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docs: Update architecture document

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## Overview
This is an architectural overview of Kata Containers, based on the 2.0 release.
Kata Containers is an open source community working to build a secure
container [runtime](#runtime) with lightweight virtual machines (VM's)
that feel and perform like standard Linux containers, but provide
stronger [workload](#workload) isolation using hardware
[virtualization](#virtualization) technology as a second layer of
defence.
The primary deliverable of the Kata Containers project is a CRI friendly shim. There is also a CRI friendly library API behind them.
Kata Containers runs on [multiple architectures](../../src/runtime/README.md#platform-support)
and supports [multiple hypervisors](../hypervisors.md).
The [Kata Containers runtime](../../src/runtime)
is compatible with the [OCI](https://github.com/opencontainers) [runtime specification](https://github.com/opencontainers/runtime-spec)
and therefore works seamlessly with the [Kubernetes\* Container Runtime Interface (CRI)](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/container-runtime-interface.md)
through the [CRI-O\*](https://github.com/kubernetes-incubator/cri-o) and
[Containerd\*](https://github.com/containerd/containerd) implementation.
Kata Containers creates a QEMU\*/KVM virtual machine for pod that `kubelet` (Kubernetes) creates respectively.
The [`containerd-shim-kata-v2` (shown as `shimv2` from this point onwards)](../../src/runtime/cmd/containerd-shim-kata-v2/)
is the Kata Containers entrypoint, which
implements the [Containerd Runtime V2 (Shim API)](https://github.com/containerd/containerd/tree/master/runtime/v2) for Kata.
Before `shimv2` (as done in [Kata Containers 1.x releases](https://github.com/kata-containers/runtime/releases)), we need to create a `containerd-shim` and a [`kata-shim`](https://github.com/kata-containers/shim) for each container and the Pod sandbox itself, plus an optional [`kata-proxy`](https://github.com/kata-containers/proxy) when VSOCK is not available. With `shimv2`, Kubernetes can launch Pod and OCI compatible containers with one shim (the `shimv2`) per Pod instead of `2N+1` shims, and no standalone `kata-proxy` process even if no VSOCK is available.
![Kubernetes integration with shimv2](arch-images/shimv2.svg)
The container process is then spawned by
[`kata-agent`](../../src/agent), an agent process running
as a daemon inside the virtual machine. `kata-agent` runs a [`ttRPC`](https://github.com/containerd/ttrpc-rust) server in
the guest using a VIRTIO serial or VSOCK interface which QEMU exposes as a socket
file on the host. `shimv2` uses a `ttRPC` protocol to communicate with
the agent. This protocol allows the runtime to send container management
commands to the agent. The protocol is also used to carry the I/O streams (stdout,
stderr, stdin) between the containers and the manage engines (e.g. CRI-O or containerd).
For any given container, both the init process and all potentially executed
commands within that container, together with their related I/O streams, need
to go through the VSOCK interface exported by QEMU.
The container workload, that is, the actual OCI bundle rootfs, is exported from the
host to the virtual machine. In the case where a block-based graph driver is
configured, `virtio-scsi` will be used. In all other cases a `virtio-fs` VIRTIO mount point
will be used. `kata-agent` uses this mount point as the root filesystem for the
container processes.
This document is a summary of the Kata Containers architecture.
## Virtualization
How Kata Containers maps container concepts to virtual machine technologies, and how this is realized in the multiple
hypervisors and VMMs that Kata supports is described within the [virtualization documentation](./virtualization.md)
For details on how Kata Containers maps container concepts to VM
technologies, and how this is realized in the multiple hypervisors and
VMMs that Kata supports see the
[virtualization documentation](./virtualization.md).
## Compatibility
The [Kata Containers runtime](../../src/runtime) is compatible with
the [OCI](https://github.com/opencontainers)
[runtime specification](https://github.com/opencontainers/runtime-spec)
and therefore works seamlessly with the
[Kubernetes Container Runtime Interface (CRI)](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-node/container-runtime-interface.md)
through the [CRI-O](https://github.com/kubernetes-incubator/cri-o)
and [containerd](https://github.com/containerd/containerd)
implementations.
Kata Containers provides a ["shimv2"](#shim-v2-architecture) compatible runtime.
## Shim v2 architecture
The Kata Containers runtime is shim v2 ("shimv2") compatible. This
section explains what this means.
### History
In the old [Kata 1.x architecture](https://github.com/kata-containers/documentation/blob/master/design/architecture.md),
the Kata [runtime](#runtime) was an executable called `kata-runtime`.
The container manager called this executable multiple times when
creating each container. Each time the runtime was called a different
OCI command-line verb was provided. This architecture was simple, but
not well suited to creating VM based containers due to the issue of
handling state between calls. Additionally, the architecture suffered
from performance issues related to continually having to spawn new
instances of the runtime binary, and
[Kata shim](https://github.com/kata-containers/shim) and
[Kata proxy](https://github.com/kata-containers/proxy) processes for systems
that did not provide VSOCK.
### An improved architecture
The
[containerd runtime shimv2 architecture](https://github.com/containerd/containerd/tree/main/runtime/v2)
or _shim API_ architecture resolves the issues with the old
architecture by defining a set of shimv2 APIs that a compatible
runtime implementation must supply. Rather than calling the runtime
binary multiple times for each new container, the shimv2 architecture
runs a single instance of the runtime binary (for any number of
containers). This improves performance and resolves the state handling
issue.
