Lifecycle of a VM pod

This document describes the lifecycle of VM pod managed by Virtlet.

This description omits the details of volume setup (using flexvolumes), handling of logs, the VM console and port forwarding (done by streaming server), or port forwarding.

Assumptions

Communication between kubelet and Virtlet goes through criproxy which directs requests to Virtlet only if the requests concern a pod that has Virtlet-specific annotation or an image that has Virtlet-specific prefix.

Lifecycle

VM Pod Startup

  • A pod is created in Kubernetes cluster, either directly by the user or via some other mechanism such as a higher-level Kubernetes object managed by kube-controller-manager (ReplicaSet, DaemonSet etc.).
  • Scheduler places the pod on a node based on the requested resources (CPU, memory, etc.) as well as pod's nodeSelector and pod/node affinity constraints, taints/tolerations and so on.
  • kubelet running on the target node accepts the pod.
  • kubelet invokes a CRI call RunPodSandbox to create the pod sandbox which will enclose all the containers in the pod definition. Note that at this point no information about the containers within the pod is passed to the call. kubelet can later request the information about the pod by means of PodSandboxStatus calls.
  • If there's a Virtlet-specific annotation kubernetes.io/target-runtime: virtlet.cloud, CRI proxy passes the call to Virtlet.
  • Virtlet saves sandbox metadata in its internal database, sets up the network namespace and then uses internal tapmanager mechanism to invoke ADD operation via the CNI plugin as specified by the CNI configuration on the node.
  • The CNI plugin configures the network namespace by setting up network interfaces, IP addresses, routes, iptables rules and so on, and returns the network configuration information to the caller as described in the CNI spec.
  • Virtlet's tapmanager mechanism adjusts the configuration of the network namespace to make it work with the VM.
  • After creating the sandbox, kubelet starts the containers defined in the pod sandbox. Currently, Virtlet supports just one container per VM pod. So, the VM pod startup steps after this one describe the startup of this single container.
  • Depending on the image pull police of the container, kubelet checks if the image needs to be pulled by means of ImageStatus call and then uses PullImage CRI call to pull the image if it doesn't exist or if imagePullPolicy: Always is used.
  • If PullImage is invoked, Virtlet resolves the image location based on the image name translation configuration, then downloads the file and stores it in the image store.
  • After the image is ready (no pull was needed or the PullImage call completed successfully), kubelet uses CreateContainer CRI call to create the container in the pod sandbox using the specified image.
  • Virtlet uses the sandbox and container metadata to generate libvirt domain definition, using vmwrapper binary as the emulator and without specifying any network configuration in the domain.
  • After CreateContainer call completes, kubelet invokes StartContainer call on the newly created container.
  • Virtlet starts the libvirt domain. libvirt invokes vmwrapper as the emulator, passing it the necessary command line arguments as well as environment variables set by Virtlet. vmwrapper uses the environment variable values passed to Virtlet to communicate with tapmanager over an Unix domain socket, retrieving a file descriptor for a tap device and/or pci address of SR-IOV device set up by tapmanager. tapmanager uses its own simple protocol to communicate with vmwrapper because it needs to send file descriptors over the socket. This is not usually supported by RPC libraries, see e.g. grpc/grpc#11417. vmwrapper then updates the command line arguments to include the network interface information and execs the actual emulator (qemu).

At this point the VM is running and accessible via the network, and the pod is in Running state as well as it's only container.

Deleting a pod

This sequence is initiated when the pod is deleted, either by means of kubectl delete or a controller manager action due to deletion or downscaling of a higher-level object.

  • kubelet notices the pod being deleted.
  • kubelet invokes StopContainer CRI calls which is getting forwared to Virtlet based on the containing pod sandbox annotations.
  • Virtlet stops the libvirt domain. libvirt sends a signal to qemu, which initiates the shutdown. If it doesn't quit in a reasonable time determined by pod's termination grace period, Virtlet will forcibly terminate the domain, thus killing the qemu process.
  • After all the containers in the pod (the single container in case of Virtlet VM pod) are stopped, kubelet invokes StopPodSandbox CRI call.
  • Virtlet asks its tapmanager to remove pod from the network by means of CNI DEL command.
  • after StopPodSandbox returns, the pod sandbox will be eventually GC'd by kubelet by means of RemovePodSandbox CRI call.
  • Upon RemovePodSandbox, Virtlet removes the pod metadata from its internal database.