前言

由前面章節可以得知，Kubernetes的Node有兩種，分別是負責決策與判斷的Master Node，以及負責執行的Worker Node。而本章節將與大家探討Master Node與其特性。

What is Master Node

Master Node主要用來管理Work Node(也就是Slave Node, Kubernetes Node)，進行工作的調配與規劃..等，也因此Master Node有著以下幾個components來完成這些功能

• API Server
• Controller Manager
• Scheduler
• etcd

• Get master node components
  利用該指令能得到master node components的狀態

$ kubectl get componentstatuses
NAME                 STATUS    MESSAGE             ERROR
etcd-0               Healthy   {"health":"true"}
etcd-1               Healthy   {"health":"true"}
controller-manager   Healthy   ok
scheduler            Healthy   ok

• Get components detail
  利用該指令能得到特定component的詳細資訊，像是api endpoint..等。

$ kubectl describe componentstatuses controller-manager                                                                                             
Name:         controller-manager
Namespace:
Labels:       <none>
Annotations:  <none>
API Version:  v1
Conditions:
  Message:  ok
  Status:   True
  Type:     Healthy
Kind:       ComponentStatus
Metadata:
  Creation Timestamp:  <nil>
  Self Link:           /api/v1/componentstatuses/controller-manager
Events:                <none>

Controller Manager

Controller manager主要透過各種controller來控制process，邏輯上每個controller都是一個獨立的process，但為了降低複雜度，controllers會被編譯在一個二進位的文件，並且在同一process運行著。

這些Controllers包含著

• Node controller: 負責當Node出現故障時的訊息通知與增減的Controller
• Replication controller: 負責管理Pods數量與狀態的Controller，但現在使用ReplicaSet居多
• Endpoints controller: 負責生成與維護所有endppoints物件(such as service)的Controller
• Service Account & Token controllers: 負責管理用戶帳戶與服務德Controller

在後面篇章會介紹k8s的worker node, replication, service, account....etc

Scheduler

Scheduler會依據機器資源、軟體資源(叢集)、調度決策、affinity and anti-affinity親和力與反親和力等多方考量，去決定Pods是否新建與Pods的數量分配(分配至Ｗorker Node)。因為每個Pods都會有自己的requirements(像是平均Memory用量超過50%新建、不建立該種Pod在Node-1上..等)，所以在叢集調度上並非易事，也因此有了該components的產生

Tips: affinity又有分成是Node affinity與 Pod affinity，affinity也是屬於種調度策略，這邊在後面篇章會詳細介紹。

Etcd

每個Cluster都會有著一個以上的etcd，etcd用一致且高可用的鍵值方式儲存Kubernetes叢集的資料，Kubernetes Cluster會預設將etcd資料作為備份。

API-Server

API-Server是Kubernetes Control Plane中的一個重要組件，透過API-Server對外曝露所有的Kubernetes API，亦可以把它當作是Kubernetes Control Plane的前端。而我們操作的Kubernetes ctl也是我們透過kubectl的方式與API-Server進行溝通，總結來說API-Server有著以下幾個功能：

• 提供API讓使用者能夠取得叢集內部各資源資訊、創建/更改/移除 各資源或下達調度策略對各資源進行調配。
• 代理群集當中一些額外組件，像是Kubernetes UI、metrics-server..etc
• 創建Kubernetes server
• 讓資源進行版本更新