The shimv2 API is similar to the
[OCI runtime](https://github.com/opencontainers/runtime-spec)
API in terms of the way the container lifecycle is split into
different verbs. Rather than calling the runtime multiple times, the
container manager creates a socket and passes it to the shimv2
runtime. The socket is a bi-directional communication channel that
uses a gRPC based protocol to allow the container manager to send API
calls to the runtime, which returns the result to the container
manager using the same channel.
The shimv2 architecture allows running several containers per VM to
support container engines that require multiple containers running
inside a pod.
With the new architecture [Kubernetes](#kubernetes-support) can
launch both Pod and OCI compatible containers with a single
[runtime](#runtime) shim per Pod, rather than `2N+1` shims. No stand
alone `kata-proxy` process is required, even if VSOCK is not
available.
### Architectural comparison
| Kata version | Kata Runtime process calls | Kata shim processes | Kata proxy processes (if no VSOCK) |
|-|-|-|-|
| 1.x | multiple per container | 1 per container connection | 1 |
| 2.x | 1 per VM (hosting any number of containers) | 0 | 0 |
> **Notes:**
>
> - A single VM can host one or more containers.
>
> - The "Kata shim processes" column refers to the old
> [Kata shim](https://github.com/kata-containers/shim) (`kata-shim` binary),
> *not* the new shimv2 runtime instance (`containerd-shim-kata-v2` binary).
The diagram below shows how the original architecture was simplified
with the advent of shimv2.
![Kubernetes integration with shimv2](arch-images/shimv2.svg)
## Root filesystem
This document uses the term _rootfs_ to refer to a root filesystem
which is mounted as the top-level directory ("`/`") and often referred
to as _slash_.
It is important to understand this term since the overall system uses
multiple different rootfs's (as explained in the
[Environments](#environments) section.
## Example command
The following containerd command creates a container. It is referred
to throughout this document to help explain various points:
```bash
$ sudo ctr run --runtime "io.containerd.kata.v2" --rm -t "quay.io/libpod/ubuntu:latest" foo sh
```
This command requests that containerd:
- Create a container (`ctr run`).
- Use the Kata [shimv2](#shim-v2-architecture) runtime (`--runtime "io.containerd.kata.v2"`).
- Delete the container when it [exits](#workload-exit) (`--rm`).
- Attach the container to the user's terminal (`-t`).
- Use the Ubuntu Linux [container image](#container-image)
to create the container [rootfs](#root-filesystem) that will become
the [container environment](#environments)
(`quay.io/libpod/ubuntu:latest`).
- Create the container with the name "`foo`".
- Run the `sh(1)` command in the Ubuntu rootfs based container
environment.
The command specified here is referred to as the [workload](#workload).
> **Note:**
>
> For the purposes of this document and to keep explanations
> simpler, we assume the user is running this command in the
> [host environment](#environments).
## Container image
In the [example command](#example-command) the user has specified the
type of container they wish to run via the container image name:
`ubuntu`. This image name corresponds to a _container image_ that can
be used to create a container with an Ubuntu Linux environment. Hence,
in our [example](#example-command), the `sh(1)` command will be run
inside a container which has an Ubuntu rootfs.
> **Note:**
>
> The term _container image_ is confusing since the image in question
> is **not** a container: it is simply a set of files (_an image_)
> that can be used to _create_ a container. The term _container
> template_ would be more accurate but the term _container image_ is
> commonly used so this document uses the standard term.
For the purposes of this document, the most important part of the
[example command line](#example-command) is the container image the
user has requested. Normally, the container manager will _pull_
(download) a container image from a remote site and store a copy
locally. This local container image is used by the container manager
to create an [OCI bundle](#oci-bundle) which will form the environment
the container will run in. After creating the OCI bundle, the
container manager launches a [runtime](#runtime) which will create the
container using the provided OCI bundle.
## OCI bundle
To understand what follows, it is important to know at a high level
how an OCI ([Open Containers Initiative](https://opencontainers.org)) compatible container is created.
An OCI compatible container is created by taking a
[container image](#container-image) and converting the embedded rootfs
into an
[OCI rootfs bundle](https://github.com/opencontainers/runtime-spec/blob/main/bundle.md),
or more simply, an _OCI bundle_.
An OCI bundle is a `tar(1)` archive normally created by a container
manager which is passed to an OCI [runtime](#runtime) which converts
it into a full container rootfs. The bundle contains two assets:
- A container image [rootfs](#root-filesystem)
This is simply a directory of files that will be used to represent
the rootfs for the container.
For the [example command](#example-command), the directory will
contain the files necessary to create a minimal Ubuntu root
filesystem.
- An [OCI configuration file](https://github.com/opencontainers/runtime-spec/blob/main/config.md)
This is a JSON file called `config.json`.
The container manager will create this file so that:
- The `root.path` value is set to the full path of the specified
container rootfs.
In [the example](#example-command) this value will be `ubuntu`.
- The `process.args` array specifies the list of commands the user
wishes to run. This is known as the [workload](#workload).
In [the example](#example-command) the workload is `sh(1)`.
