一文讀懂 Kubernetes APIServer 原理-有解無憂

前言

整個Kubernetes技術體系由宣告式API以及Controller構成，而kube-apiserver是Kubernetes的宣告式api server，并為其它組件互動提供了橋梁，因此加深對kube-apiserver的理解就顯得至關重要了，

整體組件功能

kube-apiserver作為整個Kubernetes集群操作etcd的唯一入口，負責Kubernetes各資源的認證&鑒權，校驗以及CRUD等操作，提供RESTful APIs，供其它組件呼叫：

kube-apiserver包含三種APIServer：

aggregatorServer：負責處理 apiregistration.k8s.io 組下的APIService資源請求，同時將來自用戶的請求攔截轉發給aggregated server(AA)
kubeAPIServer：負責對請求的一些通用處理，包括：認證、鑒權以及各個內建資源(pod, deployment，service and etc)的REST服務等
apiExtensionsServer：負責CustomResourceDefinition（CRD）apiResources以及apiVersions的注冊，同時處理CRD以及相應CustomResource（CR）的REST請求(如果對應CR不能被處理的話則會回傳404)，也是apiserver Delegation的最后一環

另外還包括bootstrap-controller，主要負責Kubernetes default apiserver service的創建以及管理，

接下來將對上述組件進行概覽性總結，

bootstrap-controller

apiserver bootstrap-controller創建&運行邏輯在k8s.io/kubernetes/pkg/master目錄
bootstrap-controller主要用于創建以及維護內部kubernetes default apiserver service
kubernetes default apiserver service spec.selector為空，這是default apiserver service與其它正常service的最大區別，表明了這個特殊的service對應的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的幾個主要功能如下：
- 創建 default、kube-system 和 kube-public 以及 kube-node-lease 命名空間
- 創建&維護kubernetes default apiserver service以及對應的endpoint
- 提供基于Service ClusterIP的檢查及修復功能(--service-cluster-ip-range指定范圍)
- 提供基于Service NodePort的檢查及修復功能(--service-node-port-range指定范圍)

// 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)
    }
    // 定期執行bootstrap controller主要的四個功能(reconciliation)  
    c.runner = async.NewRunner(c.RunKubernetesNamespaces, c.RunKubernetesService, repairClusterIPs.RunUntil, repairNodePorts.RunUntil)
    c.runner.Start()
}

更多代碼原理詳情，參考 kubernetes-reading-notes ，

kubeAPIServer

KubeAPIServer主要提供對內建API Resources的操作請求，為Kubernetes中各API Resources注冊路由資訊，同時暴露RESTful API，使集群中以及集群外的服務都可以通過RESTful API操作Kubernetes中的資源

另外，kubeAPIServer是整個Kubernetes apiserver的核心，下面將要講述的aggregatorServer以及apiExtensionsServer都是建立在kubeAPIServer基礎上進行擴展的(補充了Kubernetes對用戶自定義資源的能力支持)

kubeAPIServer最核心的功能是為Kubernetes內置資源添加路由，如下：

呼叫 m.InstallLegacyAPI 將核心 API Resources添加到路由中，在apiserver中即是以 /api 開頭的 resource；
呼叫 m.InstallAPIs 將擴展的 API Resources添加到路由中，在apiserver中即是以 /apis 開頭的 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:
//   KubeletClientConfig
func (c completedConfig) New(delegationTarget genericapiserver.DelegationTarget) (*Master, error) {
    ...
    // 安裝 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
        }
    }
    ...
    // 安裝 APIs(named groups apis)
    if err := m.InstallAPIs(c.ExtraConfig.APIResourceConfigSource, c.GenericConfig.RESTOptionsGetter, restStorageProviders...); err != nil {
        return nil, err
    }
    ...
    return m, nil
}

整個kubeAPIServer提供了三類API Resource介面：

core group：主要在 /api/v1 下；
named groups：其 path 為 /apis/$GROUP/$VERSION；
系統狀態的一些 API：如/metrics 、/version 等；

