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Components

Overview

KubeVirt consists of a set of services:

            |
    Cluster | (virt-controller)
            |
------------+---------------------------------------
            |
 Kubernetes | (VMI CRD)
            |
------------+---------------------------------------
            |
  DaemonSet | (virt-handler) (vm-pod M)
            |

M: Managed by KubeVirt
CRD: Custom Resource Definition

Example flow: Create and Delete a VMI

The following flow illustrates the communication flow between several (not all) components present in KubeVirt. In general the communication pattern can be considered to be a choreography, where all components act by themselves to realize the state provided by the VMI objects.

Client                     K8s API     VMI CRD  Virt Controller         VMI Handler
-------------------------- ----------- ------- ----------------------- ----------

                           listen <----------- WATCH /virtualmachines
                           listen <----------------------------------- WATCH /virtualmachines
                                                  |                       |
POST /virtualmachines ---> validate               |                       |
                           create ---> VMI ---> observe --------------> observe
                             |          |         v                       v
                           validate <--------- POST /pods              defineVMI
                           create       |         |                       |
                             |          |         |                       |
                           schedPod ---------> observe                    |
                             |          |         v                       |
                           validate <--------- PUT /virtualmachines       |
                           update ---> VMI ---------------------------> observe
                             |          |         |                    launchVMI
                             |          |         |                       |
                             :          :         :                       :
                             |          |         |                       |
DELETE /virtualmachines -> validate     |         |                       |
                           delete ----> * ---------------------------> observe
                             |                    |                    shutdownVMI
                             |                    |                       |
                             :                    :                       :

Disclaimer: The diagram above is not completely accurate, because there are temporary workarounds in place to avoid bugs and address some other stuff.

  1. A client posts a new VMI definition to the K8s API Server.
  2. The K8s API Server validates the input and creates a VMI custom resource definition (CRD) object.
  3. The virt-controller observes the creation of the new VMI object and creates a corrsponding pod.
  4. Kubernetes is scheduling the pod on a host.
  5. The virt-controller observes that a pod for the VMI got started and updates the nodeName field in theVMI object. Now that the nodeName is set, the responsibility transitions to the virt-handler for any further action.
  6. The virt-handler (DaemonSet) observes that a VMI got assigned to the host where it is running on.
  7. The virt-handler is using the VMI Specification and signals the creation of the corresponding domain using a libvirtd instance in the VMI's pod.
  8. A client deletes the VMI object through the virt-api-server.
  9. The virt-handler observes the deletion and turns off the domain.

virt-api-server

HTTP API server which serves as the entry point for all virtualization related flows.

The API Server is taking care to update the virtualization related custom resource definition (see below).

As the main entrypoint to KubeVirt it is responsible for defaulting and validation of the provided VMI CRDs.

VMI (CRD)

VMI definitions are kept as custom resource definitions inside the Kubernetes API server.

The VMI definition is defining all properties of the Virtual machine itself, for example

  • Machine type
  • CPU type
  • Amount of RAM and vCPUs
  • Number and type of NICs
  • …

virt-controller

From a high-level perspective the virt-controller has all the cluster wide virtualization functionality.

This controller is responsible for monitoring the VMI (CRDs) and managing the associated pods. Currently the controller will make sure to create and manage the life-cycle of the pods associated to the VMI objects.

A VMI object will always be associated to a pod during it's life-time, however, due to i.e. migration of a VMI the pod instance might change over time.

virt-launcher

For every VMI object one pod is created. This pod's primary container runs the virt-launcher KubeVirt component.

Kubernetes or the kubelet is not running the VMIs itself. Instead a daemon on every host in the cluster will take care to launch a VMI process for every pod which is associated to a VMI object whenever it is getting scheduled on a host.

The main purpose of the virt-launcher Pod is to provide the cgroups and namespaces, which will be used to host the VMI process.

virt-handler signals virt-launcher to start a VMI by passing the VMI's CRD object to virt-launcher. virt-launcher then uses a local libvirtd instance within its container to start the VMI. From there virt-launcher monitors the VMI process and terminates once the VMI has exited.

If the Kubernetes runtime attempts to shutdown the virt-launcher pod before the VMI has exited, virt-launcher forwards signals from Kubernetes to the VMI process and attempts to hold off the termination of the pod until the VMI has shutdown successfully.

virt-handler

Every host needs a single instance of virt-handler. It can be delivered as a DaemonSet.

Like the virt-controller, the virt-handler is also reactive and is watching for changes of the VMI object, once detected it will perform all necessary operations to change a VMI to meet the required state.

This behavior is similar to the choreography between the Kubernetes API Server and the kubelet.

The main areas which virt-handler has to cover are:

  1. Keep a cluster-level VMI spec in sync with a corresponding libvirt domain.
  2. Report domain state and spec changes to the cluster.
  3. Invoke node-centric plugins which can fulfill networking and storage requirements defined in VMI specs.

libvirtd

An instance of libvirtd is present in every VMI pod. virt-launcher uses libvirtd to manage the life-cycle of the VMI process.

Additional components

The components above are essential to deliver core virtualization functionality in your cluster. However fully featured virtual machines require more than just plain virtualization functionality. Beyond virtualization they also require reliable storage and networking functionality to be fully usable.

The components below will be providing this additional functionality if the functionality is not provided by kubernetes itself.

Storage

We will try to leverage as much of Kubernetes regarding to mounting and preparing images for VMI. However, virt-handler may provide a plugin mechanism to allow storage mounting and setup from the host, if the KubeVirt requirements do not fit into the Kubernetes storage scenarios.

Since host side preparation of storage may not be enough, a cluster-wide Storage Controller can be used to prepare storage.

Investigations are still in progress.

Storage Controller

Such a controller will not be part of KubeVirt itself.

However KubeVirt might define a Storage CRD along side with a flow description which will allow such a controller seamless integration into KubeVirt.

Networking

We will try to leverage as much of Kubernetes networking plugin mechanisms (e.g. CNI). However, virt-handler may provide a plugin mechanism to allow network setup on a host, if the KubeVirt requirements do not fit into the Kubernetes storage scenarios.

Since host side preparation of network interfaces may not be enough, a cluster-wide Network Controller can be used to prepare the network.

Investigations are still in progress.

Network Controller

Such a controller will not be part of KubeVirt itself.

However KubeVirt might define a Networking CRD along side with a flow description which will allow such a controller seamless integration into KubeVirt.