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⚠️ We are in the progress of migrating and deprecating all the in-tree providers to OOT. Please avoid making any new feature enhancements to the intree providers. Kindly make it on the OOT providers available here. More details on adding new OOT providers can be found here.


Machine Controller Manager aka MCM is a group of cooperative controllers that manage the lifecycle of the worker machines. It is inspired by the design of Kube Controller Manager in which various sub controllers manage their respective Kubernetes Clients. MCM gives you the following benefits:

  • seamlessly manage machines/nodes with a declarative API (of course, across different cloud providers)
  • integrate generically with the cluster autoscaler
  • plugin with tools such as the node-problem-detector
  • transport the immutability design principle to machine/nodes
  • implement e.g. rolling upgrades of machines/nodes

MCM supports following providers. These provider code is maintained externally (out-of-tree), and the links for the same are linked below:

It can easily be extended to support other cloud providers as well.

Example of managing machine:

kubectl create/get/delete machine vm1

Key terminologies

Nodes/Machines/VMs are different terminologies used to represent similar things. We use these terms in the following way

  1. VM: A virtual machine running on any cloud provider. It could also refer to a physical machine (PM) in case of a bare metal setup.
  2. Node: Native kubernetes node objects. The objects you get to see when you do a “kubectl get nodes”. Although nodes can be either physical/virtual machines, for the purposes of our discussions it refers to a VM.
  3. Machine: A VM that is provisioned/managed by the Machine Controller Manager.

Design of Machine Controller Manager

The design of the Machine Controller Manager is influenced by the Kube Controller Manager, where-in multiple sub-controllers are used to manage the Kubernetes clients.

Design Principles

It’s designed to run in the master plane of a Kubernetes cluster. It follows the best principles and practices of writing controllers, including, but not limited to:

  • Reusing code from kube-controller-manager
  • leader election to allow HA deployments of the controller
  • workqueues and multiple thread-workers
  • SharedInformers that limit to minimum network calls, de-serialization and provide helpful create/update/delete events for resources
  • rate-limiting to allow back-off in case of network outages and general instability of other cluster components
  • sending events to respected resources for easy debugging and overview
  • Prometheus metrics, health and (optional) profiling endpoints

Objects of Machine Controller Manager

Machine Controller Manager reconciles a set of Custom Resources namely MachineDeployment, MachineSet and Machines which are managed & monitored by their controllers MachineDeployment Controller, MachineSet Controller, Machine Controller respectively along with another cooperative controller called the Safety Controller.

Machine Controller Manager makes use of 4 CRD objects and 1 Kubernetes secret object to manage machines. They are as follows:

Custom ResourceObjectDescription
MachineClassA MachineClass represents a template that contains cloud provider specific details used to create machines.
MachineA Machine represents a VM which is backed by the cloud provider.
MachineSetA MachineSet ensures that the specified number of Machine replicas are running at a given point of time.
MachineDeploymentA MachineDeployment provides a declarative update for MachineSet and Machines.
SecretA Secret here is a Kubernetes secret that stores cloudconfig (initialization scripts used to create VMs) and cloud specific credentials.

See here for CRD API Documentation

Components of Machine Controller Manager

MachineDeployment controllerMachine Deployment controller reconciles the MachineDeployment objects and manages the lifecycle of MachineSet objects. MachineDeployment consumes provider specific MachineClass` in its spec.template.spec which is the template of the VM spec that would be spawned on the cloud by MCM.
MachineSet controllerMachineSet controller reconciles the MachineSet objects and manages the lifecycle of Machine objects.
Safety controllerThere is a Safety Controller responsible for handling the unidentified or unknown behaviours from the cloud providers. Safety Controller:
  • freezes the MachineDeployment controller and MachineSet controller if the number of Machine objects goes beyond a certain threshold on top of Spec.replicas. It can be configured by the flag --safety-up or --safety-down and also --machine-safety-overshooting-period`.
  • freezes the functionality of the MCM if either of the target-apiserver or the control-apiserver is not reachable.
  • unfreezes the MCM automatically once situation is resolved to normal. A freeze label is applied on MachineDeployment/MachineSet to enforce the freeze condition.

Along with the above Custom Controllers and Resources, MCM requires the MachineClass to use K8s Secret that stores cloudconfig (initialization scripts used to create VMs) and cloud specific credentials. All these controllers work in an co-operative manner. They form a parent-child relationship with MachineDeployment Controller being the grandparent, MachineSet Controller being the parent, and Machine Controller being the child.


To start using or developing the Machine Controller Manager, see the documentation in the /docs repository, please find the index here.


An FAQ is available here

Cluster-api Implementation


Relevant for: Users , Operators , Developers