Work on pod ¶

Create pod ¶

Create pod my-first-podl.

kubectl apply -f - << EOF
apiVersion: v1
kind: Pod
metadata:
  name: my-first-pod
spec:
  containers:
  - name: nginx
    image: nginx:mainline
    ports:
    - containerPort: 80
EOF

Verify status of the pod just created.

kubectl get pods -o wide

Track pod ¶

Check logs of the pod just created.

kubectl logs my-first-pod

In case logs or describe or any other of the output generating commands don't help us to get to the root cause of an issue, we can use use kubectl exec -it <my-pod> -- bash command to look into it ourselves.

kubectl exec -it my-first-pod -- bash
root@my-first-pod:/# ls
root@my-first-pod:/# cd bin
root@my-first-pod:/bin# ls
root@my-first-pod:/bin# exit

Execute command kubectl explain pod.spec will get details of Spec segment of Pod kind in yaml file.

We can check the official API reference of the pod resource for help or use kubectl explain pod to get a command-line based description of the resource. By appending .<field> to the resource type, the explain command will provide more details on the specified field.

kubectl explain pod.kind
kubectl explain pod.spec
kubectl explain pod.spec.containers
kubectl explain pod.spec.containers.name

Label pod ¶

Get pod's label with option --show-labels.

kubectl get pods
kubectl get pods --show-labels

Add two labels to the pod pod my-first-pod.

kubectl label pod my-first-pod nginx=mainline
kubectl label pod my-first-pod env=demo
kubectl get pods --show-labels

Search pod by labels.

kubectl get pod -l env=demo
kubectl get pod -l env=demo,nginx=mainline
kubectl get pod -l env=training

Remove label

kubectl label pods my-first-pod env-
kubectl get pods --show-labels

Describe pod.

kubectl describe pod my-first-pod

Delete pod. Run watch kubectl get pods to monitor the pod status.

kubectl delete pod my-first-pod
watch kubectl get pods

Static Pod ¶

Scenario

*Create a static pod. * kubectl will automatically check yaml file in /etc/kubernetes/manifests/ and create the static pod once it's detected.

Demo:

Some system static Pods are already in place.

ll /etc/kubernetes/manifests/

Result

-rw------- 1 root root 2292 Jul 23 10:45 etcd.yaml
-rw------- 1 root root 3889 Jul 23 10:45 kube-apiserver.yaml
-rw------- 1 root root 3395 Jul 23 10:45 kube-controller-manager.yaml
-rw------- 1 root root 1464 Jul 23 10:45 kube-scheduler.yaml

Create yaml file my-nginx.yaml in directory /etc/kubernetes/manifests/. Once the file is created, the static pod my-nginx is created automatically.

kubectl run my-nginx --image=nginx:mainline --dry-run=client -n default -oyaml | sudo tee /etc/kubernetes/manifests/my-nginx.yaml

Check status of the Pod my-nginx. The node name cka001 is part of the Pod name, which means the Pod is running on node cka001.

kubectl get pod -o wide

Result

NAME              READY   STATUS    RESTARTS   AGE   IP               NODE     NOMINATED NODE   READINESS GATES
my-nginx-cka001   1/1     Running   0          20s   10.244.228.196   cka001   <none>           <none>

Delete the yaml file /etc/kubernetes/manifests/my-nginx.yaml, the static pod will be deleted automatically.

sudo rm /etc/kubernetes/manifests/my-nginx.yaml

Multi-Container Pod ¶

Scenario:

Create Multi-Container Pod
Describe the Pod
Check the log of Pod
Check the log of Containers

Create a Pod multi-container-pod with multiple container container-1-nginx and container-2-alpine.

kubectl apply -f - << EOF
apiVersion: v1
kind: Pod
metadata:
  name: multi-container-pod
spec:
  containers:
  - name: container-1-nginx
    image: nginx
    ports:
    - containerPort: 80  
  - name: container-2-alpine
    image: alpine
    command: ["watch", "wget", "-qO-", "localhost"]
EOF

Get the status.

kubectl get pod multi-container-pod

Result

NAME                  READY   STATUS    RESTARTS   AGE
multi-container-pod   2/2     Running   0          81s

Get details of the pod.

