The Machine API is a combination of primary resources that are based on the upstream Cluster API project and custom OKD resources.
For OKD 4 clusters, the Machine API performs all node host provisioning management actions after the cluster installation finishes. Because of this system, OKD 4 offers an elastic, dynamic provisioning method on top of public or private cloud infrastructure.
The two primary resources are:
A fundamental unit that describes the host for a node. A machine has a
providerSpec specification, which describes the types of compute nodes that are offered for different cloud platforms. For example, a machine type for a worker node on Amazon Web Services (AWS) might define a specific machine type and required metadata.
- Machine sets
MachineSet resources are groups of machines. Machine sets are to machines as replica sets are to pods. If you need more machines or must scale them down, you change the replicas field on the machine set to meet your compute need.
Control plane machines cannot be managed by machine sets.
The following custom resources add more capabilities to your cluster:
- Machine autoscaler
MachineAutoscaler resource automatically scales machines in a cloud. You can set the minimum and maximum scaling boundaries for nodes in a specified machine set, and the machine autoscaler maintains that range of nodes. The
MachineAutoscaler object takes effect after a
ClusterAutoscaler object exists. Both
MachineAutoscaler resources are made available by the
- Cluster autoscaler
This resource is based on the upstream cluster autoscaler project. In the OKD implementation, it is integrated with the Machine API by extending the machine set API. You can set cluster-wide scaling limits for resources such as cores, nodes, memory, GPU, and so on. You can set the priority so that the cluster prioritizes pods so that new nodes are not brought online for less important pods. You can also set the scaling policy so that you can scale up nodes but not scale them down.
- Machine health check
MachineHealthCheck resource detects when a machine is unhealthy, deletes it, and, on supported platforms, makes a new machine.
In OKD version 3.11, you could not roll out a multi-zone architecture easily because the cluster did not manage machine provisioning. Beginning with OKD version 4.1, this process is easier. Each machine set is scoped to a single zone, so the installation program sends out machine sets across availability zones on your behalf. And then because your compute is dynamic, and in the face of a zone failure, you always have a zone for when you must rebalance your machines. In global Azure regions that do not have multiple availability zones, you can use availability sets to ensure high availability. The autoscaler provides best-effort balancing over the life of a cluster.