## 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，併為其它元件互動提供了橋樑。因此加深對kube-apiserver的理解就顯得至關重要了。

![](https://main.qcloudimg.com/raw/066f8a835dd51b6cdf0577bd2d513bbf.png)

## 整體元件功能

kube-apiserver作為整個Kubernetes叢集操作etcd的唯一入口，負責Kubernetes各資源的認證&鑑權，校驗以及CRUD等操作，提供RESTful APIs，供其它元件呼叫：

![](https://main.qcloudimg.com/raw/a4e47ea5a0052e2fdaf78706048c0342.png)

kube-apiserver包含三種APIServer：

- **aggregatorServer**：負責處理 `apiregistration.k8s.io` 組下的APIService資源請求，同時將來自使用者的請求攔截轉發給aggregated server(AA)
- **kubeAPIServer**：負責對請求的一些通用處理，包括：認證、鑑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的註冊，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返回404)，也是apiserver Delegation的最後一環

另外還包括bootstrap-controller，主要負責Kubernetes default apiserver service的建立以及管理。

接下來將對上述元件進行概覽性總結。

## bootstrap-controller

- **apiserver bootstrap-controller**建立&執行邏輯在k8s.io/kubernetes/pkg/master目錄
- **bootstrap-controller**主要用於建立以及維護內部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：
  - 建立 default、kube-system 和 kube-public 以及 kube-node-lease 名稱空間
  - 建立&維護kubernetes default apiserver service以及對應的endpoint
  - 提供基於Service ClusterIP的檢查及修復功能(`--service-cluster-ip-range`指定範圍)
  - 提供基於Service NodePort的檢查及修復功能(`--service-node-port-range`指定範圍)

```go
// 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()
}
```

更多程式碼原理詳情，參考 [kubernetes-reading-notes](https://github.com/duyanghao/kubernetes-reading-notes/tree/master/core/api-server) 。

## kubeAPIServer

KubeAPIServer主要提供對內建API Resources的操作請求，為Kubernetes中各API Resources註冊路由資訊，同時暴露RESTful API，使叢集中以及叢集外的服務都可以透過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基礎上進行擴充套件的(補充了Kubernetes對使用者自定義資源的能力支援)

kubeAPIServer最核心的功能是為Kubernetes內建資源新增路由，如下：

- 呼叫 `m.InstallLegacyAPI` 將核心 API Resources新增到路由中，在apiserver中即是以 `/api` 開頭的 resource；
- 呼叫 `m.InstallAPIs` 將擴充套件的 API Resources新增到路由中，在apiserver中即是以 `/apis` 開頭的 resource；

```go
// 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：

![](https://main.qcloudimg.com/raw/907a38287b3d85c85353beabfa3dcc51.png)

kubeAPIServer會為每種API資源建立對應的RESTStorage，RESTStorage的目的是將每種資源的訪問路徑及其後端儲存的操作對應起來：透過構造的REST Storage實現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，對每一個操作新增一個handler(handler對應REST Storage實現的相關介面)，並註冊到route，最終對外提供RESTful API，如下：

```go
// 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 實現了哪些 REST 操作介面，以此來判斷其支援的 verbs 以便為其新增路由
    // 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 新增對應的 actions(+根據是否支援 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 對映
    // 為每個操作新增對應的 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 進行繫結    
            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、新增到路由中    
            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程式碼結構整理如下：

```
1. apiserver整體啟動邏輯 k8s.io/kubernetes/cmd/kube-apiserver
2. apiserver bootstrap-controller建立&執行邏輯 k8s.io/kubernetes/pkg/master
3. API Resource對應後端RESTStorage(based on genericregistry.Store)建立k8s.io/kubernetes/pkg/registry
4. aggregated-apiserver建立&處理邏輯 k8s.io/kubernetes/staging/src/k8s.io/kube-aggregator
5. extensions-apiserver建立&處理邏輯 k8s.io/kubernetes/staging/src/k8s.io/apiextensions-apiserver
6. apiserver建立&執行 k8s.io/kubernetes/staging/src/k8s.io/apiserver/pkg/server
7. 註冊API Resource資源處理handler(InstallREST&Install®isterResourceHandlers) k8s.io/kubernetes/staging/src/k8s.io/apiserver/pkg/endpoints
8. 建立儲存後端(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：

![](https://main.qcloudimg.com/raw/1d8e08a7f5c37f8d5d29a7fe92ad1e0f.png)

更多程式碼原理詳情，參考 [kubernetes-reading-notes](https://github.com/duyanghao/kubernetes-reading-notes/tree/master/core/api-server) 。

## aggregatorServer

aggregatorServer主要用於處理擴充套件Kubernetes API Resources的第二種方式Aggregated APIServer(AA)，將CR請求代理給AA：

![](https://main.qcloudimg.com/raw/b17c4c179284b7db2cd4f6832bfcba06.jpg)

這裡結合Kubernetes官方給出的aggregated apiserver例子[sample-apiserver](https://github.com/kubernetes/sample-apiserver)，總結原理如下：

- aggregatorServer透過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的請求轉發給後端的aggregated server
  - `availableConditionController`：維護 APIServices 的可用狀態，包括其引用 Service 是否可用等；
  - `autoRegistrationController`：用於保持 API 中存在的一組特定的 APIServices；
  - `crdRegistrationController`：負責將 CRD GroupVersions 自動註冊到 APIServices 中；
  - `openAPIAggregationController`：將 APIServices 資源的變化同步至提供的 OpenAPI 文件；

- apiserviceRegistrationController負責根據APIService定義的aggregated server service構建代理，將CR的請求轉發給後端的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型別則會設定代理並將請求轉化為對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)=>透過service ClusterIP:Port訪問(預設) 或者 透過隨機選擇service endpoint backend進行訪問：

