Deploying machine health checks

About machine health checks
- Limitations when deploying machine health checks
Sample MachineHealthCheck resource
- Short-circuiting machine health check remediation
Creating a MachineHealthCheck resource
About power-based remediation of bare metal

You can configure and deploy a machine health check to automatically repair damaged machines in a machine pool.

This process is not applicable for clusters with manually provisioned machines. You can use the advanced machine management and scaling capabilities only in clusters where the Machine API is operational.

About machine health checks

Machine health checks automatically repair unhealthy machines in a particular machine pool.

To monitor machine health, create a resource to define the configuration for a controller. Set a condition to check, such as staying in the NotReady status for five minutes or displaying a permanent condition in the node-problem-detector, and a label for the set of machines to monitor.

You cannot apply a machine health check to a machine with the master role.

The controller that observes a MachineHealthCheck resource checks for the defined condition. If a machine fails the health check, the machine is automatically deleted and one is created to take its place. When a machine is deleted, you see a machine deleted event.

To limit disruptive impact of the machine deletion, the controller drains and deletes only one node at a time. If there are more unhealthy machines than the maxUnhealthy threshold allows for in the targeted pool of machines, remediation stops and therefore enables manual intervention.

Consider the timeouts carefully, accounting for workloads and requirements.

Long timeouts can result in long periods of downtime for the workload on the unhealthy machine.
Too short timeouts can result in a remediation loop. For example, the timeout for checking the NotReady status must be long enough to allow the machine to complete the startup process.

To stop the check, remove the resource.

Limitations when deploying machine health checks

There are limitations to consider before deploying a machine health check:

Only machines owned by a machine set are remediated by a machine health check.
Control plane machines are not currently supported and are not remediated if they are unhealthy.
If the node for a machine is removed from the cluster, a machine health check considers the machine to be unhealthy and remediates it immediately.
If the corresponding node for a machine does not join the cluster after the nodeStartupTimeout, the machine is remediated.
A machine is remediated immediately if the Machine resource phase is Failed.

Additional resources

For more information about the node conditions you can define in a MachineHealthCheck CR, see About listing all the nodes in a cluster.
For more information about short-circuiting, see Short-circuiting machine health check remediation.

Sample MachineHealthCheck resource

The MachineHealthCheck resource for all cloud-based installation types, and other than bare metal, resembles the following YAML file:

apiVersion: machine.openshift.io/v1beta1
kind: MachineHealthCheck
metadata:
  name: example (1)
  namespace: openshift-machine-api
spec:
  selector:
    matchLabels:
      machine.openshift.io/cluster-api-machine-role: <role> (2)
      machine.openshift.io/cluster-api-machine-type: <role> (2)
      machine.openshift.io/cluster-api-machineset: <cluster_name>-<label>-<zone> (3)
  unhealthyConditions:
  - type:    "Ready"
    timeout: "300s" (4)
    status: "False"
  - type:    "Ready"
    timeout: "300s" (4)
    status: "Unknown"
  maxUnhealthy: "40%" (5)
  nodeStartupTimeout: "10m" (6)

1	Specify the name of the machine health check to deploy.
2	Specify a label for the machine pool that you want to check.
3	Specify the machine set to track in `<cluster_name>-<label>-<zone>` format. For example, `prod-node-us-east-1a`.
4	Specify the timeout duration for a node condition. If a condition is met for the duration of the timeout, the machine will be remediated. Long timeouts can result in long periods of downtime for a workload on an unhealthy machine.
5	Specify the amount of machines allowed to be concurrently remediated in the targeted pool. This can be set as a percentage or an integer. If the number of unhealthy machines exceeds the limit set by `maxUnhealthy`, remediation is not performed.
6	Specify the timeout duration that a machine health check must wait for a node to join the cluster before a machine is determined to be unhealthy.

The matchLabels are examples only; you must map your machine groups based on your specific needs.

Short-circuiting machine health check remediation

Short circuiting ensures that machine health checks remediate machines only when the cluster is healthy. Short-circuiting is configured through the maxUnhealthy field in the MachineHealthCheck resource.