## Workload
The workload is the command the user requested to run in the
container and is specified in the [OCI bundle](#oci-bundle)'s
configuration file.
In our [example](#example-command), the workload is the `sh(1)` command.
### Workload root filesystem
For details of how the [runtime](#runtime) makes the
[container image](#container-image) chosen by the user available to
the workload process, see the
[Container creation](#container-creation) and [storage](#storage) sections.
Note that the workload is isolated from the [guest VM](#environments) environment by its
surrounding [container environment](#environments). The guest VM
environment where the container runs in is also isolated from the _outer_
[host environment](#environments) where the container manager runs.
## System overview
### Environments
The following terminology is used to describe the different or
environments (or contexts) various processes run in. It is necessary
to study this table closely to make sense of what follows:
| Type | Name | Virtualized | Containerized | rootfs | Rootfs device type | Mount type | Description |
|-|-|-|-|-|-|-|-|
| Host | Host | no `[1]` | no | Host specific | Host specific | Host specific | The environment provided by a standard, physical non virtualized system. |
| VM root | Guest VM | yes | no | rootfs inside the [guest image](#guest-image) | Hypervisor specific `[2]` | `ext4` | The first (or top) level VM environment created on a host system. |
| VM container root | Container | yes | yes | rootfs type requested by user ([`ubuntu` in the example](#example-command)) | `kataShared` | [virtio FS](#virtio-fs) | The first (or top) level container environment created inside the VM. Based on the [OCI bundle](#oci-bundle). |
**Key:**
- `[1]`: For simplicity, this document assumes the host environment
runs on physical hardware.
- `[2]`: See the [DAX](#dax) section.
> **Notes:**
>
> - The word "root" is used to mean _top level_ here in a similar
> manner to the term [rootfs](#root-filesystem).
>
> - The term "first level" prefix used above is important since it implies
> that it is possible to create multi level systems. However, they do
> not form part of a standard Kata Containers environment so will not
> be considered in this document.
The reasons for containerizing the [workload](#workload) inside the VM
are:
- Isolates the workload entirely from the VM environment.
- Provides better isolation between containers in a [pod](#kubernetes-support).
- Allows the workload to be managed and monitored through its cgroup
confinement.
### Container creation
The steps below show at a high level how a Kata Containers container is
created using the containerd container manager:
1. The user requests the creation of a container by running a command
like the [example command](#example-command).
1. The container manager daemon runs a single instance of the Kata
[runtime](#runtime).
1. The Kata runtime loads its [configuration file](#configuration).
1. The container manager calls a set of shimv2 API functions on the runtime.
1. The Kata runtime launches the configured [hypervisor](#hypervisor).
1. The hypervisor creates and starts (_boots_) a VM using the
[guest assets](#guest-assets):
- The hypervisor [DAX](#dax) shares the [guest image](#guest-image)
into the VM to become the VM [rootfs](#root-filesystem) (mounted on a `/dev/pmem*` device),
which is known as the [VM root environment](#environments).
- The hypervisor mounts the [OCI bundle](#oci-bundle), using [virtio FS](#virtio-fs),
into a container specific directory inside the VM's rootfs.
This container specific directory will become the
[container rootfs](#environments), known as the
[container environment](#environments).
1. The [agent](#agent) is started as part of the VM boot.
1. The runtime calls the agent's `CreateSandbox` API to request the
agent create a container:
1. The agent creates a [container environment](#environments)
in the container specific directory that contains the [container rootfs](#environments).
The container environment hosts the [workload](#workload) in the
[container rootfs](#environments) directory.
1. The agent spawns the workload inside the container environment.
> **Notes:**
>
> - The container environment created by the agent is equivalent to
> a container environment created by the
> [`runc`](https://github.com/opencontainers/runc) OCI runtime;
> Linux cgroups and namespaces are created inside the VM by the
> [guest kernel](#guest-kernel) to isolate the workload from the
> VM environment the container is created in. See the
> [Environments](#environments) section for an explanation of why
> this is done.
>
> - See the [guest image](#guest-image) section for details of
> exactly how the agent is started.
1. The container manager returns control of the container to the
user running the `ctr` command.
> **Note:**
>
> At this point, the container is running and:
>
> - The [workload](#workload) process ([`sh(1)` in the example](#example-command))
> is running in the [container environment](#environments).
> - The user is now able to interact with the workload
> (using the [`ctr` command in the example](#example-command)).
> - The [agent](#agent), running inside the VM is monitoring the
> [workload](#workload) process.
> - The [runtime](#runtime) is waiting for the agent's `WaitProcess` API
> call to complete.
Further details of these steps are provided in the sections below.
### Container shutdown
There are two possible ways for the container environment to be
terminated:
- When the [workload](#workload) exits.
This is the standard, or _graceful_ shutdown method.
- When the container manager forces the container to be deleted.
#### Workload exit
The [agent](#agent) will detect when the [workload](#workload) process
exits, capture its exit status (see `wait(2)`) and return that value
to the [runtime](#runtime) by specifying it as the response to the
`WaitProcess` agent API call made by the [runtime](#runtime).
The runtime then passes the value back to the container manager by the
`Wait` [shimv2 API](#shim-v2-architecture) call.
Once the workload has fully exited, the VM is no longer needed and the
runtime cleans up the environment (which includes terminating the
[hypervisor](#hypervisor) process).
