Docker 与 K8S学习笔记（二十）—— 使用Downward API向容器注入Pod信息

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Kubernetes在创建Pod时，会为Pod和容器设置一些额外的信息，比如Pod名称、Pod IP、Node IP、Label、Annotation、资源限制等，我们经常会在应用程序中使用到这些数据，比如利用Pod名称作为应用日志的字段，方便分析日志。为了能在容器内获取这些信息，我们可以使用Downward API机制来实现。

Downward API可以通过环境变量和Volume挂载这两种方式将Pod信息注入容器，我们分别来看一下：

一、环境变量方式

我们还是以Busybox为例进行演示，我们将Pod信息和Container信息以环境变量方式注入容器，在容器启动后通过env命令打印出来，我们Yaml文件内容如下：

apiVersion: v1
kind: Pod
metadata:name: busybox-pod
spec:containers:- name: busyboximage: busyboxcommand: ["/bin/sh", "-c", "env | grep VAR\_"]resources:requests:memory: "16Mi"cpu: "125m"limits:memory: "32Mi"cpu: "250m"env:- name: VAR\_NODE\_NAMEvalueFrom:fieldRef:fieldPath: spec.nodeName- name: VAR\_POD\_NAMEvalueFrom:fieldRef:fieldPath: metadata.name- name: VAR\_POD\_NAMESPACEvalueFrom:fieldRef:fieldPath: metadata.namespace- name: VAR\_POD\_IPvalueFrom:fieldRef:fieldPath: status.podIP- name: VAR\_SERVICE\_ACCOUNTvalueFrom:fieldRef:fieldPath: spec.serviceAccountName- name: VAR\_CPU\_REQUESTvalueFrom:resourceFieldRef:containerName: busyboxresource: requests.cpu- name: VAR\_CPU\_LIMITvalueFrom:resourceFieldRef:containerName: busyboxresource: limits.cpu- name: VAR\_MEM\_REQUESTvalueFrom:resourceFieldRef:containerName: busyboxresource: requests.memory- name: VAR\_MEM\_LIMITvalueFrom:resourceFieldRef:containerName: busyboxresource: limits.memoryrestartPolicy: Never

我们创建Pod并使用kubtctl logs命令打印下输出：

$ sudo kubectl apply -f busy\_pod.yaml
pod/busybox-pod created
$ sudo kubectl logs busybox-pod
VAR\_MEM\_REQUEST=16777216 # 容器内存请求值
VAR\_NODE\_NAME=ayato # 节点名称
VAR\_SERVICE\_ACCOUNT=default # Pod使用的ServiceAccount名称
VAR\_CPU\_REQUEST=1 # 容器cpu请求值
VAR\_POD\_NAME=busybox-pod # pod名称
VAR\_MEM\_LIMIT=33554432 # 容器内存限制值
VAR\_POD\_NAMESPACE=default # Pod所在命名空间
VAR\_POD\_IP=172.17.0.6 # Pod ip地址
VAR\_CPU\_LIMIT=1               # 容器cpu请求值

二、Volume挂载方式

我们接下来尝试使用Volume挂载方式，将Pod信息注入容器。还是以Busybox为例，由于Pod信息都是以文件方式注入容器，所以我们修改容器启动后执行命令：我们使用cat不断打印注入的文件，修改后的Yaml文件如下：

apiVersion: v1
kind: Pod
metadata:name: busybox-podlabels:cluster: demo-clustertype: tool-podannotations:builder: alalazy
spec:containers:- name: busyboximage: busyboxcommand: ["/bin/sh", "-c"]args:- while true; doif [[ -e /etc/podinfo/labels ]]; thenecho -en '\n\n'; cat /etc/podinfo/labels; fi;if [[ -e /etc/podinfo/annotations ]]; thenecho -en '\n\n'; cat /etc/podinfo/annotations; fi;if [[ -e /etc/podinfo/cpu\_limit ]]; thenecho -en '\n\n'; cat /etc/podinfo/cpu\_limit; fi;if [[ -e /etc/podinfo/cpu\_request ]]; thenecho -en '\n\n'; cat /etc/podinfo/cpu\_request; fi;if [[ -e /etc/podinfo/mem\_limit ]]; thenecho -en '\n\n'; cat /etc/podinfo/mem\_limit; fi;if [[ -e /etc/podinfo/mem\_request ]]; thenecho -en '\n\n'; cat /etc/podinfo/mem\_request; fi;sleep 5;done;volumeMounts:- name: podinfomountPath: /etc/podinforesources:requests:memory: "16Mi"cpu: "125m"limits:memory: "32Mi"cpu: "250m"volumes:- name: podinfodownwardAPI:items:- path: "labels"fieldRef:fieldPath: metadata.labels- path: "annotations"fieldRef:fieldPath: metadata.annotations- path: "cpu\_limit"resourceFieldRef:containerName: busyboxresource: limits.cpudivisor: 1m- path: "cpu\_request"resourceFieldRef:containerName: busyboxresource: requests.cpudivisor: 1m- path: "mem\_limit"resourceFieldRef:containerName: busyboxresource: limits.memorydivisor: 1Mi- path: "mem\_request"resourceFieldRef:containerName: busyboxresource: requests.memorydivisor: 1Mi