而API的URL大致以 /apis/{group}/{version}/namespaces/{namespace}/resource/{name} 組成，結構如下圖所示：

kubeAPIServer會為每種API資源創建對應的RESTStorage，RESTStorage的目的是將每種資源的訪問路徑及其后端存盤的操作對應起來：通過構造的REST Storage實作的介面判斷該資源可以執行哪些操作（如：create、update等），將其對應的操作存入到action中，每一個操作對應一個標準的REST method，如create對應REST method為POST，而update對應REST method為PUT，最終根據actions陣列依次遍歷，對每一個操作添加一個handler(handler對應REST Storage實作的相關介面)，并注冊到route，最終對外提供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:181
func (a *APIInstaller) registerResourceHandlers(path string, storage rest.Storage, ws *restful.WebService) (*metav1.APIResource, error) {
    ...
    // 1、判斷該 resource 實作了哪些 REST 操作介面，以此來判斷其支持的 verbs 以便為其添加路由
    // 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、為 resource 添加對應的 actions(+根據是否支持 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、從 rest.Storage 到 restful.Route 映射
    // 為每個操作添加對應的 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 與 handler 進行系結    
            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.
    }
    ...
}

kubeAPIServer代碼結構整理如下：

1. apiserver整體啟動邏輯 k8s.io/kubernetes/cmd/kube-apiserver
2. apiserver bootstrap-controller創建&運行邏輯 k8s.io/kubernetes/pkg/master
3. API Resource對應后端RESTStorage(based on genericregistry.Store)創建k8s.io/kubernetes/pkg/registry
4. aggregated-apiserver創建&處理邏輯 k8s.io/kubernetes/staging/src/k8s.io/kube-aggregator
5. extensions-apiserver創建&處理邏輯 k8s.io/kubernetes/staging/src/k8s.io/apiextensions-apiserver
6. apiserver創建&運行 k8s.io/kubernetes/staging/src/k8s.io/apiserver/pkg/server
7. 注冊API Resource資源處理handler(InstallREST&Install?isterResourceHandlers) k8s.io/kubernetes/staging/src/k8s.io/apiserver/pkg/endpoints
8. 創建存盤后端(etcdv3) k8s.io/kubernetes/staging/src/k8s.io/apiserver/pkg/storage
9. genericregistry.Store.CompleteWithOptions初始化 k8s.io/kubernetes/staging/src/k8s.io/apiserver/pkg/registry

呼叫鏈整理如下：

更多代碼原理詳情，參考 kubernetes-reading-notes ，

aggregatorServer

aggregatorServer主要用于處理擴展Kubernetes API Resources的第二種方式Aggregated APIServer(AA)，將CR請求代理給AA：

這里結合Kubernetes官方給出的aggregated apiserver例子sample-apiserver，總結原理如下：

aggregatorServer通過APIServices物件關聯到某個Service來進行請求的轉發，其關聯的Service型別進一步決定了請求轉發的形式，aggregatorServer包括一個GenericAPIServer和維護自身狀態的Controller，其中GenericAPIServer主要處理apiregistration.k8s.io組下的APIService資源請求，而Controller包括：
- apiserviceRegistrationController：負責根據APIService定義的aggregated server service構建代理，將CR的請求轉發給后端的aggregated server
- availableConditionController：維護 APIServices 的可用狀態，包括其參考 Service 是否可用等；
- autoRegistrationController：用于保持 API 中存在的一組特定的 APIServices；
- crdRegistrationController：負責將 CRD GroupVersions 自動注冊到 APIServices 中；
- openAPIAggregationController：將 APIServices 資源的變化同步至提供的 OpenAPI 檔案；

apiserviceRegistrationController負責根據APIService定義的aggregated server service構建代理，將CR的請求轉發給后端的aggregated server，apiService有兩種型別：Local(Service為空)以及Service(Service非空)，apiserviceRegistrationController負責對這兩種型別apiService設定代理：Local型別會直接路由給kube-apiserver進行處理；而Service型別則會設定代理并將請求轉化為對aggregated Service的請求(proxyPath := "/apis/" + apiService.Spec.Group + "/" + apiService.Spec.Version)，而請求的負載均衡策略則是優先本地訪問kube-apiserver(如果service為kubernetes default apiserver service:443)=>通過service ClusterIP:Port訪問(默認) 或者通過隨機選擇service endpoint backend進行訪問：

func (s *APIAggregator) AddAPIService(apiService *v1.APIService) error {
  ...
    proxyPath := "/apis/" + apiService.Spec.Group + "/" + apiService.Spec.Version
    // v1. is a special case for the legacy API.  It proxies to a wider set of endpoints.
    if apiService.Name == legacyAPIServiceName {
        proxyPath = "/api"
    }
    // register the proxy handler
    proxyHandler := &proxyHandler{
        localDelegate:   s.delegateHandler,
        proxyClientCert: s.proxyClientCert,
        proxyClientKey:  s.proxyClientKey,
        proxyTransport:  s.proxyTransport,
        serviceResolver: s.serviceResolver,
        egressSelector:  s.egressSelector,
    }
  ...
    s.proxyHandlers[apiService.Name] = proxyHandler
    s.GenericAPIServer.Handler.NonGoRestfulMux.Handle(proxyPath, proxyHandler)
    s.GenericAPIServer.Handler.NonGoRestfulMux.UnlistedHandlePrefix(proxyPath+"/", proxyHandler)
  ...
    // it's time to register the group aggregation endpoint
    groupPath := "/apis/" + apiService.Spec.Group
    groupDiscoveryHandler := &apiGroupHandler{
        codecs:    aggregatorscheme.Codecs,
        groupName: apiService.Spec.Group,
        lister:    s.lister,
        delegate:  s.delegateHandler,
    }
    // aggregation is protected
    s.GenericAPIServer.Handler.NonGoRestfulMux.Handle(groupPath, groupDiscoveryHandler)
    s.GenericAPIServer.Handler.NonGoRestfulMux.UnlistedHandle(groupPath+"/", groupDiscoveryHandler)
    s.handledGroups.Insert(apiService.Spec.Group)
    return nil
}
// k8s.io/kubernetes/staging/src/k8s.io/kube-aggregator/pkg/apiserver/handler_proxy.go:109
func (r *proxyHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
    // 加載roxyHandlingInfo處理請求  
    value := r.handlingInfo.Load()
    if value =https://www.cnblogs.com/tencent-cloud-native/p/= nil {
        r.localDelegate.ServeHTTP(w, req)
        return
    }
    handlingInfo := value.(proxyHandlingInfo)
  ...
    // 判斷APIService服務是否正常
    if !handlingInfo.serviceAvailable {
        proxyError(w, req,"service unavailable", http.StatusServiceUnavailable)
        return
    }
    // 將原始請求轉化為對APIService的請求
    // write a new location based on the existing request pointed at the target service
    location := &url.URL{}
    location.Scheme = "https"
    rloc, err := r.serviceResolver.ResolveEndpoint(handlingInfo.serviceNamespace, handlingInfo.serviceName, handlingInfo.servicePort)
    if err != nil {
        klog.Errorf("error resolving %s/%s: %v", handlingInfo.serviceNamespace, handlingInfo.serviceName, err)
        proxyError(w, req, "service unavailable", http.StatusServiceUnavailable)
        return
    }
    location.Host = rloc.Host
    location.Path = req.URL.Path
    location.RawQuery = req.URL.Query().Encode()
    newReq, cancelFn := newRequestForProxy(location, req)
    defer cancelFn()
   ...
    proxyRoundTripper = transport.NewAuthProxyRoundTripper(user.GetName(), user.GetGroups(), user.GetExtra(), proxyRoundTripper)
    handler := proxy.NewUpgradeAwareHandler(location, proxyRoundTripper, true, upgrade, &responder{w: w})
    handler.ServeHTTP(w, newReq)
}

$ kubectl get APIService           
NAME                                   SERVICE                      AVAILABLE   AGE
...
v1.apps                                Local                        True        50d
...
v1beta1.metrics.k8s.io                 kube-system/metrics-server   True        50d
...

# default APIServices
$ kubectl get -o yaml APIService/v1.apps
apiVersion: apiregistration.k8s.io/v1
kind: APIService
metadata:
  labels:
    kube-aggregator.kubernetes.io/automanaged: onstart
  name: v1.apps
  selfLink: /apis/apiregistration.k8s.io/v1/apiservices/v1.apps
spec:
  group: apps
  groupPriorityMinimum: 17800
  version: v1
  versionPriority: 15
status:
  conditions:
  - lastTransitionTime: "2020-10-20T10:39:48Z"
    message: Local APIServices are always available
    reason: Local
    status: "True"
    type: Available