> **Note:**
>
> When [agent tracing is enabled](../tracing.md#agent-shutdown-behaviour),
> the shutdown behaviour is different.
#### Container manager requested shutdown
If the container manager requests the container be deleted, the
[runtime](#runtime) will signal the agent by sending it a
`DestroySandbox` [ttRPC API](../../src/agent/protocols/protos/agent.proto) request.
## Guest assets
The hypervisor will launch a virtual machine which includes a minimal guest kernel
and a guest image.
Kata Containers creates a VM in which to run one or more containers. It
does this by launching a [hypervisor](#hypervisor) to create the VM.
The hypervisor needs two assets for this task: a Linux kernel and a
small root filesystem image to boot the VM.
### Guest kernel
The guest kernel is passed to the hypervisor and used to boot the virtual
machine. The default kernel provided in Kata Containers is highly optimized for
kernel boot time and minimal memory footprint, providing only those services
required by a container workload. This is based on a very current upstream Linux
kernel.
The [guest kernel](../../tools/packaging/kernel)
is passed to the hypervisor and used to boot the VM.
The default kernel provided in Kata Containers is highly optimized for
kernel boot time and minimal memory footprint, providing only those
services required by a container workload. It is based on the latest
Linux LTS (Long Term Support) [kernel](https://www.kernel.org).
### Guest image
Kata Containers supports both an `initrd` and `rootfs` based minimal guest image.
The hypervisor uses an image file which provides a minimal root
filesystem used by the guest kernel to boot the VM and host the Kata
Container. Kata Containers supports both initrd and rootfs based
minimal guest images. The [default packages](../install/) provide both
an image and an initrd, both of which are created using the
[`osbuilder`](../../tools/osbuilder) tool.
> **Notes:**
>
> - Although initrd and rootfs based images are supported, not all
> [hypervisors](#hypervisor) support both types of image.
>
> - The guest image is *unrelated* to the image used in a container
> workload.
>
> For example, if a user creates a container that runs a shell in a
> BusyBox image, they will run that shell in a BusyBox environment.
> However, the guest image running inside the VM that is used to
> *host* that BusyBox image could be running Clear Linux, Ubuntu,
> Fedora or any other distribution potentially.
>
> The `osbuilder` tool provides
> [configurations for various common Linux distributions](../../tools/osbuilder/rootfs-builder)
> which can be built into either initrd or rootfs guest images.
>
> - If you are using a [packaged version of Kata
> Containers](../install), you can see image details by running the
> [`kata-collect-data.sh`](../../src/runtime/data/kata-collect-data.sh.in)
> script as `root` and looking at the "Image details" section of the
> output.
#### Root filesystem image
The default packaged root filesystem image, sometimes referred to as the "mini O/S", is a
highly optimized container bootstrap system based on [Clear Linux](https://clearlinux.org/). It provides an extremely minimal environment and
has a highly optimized boot path.
The default packaged rootfs image, sometimes referred to as the _mini
O/S_, is a highly optimized container bootstrap system.
The only services running in the context of the mini O/S are the init daemon
(`systemd`) and the [Agent](#agent). The real workload the user wishes to run
is created using libcontainer, creating a container in the same manner that is done
by `runc`.
If this image type is [configured](#configuration), when the user runs
the [example command](#example-command):
For example, when `ctr run -ti ubuntu date` is run:
- The [runtime](#runtime) will launch the configured [hypervisor](#hypervisor).
- The hypervisor will boot the mini-OS image using the [guest kernel](#guest-kernel).
- The kernel will start the init daemon as PID 1 (`systemd`) inside the VM root environment.
- `systemd`, running inside the mini-OS context, will launch the [agent](#agent)
in the root context of the VM.
- The agent will create a new container environment, setting its root
filesystem to that requested by the user (Ubuntu in [the example](#example-command)).
- The agent will then execute the command (`sh(1)` in [the example](#example-command))
inside the new container.
- The hypervisor will boot the mini-OS image using the guest kernel.
- `systemd`, running inside the mini-OS context, will launch the `kata-agent` in
the same context.
- The agent will create a new confined context to run the specified command in
(`date` in this example).
- The agent will then execute the command (`date` in this example) inside this
new context, first setting the root filesystem to the expected Ubuntu\* root
filesystem.
The table below summarises the default mini O/S showing the
environments that are created, the services running in those
environments (for all platforms) and the root filesystem used by
each service:
| Process | Environment | systemd service? | rootfs | User accessible | Notes |
|-|-|-|-|-|-|
| systemd | VM root | n/a | [VM guest image](#guest-image)| [debug console][debug-console] | The init daemon, running as PID 1 |
| [Agent](#agent) | VM root | yes | [VM guest image](#guest-image)| [debug console][debug-console] | Runs as a systemd service |
| `chronyd` | VM root | yes | [VM guest image](#guest-image)| [debug console][debug-console] | Used to synchronise the time with the host |
| container workload (`sh(1)` in [the example](#example-command)) | VM container | no | User specified (Ubuntu in [the example](#example-command)) | [exec command](#exec-command) | Managed by the agent |
See also the [process overview](#process-overview).
> **Notes:**
>
> - The "User accessible" column shows how an administrator can access
> the environment.
