在k8s中除了使用secret和configMap向容器注入配置和敏感信息外，应用程序有时还需要基于其所运行的外在环境（如运行在哪个宿主机之上，宿主机的IP地址等）来了解自己的运行特性。
downwardAPI存储卷类型，从严格意义上来说，downwardAPI不是存储卷，它自身就存在。原因在于，它引用的是Pod自身的运行环境信息，这些信息在Pod启动后就存在。
类似于ConfigMap或Secret资源，容器能够在环境变量中在valueFrom字段中嵌套fieldRef或resourceFieldRef字段来引用其所属Pod对象的元数据信息。不过，通常只有常量类型的属性才能够通过环境变量注入到容器中，毕竟，在进程启动完成后无法再向其告知变量值的变动，于是，环境变量也就不支持中途的更新操作。
DownwardAPI引用 DownwardAPI的引用分为两类
第一类 容器规范中可在环境变量配置中的valueFrom通过内嵌字段fieldRef引用的信息包括如下这些：
1 2 3 4 5 metadata.name：Pod对象的名称； metadata.namespace：Pod对象隶属的名称空间； metadata.uid：Pod对象的UID； metadata.labels['<KEY>' ]：Pod对象标签中的指定键的值，例如metadata.labels['mylabel' ]，仅Kubernetes 1.9及之后的版本才支持； metadata.annotations['<KEY>' ]：Pod对象注解信息中的指定键的值，仅Kubernetes 1.9及之后的版本才支持。
第二类 容器上的计算资源需求和资源限制相关的信息，以及临时存储资源需求和资源限制相关的信息可通过容器规范中的resourceFieldRef字段引用，相关字段包括requests.cpu、limits.cpu、requests.memory和limits.memory等。另外，可通过环境变量引用的信息有如下几个：
1 2 3 4 status.podIP：Pod对象的IP地址 spec.serviceAccountName：Pod对象使用的ServiceAccount资源名称 spec.nodeName：节点名称 status.hostIP：节点IP地址
另外，还可以通过resourceFieldRef字段引用当前容器的资源请求及资源限额的定义，因此它们包括requests.cpu、requests.memory、requests.ephemeral-storage、limits.cpu、limits.memory和limits.ephemeral-storage这6项.
使用downwardAPI变量示例 1.编写资源清单
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 root@k8s-master01:~/yaml/chapter06 apiVersion: v1 kind: Pod metadata: name: downwardapi-env-demo labels: app: demoapp spec: containers: - name: demoapp image: ikubernetes/demoapp:v1.0 command : ["/bin/sh" ,"-c" ,"env" ] resources: requests: memory: "32Mi" cpu: "250m" limits: memory: "64Mi" cpu: "500m" env: - name: THIS_POD_NAME valueFrom: fieldRef: fieldPath: metadata.name - name: THIS_POD_NAMESPACE valueFrom: fieldRef: fieldPath: metadata.namespace - name: THIS_APP_LABEL valueFrom: fieldRef: fieldPath: metadata.labels['app' ] - name: THIS_CPU_LIMIT valueFrom: resourceFieldRef: resource: limits.cpu - name: THIS_MEM_REQUEST valueFrom: resourceFieldRef: resource: requests.memory divisor: 1Mi restartPolicy: Never
2.应用资源清单
1 2 3 4 5 6 7 root@k8s-master01:~/yaml/chapter06 pod/downwardapi-env-demo created root@k8s-master01:~/yaml/chapter06 NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES downwardapi-env-demo 0/1 Completed 0 40m 10.244.1.15 k8s-node01 <none> <none>
3.查看容器日志
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 root@k8s-master01:~/yaml/chapter06 KUBERNETES_SERVICE_PORT=443 KUBERNETES_PORT=tcp://10.96.0.1:443 HOSTNAME=downwardapi-env-demo THIS_CPU_LIMIT=1 THIS_APP_LABEL=demoapp SHLVL=1 HOME=/root THIS_MEM_REQUEST=32 PS1=[\u@\h \w]\$ KUBERNETES_PORT_443_TCP_ADDR=10.96.0.1 PATH=/usr/local /sbin:/usr/local /bin:/usr/sbin:/usr/bin:/sbin:/bin THIS_POD_NAME=downwardapi-env-demo KUBERNETES_PORT_443_TCP_PORT=443 KUBERNETES_PORT_443_TCP_PROTO=tcp DEPLOYENV=Production KUBERNETES_SERVICE_PORT_HTTPS=443 KUBERNETES_PORT_443_TCP=tcp://10.96.0.1:443 RELEASE=Stable KUBERNETES_SERVICE_HOST=10.96.0.1 PWD=/ THIS_POD_NAMESPACE=default
downwardAPI存储卷示例 1.编写配置清单
