# VirtualMachineInstanceReplicaSet A *VirtualMachineInstanceReplicaSet* tries to ensures that a specified number of VirtualMachineInstance replicas are running at any time. In other words, a *VirtualMachineInstanceReplicaSet* makes sure that a VirtualMachineInstance or a homogeneous set of VirtualMachineInstances is always up and ready. It is very similar to a [Kubernetes ReplicaSet](https://kubernetes.io/docs/concepts/workloads/controllers/replicaset/). No state is kept and no guarantees about the maximum number of VirtualMachineInstance replicas which are up are given. For example, the *VirtualMachineInstanceReplicaSet* may decide to create new replicas if possibly still running VMs are entering an unknown state. ## Using VirtualMachineInstanceReplicaSet The *VirtualMachineInstanceReplicaSet* allows us to specify a *VirtualMachineInstanceTemplate* in `spec.template`. It consists of `ObjectMetadata` in `spec.template.metadata`, and a `VirtualMachineInstanceSpec` in `spec.template.spec`. The specification of the virtual machine is equal to the specification of the virtual machine in the `VirtualMachineInstance` workload. `spec.replicas` can be used to specify how many replicas are wanted. If unspecified, the default value is 1. This value can be updated anytime. The controller will react to the changes. `spec.selector` is used by the controller to keep track of managed virtual machines. The selector specified there must be able to match the virtual machine labels as specified in `spec.template.metadata.labels`. If the selector does not match these labels, or they are empty, the controller will simply do nothing except from logging an error. The user is responsible for not creating other virtual machines or *VirtualMachineInstanceReplicaSets* which conflict with the selector and the template labels. ## Exposing a VirtualMachineInstanceReplicaSet as a Service A VirtualMachineInstanceReplicaSet could be exposed as a service. When this is done, one of the VirtualMachineInstances replicas will be picked for the actual delivery of the service. For example, exposing SSH port (22) as a ClusterIP service using virtctl on a VirtualMachineInstanceReplicaSet: ``` $ virtctl expose vmirs vmi-ephemeral --name vmiservice --port 27017 --target-port 22 ``` All service exposure options that apply to a VirtualMachineInstance apply to a VirtualMachineInstanceReplicaSet. See [Exposing VirtualMachineInstance](http://kubevirt.io/user-guide/#/workloads/virtual-machines/expose-service) for more details. ## When to use a VirtualMachineInstanceReplicaSet > **Note:** The base assumption is that referenced disks are read-only or that the VMIs are writing internally to a tmpfs. The most obvious volume sources for VirtualMachineInstanceReplicaSets which KubeVirt supports are referenced below. If other types are used **data corruption** is possible. Using VirtualMachineInstanceReplicaSet is the right choice when one wants many identical VMs and does not care about maintaining any disk state after the VMs are terminated. [Volume types](https://kubevirtlegacy.gitbook.io/user-guide/docs/virtual_machines/disks_and_volumes) which work well in combination with a VirtualMachineInstanceReplicaSet are: * **cloudInitNoCloud** * **ephemeral** * **containerDisk** * **emptyDisk** * **configMap** * **secret** * any other type, if the VMI writes internally to a tmpfs ### Fast starting ephemeral Virtual Machines This use-case involves small and fast booting VMs with little provisioning performed during initialization. In this scenario, migrations are not important. Redistributing VM workloads between Nodes can be achieved simply by deleting managed VirtualMachineInstances which are running on an overloaded Node. The `eviction` of such a VirtualMachineInstance can happen by directly deleting the VirtualMachineInstance instance (KubeVirt aware workload redistribution) or by deleting the corresponding Pod where the Virtual Machine runs in (Only Kubernetes aware workload redistribution). ### Slow starting ephemeral Virtual Machines In this use-case one