通過kubectl proxy，我們能夠得知目前叢集最上層的api path為何

$ kubectl proxy
Starting to serve on 127.0.0.1:8001

{
  "paths": [
    "/api",
    "/api/v1",
    "/apis",
    "/apis/",
    "/apis/admissionregistration.k8s.io",
    "/apis/admissionregistration.k8s.io/v1",
    "/apis/admissionregistration.k8s.io/v1beta1",
    "/apis/apiextensions.k8s.io",
    "/apis/apiextensions.k8s.io/v1",
    "/apis/apiextensions.k8s.io/v1beta1",
    "/apis/apiregistration.k8s.io",
    "/apis/apiregistration.k8s.io/v1",
    "/apis/apiregistration.k8s.io/v1beta1",
    "/apis/apps",
    "/apis/apps/v1",
    "/apis/authentication.k8s.io",
    "/apis/authentication.k8s.io/v1",
    "/apis/authentication.k8s.io/v1beta1",
    "/apis/authorization.k8s.io",
    "/apis/authorization.k8s.io/v1",
    "/apis/authorization.k8s.io/v1beta1",
    "/apis/autoscaling",
    "/apis/autoscaling/v1",
    "/apis/autoscaling/v2beta1",
    "/apis/autoscaling/v2beta2",
    "/apis/batch",
    "/apis/batch/v1",
    "/apis/batch/v1beta1",
    "/apis/certificates.k8s.io",
    "/apis/certificates.k8s.io/v1beta1",
    "/apis/cloud.google.com",
    "/apis/cloud.google.com/v1",
    "/apis/cloud.google.com/v1beta1",
    "/apis/coordination.k8s.io",
    "/apis/coordination.k8s.io/v1",
    "/apis/coordination.k8s.io/v1beta1",
    "/apis/discovery.k8s.io",
    "/apis/discovery.k8s.io/v1beta1",
    "/apis/extensions",
    "/apis/extensions/v1beta1",
    "/apis/metrics.k8s.io",
    "/apis/metrics.k8s.io/v1beta1",
    "/apis/migration.k8s.io",
    "/apis/migration.k8s.io/v1alpha1",
    "/apis/networking.gke.io",
    "/apis/networking.gke.io/v1beta1",
    "/apis/networking.gke.io/v1beta2",
    "/apis/networking.k8s.io",
    "/apis/networking.k8s.io/v1",
    "/apis/networking.k8s.io/v1beta1",
    "/apis/node.k8s.io",
    "/apis/node.k8s.io/v1beta1",
    "/apis/nodemanagement.gke.io",
    "/apis/nodemanagement.gke.io/v1alpha1",
    "/apis/policy",
    "/apis/policy/v1beta1",
    "/apis/rbac.authorization.k8s.io",
    "/apis/rbac.authorization.k8s.io/v1",
    "/apis/rbac.authorization.k8s.io/v1beta1",
    "/apis/scheduling.k8s.io",
    "/apis/scheduling.k8s.io/v1",
    "/apis/scheduling.k8s.io/v1beta1",
    "/apis/snapshot.storage.k8s.io",
    "/apis/snapshot.storage.k8s.io/v1beta1",
    "/apis/storage.k8s.io",
    "/apis/storage.k8s.io/v1",
    "/apis/storage.k8s.io/v1beta1",
    "/healthz",
    "/healthz/SSH Tunnel Check",
    "/healthz/autoregister-completion",
    "/healthz/etcd",
    "/healthz/log",
    "/healthz/ping",
    "/healthz/poststarthook/apiservice-openapi-controller",
    "/healthz/poststarthook/apiservice-registration-controller",
    "/healthz/poststarthook/apiservice-status-available-controller",
    "/healthz/poststarthook/bootstrap-controller",
    "/healthz/poststarthook/crd-informer-synced",
    "/healthz/poststarthook/generic-apiserver-start-informers",
    "/healthz/poststarthook/kube-apiserver-autoregistration",
    "/healthz/poststarthook/rbac/bootstrap-roles",
    "/healthz/poststarthook/scheduling/bootstrap-system-priority-classes",
    "/healthz/poststarthook/start-apiextensions-controllers",
    "/healthz/poststarthook/start-apiextensions-informers",
    "/healthz/poststarthook/start-cluster-authentication-info-controller",
    "/healthz/poststarthook/start-kube-aggregator-informers",
    "/healthz/poststarthook/start-kube-apiserver-admission-initializer",
    "/livez",
    "/livez/SSH Tunnel Check",
    "/livez/autoregister-completion",
    "/livez/etcd",
    "/livez/log",
    "/livez/ping",
    "/livez/poststarthook/apiservice-openapi-controller",
    "/livez/poststarthook/apiservice-registration-controller",
    "/livez/poststarthook/apiservice-status-available-controller",
    "/livez/poststarthook/bootstrap-controller",
    "/livez/poststarthook/crd-informer-synced",
    "/livez/poststarthook/generic-apiserver-start-informers",
    "/livez/poststarthook/kube-apiserver-autoregistration",
    "/livez/poststarthook/rbac/bootstrap-roles",
    "/livez/poststarthook/scheduling/bootstrap-system-priority-classes",
    "/livez/poststarthook/start-apiextensions-controllers",
    "/livez/poststarthook/start-apiextensions-informers",
    "/livez/poststarthook/start-cluster-authentication-info-controller",
    "/livez/poststarthook/start-kube-aggregator-informers",
    "/livez/poststarthook/start-kube-apiserver-admission-initializer",
    "/logs",
    "/metrics",
    "/openapi/v2",
    "/readyz",
    "/readyz/SSH Tunnel Check",
    "/readyz/autoregister-completion",
    "/readyz/etcd",
    "/readyz/informer-sync",
    "/readyz/log",
    "/readyz/ping",
    "/readyz/poststarthook/apiservice-openapi-controller",
    "/readyz/poststarthook/apiservice-registration-controller",
    "/readyz/poststarthook/apiservice-status-available-controller",
    "/readyz/poststarthook/bootstrap-controller",
    "/readyz/poststarthook/crd-informer-synced",
    "/readyz/poststarthook/generic-apiserver-start-informers",
    "/readyz/poststarthook/kube-apiserver-autoregistration",
    "/readyz/poststarthook/rbac/bootstrap-roles",
    "/readyz/poststarthook/scheduling/bootstrap-system-priority-classes",
    "/readyz/poststarthook/start-apiextensions-controllers",
    "/readyz/poststarthook/start-apiextensions-informers",
    "/readyz/poststarthook/start-cluster-authentication-info-controller",
    "/readyz/poststarthook/start-kube-aggregator-informers",
    "/readyz/poststarthook/start-kube-apiserver-admission-initializer",
    "/readyz/shutdown",
    "/version"
  ]
}

Kubernetes Control Plane

最後再來看一次k8s cluster架構，今天介紹的Master Node部分就是扮演著Cluster中 Control Plane的角色，負責K8s大小事的決策以及調度，透過api-server進行溝通最後再將這些資料都存在etcd當中。

小結

這章節主要告訴大家Master Node的基本原理與重要組件，希望能帶給剛認識Kubernetes的讀者一個基礎的觀念。那再下一章節會繼續為大家帶來Worker Node(Slave Node)

Reference

https://kubernetes.io/docs/concepts/overview/components/