Project: KubeVirt (App Definition and Development / Application Definition & Image Build)
File: components.md

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 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 CR 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. A client posts a new VMI definition to the K8s API Server. The K8s API Server validates the input and creates a `VMI` custom resource. The `virt-controller` observes the creation of the new `VMI` object and creates a corresponding pod. Kubernetes is scheduling the pod on a host. The `virt-controller` observes that a pod for the `VMI` got started and updates the `nodeName` field in the`VMI` object. Now that the `nodeName` is set, the responsibility transitions to the `virt-handler` for any further action. The `virt-handler` (DaemonSet) observes that a `VMI` got assigned to the host where it is running on. 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. A client deletes the `VMI` object through the `virt-api-server`. The `virt-handler` observes the deletion and turns off the domain. 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 definitions are kept as custom resources 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 From a high-level perspective the virt-controller has all the cluster wide virtualization functionality. This controller is responsible for monitoring the VMI (CRs) 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. 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

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. 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: Keep a cluster-level VMI spec in sync with a corresponding libvirt domain. Report domain state and spec changes to the cluster. Invoke node-centric plugins which can fulfill networking and storage requirements defined in VMI specs. An instance of `libvirtd` is present in every VMI pod. `virt-launcher` uses libvirtd to manage the life-cycle of the VMI process. 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. 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 can be used to prepare storage. Investigations are still in progress. 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. KubeVirt aims to seamlessly integrate into Kubernetes networking, with VMs connected to the same network as Pods, using the same resources and same APIs. Without any external extensions, VMs in KubeVirt are capable of using the pod network, Services, or NetworkPolicies. To implement this, VMs are only bound to network resources that are made available to the virt-launcher Pod's network namespace. With this model, a wide variety of networking plugins can be supported. We also care for advanced networking scenarios, such as connecting to service meshes, multiple network interfaces, SR-IOV, or MAC address pooling. To implement integration with these technologies, we are following the [KubeVirt Razor](architecture.md#the-razor), attempting to reuse existing container-focused technologies as much as is possible. You can find more information about this topic, including examples of specific plugins and extensions, in the user-guide: