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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 makes use of 4 CRD objects and 1 Kubernetes secret object to manage machines. They are as follows,

  1. MachineClass: Represents a template that contains cloud provider specific details used to create machines.
  2. Machine: Represents a VM which is backed by the cloud provider.
  3. MachineSet: Represents a group of machines managed by the Machine Controller Manager.
  4. MachineDeployment: Represents a group of machine-sets managed by the Machine Controller Manager to allow updating machines.
  5. Secret: Represents a Kubernetes secret that stores cloudconfig (initialization scripts used to create VMs) and cloud specific credentials

Components of Machine Controller Manager

Machine Controller Manager is made up of 3 sub-controllers as of now. They are -

  1. Machine Controller: Used to create/update/delete machines. It is the only controller which actually talks to the cloud providers.
  2. MachineSet Controller: Used to manage MachineSets. This controller ensures that desired number of machines are always up and running healthy.
  3. MachineDeployment Controller: Used to update machines from one version to another by manipulating the MachineSet objects.
  4. Machine Safety Controller: A safety net controller that terminates orphan VMs and freezes MachineSet/MachineDeployment objects which are overshooting or timing out while trying to join nodes to the cluster.

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.