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You can use the Control Plane Machine Set Operator to improve the resiliency of the control plane for your OKD cluster.

High availability and fault tolerance with failure domains

When possible, the control plane machine set spreads the control plane machines across multiple failure domains. This configuration provides high availability and fault tolerance within the control plane. This strategy can help protect the control plane when issues arise within the infrastructure provider.

Failure domain platform support and configuration

The control plane machine set concept of a failure domain is analogous to existing concepts on cloud providers. Not all platforms support the use of failure domains.

Table 1. Failure domain support matrix
Cloud provider Support for failure domains Provider nomenclature

Amazon Web Services (AWS)

X

Availability Zone (AZ)

Microsoft Azure

X

Azure availability zone

VMware vSphere

Not applicable

The failure domain configuration in the control plane machine set custom resource (CR) is platform-specific. For more information about failure domain parameters in the CR, see the sample failure domain configuration for your provider.

Balancing control plane machines

The control plane machine set balances control plane machines across the failure domains that are specified in the custom resource (CR).

When possible, the control plane machine set uses each failure domain equally to ensure appropriate fault tolerance. If there are fewer failure domains than control plane machines, failure domains are selected for reuse alphabetically by name. For clusters with no failure domains specified, all control plane machines are placed within a single failure domain.

Some changes to the failure domain configuration cause the control plane machine set to rebalance the control plane machines. For example, if you add failure domains to a cluster with fewer failure domains than control plane machines, the control plane machine set rebalances the machines across all available failure domains.

Recovery of failed control plane machines

The Control Plane Machine Set Operator automates the recovery of control plane machines. When a control plane machine is deleted, the Operator creates a replacement with the configuration that is specified in the ControlPlaneMachineSet custom resource (CR).

For clusters that use control plane machine sets, you can configure a machine health check. The machine health check deletes unhealthy control plane machines so that they are replaced.

If you configure a MachineHealthCheck resource for the control plane, set the value of maxUnhealthy to 1.

This configuration ensures that the machine health check takes no action when multiple control plane machines appear to be unhealthy. Multiple unhealthy control plane machines can indicate that the etcd cluster is degraded or that a scaling operation to replace a failed machine is in progress.

If the etcd cluster is degraded, manual intervention might be required. If a scaling operation is in progress, the machine health check should allow it to finish.

Additional resources