>
> - The container workload is running inside a full container
> environment which itself is running within a VM environment.
>
> - See the [configuration files for the `osbuilder` tool](../../tools/osbuilder/rootfs-builder)
> for details of the default distribution for platforms other than
> Intel x86_64.
#### Initrd image
A compressed `cpio(1)` archive, created from a rootfs which is loaded into memory and used as part of the Linux startup process. During startup, the kernel unpacks it into a special instance of a `tmpfs` that becomes the initial root filesystem.
The initrd image is a compressed `cpio(1)` archive, created from a
rootfs which is loaded into memory and used as part of the Linux
startup process. During startup, the kernel unpacks it into a special
instance of a `tmpfs` mount that becomes the initial root filesystem.
The only service running in the context of the initrd is the [Agent](#agent) as the init daemon. The real workload the user wishes to run is created using libcontainer, creating a container in the same manner that is done by `runc`.
If this image type is [configured](#configuration), when the user runs
the [example command](#example-command):
- The [runtime](#runtime) will launch the configured [hypervisor](#hypervisor).
- The hypervisor will boot the mini-OS image using the [guest kernel](#guest-kernel).
- The kernel will start the init daemon as PID 1 (the [agent](#agent))
inside the VM root environment.
- The [agent](#agent) will create a new container environment, setting its root
filesystem to that requested by the user (`ubuntu` in
[the example](#example-command)).
- The agent will then execute the command (`sh(1)` in [the example](#example-command))
inside the new container.
The table below summarises the default mini O/S showing the environments that are created,
the processes running in those environments (for all platforms) and
the root filesystem used by each service:
| Process | Environment | rootfs | User accessible | Notes |
|-|-|-|-|-|
| [Agent](#agent) | VM root | [VM guest image](#guest-image) | [debug console][debug-console] | Runs as the init daemon (PID 1) |
| container workload | VM container | User specified (Ubuntu in this example) | [exec command](#exec-command) | Managed by the agent |
> **Notes:**
>
> - The "User accessible" column shows how an administrator can access
> the environment.
>
> - It is possible to use a standard init daemon such as systemd with
> an initrd image if this is desirable.
See also the [process overview](#process-overview).
#### Image summary
| Image type | Default distro | Init daemon | Reason | Notes |
|-|-|-|-|-|
| [image](#root-filesystem-image) | [Clear Linux](https://clearlinux.org) (for x86_64 systems)| systemd | Minimal and highly optimized | systemd offers flexibility |
| [initrd](#initrd-image) | [Alpine Linux](https://alpinelinux.org) | Kata [agent](#agent) (as no systemd support) | Security hardened and tiny C library |
See also:
- The [osbuilder](../../tools/osbuilder) tool
This is used to build all default image types.
- The [versions database](../../versions.yaml)
The `default-image-name` and `default-initrd-name` options specify
the default distributions for each image type.
## Hypervisor
The [hypervisor](../hypervisors.md) specified in the
[configuration file](#configuration) creates a VM to host the
[agent](#agent) and the [workload](#workload) inside the
[container environment](#environments).
> **Note:**
>
> The hypervisor process runs inside an environment slightly different
> to the host environment:
>
> - It is run in a different cgroup environment to the host.
> - It is given a separate network namespace from the host.
> - If the [OCI configuration specifies a SELinux label](https://github.com/opencontainers/runtime-spec/blob/main/config.md#linux-process),
> the hypervisor process will run with that label (*not* the workload running inside the hypervisor's VM).
## Agent
[`kata-agent`](../../src/agent) is a process running in the guest as a supervisor for managing containers and processes running within those containers.
The Kata Containers agent ([`kata-agent`](../../src/agent)), written
in the [Rust programming language](https://www.rust-lang.org), is a
long running process that runs inside the VM. It acts as the
supervisor for managing the containers and the [workload](#workload)
running within those containers. Only a single agent process is run
for each VM created.
For the 2.0 release, the `kata-agent` is rewritten in the [RUST programming language](https://www.rust-lang.org/) so that we can minimize its memory footprint while keeping the memory safety of the original GO version of [`kata-agent` used in Kata Container 1.x](https://github.com/kata-containers/agent). This memory footprint reduction is pretty impressive, from tens of megabytes down to less than 100 kilobytes, enabling Kata Containers in more use cases like functional computing and edge computing.
### Agent communications protocol
The `kata-agent` execution unit is the sandbox. A `kata-agent` sandbox is a container sandbox defined by a set of namespaces (NS, UTS, IPC and PID). `shimv2` can
run several containers per VM to support container engines that require multiple
containers running inside a pod.
The agent communicates with the other Kata components (primarily the
[runtime](#runtime)) using a
[`ttRPC`](https://github.com/containerd/ttrpc-rust) based
[protocol](../../src/agent/protocols/protos).
`kata-agent` communicates with the other Kata components over `ttRPC`.
> **Note:**
>
> If you wish to learn more about this protocol, a practical way to do
> so is to experiment with the
> [agent control tool](#agent-control-tool) on a test system.
> This tool is for test and development purposes only and can send
> arbitrary ttRPC agent API commands to the [agent](#agent).
