Mirantis/virtlet

# Overview

The following figure provides a general view of Virtlet architecture:

![Virtlet Architecture](img/virtlet.png)

Virtlet consists of the following components:

* [Virtlet manager](https://github.com/Mirantis/virtlet/tree/master/cmd/virtlet)
  which implements CRI interface for virtualization and image handling
* [libvirt](http://libvirt.org) instance
* [vmwrapper](https://github.com/Mirantis/virtlet/tree/master/cmd/vmwrapper)
  which is responsible for setting up the environment for emulator
* the emulator, currently [qemu](http://www.qemu-project.org/) with
  KVM support (with a possibility to disable KVM)

In addition to the above, our example setup uses
[CRI proxy](https://github.com/Mirantis/criproxy) which provides the
possibility to mix dockershim and VM based workloads on the same k8s
node.

## Virtlet manager

The main binary is responsible for providing API fullfiling
[CRI specification](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/container-runtime-interface.md).
It serves the requests from kubelet by doing the following:

* setting up libvirt VM environment (virtual drives, network
  interfaces, trimming resources like RAM, CPU)
* performing CNI setup
* telling libvirt to call vmwrapper instead of using emulator directly
* querying libvirt for VM statuses
* instructing libvirt to stop VMs
* performing CNI teardown
* and finally calling libvirt to tear down VM environment.

## vmwrapper

`vmrapper` is run by libvirt and wraps the emulator (QEMU/KVM).  It
requests tap file descriptor from Virtlet, adds command line
arguments needed by the emulator to use the tap device and then
`exec`s the emulator.

## CRI Proxy

[CRI Proxy](https://github.com/Mirantis/criproxy) is an external
project provides a way to run multiple CRI implementations on the same
node, e.g. Virtlet and dockershim. This is handy for running
infrastructure pods such as kube-proxy. It's possible to reuse
dockershim component from kubelet to make it possible to have Docker
as one of CRI implementations on the multi-runtime nodes.