Kubernetes Downward API

在Kubernetes环境中，因为容器是被动态分配到节点上的，也就是说在容器启动之前，无法知道运行环境的具体信息

那如何在程序中获得当前容器的 pod ip 呢？又或者是 当前容器的 内存限制呢？

Kubernetes 使用 Downward API 通过环境变量和文件，将有关Pod的元数据注入到容器和集群中。

apiVersion: v1
kind: Pod
metadata:
  name: getpodip
spec:
  containers:
  - image: nginx
    name: getpodip
    env: 
    - name: POD_IP
      valueFrom:
        fieldRef: 
          fieldPath: status.podIP
    - name: MEM_LIMITS
      valueFrom:
        resourceFieldRef: 
          containerName: getpodip
          resource: limits.memory

不过由于环境变量无法在容器启动之后更改，也可以将信息挂载到磁盘卷上，这样每次修改之后，程序可以通过重新读取文件内容，更新这些信息

apiVersion: v1
kind: Pod
metadata:
  name: getpodip
spec:
  containers:
  - image: nginx
    name: getpodip
    volumeMounts:
      - name: pod-info
        mountPath: /pod-info
        readOnly: true
      - name: pod-resource
        mountPath: /pod-resource
        readOnly: true
  volumes:
    - name: pod-info
      downwardAPI:
        items:
          - path: POD_IP
            fieldRef: 
              fieldPath: metadata.name
    - name: pod-resource
      downwardAPI:
        items:
          - path: "cpu_limit"
            resourceFieldRef:
              containerName: getpodip
              resource: limits.cpu
              divisor: 1m
          - path: "cpu_request"
            resourceFieldRef:
              containerName: getpodip
              resource: requests.cpu
              divisor: 1m
          - path: "mem_limit"
            resourceFieldRef:
              containerName: getpodip
              resource: limits.memory
              divisor: 1Mi
          - path: "mem_request"
            resourceFieldRef:
              containerName: getpodip
              resource: requests.memory
              divisor: 1Mi

不过不知道为什么这里 metadata.name 无法替换为 status.podIP

The Pod "getpodip" is invalid: 
* spec.volumes[0].downwardAPI.fieldRef.fieldPath: Unsupported value: "status.podIP": supported values: "metadata.annotations", "metadata.labels", "metadata.name", "metadata.namespace", "metadata.uid"
* spec.containers[0].volumeMounts[0].name: Not found: "pod-info"

• Env vars using valueFrom.fieldRef:
  •  spec.nodeName – the node’s name
  •  status.hostIP – the node’s IP
  •  metadata.name – the pod’s name
  •  metadata.namespace – the pod’s namespace
  •  status.podIP – the pod’s IP address
  •  spec.serviceAccountName – the pod’s service account name
  •  metadata.uid – the pod’s UID
  •  metadata.labels[”] – the value of the pod’s label (for example, metadata.labels[‘mylabel’])
  •  metadata.annotations[”] – the value of the pod’s annotation (for example, metadata.annotations[‘myannotation’])
• DownwardAPI volumes via items of fieldRef:
  •  spec.nodeName – the node’s name
  •  status.hostIP – the node’s IP
  •  metadata.name – the pod’s name
  •  metadata.namespace – the pod’s namespace
  •  status.podIP – the pod’s IP address
  •  spec.serviceAccountName – the pod’s service account name
  •  metadata.uid – the pod’s UID
  •  metadata.labels- the value of the pod’s label (for example, metadata.labels[‘mylabel’])
  •  metadata.annotations – the value of the pod’s annotation (for example, metadata.annotations[‘myannotation’])