kubectl describe pod multi-container-pod

Result

.......
Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  41s   default-scheduler  Successfully assigned dev/multi-container-pod to cka002
  Normal  Pulling    40s   kubelet            Pulling image "nginx"
  Normal  Pulled     23s   kubelet            Successfully pulled image "nginx" in 16.767129903s
  Normal  Created    22s   kubelet            Created container container-1-nginx
  Normal  Started    22s   kubelet            Started container container-1-nginx
  Normal  Pulling    22s   kubelet            Pulling image "alpine"
  Normal  Pulled     14s   kubelet            Successfully pulled image "alpine" in 7.776104353s
  Normal  Created    14s   kubelet            Created container container-2-alpine
  Normal  Started    14s   kubelet            Started container container-2-alpine

For multi-container pod, container name is needed if we want to get log of pod via command kubectl logs <pod_name> <container_name>.

Without the container name, we receive error.

kubectl logs multi-container-pod

Result

error: a container name must be specified for pod multi-container-pod, choose one of: [container-1-nginx container-2-alpine]

With specified container name, we get the log info.

kubectl logs multi-container-pod container-1-nginx

Result

......
::1 - - [23/Jul/2022:04:06:37 +0000] "GET / HTTP/1.1" 200 615 "-" "Wget" "-"

Same if we need specify container name to login into the pod via command kubectl exec -it <pod_name> -c <container_name> -- <commands>.

kubectl exec -it multi-container-pod -c container-1-nginx -- /bin/bash
root@multi-container-pod:/# ls

Clean up

kubectl delete pod multi-container-pod

Quick reference of a simple yaml file to create Multiple Containers.

kubectl apply -f - << EOF
apiVersion: v1
kind: Pod
metadata:
  name: my-multi-pod
spec:
  containers:
  - image: nginx
    name: nginx
  - image: memcached
    name: memcached
  - image: redis
    name: redis
EOF

Scenario:

Create a Pod my-busybox with a container container-1-busybox. The container will write message to file /var/log/my-pod-busybox.log.
Add another container container-2-busybox (Sidecar) to the Pod my-busybox. The sidecar container read message from file /var/log/my-pod-busybox.log.
Tips: create a Volume to store the log file, which is shared with two containers.

Demo:

Create a Pod my-busybox with a container container-1-busybox.

kubectl apply -f - << EOF
apiVersion: v1
kind: Pod
metadata:
  name: my-busybox
spec:
  containers:
  - name: container-1-busybox
    image: busybox
    args:
    - /bin/sh
    - -c
    - >
      i=0;
      while true;
      do
        echo "Hello message from container-1: $i" >> /var/log/my-pod-busybox.log;
        i=$((i+1));
        sleep 1;
      done
EOF

Search emptyDir in the Kubernetes documetation. Refer to below template for emptyDir via https://kubernetes.io/zh-cn/docs/concepts/storage/volumes/

Add below new features into the Pod

Volume:
volume name: logfile
type: emptyDir
Update existing container:
name: container-1-busybox
volumeMounts
- name: logfile
- mounthPath: /var/log
Add new container:
name: container-2-busybox
image: busybox
args: ['/bin/sh', '-c', 'tail -n+1 -f /var/log/my-pod-busybox.log']
volumeMounts:
- name: logfile
- mountPath: /var/log

kubectl get pod my-busybox -o yaml > my-busybox.yaml
vi my-busybox.yaml
kubectl delete pod my-busybox 
kubectl apply -f my-busybox.yaml
kubectl logs my-busybox -c container-2-busybox

The final file my-busybox.yaml looks like below.