# aggregated server    
$ kubectl get -o yaml APIService/v1beta1.metrics.k8s.io
apiVersion: apiregistration.k8s.io/v1
kind: APIService
metadata:
  labels:
    addonmanager.kubernetes.io/mode: Reconcile
    kubernetes.io/cluster-service: "true"
  name: v1beta1.metrics.k8s.io
  selfLink: /apis/apiregistration.k8s.io/v1/apiservices/v1beta1.metrics.k8s.io
spec:
  group: metrics.k8s.io
  groupPriorityMinimum: 100
  insecureSkipTLSVerify: true
  service:
    name: metrics-server
    namespace: kube-system
    port: 443
  version: v1beta1
  versionPriority: 100
status:
  conditions:
  - lastTransitionTime: "2020-12-05T00:50:48Z"
    message: all checks passed
    reason: Passed
    status: "True"
    type: Available

# CRD
$ kubectl get -o yaml APIService/v1.duyanghao.example.com
apiVersion: apiregistration.k8s.io/v1
kind: APIService
metadata:
  labels:
    kube-aggregator.kubernetes.io/automanaged: "true"
  name: v1.duyanghao.example.com
  selfLink: /apis/apiregistration.k8s.io/v1/apiservices/v1.duyanghao.example.com
spec:
  group: duyanghao.example.com
  groupPriorityMinimum: 1000
  version: v1
  versionPriority: 100
status:
  conditions:
  - lastTransitionTime: "2020-12-11T08:45:37Z"
    message: Local APIServices are always available
    reason: Local
    status: "True"
    type: Available

aggregatorServer創建程序中會根據所有kube-apiserver定義的API資源創建默認的APIService串列，名稱即是$VERSION.$GROUP，這些APIService都會有標簽kube-aggregator.kubernetes.io/automanaged: onstart，例如：v1.apps apiService，autoRegistrationController創建并維護這些串列中的APIService，也即我們看到的Local apiService；對于自定義的APIService(aggregated server)，則不會對其進行處理
aggregated server實作CR(自定義API資源) 的CRUD API介面，并可以靈活選擇后端存盤，可以與core kube-apiserver一起公用etcd，也可自己獨立部署etcd資料庫或者其它資料庫，aggregated server實作的CR API路徑為：/apis/$GROUP/$VERSION，具體到sample apiserver為：/apis/wardle.example.com/v1alpha1，下面的資源型別有：flunders以及fischers
aggregated server通過部署APIService型別資源，service fields指向對應的aggregated server service實作與core kube-apiserver的集成與互動

sample-apiserver目錄結構如下，可參考撰寫自己的aggregated server：

staging/src/k8s.io/sample-apiserver
├── artifacts
│   ├── example
│   │   ├── apiservice.yaml
      ...
├── hack
├── main.go
└── pkg
├── admission
├── apis
├── apiserver
├── cmd
├── generated
│   ├── clientset
│   │   └── versioned
              ...
│   │       └── typed
│   │           └── wardle
│   │               ├── v1alpha1
│   │               └── v1beta1
│   ├── informers
│   │   └── externalversions
│   │       └── wardle
│   │           ├── v1alpha1
│   │           └── v1beta1
│   ├── listers
│   │   └── wardle
│   │       ├── v1alpha1
│   │       └── v1beta1
└── registry

其中，artifacts用于部署yaml示例
hack目錄存放自動腳本(eg: update-codegen)
main.go是aggregated server啟動入口；pkg/cmd負責啟動aggregated server具體邏輯；pkg/apiserver用于aggregated server初始化以及路由注冊
pkg/apis負責相關CR的結構體定義，自動生成(update-codegen)
pkg/admission負責準入的相關代碼
pkg/generated負責生成訪問CR的clientset，informers，以及listers
pkg/registry目錄負責CR相關的RESTStorage實作

更多代碼原理詳情，參考 kubernetes-reading-notes ，

apiExtensionsServer

apiExtensionsServer主要負責CustomResourceDefinition（CRD）apiResources以及apiVersions的注冊，同時處理CRD以及相應CustomResource（CR）的REST請求(如果對應CR不能被處理的話則會回傳404)，也是apiserver Delegation的最后一環

