容器运行时接口（CRI）
容器运行时接口（Container Runtime Interface），简称 CRI。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，基于 gRPC，在 Kubernetes v1.10 + 版本中是在 pkg/kubelet/apis/cri/runtime/v1alpha2 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。
启用 CRI
除非集成了 rktnetes，否则 CRI 都是被默认启用了，从 Kubernetes 1.7 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 socket，windows 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。
CRI 接口
Kubernetes 1.9 中的 CRI 接口在 api.proto 中的定义如下：
// 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 Kubernetes 1.5 开始已经支持 CRI，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 CRI 的后端有：
- cri-o：cri-o 是 Kubernetes 的 CRI 标准的实现，并且允许 Kubernetes 间接使用 OCI 兼容的容器运行时，可以把 cri-o 看成 Kubernetes 使用 OCI 兼容的容器运行时的中间层。
- cri-containerd：基于 Containerd 的 Kubernetes CRI 实现
- rkt：由 CoreOS 主推的用来跟 docker 抗衡的容器运行时
- frakti：基于 hypervisor 的 CRI
- docker：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 标准
CRI 是由 SIG-Node 来维护的。
当前通过 CRI-O 间接支持 CRI 的后端
当前同样存在一些只实现了 OCI 标准的容器，但是它们可以通过 CRI-O 来作为 Kubernetes 的容器运行时。CRI-O 是 Kubernetes 的 CRI 标准的实现，并且允许 Kubernetes 间接使用 OCI 兼容的容器运行时。
- Clear Containers：由 Intel 推出的兼容 OCI 容器运行时，可以通过 CRI-O 来兼容 CRI。
- Kata Containers：符合 OCI 规范，可以通过 CRI-O 或 Containerd CRI Plugin 来兼容 CRI。
- gVisor：由谷歌推出的容器运行时沙箱 (Experimental)，可以通过 CRI-O 来兼容 CRI。