```go
  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) {
      // 載入roxyHandlingInfo處理請求  
      value := r.handlingInfo.Load()
      if value == 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
  ...
  ```

```yaml
  # 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建立過程中會根據所有kube-apiserver定義的API資源建立預設的APIService列表，名稱即是`$VERSION.$GROUP`，這些APIService都會有標籤`kube-aggregator.kubernetes.io/automanaged: onstart`，例如：v1.apps apiService。autoRegistrationController建立並維護這些列表中的APIService，也即我們看到的Local apiService；對於自定義的APIService(aggregated server)，則不會對其進行處理

- aggregated server實現CR(自定義API資源) 的CRUD API介面，並可以靈活選擇後端儲存，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實現的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透過部署APIService型別資源，service fields指向對應的aggregated server service實現與core kube-apiserver的整合與互動

- sample-apiserver目錄結構如下，可參考編寫自己的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用於部署yaml示例
  - hack目錄存放自動指令碼(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用於aggregated server初始化以及路由註冊
  - pkg/apis負責相關CR的結構體定義，自動生成(update-codegen)
  - pkg/admission負責准入的相關程式碼
  - 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實現

更多程式碼原理詳情，參考 [kubernetes-reading-notes](https://github.com/duyanghao/kubernetes-reading-notes/tree/master/core/api-server) 。

## apiExtensionsServer

apiExtensionsServer主要負責CustomResourceDefinition（CRD）apiResources以及apiVersions的註冊，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返回404)，也是apiserver Delegation的最後一環

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內建的各種資源型別，例如Pod、Service等。利用CR我們可以定義任何想要的資源型別

- CRD透過yaml檔案的形式向Kubernetes註冊CR實現自定義api-resources，屬於第二種擴充套件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的註冊，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返回404)，也是apiserver Delegation的最後一環

- `crdRegistrationController`負責將CRD GroupVersions自動註冊到APIServices中。具體邏輯為：列舉所有CRDs，然後根據CRD定義的crd.Spec.Group以及crd.Spec.Versions欄位構建APIService，並新增到autoRegisterController.apiServicesToSync中，由autoRegisterController進行建立以及維護操作。這也是為什麼建立完CRD後會產生對應的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 文件，可透過訪問 `/openapi/v2` 進行檢視；

- `crdController`：負責將 crd 資訊註冊到 apiVersions 和 apiResources 中，兩者的資訊可透過 `kubectl api-versions` 和 `kubectl api-resources` 檢視；

- `kubectl api-versions`命令返回所有Kubernetes叢集資源的版本資訊（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`，並在最後將兩個請求的返回結果進行了合併）

```
    $ 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，然後對每一個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 中是否有命名衝突，可在 crd `.status.conditions` 中檢視；

- `establishingController`：檢查 crd 是否處於正常狀態，可在 crd `.status.conditions` 中檢視；

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

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

- `finalizingController`：類似於 finalizes 的功能，與 CRs 的刪除有關；

- 總結CR CRUD APIServer處理邏輯如下：

- createAPIExtensionsServer=>NewCustomResourceDefinitionHandler=>crdHandler=>註冊CR CRUD API介面：

```go
    // 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：

- 解析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))

- 透過crd.UID以及crd.Name獲取crdInfo，若不存在則建立對應的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)等建立完成，由於CR在Kubernetes程式碼中並沒有具體結構體定義，所以這裡會先初始化一個範型結構體Unstructured(用於儲存所有型別的Custom Resource)，並對該結構體進行SetGroupVersionKind操作(設定具體Custom Resource Type)

- 從customresource.REST storage獲取Unstructured結構體後會對其進行相應轉換然後返回

```go
      // 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
        }
      }
      
      ```

更多程式碼原理詳情，參考 [kubernetes-reading-notes](https://github.com/duyanghao/kubernetes-reading-notes/tree/master/core/api-server) 。

## 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。透過閱讀本文可以對apiserver內部原理有一個大致的理解，另外也有助於後續深入研究

## Refs

- [kubernetes-reading-notes](https://github.com/duyanghao/kubernetes-reading-notes/tree/master/core/api-server)

一文讀懂 Kubernetes APIServer 原理

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