原理總結如下：

Custom Resource，簡稱CR，是Kubernetes自定義資源型別，與之相對應的就是Kubernetes內置的各種資源型別，例如Pod、Service等，利用CR我們可以定義任何想要的資源型別
CRD通過yaml檔案的形式向Kubernetes注冊CR實作自定義api-resources，屬于第二種擴展Kubernetes API資源的方式，也是普遍使用的一種
APIExtensionServer負責CustomResourceDefinition（CRD）apiResources以及apiVersions的注冊，同時處理CRD以及相應CustomResource（CR）的REST請求(如果對應CR不能被處理的話則會回傳404)，也是apiserver Delegation的最后一環
crdRegistrationController負責將CRD GroupVersions自動注冊到APIServices中，具體邏輯為：列舉所有CRDs，然后根據CRD定義的crd.Spec.Group以及crd.Spec.Versions欄位構建APIService，并添加到autoRegisterController.apiServicesToSync中，由autoRegisterController進行創建以及維護操作，這也是為什么創建完CRD后會產生對應的APIService物件

APIExtensionServer包含的controller以及功能如下所示：

openapiController：將 crd 資源的變化同步至提供的 OpenAPI 檔案，可通過訪問 /openapi/v2 進行查看；
crdController：負責將 crd 資訊注冊到 apiVersions 和 apiResources 中，兩者的資訊可通過 kubectl api-versions 和 kubectl api-resources 查看；

kubectl api-versions命令回傳所有Kubernetes集群資源的版本資訊（實際發出了兩個請求，分別是https://127.0.0.1:6443/api以及https://127.0.0.1:6443/apis，并在最后將兩個請求的回傳結果進行了合并）

$ kubectl -v=8 api-versions 
I1211 11:44:50.276446   22493 loader.go:375] Config loaded from file:  /root/.kube/config
I1211 11:44:50.277005   22493 round_trippers.go:420] GET https://127.0.0.1:6443/api?timeout=32s
...
I1211 11:44:50.290265   22493 request.go:1068] Response Body: {"kind":"APIVersions","versions":["v1"],"serverAddressByClientCIDRs":[{"clientCIDR":"0.0.0.0/0","serverAddress":"x.x.x.x:6443"}]}
I1211 11:44:50.293673   22493 round_trippers.go:420] GET https://127.0.0.1:6443/apis?timeout=32s
...
I1211 11:44:50.298360   22493 request.go:1068] Response Body: {"kind":"APIGroupList","apiVersion":"v1","groups":[{"name":"apiregistration.k8s.io","versions":[{"groupVersion":"apiregistration.k8s.io/v1","version":"v1"},{"groupVersion":"apiregistration.k8s.io/v1beta1","version":"v1beta1"}],"preferredVersion":{"groupVersion":"apiregistration.k8s.io/v1","version":"v1"}},{"name":"extensions","versions":[{"groupVersion":"extensions/v1beta1","version":"v1beta1"}],"preferredVersion":{"groupVersion":"extensions/v1beta1","version":"v1beta1"}},{"name":"apps","versions":[{"groupVersion":"apps/v1","version":"v1"}],"preferredVersion":{"groupVersion":"apps/v1","version":"v1"}},{"name":"events.k8s.io","versions":[{"groupVersion":"events.k8s.io/v1beta1","version":"v1beta1"}],"preferredVersion":{"groupVersion":"events.k8s.io/v1beta1","version":"v1beta1"}},{"name":"authentication.k8s.io","versions":[{"groupVersion":"authentication.k8s.io/v1","version":"v1"},{"groupVersion":"authentication.k8s.io/v1beta1","version":"v1beta1"}],"preferredVersion":{"groupVersion":"authentication.k8s.io/v1"," [truncated 4985 chars]
apiextensions.k8s.io/v1
apiextensions.k8s.io/v1beta1
apiregistration.k8s.io/v1
apiregistration.k8s.io/v1beta1
apps/v1
authentication.k8s.io/v1beta1
...
storage.k8s.io/v1
storage.k8s.io/v1beta1
v1

