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Technical Overview

Kubernetes allows for extensions to its architecture via custom resources, which add a new endpoint in the Kubernetes API that stores and retrieves a collection API objects of a certain kind. The custom resources by themselves only enable store and retrieve structured data; to add business logic and specific functionality, custom controllers are needed. Controllers are clients of the Kubernetes API-Server that typically read an object's .spec, possibly do things, and then update the object's .status.

KubeVirt uses CRDs, controllers and other Kubernetes features, to represent and manage traditional virtual machines side by side with containers.

KubeVirt's primary CRD is the VirtualMachine (VM) resource, which manages the lifecycle of a VirtualMachineInstance (VMI) object that represents a single virtualized workload that executes once until completion (i.e., powered off).

Project Components

The key KubeVirt components are the virt-api, the virt-controller, the virt-handler, and the virt-launcher.

KubeVirt components

  • virt-api: This component provides a HTTP RESTful entrypoint to manage the virtual machines within the cluster.
  • virt-controller: This component is a Kubernetes controller that manages the lifecycle of VMs within the Kubernetes cluster.
  • virt-handler: This is a daemon that runs on each Kubernetes node. It is responsible for monitoring the state of VMIs according to Kubernetes and ensuring the corresponding libvirt domain is booted or halted accordingly. To perform these operations, the virt-handler has a communication channel with each virt-launcher that is used to manage the lifecycle of the qemu process within the virt-launcher pod.
  • virt-launcher: There is one per running VMI. This component directly manages the lifecycle of the qemu process within the VMI's pod and receives lifecycle commands from virt-handler.

Scripts

  • cluster-up/kubectl.sh: This is a wrapper around Kubernetes' kubectl command so that it can be run directly from this checkout without logging into a node.
  • cluster-up/virtctl.sh is a wrapper around virtctl. virtctl brings all virtual machine specific commands with it. It is supplement to kubectl. e.g. cluster-up/virtctl.sh console testvm.
  • cluster-up/cli.sh helps you creating ephemeral kubernetes and openshift clusters for testing. This is helpful when direct management or access to cluster nodes is necessary. e.g. cluster-up/cli.sh ssh node01.

Makefile Commands

  • make cluster-up: This will deploy a fresh environment, the contents of KUBE_PROVIDER will be used to determine which provider from the cluster directory will be deployed.
  • make cluster-sync: After deploying a fresh environment, or after making changes to code in this tree, this command will sync the Pods and DaemonSets in the running KubeVirt environment with the state of this tree.
  • make cluster-down: This will tear down a running KubeVirt environment.