前言

整个Kubernetes技术体系由声明式API以及Controller构成，而kube-apiserver是Kubernetes的声明式api server，并为其它组件交互提供了桥梁。因此加深对kube-apiserver的理解就显得至关重要了。

整体组件功能

kube-apiserver作为整个Kubernetes集群操作etcd的唯一入口，负责Kubernetes各资源的认证&鉴权，校验以及CRUD等操作，提供RESTful APIs，供其它组件调用：

kube-apiserver包含三种APIServer：

aggregatorServer：负责处理 apiregistration.k8s.io 组下的APIService资源请求，同时将来自 域名交易 用户的请求拦截转发给aggregated server(AA)

kubeAPIServer：负责对请求的一些通用处理，包括：认证、鉴权以及各个内建资源(pod, deployment，service and etc)的REST服务等

apiExtensionsServer：负责CustomResourceDefinition（CRD）apiResources以及apiVersions的注册，同时处理CRD以及相应CustomResource（CR）的REST请求(如果对应CR不能被处理的话则会返回404)，也是apiserver Delegation的最后一环

另外还包括bootstrap-controller，主要负责Kubernetes default apiserver service的创建以及管理。

接下来将对上述组件进行概览性总结。

bootstrap-controller

apiserver bootstrap-controller创建&运行逻辑在k8s.io/kubernetes/pkg/master目录

bootstrap-controller主要用于创建以及维护内部kubernetes default apiserver service

kubernetes default apiserver service spec.selector为空，这是default apiserver service与其它正常service的最大区别，表明了这个特殊的service对应的endpoints不由endpoints controller控制，而是直接受kube-apiserver bootstrap-controller管理(maintained by this code, not by the pod selector)

bootstrap-controller的几个主要功能如下：

创建 default、kube-system 和 kube-public 以及 kube-node-lease 命名空间

创建&维护kubernetes default apiserver service以及对应的endpoint

提供基于Service ClusterIP的检查及修复功能(--service-cluster-ip-range指定范围)

提供基于Service NodePort的检查及修复功能(--service-node-port-range指定范围)

// k8s.io/kubernetes/pkg/master/controller.go:142// Start begins the core controller loops that must exist for bootstrapping// a cluster.func (c *Controller) Start() {

    if c.runner != nil {

        return

    }

    // Reconcile during first run removing itself until server is ready.

    endpointPorts := createEndpointPortSpec(c.PublicServicePort, "https", c.ExtraEndpointPorts)

    if err := c.EndpointReconciler.RemoveEndpoints(kubernetesServiceName, c.PublicIP, endpointPorts); err != nil {

        klog.Errorf("Unable to remove old endpoints from kubernetes service: %v", err)

    }

    repairClusterIPs := servicecontroller.NewRepair(c.ServiceClusterIPInterval, c.ServiceClient, c.EventClient, &c.ServiceClusterIPRange, c.ServiceClusterIPRegistry, &c.SecondaryServiceClusterIPRange, c.SecondaryServiceClusterIPRegistry)

    repairNodePorts := portallocatorcontroller.NewRepair(c.ServiceNodePortInterval, c.ServiceClient, c.EventClient, c.ServiceNodePortRange, c.ServiceNodePortRegistry)

    // run all of the controllers once prior to returning from Start.

    if err := repairClusterIPs.RunOnce(); err != nil {

        // If we fail to repair cluster IPs apiserver is useless. We should restart and retry.

        klog.Fatalf("Unable to perform initial IP allocation check: %v", err)

    }

    if err := repairNodePorts.RunOnce(); err != nil {

        // If we fail to repair node ports apiserver is useless. We should restart and retry.

        klog.Fatalf("Unable to perform initial service nodePort check: %v", err)

    }

    // 定期执行bootstrap controller主要的四个功能(reconciliation)  

    c.runner = async.NewRunner(c.RunKubernetesNamespaces, c.RunKubernetesService, repairClusterIPs.RunUntil, repairNodePorts.RunUntil)

    c.runner.Start()

KubeAPIServer主要提供对内建API Resources的操作请求，为Kubernetes中各API Resources注册路由信息，同时暴露RESTful API，使集群中以及集群外的服务都可以通过RESTful API操作Kubernetes中的资源

另外，kubeAPIServer是整个Kubernetes apiserver的核心，下面将要讲述的aggregatorServer以及apiExtensionsServer都是建立在kubeAPIServer基础上进行扩展的(补充了Kubernetes对用户自定义资源的能力支持)

kubeAPIServer最核心的功能是为Kubernetes内置资源添加路由，如下：

调用 m.InstallLegacyAPI 将核心 API Resources添加到路由中，在apiserver中即是以 /api 开头的 resource；

调用 m.InstallAPIs 将扩展的 API Resources添加到路由中，在apiserver中即是以 /apis 开头的 resource；

// k8s.io/kubernetes/pkg/master/master.go:332// New returns a new instance of Master from the given config.// Certain config fields will be set to a default value if unset.// Certain config fields must be specified, including://   KubeletClientConfigfunc (c completedConfig) New(delegationTarget genericapiserver.DelegationTarget) (*Master, error) {

    ...