kubectl api-resources命令就是先獲取所有API版本資訊，然后對每一個API版本呼叫介面獲取該版本下的所有API資源型別

$ kubectl -v=8 api-resources
 5077 loader.go:375] Config loaded from file:  /root/.kube/config
 I1211 15:19:47.593450   15077 round_trippers.go:420] GET https://127.0.0.1:6443/api?timeout=32s
 I1211 15:19:47.602273   15077 request.go:1068] Response Body: {"kind":"APIVersions","versions":["v1"],"serverAddressByClientCIDRs":[{"clientCIDR":"0.0.0.0/0","serverAddress":"x.x.x.x:6443"}]}
 I1211 15:19:47.606279   15077 round_trippers.go:420] GET https://127.0.0.1:6443/apis?timeout=32s
 I1211 15:19:47.610333   15077 request.go:1068] Response Body: {"kind":"APIGroupList","apiVersion":"v1","groups":[{"name":"apiregistration.k8s.io","versions":[{"groupVersion":"apiregistration.k8s.io/v1","version":"v1"},{"groupVersion":"apiregistration.k8s.io/v1beta1","version":"v1beta1"}],"preferredVersion":{"groupVersion":"apiregistration.k8s.io/v1","version":"v1"}},{"name":"extensions","versions":[{"groupVersion":"extensions/v1beta1","version":"v1beta1"}],"preferredVersion":{"groupVersion":"extensions/v1beta1","version":"v1beta1"}},{"name":"apps","versions":[{"groupVersion":"apps/v1","version":"v1"}],"preferredVersion":{"groupVersion":"apps/v1","version":"v1"}},{"name":"events.k8s.io","versions":[{"groupVersion":"events.k8s.io/v1beta1","version":"v1beta1"}],"preferredVersion":{"groupVersion":"events.k8s.io/v1beta1","version":"v1beta1"}},{"name":"authentication.k8s.io","versions":[{"groupVersion":"authentication.k8s.io/v1","version":"v1"},{"groupVersion":"authentication.k8s.io/v1beta1","version":"v1beta1"}],"preferredVersion":{"groupVersion":"authentication.k8s.io/v1"," [truncated 4985 chars]
 I1211 15:19:47.614700   15077 round_trippers.go:420] GET https://127.0.0.1:6443/apis/batch/v1?timeout=32s
 I1211 15:19:47.614804   15077 round_trippers.go:420] GET https://127.0.0.1:6443/apis/authentication.k8s.io/v1?timeout=32s
 I1211 15:19:47.615687   15077 round_trippers.go:420] GET https://127.0.0.1:6443/apis/auth.tkestack.io/v1?timeout=32s
 https://127.0.0.1:6443/apis/authentication.k8s.io/v1beta1?timeout=32s
 I1211 15:19:47.616794   15077 round_trippers.go:420] GET https://127.0.0.1:6443/apis/coordination.k8s.io/v1?timeout=32s
 I1211 15:19:47.616863   15077 round_trippers.go:420] GET https://127.0.0.1:6443/apis/apps/v1?timeout=32s
 ...
 NAME                              SHORTNAMES   APIGROUP                       NAMESPACED   KIND
 bindings                                                                      true         Binding
 endpoints                         ep                                          true         Endpoints
 events                            ev                                          true         Event
 limitranges                       limits                                      true         LimitRange
 namespaces                        ns                                          false        Namespace
 nodes                             no                                          false        Node
 ...

	 

 - `namingController`：檢查 crd obj 中是否有命名沖突，可在 crd `.status.conditions` 中查看；

 - `establishingController`：檢查 crd 是否處于正常狀態，可在 crd `.status.conditions` 中查看；

 - `nonStructuralSchemaController`：檢查 crd obj 結構是否正常，可在 crd `.status.conditions` 中查看；

 - `apiApprovalController`：檢查 crd 是否遵循 Kubernetes API 宣告策略，可在 crd `.status.conditions` 中查看；

 - `finalizingController`：類似于 finalizes 的功能，與 CRs 的洗掉有關；

總結CR CRUD APIServer處理邏輯如下：

createAPIExtensionsServer=>NewCustomResourceDefinitionHandler=>crdHandler=>注冊CR CRUD API介面：