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 root@k8s-master01:~/yaml/chapter06 apiVersion: v1 kind: Pod metadata: name: downwardapi-volume-demo labels: zone: zone1 rack: rack100 app: demoapp annotations: region: ease-cn spec: containers: - name: demoapp image: ikubernetes/demoapp:v1.0 resources: requests: memory: "32Mi" cpu: "250m" limits: memory: "64Mi" cpu: "500m" volumeMounts: - name: podinfo mountPath: /etc/podinfo readOnly: false volumes: - name: podinfo downwardAPI: defaultMode: 420 items: - fieldRef: fieldPath: metadata.namespace path: pod_namespace - fieldRef: fieldPath: metadata.labels path: pod_labels - fieldRef: fieldPath: metadata.annotations path: pod_annotations - resourceFieldRef: containerName: demoapp resource: limits.cpu path: "cpu_limit" - resourceFieldRef: containerName: demoapp resource: requests.memory divisor: "1Mi" path: "mem_request"
2.应用配置清单
1 2 3 4 5 root@k8s-master01:~/yaml/chapter06 pod/downwardapi-volume-demo created root@k8s-master01:~/yaml/chapter06 NAME READY STATUS RESTARTS AGE downwardapi-volume-demo 1/1 Running 0 19s
3.查看downwardAPI在/etc/podinfo所挂载的信息
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 root@k8s-master01:~/yaml/chapter06 [root@downwardapi-volume-demo /] [root@downwardapi-volume-demo /etc/podinfo] total 0 lrwxrwxrwx 1 root root 16 Jul 19 09:17 cpu_limit -> ..data/cpu_limit lrwxrwxrwx 1 root root 18 Jul 19 09:17 mem_request -> ..data/mem_request lrwxrwxrwx 1 root root 22 Jul 19 09:17 pod_annotations -> ..data/pod_annotations lrwxrwxrwx 1 root root 17 Jul 19 09:17 pod_labels -> ..data/pod_labels lrwxrwxrwx 1 root root 20 Jul 19 09:17 pod_namespace -> ..data/pod_namespace [root@downwardapi-volume-demo /etc/podinfo] 1 32 kubectl.kubernetes.io/last-applied-configuration="{\"apiVersion\":\"v1\",\"kind\":\"Pod\",\"metadata\":{\"annotations\":{\"region\":\"ease-cn\"},\"labels\":{\"app\":\"demoapp\",\"rack\":\"rack100\",\"zone\":\"zone1\"},\"name\":\"downwardapi-volume-demo\",\"namespace\":\"default\"},\"spec\":{\"containers\":[{\"image\":\"ikubernetes/demoapp:v1.0\",\"name\":\"demoapp\",\"resources\":{\"limits\":{\"cpu\":\"500m\",\"memory\":\"64Mi\"},\"requests\":{\"cpu\":\"250m\",\"memory\":\"32Mi\"}},\"volumeMounts\":[{\"mountPath\":\"/etc/podinfo\",\"name\":\"podinfo\",\"readOnly\":false}]}],\"volumes\":[{\"downwardAPI\":{\"defaultMode\":420,\"items\":[{\"fieldRef\":{\"fieldPath\":\"metadata.namespace\"},\"path\":\"pod_namespace\"},{\"fieldRef\":{\"fieldPath\":\"metadata.labels\"},\"path\":\"pod_labels\"},{\"fieldRef\":{\"fieldPath\":\"metadata.annotations\"},\"path\":\"pod_annotations\"},{\"path\":\"cpu_limit\",\"resourceFieldRef\":{\"containerName\":\"demoapp\",\"resource\":\"limits.cpu\"}},{\"path\":\"mem_request\",\"resourceFieldRef\":{\"containerName\":\"demoapp\",\"divisor\":\"1Mi\",\"resource\":\"requests.memory\"}}]},\"name\":\"podinfo\"}]}}\n" kubernetes.io/config.seen="2021-07-19T09:17:04.392801370Z" kubernetes.io/config.source="api" region="ease-cn" app="demoapp" rack="rack100"
