Downward API从容器内获取Pod信息

将Pod信息注入为环境变量

通过Downward API将Pod的IP、 名称和所在Namespace注入容器的环境变量中， 容器应用使用env命令将全部环境变量打印到标准输出中

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apiVersion: v1
kind: Pod
metadata:
  name: dapi-test-pod
spec:
  containers:
  - name: test-container
    image: busybox
    command: [ "/bin/sh", "-c", "env" ]
    env:
      - name: MY_POD_NAME
        valueFrom:
          fieldRef:
            fieldPath: metadata.name
      - name: MY_POD_NAMESPACE
        valueFrom:
          fieldRef:
            fieldPath: metadata.namespace
      - name: MY_POD_IP
        valueFrom:
          fieldRef:
            fieldPath: status.podIP
  restartPolicy: Never

将容器资源信息注入为环境变量

通过Downward API将Container的资源请求和限制信息注入容器的环境变量中， 容器应用使用printenv命令将设置的资源请求和资源限制环境变量打印到标准输出中 

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apiVersion: v1
kind: Pod
metadata:
  name: dapi-test-pod-container-vars 
spec:
  containers:
    - name: test-container
      image: busybox
      imagePullPolicy: Never
      command: [ "sh", "-c"]
      args:
      - while true; do
          echo -en '\n';
          printenv MY_CPU_REQUEST MY_CPU_LIMIT;
          printenv MY_MEM_REQUEST MY_MEM_LIMIT;
          sleep 3600;
        done;
      resources:
        requests:
          memory: "32Mi"
          cpu: "125m"
        limits:
          memory: "64Mi"
          cpu: "250m"
      env:
        - name: MY_CPU_REQUEST
          valueFrom:
            resourceFieldRef:
              containerName: test-container
              resource: requests.cpu
        - name: MY_CPU_LIMIT
          valueFrom:
            resourceFieldRef:
              containerName: test-container
              resource: limits.cpu
        - name: MY_MEM_REQUEST
          valueFrom:
            resourceFieldRef:
              containerName: test-container
              resource: requests.memory
        - name: MY_MEM_LIMIT
          valueFrom:
            resourceFieldRef:
              containerName: test-container
              resource: limits.memory
  restartPolicy: Never

Volume挂载方式

通过Downward API将Pod的Label、 Annotation列表通过Volume挂载为容器中的一个文件， 容器应用使用echo命令将文件的内容打印到标准输出中

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apiVersion: v1 
kind: Pod 
metadata:
  name: dapi-test-pod-volume
  labels:
    zone: us-est-coast
    cluster: test-cluster1
    rack: rack-22
  annotations:
    build: two
    builder: john-doe
spec:
  containers:
    - name: test-container
      image: busybox:latest
      command: ["sh", "-c"]
      args:
      - while true; do
          if [[ -e /etc/podinfo/labels ]]; then
            echo -en '\n\n'; cat /etc/podinfo/labels; fi;
          if [[ -e /etc/podinfo/annotations ]]; then
            echo -en '\n\n'; cat /etc/podinfo/annotations; fi;
          sleep 60;
        done;
      volumeMounts:
        - name: podinfo
          mountPath: /etc/podinfo
          readOnly: false
  volumes:
    - name: podinfo
      downwardAPI:
        items:
          - path: "labels"
            fieldRef:
              fieldPath: metadata.labels
          - path: "annotations"
            fieldRef:
              fieldPath: metadata.annotations

根据设置， 系统将在容器内生成/etc/labels和/etc/annotations两个文件。 在/etc/labels文件中将包含metadata.labels的全部Label列表， 在/etc/annotations文件中将包含metadata.annotations的全部Label列表。

Downward API的作用

程绑定的IP地址等信息事先写入配置文件中， 进程在启动时会读取这些信息， 然后将这些信息发布到某个类似服务注册中心的地方， 以实现集群节点的自动发现功能。