## Runtime
`containerd-shim-kata-v2` is a [containerd runtime shimv2](https://github.com/containerd/containerd/blob/v1.4.1/runtime/v2/README.md) implementation and is responsible for handling the `runtime v2 shim APIs`, which is similar to [the OCI runtime specification](https://github.com/opencontainers/runtime-spec) but simplifies the architecture by loading the runtime once and making RPC calls to handle the various container lifecycle commands. This refinement is an improvement on the OCI specification which requires the container manager call the runtime binary multiple times, at least once for each lifecycle command.
The Kata Containers runtime (the [`containerd-shim-kata-v2`](../../src/runtime/cmd/containerd-shim-kata-v2
) binary) is a [shimv2](#shim-v2-architecture) compatible runtime.
`containerd-shim-kata-v2` heavily utilizes the
[virtcontainers package](../../src/runtime/virtcontainers/), which provides a generic, runtime-specification agnostic, hardware-virtualized containers library.
> **Note:**
>
> The Kata Containers runtime is sometimes referred to as the Kata
> _shim_. Both terms are correct since the `containerd-shim-kata-v2`
> is a container runtime, and that runtime implements the containerd
> shim v2 API.
The runtime makes heavy use of the [`virtcontainers`
package](../../src/runtime/virtcontainers), which provides a generic,
runtime-specification agnostic, hardware-virtualized containers
library.
The runtime is responsible for starting the [hypervisor](#hypervisor)
and it's VM, and communicating with the [agent](#agent) using a
[ttRPC based protocol](#agent-communications-protocol) over a VSOCK
socket that provides a communications link between the VM and the
host.
This protocol allows the runtime to send container management commands
to the agent. The protocol is also used to carry the standard I/O
streams (`stdout`, `stderr`, `stdin`) between the containers and
container managers (such as CRI-O or containerd).
## Utility program
The `kata-runtime` binary is a utility program that provides
administrative commands to manipulate and query a Kata Containers
installation.
> **Note:**
>
> In Kata 1.x, this program also acted as the main
> [runtime](#runtime), but this is no longer required due to the
> improved shimv2 architecture.
### exec command
The `exec` command allows an administrator or developer to enter the
[VM root environment](#environments) which is not accessible by the container
[workload](#workload).
See [the developer guide](../Developer-Guide.md#connect-to-debug-console) for further details.
### Configuration
The runtime uses a TOML format configuration file called `configuration.toml`. By default this file is installed in the `/usr/share/defaults/kata-containers` directory and contains various settings such as the paths to the hypervisor, the guest kernel and the mini-OS image.
The actual configuration file paths can be determined by running:
```
$ kata-runtime --show-default-config-paths
```
Most users will not need to modify the configuration file.
The file is well commented and provides a few "knobs" that can be used to modify the behavior of the runtime and your chosen hypervisor.
See the [configuration file details](../../src/runtime/README.md#configuration).
The configuration file is also used to enable runtime [debug output](../Developer-Guide.md#enable-full-debug).
## Process overview
The table below shows an example of the main processes running in the
different [environments](#environments) when a Kata Container is
created with containerd using our [example command](#example-command):
| Description | Host | VM root environment | VM container environment |
|-|-|-|-|
| Container manager | `containerd` | |
| Kata Containers | [runtime](#runtime), [`virtiofsd`](#virtio-fs), [hypervisor](#hypervisor) | [agent](#agent) |
| User [workload](#workload) | | | [`ubuntu sh`](#example-command) |
## Networking
Containers will typically live in their own, possibly shared, networking namespace.
@ -135,8 +647,10 @@ In order to do so, container engines will usually add one end of a virtual
ethernet (`veth`) pair into the container networking namespace. The other end of
the `veth` pair is added to the host networking namespace.
This is a very namespace-centric approach as many hypervisors/VMMs cannot handle `veth`
interfaces. Typically, `TAP` interfaces are created for VM connectivity.
This is a very namespace-centric approach as many hypervisors or VM
Managers (VMMs) such as `virt-manager` cannot handle `veth`
interfaces. Typically, `TAP` interfaces are created for VM
connectivity.
To overcome incompatibility between typical container engines expectations
and virtual machines, Kata Containers networking transparently connects `veth`
@ -156,7 +670,7 @@ on par with MACVTAP.
Kata Containers has deprecated support for bridge due to lacking performance relative to TC-filter and MACVTAP.
Kata Containers supports both
Kata Containers supports both
[CNM](https://github.com/docker/libnetwork/blob/master/docs/design.md#the-container-network-model)
and [CNI](https://github.com/containernetworking/cni) for networking management.
@ -170,47 +684,91 @@ The following diagram illustrates the Kata Containers network hotplug workflow.
![Network Hotplug](arch-images/kata-containers-network-hotplug.png)
## Storage
Container workloads are shared with the virtualized environment through [virtio-fs](https://virtio-fs.gitlab.io/).
The [devicemapper `snapshotter`](https://github.com/containerd/containerd/tree/master/snapshots/devmapper) is a special case. The `snapshotter` uses dedicated block devices rather than formatted filesystems, and operates at the block level rather than the file level. This knowledge is used to directly use the underlying block device instead of the overlay file system for the container root file system. The block device maps to the top read-write layer for the overlay. This approach gives much better I/O performance compared to using `virtio-fs` to share the container file system.
### virtio SCSI
Kata Containers has the ability to hotplug and remove block devices, which makes it possible to use block devices for containers started after the VM has been launched.
