CRI - Container Runtime Interface

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- [Kubernetes架构](concepts/index.md) - [Kubernetes架构](concepts/index.md)
- [Etcd解析](concepts/etcd.md) - [Etcd解析](concepts/etcd.md)
- [开放接口](concepts/open-interfaces.md) - [开放接口](concepts/open-interfaces.md)
- [CRI - Container Runtime Interface容器运行时接口](concepts/cri.md)
- [设计理念](concepts/concepts.md) - [设计理念](concepts/concepts.md)
- [资源对象与基本概念解析](concepts/objects.md) - [资源对象与基本概念解析](concepts/objects.md)
- [Pod](concepts/pod-overview.md) - [Pod](concepts/pod-overview.md)

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# CRI - Container Runtime Interface容器运行时接口
CRI中定义了**容器**和**镜像**的服务的接口,因为容器运行时与镜像的生命周期是彼此隔离的,因此需要定义两个服务。该接口使用[Protocol Buffer](https://developers.google.com/protocol-buffers/),基于[gRPC](https://grpc.io/)在kubernetes v1.7+版本中是在[pkg/kubelet/apis/cri/v1alpha1/runtime](https://github.com/kubernetes/kubernetes/tree/master/pkg/kubelet/apis/cri/v1alpha1/runtime)的`api.proto`中定义的。
## CRI架构
Container Runtime实现了CRI gRPC Server包括`RuntimeService`和`ImageService`。该gRPC Server需要监听本地的Unix socket而kubelet则作为gRPC Client运行。
![CRI架构-图片来自kubernetes blog](../images/cri-architecture.png)
## 启用CRI
除非集成了rktnetes否则CRI都是被默认启用了kubernetes1.7版本开始旧的预集成的docker CRI已经被移除。
要想启用CRI只需要在kubelet的启动参数重传入此参数`--container-runtime-endpoint`远程运行时服务的端点。当前Linux上支持unix socketwindows上支持tcp。例如`unix:///var/run/dockershim.sock`、 `tcp://localhost:373`,默认是`unix:///var/run/dockershim.sock`即默认使用本地的docker作为容器运行时。
关于CRI的详细进展请参考[CRI: the Container Runtime Interface](https://github.com/kubernetes/community/blob/master/contributors/devel/container-runtime-interface.md)。
## CRI接口
Kubernetes1.9中的CRI接口在`api.proto`中的定义如下:
```protobuf
// Runtime service defines the public APIs for remote container runtimes
service RuntimeService {
// Version returns the runtime name, runtime version, and runtime API version.
rpc Version(VersionRequest) returns (VersionResponse) {}
// RunPodSandbox creates and starts a pod-level sandbox. Runtimes must ensure
// the sandbox is in the ready state on success.
rpc RunPodSandbox(RunPodSandboxRequest) returns (RunPodSandboxResponse) {}
// StopPodSandbox stops any running process that is part of the sandbox and
// reclaims network resources (e.g., IP addresses) allocated to the sandbox.
// If there are any running containers in the sandbox, they must be forcibly
// terminated.
// This call is idempotent, and must not return an error if all relevant
// resources have already been reclaimed. kubelet will call StopPodSandbox
// at least once before calling RemovePodSandbox. It will also attempt to
// reclaim resources eagerly, as soon as a sandbox is not needed. Hence,
// multiple StopPodSandbox calls are expected.
rpc StopPodSandbox(StopPodSandboxRequest) returns (StopPodSandboxResponse) {}
// RemovePodSandbox removes the sandbox. If there are any running containers
// in the sandbox, they must be forcibly terminated and removed.
// This call is idempotent, and must not return an error if the sandbox has
// already been removed.
rpc RemovePodSandbox(RemovePodSandboxRequest) returns (RemovePodSandboxResponse) {}
// PodSandboxStatus returns the status of the PodSandbox. If the PodSandbox is not
// present, returns an error.
rpc PodSandboxStatus(PodSandboxStatusRequest) returns (PodSandboxStatusResponse) {}
// ListPodSandbox returns a list of PodSandboxes.
rpc ListPodSandbox(ListPodSandboxRequest) returns (ListPodSandboxResponse) {}
// CreateContainer creates a new container in specified PodSandbox
rpc CreateContainer(CreateContainerRequest) returns (CreateContainerResponse) {}
// StartContainer starts the container.
rpc StartContainer(StartContainerRequest) returns (StartContainerResponse) {}
// StopContainer stops a running container with a grace period (i.e., timeout).
// This call is idempotent, and must not return an error if the container has
// already been stopped.
// TODO: what must the runtime do after the grace period is reached?
rpc StopContainer(StopContainerRequest) returns (StopContainerResponse) {}
// RemoveContainer removes the container. If the container is running, the
// container must be forcibly removed.
// This call is idempotent, and must not return an error if the container has