我们创建此Pod，并通过kubectl logs查看输出：

$ sudo kubectl apply -f busy\_pod.yaml
pod/busybox-pod created
$ sudo kubectl logs busybox-pod
cluster="demo-cluster"
type="tool-pod"builder="alalazy"
kubectl.kubernetes.io/last-applied-configuration="{\"apiVersion\":\"v1\",\"kind\":\"Pod\",\"metadata\":{\"annotations\":{\"builder\":\"alalazy\"},\"labels\":{\"cluster\":\"demo-cluster\",\"type\":\"tool-pod\"},\"name\":\"busybox-pod\",\"namespace\":\"default\"},\"spec\":{\"containers\":[{\"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; if [[ -e /etc/podinfo/cpu\_limit ]]; then echo -en '\\\\n\\\\n'; cat /etc/podinfo/cpu\_limit; fi; if [[ -e /etc/podinfo/cpu\_request ]]; then echo -en '\\\\n\\\\n'; cat /etc/podinfo/cpu\_request; fi; if [[ -e /etc/podinfo/mem\_limit ]]; then echo -en '\\\\n\\\\n'; cat /etc/podinfo/mem\_limit; fi; if [[ -e /etc/podinfo/mem\_request ]]; then echo -en '\\\\n\\\\n'; cat /etc/podinfo/mem\_request; fi; sleep 5; done;\"],\"command\":[\"/bin/sh\",\"-c\"],\"image\":\"busybox\",\"name\":\"busybox\",\"resources\":{\"limits\":{\"cpu\":\"250m\",\"memory\":\"32Mi\"},\"requests\":{\"cpu\":\"125m\",\"memory\":\"16Mi\"}},\"volumeMounts\":[{\"mountPath\":\"/etc/podinfo\",\"name\":\"podinfo\"}]}],\"volumes\":[{\"downwardAPI\":{\"items\":[{\"fieldRef\":{\"fieldPath\":\"metadata.labels\"},\"path\":\"labels\"},{\"fieldRef\":{\"fieldPath\":\"metadata.annotations\"},\"path\":\"annotations\"},{\"path\":\"cpu\_limit\",\"resourceFieldRef\":{\"containerName\":\"busybox\",\"divisor\":\"1m\",\"resource\":\"limits.cpu\"}},{\"path\":\"cpu\_request\",\"resourceFieldRef\":{\"containerName\":\"busybox\",\"divisor\":\"1m\",\"resource\":\"requests.cpu\"}},{\"path\":\"mem\_limit\",\"resourceFieldRef\":{\"containerName\":\"busybox\",\"divisor\":\"1Mi\",\"resource\":\"limits.memory\"}},{\"path\":\"mem\_request\",\"resourceFieldRef\":{\"containerName\":\"busybox\",\"divisor\":\"1Mi\",\"resource\":\"requests.memory\"}}]},\"name\":\"podinfo\"}]}}\n"
kubernetes.io/config.seen="2022-01-15T05:33:53.379386410Z"
kubernetes.io/config.source="api"2501253216

我们进入容器查看下挂载的文件：

$  sudo kubectl exec -it busybox-pod -- sh
/ # cd /etc/podinfo/
/etc/podinfo # ls
annotations cpu\_limit cpu\_request labels mem\_limit mem\_request

三、Downward API的能力

我们可以通过Downward API向容器注入如下信息：

1）可通过fieldRef获得的信息：

metadata.name：Pod 名称
metadata.namespace：Pod 名字空间
metadata.uid：Pod 的 UID
metadata.labels[’’]：Pod标签的值 (例如, metadata.labels[‘mylabel’]）
metadata.annotations[’’]：Pod 的注解的值（例如, metadata.annotations[‘myannotation’]）
metadata.labels：获取所有标签
metadata.annotations：获取所有注解
status.podIP：节点 IP
spec.serviceAccountName：Pod 服务帐号名称, 版本要求v1.4.0-alpha.3
spec.nodeName：节点名称, 版本要求 v1.4.0-alpha.3
status.hostIP：节点 IP, 版本要求 v1.7.0-alpha.1

2）可通过 resourceFieldRef 获得的信息：

容器的 CPU 约束值
容器的 CPU 请求值
容器的内存约束值
容器的内存请求值
容器的巨页限制值（前提是启用了DownwardAPIHugePages 特性门控）
容器的巨页请求值（前提是启用了DownwardAPIHugePages 特性门控）
容器的临时存储约束值
容器的临时存储请求值