If a block-based graph driver is [configured](#configuration),
`virtio-scsi` is used to _share_ the workload image (such as
`busybox:latest`) into the container's environment inside the VM.
Users can check to see if the container uses the devicemapper block device as its rootfs by calling `mount(8)` within the container. If the devicemapper block device
is used, `/` will be mounted on `/dev/vda`. Users can disable direct mounting of the underlying block device through the runtime configuration.
### virtio FS
If a block-based graph driver is _not_ [configured](#configuration), a
[`virtio-fs`](https://virtio-fs.gitlab.io) (`VIRTIO`) overlay
filesystem mount point is used to _share_ the workload image instead. The
[agent](#agent) uses this mount point as the root filesystem for the
container processes.
For virtio-fs, the [runtime](#runtime) starts one `virtiofsd` daemon
(that runs in the host context) for each VM created.
### Devicemapper
The
[devicemapper `snapshotter`](https://github.com/containerd/containerd/tree/master/snapshots/devmapper)
is a special case. The `snapshotter` uses dedicated block devices
rather than formatted filesystems, and operates at the block level
rather than the file level. This knowledge is used to directly use the
underlying block device instead of the overlay file system for the
container root file system. The block device maps to the top
read-write layer for the overlay. This approach gives much better I/O
performance compared to using `virtio-fs` to share the container file
system.
#### Hot plug and unplug
Kata Containers has the ability to hot plug add and hot plug remove
block devices. This makes it possible to use block devices for
containers started after the VM has been launched.
Users can check to see if the container uses the `devicemapper` block
device as its rootfs by calling `mount(8)` within the container. If
the `devicemapper` block device is used, the root filesystem (`/`)
will be mounted from `/dev/vda`. Users can disable direct mounting of
the underlying block device through the runtime
[configuration](#configuration).
## Kubernetes support
[Kubernetes\*](https://github.com/kubernetes/kubernetes/) is a popular open source
[Kubernetes](https://github.com/kubernetes/kubernetes/), or K8s, is a popular open source
container orchestration engine. In Kubernetes, a set of containers sharing resources
such as networking, storage, mount, PID, etc. is called a
[Pod](https://kubernetes.io/docs/user-guide/pods/).
[pod](https://kubernetes.io/docs/user-guide/pods/).
A node can have multiple pods, but at a minimum, a node within a Kubernetes cluster
only needs to run a container runtime and a container agent (called a
[Kubelet](https://kubernetes.io/docs/admin/kubelet/)).
A Kubernetes cluster runs a control plane where a scheduler (typically running on a
dedicated master node) calls into a compute Kubelet. This Kubelet instance is
responsible for managing the lifecycle of pods within the nodes and eventually relies
on a container runtime to handle execution. The Kubelet architecture decouples
lifecycle management from container execution through the dedicated
`gRPC` based [Container Runtime Interface (CRI)](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node/container-runtime-interface-v1.md).
Kata Containers represents a Kubelet pod as a VM.
In other words, a Kubelet is a CRI client and expects a CRI implementation to
handle the server side of the interface.
[CRI-O\*](https://github.com/kubernetes-incubator/cri-o) and [Containerd\*](https://github.com/containerd/containerd/) are CRI implementations that rely on [OCI](https://github.com/opencontainers/runtime-spec)
compatible runtimes for managing container instances.
A Kubernetes cluster runs a control plane where a scheduler (typically
running on a dedicated master node) calls into a compute Kubelet. This
Kubelet instance is responsible for managing the lifecycle of pods
within the nodes and eventually relies on a container runtime to
handle execution. The Kubelet architecture decouples lifecycle
management from container execution through a dedicated gRPC based
[Container Runtime Interface (CRI)](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/node/container-runtime-interface-v1.md).
Kata Containers is an officially supported CRI-O and Containerd runtime. Refer to the following guides on how to set up Kata Containers with Kubernetes:
In other words, a Kubelet is a CRI client and expects a CRI
implementation to handle the server side of the interface.
[CRI-O](https://github.com/kubernetes-incubator/cri-o) and
[containerd](https://github.com/containerd/containerd/) are CRI
implementations that rely on
[OCI](https://github.com/opencontainers/runtime-spec) compatible
runtimes for managing container instances.
- [How to use Kata Containers and Containerd](../how-to/containerd-kata.md)
Kata Containers is an officially supported CRI-O and containerd
runtime. Refer to the following guides on how to set up Kata
Containers with Kubernetes:
- [How to use Kata Containers and containerd](../how-to/containerd-kata.md)
- [Run Kata Containers with Kubernetes](../how-to/run-kata-with-k8s.md)
#### OCI annotations
In order for the Kata Containers runtime (or any virtual machine based OCI compatible
runtime) to be able to understand if it needs to create a full virtual machine or if it
has to create a new container inside an existing pod's virtual machine, CRI-O adds
In order for the Kata Containers [runtime](#runtime) (or any VM based OCI compatible
runtime) to be able to understand if it needs to create a full VM or if it
has to create a new container inside an existing pod's VM, CRI-O adds
specific annotations to the OCI configuration file (`config.json`) which is passed to
the OCI compatible runtime.