    // 安装 LegacyAPI(core API)

    // install legacy rest storage

    if c.ExtraConfig.APIResourceConfigSource.VersionEnabled(apiv1.SchemeGroupVersion) {

        legacyRESTStorageProvider := corerest.LegacyRESTStorageProvider{

            StorageFactory:              c.ExtraConfig.StorageFactory,

            ProxyTransport:              c.ExtraConfig.ProxyTransport,

            KubeletClientConfig:         c.ExtraConfig.KubeletClientConfig,

            EventTTL:                    c.ExtraConfig.EventTTL,

            ServiceIPRange:              c.ExtraConfig.ServiceIPRange,

            SecondaryServiceIPRange:     c.ExtraConfig.SecondaryServiceIPRange,

            ServiceNodePortRange:        c.ExtraConfig.ServiceNodePortRange,

            LoopbackClientConfig:        c.GenericConfig.LoopbackClientConfig,

            ServiceAccountIssuer:        c.ExtraConfig.ServiceAccountIssuer,

            ServiceAccountMaxExpiration: c.ExtraConfig.ServiceAccountMaxExpiration,

            APIAudiences:                c.GenericConfig.Authentication.APIAudiences,

        }

        if err := m.InstallLegacyAPI(&c, c.GenericConfig.RESTOptionsGetter, legacyRESTStorageProvider); err != nil {

            return nil, err

        }

    }

    ...

    // 安装 APIs(named groups apis)

    if err := m.InstallAPIs(c.ExtraConfig.APIResourceConfigSource, c.GenericConfig.RESTOptionsGetter, restStorageProviders...); err != nil {

        return nil, err

    }

    ...

    return m, nil

}

整个kubeAPIServer提供了三类API Resource接口：

core group：主要在 /api/v1 下；

named groups：其 path 为 /apis/$GROUP/$VERSION；

系统状态的一些 API：如/metrics 、/version 等；

kubeAPIServer会为每种API资源创建对应的RESTStorage，RESTStorage的目的是将每种资源的访问路径及其后端存储的操作对应起来：通过构造的REST Storage实现的接口判断该资源可以执行哪些操作（如：create、update等），将其对应的操作存入到action中，每一个操作对应一个标准的REST method，如create对应REST method为POST，而update对应REST method为PUT。最终根据actions数组依次遍历，对每一个操作添加一个handler(handler对应REST Storage实现的相关接口)，并注册到route，最终对外提供RESTful API，如下：

// m.GenericAPIServer.InstallLegacyAPIGroup --> s.installAPIResources --> apiGroupVersion.InstallREST --> installer.Install --> a.registerResourceHandlers// k8s.io/kubernetes/staging/src/k8s.io/apiserver/pkg/endpoints/installer.go:181func (a *APIInstaller) registerResourceHandlers(path string, storage rest.Storage, ws *restful.WebService) (*metav1.APIResource, error) {

    ...

    // 1、判断该 resource 实现了哪些 REST 操作接口，以此来判断其支持的 verbs 以便为其添加路由

    // what verbs are supported by the storage, used to know what verbs we support per path

    creater, isCreater := storage.(rest.Creater)

    namedCreater, isNamedCreater := storage.(rest.NamedCreater)

    lister, isLister := storage.(rest.Lister)

    getter, isGetter := storage.(rest.Getter)

    ...

    // 2、为 resource 添加对应的 actions(+根据是否支持 namespace)

    // Get the list of actions for the given scope.

    switch {

    case !namespaceScoped:

        // Handle non-namespace scoped resources like nodes.

        resourcePath := resource

        resourceParams := params

        itemPath := resourcePath + "/{name}"

        nameParams := append(params, nameParam)

        proxyParams := append(nameParams, pathParam)

        ...

        // Handler for standard REST verbs (GET, PUT, POST and DELETE).

        // Add actions at the resource path: /api/apiVersion/resource

        actions = appendIf(actions, action{"LIST", resourcePath, resourceParams, namer, false}, isLister)

        actions = appendIf(actions, action{"POST", resourcePath, resourceParams, namer, false}, isCreater)

        ...

    }

    ...

    // 3、从 rest.Storage 到 restful.Route 映射

    // 为每个操作添加对应的 handler

    for _, action := range actions {

        ...

        switch action.Verb {

        ...

        case "POST": // Create a resource.

            var handler restful.RouteFunction

            // 4、初始化 handler

            if isNamedCreater {

                handler = restfulCreateNamedResource(namedCreater, reqScope, admit)

            } else {

                handler = restfulCreateResource(creater, reqScope, admit)

            }

            handler = metrics.InstrumentRouteFunc(action.Verb, group, version, resource, subresource, requestScope, metrics.APIServerComponent, handler)

            ...

            // 5、route 与 handler 进行绑定    

            route := ws.POST(action.Path).To(handler).

                Doc(doc).

                Param(ws.QueryParameter("pretty", "If 'true', then the output is pretty printed.")).

                Operation("create"+namespaced+kind+strings.Title(subresource)+operationSuffix).

                Produces(append(storageMeta.ProducesMIMETypes(action.Verb), mediaTypes...)...).

                Returns(http.StatusOK, "OK", producedObject).

                // TODO: in some cases, the API may return a v1.Status instead of the versioned object

                // but currently go-restful can't handle multiple different objects being returned.

                Returns(http.StatusCreated, "Created", producedObject).

                Returns(http.StatusAccepted, "Accepted", producedObject).

                Reads(defaultVersionedObject).

                Writes(producedObject)

            if err := AddObjectParams(ws, route, versionedCreateOptions); err != nil {

                return nil, err

            }

            addParams(route, action.Params)

            // 6、添加到路由中    

            routes = append(routes, route)

        case "DELETE": // Delete a resource.

        ...

        default:

            return nil, fmt.Errorf("unrecognized action verb: %s", action.Verb)

        }

        for _, route := range routes {

            route.Metadata(ROUTE_META_GVK, metav1.GroupVersionKind{

                Group:   reqScope.Kind.Group,

                Version: reqScope.Kind.Version,

                Kind:    reqScope.Kind.Kind,

            })

            route.Metadata(ROUTE_META_ACTION, strings.ToLower(action.Verb))

            ws.Route(route)

        }

        // Note: update GetAuthorizerAttributes() when adding a custom handler .