURL changes, it will be passed to the pivot command which is included in Red Hat CoreOS, and in turn takes care of passing it to rpm-ostree.

Once an update is prepared (in terms of a new bootloader entry which points to a new OSTree "deployment" or filesystem tree), then the MachineConfigDaemon will reboot.

Verfication

Upon start, MachineConfigDaemon queries rpm-ostree to determine the booted system version and verifies it matches the expected config.

systemd unit updates

MachineConfigDaemon replaces the unit service files on disk. The updated systemd services run after machine reboot.

The daemon should prune all the systemd units that don't exist in the desiredConfig but existed before. Diff the current config and desired config, then remove the units that were removed.

Verification

  1. MachineConfigDaemon verifies that contents and existence of the systemd unit files.

  2. MachineConfigDaemon also verifies that the systemd service is enabled when specified in Ignition config.

Directory / File updates

MachineConfigDaemon replaces the file contents on disk with the contents of the file from the desiredConfig.

The daemon should apply any change in permissions on file / directories.

The daemon should prune all the files and directories that don't exist in the desiredConfig but existed before. Diff the current config and desired config, then remove the nodes that were removed.

Verification

When starting, MachineConfigDaemon verifies that contents and existence of the files and directories match the current configuration. If the MachineConfigDaemon is coming up after applying a "pending" configuration, it will become current, and then verification will proceed.

Machine reboot

MachineConfigDaemon reboots the machine after applying the updated machine configuration.

Node drain

The daemon performs best-effort node drain before rebooting.

The node drain behavior:

  1. Should not try to remove static pods.

  2. Should respect pod disruption policy for evictions.

  3. Should not evict pods marked with critical annotation.

  4. Should not evict itself from the node.

Node drain on master nodes

The draining on master nodes should not be different from worker node as the control plane is self-hosted.

Node drain master in single master

The draining of pods on the only master node will not evict the control plane as they have critical pod annotation. After rebooting the only master, the pod-checkpointer brings back the components responsible for restarting the control plane.

Node drain etcd static pods on masters

Etcd is co-located on master nodes as static pods. The draining behavior defined above prevents draining of static pods to prevent interference to etcd cluster by the daemon.

Annotating on SSH access

RHCOS nodes in Openshift are not meant to be manually accessed via SSH. MCD uses logind to watch for login sessions, which, upon detection, warns the user and annotates the node with machineconfiguration.openshift.io/ssh=accessed. This in turn will be used to warn cluster admins.