// New returns a new instance of CustomResourceDefinitions from the given config.
func (c completedConfig) New(delegationTarget genericapiserver.DelegationTarget) (*CustomResourceDefinitions, error) {
  ...
    crdHandler, err := NewCustomResourceDefinitionHandler(
      versionDiscoveryHandler,
        groupDiscoveryHandler,
      s.Informers.Apiextensions().V1().CustomResourceDefinitions(),
        delegateHandler,
      c.ExtraConfig.CRDRESTOptionsGetter,
        c.GenericConfig.AdmissionControl,
      establishingController,
        c.ExtraConfig.ServiceResolver,
      c.ExtraConfig.AuthResolverWrapper,
        c.ExtraConfig.MasterCount,
        s.GenericAPIServer.Authorizer,
        c.GenericConfig.RequestTimeout,
        time.Duration(c.GenericConfig.MinRequestTimeout)*time.Second,
        apiGroupInfo.StaticOpenAPISpec,
        c.GenericConfig.MaxRequestBodyBytes,
    )
    if err != nil {
        return nil, err
    }
    s.GenericAPIServer.Handler.NonGoRestfulMux.Handle("/apis", crdHandler)
    s.GenericAPIServer.Handler.NonGoRestfulMux.HandlePrefix("/apis/", crdHandler)
    ...
    return s, nil
}

crdHandler處理邏輯如下：

決議req(GET /apis/duyanghao.example.com/v1/namespaces/default/students)，根據請求路徑中的group(duyanghao.example.com)，version(v1)，以及resource欄位(students)獲取對應CRD內容(crd, err := r.crdLister.Get(crdName))
通過crd.UID以及crd.Name獲取crdInfo，若不存在則創建對應的crdInfo(crdInfo, err := r.getOrCreateServingInfoFor(crd.UID, crd.Name))，crdInfo中包含了CRD定義以及該CRD對應Custom Resource的customresource.REST storage
customresource.REST storage由CR對應的Group(duyanghao.example.com)，Version(v1)，Kind(Student)，Resource(students)等創建完成，由于CR在Kubernetes代碼中并沒有具體結構體定義，所以這里會先初始化一個范型結構體Unstructured(用于保存所有型別的Custom Resource)，并對該結構體進行SetGroupVersionKind操作(設定具體Custom Resource Type)

從customresource.REST storage獲取Unstructured結構體后會對其進行相應轉換然后回傳

// k8s.io/kubernetes/staging/src/k8s.io/apiextensions-apiserver/pkg/apiserver/customresource_handler.go:223
func (r *crdHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
  ctx := req.Context()
  requestInfo, ok := apirequest.RequestInfoFrom(ctx)
  ...
  crdName := requestInfo.Resource + "." + requestInfo.APIGroup
  crd, err := r.crdLister.Get(crdName)
  ...
  crdInfo, err := r.getOrCreateServingInfoFor(crd.UID, crd.Name)
  verb := strings.ToUpper(requestInfo.Verb)
  resource := requestInfo.Resource
  subresource := requestInfo.Subresource
  scope := metrics.CleanScope(requestInfo)
  ...
  switch {
  case subresource == "status" && subresources != nil && subresources.Status != nil:
      handlerFunc = r.serveStatus(w, req, requestInfo, crdInfo, terminating, supportedTypes)
  case subresource == "scale" && subresources != nil && subresources.Scale != nil:
      handlerFunc = r.serveScale(w, req, requestInfo, crdInfo, terminating, supportedTypes)
  case len(subresource) == 0:
      handlerFunc = r.serveResource(w, req, requestInfo, crdInfo, terminating, supportedTypes)
  default:
      responsewriters.ErrorNegotiated(
          apierrors.NewNotFound(schema.GroupResource{Group: requestInfo.APIGroup, Resource: requestInfo.Resource}, requestInfo.Name),
          Codecs, schema.GroupVersion{Group: requestInfo.APIGroup, Version: requestInfo.APIVersion}, w, req,
      )
  }
  if handlerFunc != nil {
      handlerFunc = metrics.InstrumentHandlerFunc(verb, requestInfo.APIGroup, requestInfo.APIVersion, resource, subresource, scope, metrics.APIServerComponent, handlerFunc)
      handler := genericfilters.WithWaitGroup(handlerFunc, longRunningFilter, crdInfo.waitGroup)
      handler.ServeHTTP(w, req)
      return
  }
}

更多代碼原理詳情，參考 kubernetes-reading-notes ，

Conclusion

本文從原始碼層面對Kubernetes apiserver進行了一個概覽性總結，包括：aggregatorServer，kubeAPIServer，apiExtensionsServer以及bootstrap-controller等，通過閱讀本文可以對apiserver內部原理有一個大致的理解，另外也有助于后續深入研究

Refs

kubernetes-reading-notes

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