@ -221,70 +779,86 @@ or `container`. Kata Containers will then use this annotation to decide if it ne
respectively create a virtual machine or a container inside a virtual machine associated
with a Kubernetes pod:
```Go
containerType, err := ociSpec.ContainerType()
if err != nil {
return err
}
handleFactory(ctx, runtimeConfig)
disableOutput := noNeedForOutput(detach, ociSpec.Process.Terminal)
var process vc.Process
switch containerType {
case vc.PodSandbox:
process, err = createSandbox(ctx, ociSpec, runtimeConfig, containerID, bundlePath, console, disableOutput, systemdCgroup)
if err != nil {
return err
}
case vc.PodContainer:
process, err = createContainer(ctx, ociSpec, containerID, bundlePath, console, disableOutput)
if err != nil {
return err
}
}
```
| Annotation value | Kata VM created? | Kata container created? |
|-|-|-|
| `sandbox` | yes | yes (inside new VM) |
| `container`| no | yes (in existing VM) |
#### Mixing VM based and namespace based runtimes
> **Note:** Since Kubernetes 1.12, the [`Kubernetes RuntimeClass`](https://kubernetes.io/docs/concepts/containers/runtime-class/)
> has been supported and the user can specify runtime without the non-standardized annotations.
With `RuntimeClass`, users can define Kata Containers as a `RuntimeClass` and then explicitly specify that a pod being created as a Kata Containers pod. For details, please refer to [How to use Kata Containers and Containerd](../../docs/how-to/containerd-kata.md).
With `RuntimeClass`, users can define Kata Containers as a
`RuntimeClass` and then explicitly specify that a pod must be created
as a Kata Containers pod. For details, please refer to [How to use
Kata Containers and containerd](../../docs/how-to/containerd-kata.md).
## Tracing
The [tracing document](../tracing.md) provides details on the tracing
architecture.
# Appendices
## DAX
Kata Containers utilizes the Linux kernel DAX [(Direct Access filesystem)](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/dax.rst?h=v5.14)
feature to efficiently map some host-side files into the guest VM space.
In particular, Kata Containers uses the QEMU NVDIMM feature to provide a
memory-mapped virtual device that can be used to DAX map the virtual machine's
root filesystem into the guest memory address space.
Kata Containers utilizes the Linux kernel DAX
[(Direct Access filesystem)](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/dax.rst?h=v5.14)
feature to efficiently map the [guest image](#guest-image) in the
[host environment](#environments) into the
[guest VM environment](#environments) to become the VM's
[rootfs](#root-filesystem).
Mapping files using DAX provides a number of benefits over more traditional VM
file and device mapping mechanisms:
If the [configured](#configuration) [hypervisor](#hypervisor) is set
to either QEMU or Cloud Hypervisor, DAX is used with the feature shown
in the table below:
| Hypervisor | Feature used | rootfs device type |
|-|-|-|
| Cloud Hypervisor (CH) | `dax` `FsConfig` configuration option | PMEM (emulated Persistent Memory device) |
| QEMU | NVDIMM memory device with a memory file backend | NVDIMM (emulated Non-Volatile Dual In-line Memory Module device) |
The features in the table above are equivalent in that they provide a memory-mapped
virtual device which is used to DAX map the VM's
[rootfs](#root-filesystem) into the [VM guest](#environments) memory
address space.
The VM is then booted, specifying the `root=` kernel parameter to make
the [guest kernel](#guest-kernel) use the appropriate emulated device
as its rootfs.
### DAX advantages
Mapping files using [DAX](#dax) provides a number of benefits over
more traditional VM file and device mapping mechanisms:
- Mapping as a direct access device allows the guest to directly
access the host memory pages (such as via Execute In Place (XIP)),
bypassing the [guest kernel](#guest-kernel)'s page cache. This
zero copy provides both time and space optimizations.
- Mapping as a direct access devices allows the guest to directly access
the host memory pages (such as via Execute In Place (XIP)), bypassing the guest
page cache. This provides both time and space optimizations.
- Mapping as a direct access device inside the VM allows pages from the
host to be demand loaded using page faults, rather than having to make requests
via a virtualized device (causing expensive VM exits/hypercalls), thus providing
a speed optimization.
- Utilizing `MAP_SHARED` shared memory on the host allows the host to efficiently
share pages.
Kata Containers uses the following steps to set up the DAX mappings:
1. QEMU is configured with an NVDIMM memory device, with a memory file
backend to map in the host-side file into the virtual NVDIMM space.
2. The guest kernel command line mounts this NVDIMM device with the DAX
feature enabled, allowing direct page mapping and access, thus bypassing the
guest page cache.
- Utilizing `mmap(2)`'s `MAP_SHARED` shared memory option on the host
allows the host to efficiently share pages.
![DAX](arch-images/DAX.png)
Information on the use of NVDIMM via QEMU is available in the [QEMU source code](http://git.qemu-project.org/?p=qemu.git;a=blob;f=docs/nvdimm.txt;hb=HEAD)
For further details of the use of NVDIMM with QEMU, see the [QEMU
project documentation](https://www.qemu.org).
## Agent control tool
The [agent control tool](../../src/tools/agent-ctl) is a test and
development tool that can be used to learn more about a Kata
Containers system.
## Terminology
See the [project glossary](../../Glossary.md).
[debug-console]: ../Developer-Guide.md#connect-to-debug-console