• CRI - Container Runtime Interface（容器运行时接口）
    • CRI架构
    • 启用CRI
    • CRI接口
    • 当前支持的CRI后端
    • 参考

    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，基于gRPC，在kubernetes v1.7+版本中是在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

    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 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。

    关于CRI的详细进展请参考CRI: the Container Runtime Interface。

    CRI接口

    Kubernetes1.9中的CRI接口在api.proto中的定义如下：

    1. // Runtime service defines the public APIs for remote container runtimes
    2. service RuntimeService {
    3.     // Version returns the runtime name, runtime version, and runtime API version.
    4.     rpc Version(VersionRequest) returns (VersionResponse) {}
    6.     // RunPodSandbox creates and starts a pod-level sandbox. Runtimes must ensure
    7.     // the sandbox is in the ready state on success.
    8.     rpc RunPodSandbox(RunPodSandboxRequest) returns (RunPodSandboxResponse) {}
    9.     // StopPodSandbox stops any running process that is part of the sandbox and
    10.     // reclaims network resources (e.g., IP addresses) allocated to the sandbox.
    11.     // If there are any running containers in the sandbox, they must be forcibly
    12.     // terminated.
    13.     // This call is idempotent, and must not return an error if all relevant
    14.     // resources have already been reclaimed. kubelet will call StopPodSandbox
    15.     // at least once before calling RemovePodSandbox. It will also attempt to
    16.     // reclaim resources eagerly, as soon as a sandbox is not needed. Hence,
    17.     // multiple StopPodSandbox calls are expected.
    18.     rpc StopPodSandbox(StopPodSandboxRequest) returns (StopPodSandboxResponse) {}
    19.     // RemovePodSandbox removes the sandbox. If there are any running containers
    20.     // in the sandbox, they must be forcibly terminated and removed.
    21.     // This call is idempotent, and must not return an error if the sandbox has
    22.     // already been removed.
    23.     rpc RemovePodSandbox(RemovePodSandboxRequest) returns (RemovePodSandboxResponse) {}
    24.     // PodSandboxStatus returns the status of the PodSandbox. If the PodSandbox is not
    25.     // present, returns an error.
    26.     rpc PodSandboxStatus(PodSandboxStatusRequest) returns (PodSandboxStatusResponse) {}
    27.     // ListPodSandbox returns a list of PodSandboxes.
    28.     rpc ListPodSandbox(ListPodSandboxRequest) returns (ListPodSandboxResponse) {}
    30.     // CreateContainer creates a new container in specified PodSandbox
    31.     rpc CreateContainer(CreateContainerRequest) returns (CreateContainerResponse) {}
    32.     // StartContainer starts the container.
    33.     rpc StartContainer(StartContainerRequest) returns (StartContainerResponse) {}
    34.     // StopContainer stops a running container with a grace period (i.e., timeout).
    35.     // This call is idempotent, and must not return an error if the container has
    36.     // already been stopped.
    37.     // TODO: what must the runtime do after the grace period is reached?
    38.     rpc StopContainer(StopContainerRequest) returns (StopContainerResponse) {}
    39.     // RemoveContainer removes the container. If the container is running, the
    40.     // container must be forcibly removed.
    41.     // This call is idempotent, and must not return an error if the container has
    42.     // already been removed.
    43.     rpc RemoveContainer(RemoveContainerRequest) returns (RemoveContainerResponse) {}
    44.     // ListContainers lists all containers by filters.
    45.     rpc ListContainers(ListContainersRequest) returns (ListContainersResponse) {}
    46.     // ContainerStatus returns status of the container. If the container is not
    47.     // present, returns an error.
    48.     rpc ContainerStatus(ContainerStatusRequest) returns (ContainerStatusResponse) {}
    49.     // UpdateContainerResources updates ContainerConfig of the container.
    50.     rpc UpdateContainerResources(UpdateContainerResourcesRequest) returns (UpdateContainerResourcesResponse) {}
    52.     // ExecSync runs a command in a container synchronously.
    53.     rpc ExecSync(ExecSyncRequest) returns (ExecSyncResponse) {}
    54.     // Exec prepares a streaming endpoint to execute a command in the container.
    55.     rpc Exec(ExecRequest) returns (ExecResponse) {}
    56.     // Attach prepares a streaming endpoint to attach to a running container.
    57.     rpc Attach(AttachRequest) returns (AttachResponse) {}
    58.     // PortForward prepares a streaming endpoint to forward ports from a PodSandbox.
    59.     rpc PortForward(PortForwardRequest) returns (PortForwardResponse) {}
    61.     // ContainerStats returns stats of the container. If the container does not
    62.     // exist, the call returns an error.
    63.     rpc ContainerStats(ContainerStatsRequest) returns (ContainerStatsResponse) {}
    64.     // ListContainerStats returns stats of all running containers.
    65.     rpc ListContainerStats(ListContainerStatsRequest) returns (ListContainerStatsResponse) {}
    67.     // UpdateRuntimeConfig updates the runtime configuration based on the given request.
    68.     rpc UpdateRuntimeConfig(UpdateRuntimeConfigRequest) returns (UpdateRuntimeConfigResponse) {}
    70.     // Status returns the status of the runtime.
    71.     rpc Status(StatusRequest) returns (StatusResponse) {}
    72. }
    74. // ImageService defines the public APIs for managing images.
    75. service ImageService {
    76.     // ListImages lists existing images.
    77.     rpc ListImages(ListImagesRequest) returns (ListImagesResponse) {}
    78.     // ImageStatus returns the status of the image. If the image is not
    79.     // present, returns a response with ImageStatusResponse.Image set to
    80.     // nil.
    81.     rpc ImageStatus(ImageStatusRequest) returns (ImageStatusResponse) {}
    82.     // PullImage pulls an image with authentication config.
    83.     rpc PullImage(PullImageRequest) returns (PullImageResponse) {}
    84.     // RemoveImage removes the image.
    85.     // This call is idempotent, and must not return an error if the image has
    86.     // already been removed.
    87.     rpc RemoveImage(RemoveImageRequest) returns (RemoveImageResponse) {}
    88.     // ImageFSInfo returns information of the filesystem that is used to store images.
    89.     rpc ImageFsInfo(ImageFsInfoRequest) returns (ImageFsInfoResponse) {}
    90. }

    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：同时兼容OCI和CRI的容器运行时
    • cri-containerd：基于Containerd的Kubernetes CNI实现
    • rkt：由于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
    • clear-containers：由Intel推出的同时兼容OCI和CRI的容器运行时
    • kata-containers：符合OCI规范同时兼容CRI

    CRI是由SIG-Node来维护的。

    参考

    • Kubernetes CRI and Minikube
    • Kubernetes container runtime interface
    • CRI-O and Alternative Runtimes in Kubernetes
    • Docker、Containerd、RunC…：你应该知道的所有
    • Introducing Container Runtime Interface (CRI) in Kubernetes