If the user defines a value for the maxUnhealthy field, before remediating any machines, the MachineHealthCheck compares the value of maxUnhealthy with the number of machines within its target pool that it has determined to be unhealthy. Remediation is not performed if the number of unhealthy machines exceeds the maxUnhealthy limit.

If maxUnhealthy is not set, the value defaults to 100% and the machines are remediated regardless of the state of the cluster.

The appropriate maxUnhealthy value depends on the scale of the cluster you deploy and how many machines the MachineHealthCheck covers. For example, you can use the maxUnhealthy value to cover multiple machine sets across multiple availability zones so that if you lose an entire zone, your maxUnhealthy setting prevents further remediation within the cluster.

The maxUnhealthy field can be set as either an integer or percentage. There are different remediation implementations depending on the maxUnhealthy value.

Setting `maxUnhealthy` by using an absolute value

If maxUnhealthy is set to 2:

Remediation will be performed if 2 or fewer nodes are unhealthy
Remediation will not be performed if 3 or more nodes are unhealthy

These values are independent of how many machines are being checked by the machine health check.

Setting `maxUnhealthy` by using percentages

If maxUnhealthy is set to 40% and there are 25 machines being checked:

Remediation will be performed if 10 or fewer nodes are unhealthy
Remediation will not be performed if 11 or more nodes are unhealthy

If maxUnhealthy is set to 40% and there are 6 machines being checked:

Remediation will be performed if 2 or fewer nodes are unhealthy
Remediation will not be performed if 3 or more nodes are unhealthy

The allowed number of machines is rounded down when the percentage of maxUnhealthy machines that are checked is not a whole number.

Creating a MachineHealthCheck resource

You can create a MachineHealthCheck resource for all MachineSets in your cluster. You should not create a MachineHealthCheck resource that targets control plane machines.

Prerequisites

Install the oc command line interface.

Procedure

Create a healthcheck.yml file that contains the definition of your machine health check.
Apply the healthcheck.yml file to your cluster:
```
$ oc apply -f healthcheck.yml
```

You can configure and deploy a machine health check to detect and repair unhealthy bare metal nodes.

About power-based remediation of bare metal

In a bare metal cluster, remediation of nodes is critical to ensuring the overall health of the cluster. Physically remediating a cluster can be challenging and any delay in putting the machine into a safe or an operational state increases the time the cluster remains in a degraded state, and the risk that subsequent failures might bring the cluster offline. Power-based remediation helps counter such challenges.

Instead of reprovisioning the nodes, power-based remediation uses a power controller to power off an inoperable node. This type of remediation is also called power fencing.

OKD uses the MachineHealthCheck controller to detect faulty bare metal nodes. Power-based remediation is fast and reboots faulty nodes instead of removing them from the cluster.

Power-based remediation provides the following capabilities:

Allows the recovery of control plane nodes
Reduces the risk data loss in hyperconverged environments
Reduces the downtime associated with recovering physical machines

MachineHealthChecks on bare metal

Machine deletion on bare metal cluster triggers reprovisioning of a bare metal host. Usually bare metal reprovisioning is a lengthy process, during which the cluster is missing compute resources and applications might be interrupted. To change the default remediation process from machine deletion to host power-cycle, annotate the MachineHealthCheck resource with the machine.openshift.io/remediation-strategy: external-baremetal annotation.

After you set the annotation, unhealthy machines are power-cycled by using BMC credentials.

Understanding the remediation process

The remediation process operates as follows:

The MachineHealthCheck (MHC) controller detects that a node is unhealthy.
The MHC notifies the bare metal machine controller which requests to power-off the unhealthy node.
After the power is off, the node is deleted, which allows the cluster to reschedule the affected workload on other nodes.
The bare metal machine controller requests to power on the node.
After the node is up, the node re-registers itself with the cluster, resulting in the creation of a new node.
After the node is recreated, the bare metal machine controller restores the annotations and labels that existed on the unhealthy node before its deletion.

If the power operations did not complete, the bare metal machine controller triggers the reprovisioning of the unhealthy node unless this is a control plane node (also known as the master node) or a node that was provisioned externally.

Creating a MachineHealthCheck resource for bare metal