// already been removed.
rpc RemoveContainer(RemoveContainerRequest) returns (RemoveContainerResponse) {}
// ListContainers lists all containers by filters.
rpc ListContainers(ListContainersRequest) returns (ListContainersResponse) {}
// ContainerStatus returns status of the container. If the container is not
// present, returns an error.
rpc ContainerStatus(ContainerStatusRequest) returns (ContainerStatusResponse) {}
// UpdateContainerResources updates ContainerConfig of the container.
rpc UpdateContainerResources(UpdateContainerResourcesRequest) returns (UpdateContainerResourcesResponse) {}
// ExecSync runs a command in a container synchronously.
rpc ExecSync(ExecSyncRequest) returns (ExecSyncResponse) {}
// Exec prepares a streaming endpoint to execute a command in the container.
rpc Exec(ExecRequest) returns (ExecResponse) {}
// Attach prepares a streaming endpoint to attach to a running container.
rpc Attach(AttachRequest) returns (AttachResponse) {}
// PortForward prepares a streaming endpoint to forward ports from a PodSandbox.
rpc PortForward(PortForwardRequest) returns (PortForwardResponse) {}
// ContainerStats returns stats of the container. If the container does not
// exist, the call returns an error.
rpc ContainerStats(ContainerStatsRequest) returns (ContainerStatsResponse) {}
// ListContainerStats returns stats of all running containers.
rpc ListContainerStats(ListContainerStatsRequest) returns (ListContainerStatsResponse) {}
// UpdateRuntimeConfig updates the runtime configuration based on the given request.
rpc UpdateRuntimeConfig(UpdateRuntimeConfigRequest) returns (UpdateRuntimeConfigResponse) {}
// Status returns the status of the runtime.
rpc Status(StatusRequest) returns (StatusResponse) {}
}
// ImageService defines the public APIs for managing images.
service ImageService {
// ListImages lists existing images.
rpc ListImages(ListImagesRequest) returns (ListImagesResponse) {}
// ImageStatus returns the status of the image. If the image is not
// present, returns a response with ImageStatusResponse.Image set to
// nil.
rpc ImageStatus(ImageStatusRequest) returns (ImageStatusResponse) {}
// PullImage pulls an image with authentication config.
rpc PullImage(PullImageRequest) returns (PullImageResponse) {}
// RemoveImage removes the image.
// This call is idempotent, and must not return an error if the image has
// already been removed.
rpc RemoveImage(RemoveImageRequest) returns (RemoveImageResponse) {}
// ImageFSInfo returns information of the filesystem that is used to store images.
rpc ImageFsInfo(ImageFsInfoRequest) returns (ImageFsInfoResponse) {}
}
```
这其中包含了两个gRPC服务
- **RuntimeService**容器和Sandbox运行时管理
- **ImageService**提供了从镜像仓库拉取、查看、和移除镜像的RPC。
## 当前支持的CRI后端
我们最初在使用Kubernetes时通常会默认使用Docker作为容器运行时其实从Kubernetes1.5开始已经开始支持CRI目前是处于Alpha版本通过CRI接口可以指定使用其它容器运行时作为Pod的后端目前支持CNI的后端有
- [cri-o](https://github.com/kubernetes-incubator/cri-o)同时兼容OCI和CRI的容器运行时
- [cri-containerd](https://github.com/containerd/cri-containerd):基于[Containerd](https://github.com/containerd/containerd)的Kubernetes CNI实现
- [rkt](https://coreos.com/rkt/)由于CoreOS主推的用来跟docker抗衡的容器运行时
- [frakti](https://github.com/kubernetes/frakti)基于hypervisor的CRI
- [docker](https://www.docker.com)kuberentes最初就开始支持的容器运行时目前还没完全从kubelet中解耦docker公司同时推广了[OCI](https://www.opencontainers.org/)标准
- [clear-containers](https://github.com/clearcontainers)由Intel推出的同时兼容OCI和CRI的容器运行时
- [kata-containers](https://katacontainers.io/)符合OCI规范同时兼容CRI
CRI是由[SIG-Node](https://kubernetes.slack.com/archives/sig-node)来维护的。
## 参考
- [Kubernetes CRI and Minikube](https://sreeninet.wordpress.com/2017/02/11/kubernetes-cri-and-minikube/)
- [Kubernetes container runtime interface](https://feisky.xyz/2016/09/24/Kubernetes-container-runtime-interface/)
- [CRI-O and Alternative Runtimes in Kubernetes](https://www.projectatomic.io/blog/2017/02/crio-runtimes/)
- [Docker、Containerd、RunC...:你应该知道的所有](http://www.infoq.com/cn/news/2017/02/Docker-Containerd-RunC)
- [Introducing Container Runtime Interface (CRI) in Kubernetes](http://blog.kubernetes.io/2016/12/container-runtime-interface-cri-in-kubernetes.html)

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