Inject data to Pod

Inject data to pod via Environmental variable

We will create a Coffee Pod

$ kubectl run tea --image=ansilh/demo-tea --env=MY_NODE_NAME=scratch --restart=Never --dry-run -o yaml >pod-with-env.yaml

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: tea
  name: tea
spec:
  containers:
  - env:
    - name: MY_NODE_NAME
      value: scratch
    image: ansilh/demo-tea
    name: coffee-new
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Never
status: {}

Lets run this Pod

$ kubectl create -f pod-with-env.yaml

$ kubectl get pods
NAME         READY   STATUS              RESTARTS   AGE
tea          1/1     Running   0          7s

Lets expose the pod as NodePort

$ kubectl expose pod tea --port=80 --target-port=8080 --type=NodePort

$ kubectl get svc tea
NAME   TYPE       CLUSTER-IP      EXTERNAL-IP   PORT(S)        AGE
tea    NodePort   192.168.10.37   <none>        80:32258/TCP   42s

Access the service using browser uisng node IP and port 32258

You will see below in Page Node:scratch

Expose Pod fields to containers

Lets extract the nodeName from spec ( Excuse me ? yeah we will see that in a moment )

k8s@k8s-master-01:~$ kubectl get pods tea -o=jsonpath='{.spec.nodeName}' && echo
k8s-worker-01
k8s@k8s-master-01:~$ kubectl get pods tea -o=jsonpath='{.status.hostIP}' && echo
192.168.56.202
k8s@k8s-master-01:~$ kubectl get pods tea -o=jsonpath='{.status.podIP}' && echo
10.10.1.23
k8s@k8s-master-01:~$

To get the JSON path , first we need to get the entire object output in JSON. We have used output in YAML so far because its easy . But internally kubectl convers YAML to JSON

$ kubectl get pod tea -o json

{
    "apiVersion": "v1",
    "kind": "Pod",
    "metadata": {
        "annotations": {
            "cni.projectcalico.org/podIP": "10.10.1.23/32"
        },
        "creationTimestamp": "2019-01-06T15:09:36Z",
        "labels": {
            "run": "tea"
        },
        "name": "tea",
        "namespace": "default",
        "resourceVersion": "218696",
        "selfLink": "/api/v1/namespaces/default/pods/tea",
        "uid": "14c1715b-11c5-11e9-9f0f-0800276a1bd2"
    },
    "spec": {
        "containers": [
            {
                "env": [
                    {
                        "name": "MY_NODE_NAME",
                        "value": "scratch"
                    }
                ],
                "image": "ansilh/demo-tea",
                "imagePullPolicy": "Always",
                "name": "coffee-new",
                "resources": {},
                "terminationMessagePath": "/dev/termination-log",
                "terminationMessagePolicy": "File",
                "volumeMounts": [
                    {
                        "mountPath": "/var/run/secrets/kubernetes.io/serviceaccount",
                        "name": "default-token-72pzg",
                        "readOnly": true
                    }
                ]
            }
        ],
        "dnsPolicy": "ClusterFirst",
        "enableServiceLinks": true,
        "nodeName": "k8s-worker-01",
        "priority": 0,
        "restartPolicy": "Never",
        "schedulerName": "default-scheduler",
        "securityContext": {},
        "serviceAccount": "default",
        "serviceAccountName": "default",
        "terminationGracePeriodSeconds": 30,
        "tolerations": [
            {
                "effect": "NoExecute",
                "key": "node.kubernetes.io/not-ready",
                "operator": "Exists",
                "tolerationSeconds": 300
            },
            {
                "effect": "NoExecute",
                "key": "node.kubernetes.io/unreachable",
                "operator": "Exists",
                "tolerationSeconds": 300
            }
        ],
        "volumes": [
            {
                "name": "default-token-72pzg",
                "secret": {
                    "defaultMode": 420,
                    "secretName": "default-token-72pzg"
                }
            }
        ]
    },
    "status": {
        "conditions": [
            {
                "lastProbeTime": null,
                "lastTransitionTime": "2019-01-06T15:09:36Z",
                "status": "True",
                "type": "Initialized"
            },
            {
                "lastProbeTime": null,
                "lastTransitionTime": "2019-01-06T15:09:42Z",
                "status": "True",
                "type": "Ready"
            },
            {
                "lastProbeTime": null,
                "lastTransitionTime": "2019-01-06T15:09:42Z",
                "status": "True",
                "type": "ContainersReady"
            },
            {
                "lastProbeTime": null,
                "lastTransitionTime": "2019-01-06T15:09:36Z",
                "status": "True",
                "type": "PodScheduled"
            }
        ],
        "containerStatuses": [
            {
                "containerID": "docker://291a72e7fdab6a9f7afc47c640126cf596f5e071903b6a9055b44ef5bcb1c104",
                "image": "ansilh/demo-tea:latest",
                "imageID": "docker-pullable://ansilh/demo-tea@sha256:998d07a15151235132dae9781f587ea4d2822c62165778570145b0f659dda7bb",
                "lastState": {},
                "name": "coffee-new",
                "ready": true,
                "restartCount": 0,
                "state": {
                    "running": {
                        "startedAt": "2019-01-06T15:09:42Z"
                    }
                }
            }
        ],
        "hostIP": "192.168.56.202",
        "phase": "Running",
        "podIP": "10.10.1.23",
        "qosClass": "BestEffort",
        "startTime": "2019-01-06T15:09:36Z"
    }
}

Remove below from pod-with-env.yaml

  - name: MY_NODE_NAME
    value: scratch

Add below Pod spec

- name: MY_NODE_NAME
  valueFrom:
   fieldRef:
    fieldPath: spec.nodeName

Resulting Pod Yaml

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: tea
  name: tea
spec:
  containers:
  - env:
    - name: MY_NODE_NAME
      valueFrom:
       fieldRef:
        fieldPath: spec.nodeName
    image: ansilh/demo-tea
    name: coffee-new
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Never
status: {}

Delete the running pod files

$ kubectl delete pod tea

Create the pod with modified yaml file

$ kubectl create -f pod-with-env.yaml

Make sure endpoint is up in service

$ kubectl get ep tea
NAME   ENDPOINTS         AGE
tea    10.10.1.26:8080   31m

Refresh the browser page. This time you will see Node:k8s-worker-01

Lets do a cleanup on default namespace.

$ kubectl delete --all pods
$ kubectl delete --all services

Now you know

How to use export Objects in Yaml and Json format
How to access each fields using jsonpath
How to inject environmental variables to Pod
How to inject system generated fields to Pod using environmental variables