has big and slow booting VMs, and complex or resource intensive provisioning is done during boot. More specifically, the timespan between the creation of a new VM and it entering the ready state is long. In this scenario, one still does not care about the state, but since re-provisioning VMs is expensive, migrations are important. Workload redistribution between Nodes can be achieved by migrating VirtualMachineInstances to different Nodes. A workload redistributor needs to be aware of KubeVirt and create migrations, instead of `evicting` VirtualMachineInstances by deletion. > **Note:** The simplest form of having a migratable ephemeral VirtualMachineInstance, will be to use local storage based on `ContainerDisks` in combination with a file based backing store. However, migratable backing store support has not officially landed yet in KubeVirt and is untested. #### Example ``` apiVersion: kubevirt.io/v1alpha3 kind: VirtualMachineInstanceReplicaSet metadata: name: testreplicaset spec: replicas: 3 selector: matchLabels: myvmi: myvmi template: metadata: name: test labels: myvmi: myvmi spec: domain: devices: disks: - disk: name: containerdisk resources: requests: memory: 64M volumes: - name: containerdisk containerDisk: image: kubevirt/cirros-container-disk-demo:latest ``` Saving this manifest into `testreplicaset.yaml` and submitting it to Kubernetes will create three virtual machines based on the template. ``` $ kubectl create -f testreplicaset.yaml virtualmachineinstancereplicaset "testreplicaset" created $ kubectl describe vmirs testreplicaset Name: testreplicaset Namespace: default Labels: Annotations: API Version: kubevirt.io/v1alpha3 Kind: VirtualMachineInstanceReplicaSet Metadata: Cluster Name: Creation Timestamp: 2018-01-03T12:42:30Z Generation: 0 Resource Version: 6380 Self Link: /apis/kubevirt.io/v1alpha3/namespaces/default/virtualmachineinstancereplicasets/testreplicaset UID: 903a9ea0-f083-11e7-9094-525400ee45b0 Spec: Replicas: 3 Selector: Match Labels: Myvmi: myvmi Template: Metadata: Creation Timestamp: Labels: Myvmi: myvmi Name: test Spec: Domain: Devices: Disks: Disk: Name: containerdisk Volume Name: containerdisk Resources: Requests: Memory: 64M Volumes: Name: containerdisk Container Disk: Image: kubevirt/cirros-container-disk-demo:latest Status: Conditions: Ready Replicas: 2 Replicas: 3 Events: Type Reason Age From Message ---- ------ ---- ---- ------- Normal SuccessfulCreate 13s virtualmachineinstancereplicaset-controller Created virtual machine: testh8998 Normal SuccessfulCreate 13s virtualmachineinstancereplicaset-controller Created virtual machine: testf474w Normal SuccessfulCreate 13s virtualmachineinstancereplicaset-controller Created virtual machine: test5lvkd ``` `Replicas` is `3` and `Ready Replicas` is `2`. This means that at the moment when showing the status, three Virtual Machines were already created, but only two are running and ready. ### Scaling via the Scale Subresource > **Note:** This requires the `CustomResourceSubresources` [feature gate](https://kubevirtlegacy.gitbook.io/user-guide/operations/activating_feature_gates#how-to-activate-a-feature-gate) to be enabled for clusters prior to 1.11. The `VirtualMachineInstanceReplicaSet` supports the `scale` subresource. As a consequence it is possible to scale it via `kubectl`: ``` $ kubectl scale vmirs myvmirs --replicas 5 ``` ### Using the Horizontal Pod Autoscaler > **Note:** This requires at cluster newer or equal to 1.11. The [HorizontalPodAutoscaler](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/) (HPA) can be used with a `VirtualMachineInstanceReplicaSet`. Simply reference it in the spec of the autoscaler: ``` apiVersion: autoscaling/v1 kind: HorizontalPodAutoscaler metadata: name: myhpa spec: scaleTargetRef: kind: VirtualMachineInstanceReplicaSet name: vmi-replicaset-cirros apiVersion: kubevirt.io/v1alpha3 minReplicas: 3 maxReplicas: 10 targetCPUUtilizationPercentage: 50 ``` or use `kubectl autoscale` to define the HPA via the commandline: ``` $ kubectl autoscale vmirs vmi-replicaset-cirros --min=3 --max=10 ```