Kubernetes - DaemonSet

# 前言

本文為閱讀 Kubernetes DaemonSet 官方文件, 並將重點摘取整理為 Q&A 方式

Kubernetes DaemonSet, Pod 會在什麼時候被 schedule 到 Node 上？

所有, 或指定的 Node 被建立時

Kubernetes DaemonSet, Pod 會在什麼時候從 Node 上被移除？

當 Node 從 cluster 被移除時, 或刪除 DaemonSet

Kubernetes DaemonSet, 有哪三種普遍的應用？

• 執行一個 cluster storage daemon
• 在每一個 Node 上執行 logs collection
• 在每一個 Node 上執行 monitoring

以下的 Kubernetes example code 代表什麼意思？

• Example:

  apiVersion: apps/v1
  kind: DaemonSet
  metadata:
    name: fluentd-elasticsearch
    namespace: kube-system
    labels:
      k8s-app: fluentd-logging
  spec:
    selector:
      matchLabels:
        name: fluentd-elasticsearch
    template:
      metadata:
        labels:
          name: fluentd-elasticsearch
      spec:
        tolerations:
        - key: node-role.kubernetes.io/master
          effect: NoSchedule
        containers:
        - name: fluentd-elasticsearch
          image: quay.io/fluentd_elasticsearch/fluentd:v2.5.2
          resources:
            limits:
              memory: 200Mi
            requests:
              cpu: 100m
              memory: 200Mi
          volumeMounts:
          - name: varlog
            mountPath: /var/log
          - name: varlibdockercontainers
            mountPath: /var/lib/docker/containers
            readOnly: true
        terminationGracePeriodSeconds: 30
        volumes:
        - name: varlog
          hostPath:
            path: /var/log
        - name: varlibdockercontainers
          hostPath:
            path: /var/lib/docker/containers

• Answer:

  # 結構上跟 Deployment 十分類似
  # 總結一下, 該 DaemonSet 會在每一個 Node 包含 Master Node
  # 上部署 fluentd-elasticsearch 做 Log 處理
  apiVersion: apps/v1
  kind: DaemonSet
  metadata:
      name: fluentd-elasticsearch
      namespace: kube-system
      labels:
          k8s-app: fluentd-logging
  spec:
      selector:
          matchLabels:
              name: fluentd-elasticsearch
      template:
          metadata:
              labels:
                  name: fluentd-elasticsearch
          spec:
              tolerations:
                  # DaemonSet 預設不會執行在 Master Node 上, 使用 tolerations 可以允許
                  # Master Node 運行 DaemonSet pod
                  - key: node-role.kubernetes.io/master
                    effect: NoSchedule
              containers:
                  - name: fluentd-elasticsearch
                    image: quay.io/fluentd_elasticsearch/fluentd:v2.5.2
                    resources:
                        limits:
                            memory: 200Mi
                        requests:
                            cpu: 100m
                            memory: 200Mi
                    volumeMounts:
                        - name: varlog
                          mountPath: /var/log
                        - name: varlibdockercontainers
                          mountPath: /var/lib/docker/containers
                          readOnly: true
              # 當 Pod 要被關閉時, 會有 30 秒的優雅退出
              terminationGracePeriodSeconds: 30
              volumes:
                  - name: varlog
                    hostPath:
                        path: /var/log
                  - name: varlibdockercontainers
                    hostPath:
                        path: /var/lib/docker/containers

# Required Fields

何謂 valid DNS subdomain name?

• 不可超過 253 characters
• alphanumeric character 開頭
• alphanumeric character 結尾
• 只可含有 lowercase alphanumeric, - 或 .

Kubernetes DaemonSet 的 name 必須合乎哪種規範？

valid DNS subdomain name

# Pod Template

Kubernetes DaemonSet 中, RestartPolicy 必須為？

Always