apiVersion: v1
kind: Pod
metadata:
  annotations:
    cni.projectcalico.org/containerID: 89644b6b073cd152f94b4cae7bdea6bbc3292cf59afd4f28102bd74f0205c9e4
    cni.projectcalico.org/podIP: 10.244.102.20/32
    cni.projectcalico.org/podIPs: 10.244.102.20/32
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"name":"my-busybox","namespace":"dev"},"spec":{"containers":[{"args":["/bin/sh","-c","i=0; while true; do\n  echo \"Hello message from container-1: \" \u003e\u003e /var/log/my-pod-busybox.log;\n  i=1;\n  sleep 1;\ndone\n"],"image":"busybox","name":"container-1-busybox"}]}}
  creationTimestamp: "2022-07-29T22:58:27Z"
  name: my-busybox
  namespace: dev
  resourceVersion: "1185720"
  uid: c5e62a16-4459-4828-a441-7d1471b89a56
spec:
  containers:
  - name: container-2-busybox
    image: busybox
    args: ['/bin/sh', '-c', 'tail -n+1 -f /var/log/my-pod-busybox.log']
    volumeMounts:
    - name: logfile
      mountPath: /var/log
  - args:
    - /bin/sh
    - -c
    - |
      i=0; while true; do
        echo "Hello message from container-1: $i" >> /var/log/my-pod-busybox.log;
        i=1;
        sleep 1;
      done
    image: busybox
    imagePullPolicy: Always
    name: container-1-busybox
    resources: {}
    terminationMessagePath: /dev/termination-log
    terminationMessagePolicy: File
    volumeMounts:
    - name: logfile
      mountPath: /var/log
    - mountPath: /var/run/secrets/kubernetes.io/serviceaccount
      name: kube-api-access-mhxlf
      readOnly: true
  dnsPolicy: ClusterFirst
  enableServiceLinks: true
  nodeName: cka003
  preemptionPolicy: PreemptLowerPriority
  priority: 0
  restartPolicy: Always
  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: logfile
    emptyDir: {}
  - name: kube-api-access-mhxlf
    projected:
      defaultMode: 420
      sources:
      - serviceAccountToken:
          expirationSeconds: 3607
          path: token
      - configMap:
          items:
          - key: ca.crt
            path: ca.crt
          name: kube-root-ca.crt
      - downwardAPI:
          items:
          - fieldRef:
              apiVersion: v1
              fieldPath: metadata.namespace
            path: namespace
status:
  conditions:
  - lastProbeTime: null
    lastTransitionTime: "2022-07-29T22:58:27Z"
    status: "True"
    type: Initialized
  - lastProbeTime: null
    lastTransitionTime: "2022-07-29T22:58:30Z"
    status: "True"
    type: Ready
  - lastProbeTime: null
    lastTransitionTime: "2022-07-29T22:58:30Z"
    status: "True"
    type: ContainersReady
  - lastProbeTime: null
    lastTransitionTime: "2022-07-29T22:58:27Z"
    status: "True"
    type: PodScheduled
  containerStatuses:
  - containerID: containerd://fd42d4ba4d94d8918d8846327b1db2328be13c5f93f381877ff0228ed7b5468d
    image: docker.io/library/busybox:latest
    imageID: docker.io/library/busybox@sha256:0e97a8ca6955f22dbc7db9e9dbe970971f423541e52c34b8cb96ccc88d6a3883
    lastState: {}
    name: container-1-busybox
    ready: true
    restartCount: 0
    started: true
    state:
      running:
        startedAt: "2022-07-29T22:58:30Z"
  hostIP: <cka003_ip>
  phase: Running
  podIP: 10.244.102.20
  podIPs:
  - ip: 10.244.102.20
  qosClass: BestEffort
  startTime: "2022-07-29T22:58:27Z"

Clean up:

kubectl delete pod my-busybox

initContainer Pod ¶

Scenario:

Create Pod myapp-pod that has two init containers.
myapp-container
init-mydb
Create two Services.
myservice
mydb

Conclusion:

myapp-container waits for Service myservice up in order to resolve the name myservice.dev.svc.cluster.local
init-mydb waits for Service mydb up in order to resolve the name mydb.dev.svc.cluster.local.

Demo:

Create Pod myapp-pod with below yaml file.

Create yaml file myapp-pod.yaml and add below content. Note: Due to the command $(cat /var/..... will be treated as host variable, we can not use echo to generate the file. It's the variabel in container itself.

vi myapp-pod.yaml

Content

apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp
spec:
  containers:
  - name: myapp-container
    image: busybox:1.28
    command: ['sh', '-c', 'echo The app is running! && sleep 3600']
  initContainers:
  - name: init-myservice
    image: busybox:1.28
    command: ['sh', '-c', "until nslookup myservice.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for myservice; sleep 2; done"]
  - name: init-mydb
    image: busybox:1.28
    command: ['sh', '-c', "until nslookup mydb.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for mydb; sleep 2; done"]

Apply the yaml file to create the Pod.

kubectl apply -f myapp-pod.yaml

Check Pod status.

kubectl get pod myapp-pod

Result

NAME        READY   STATUS     RESTARTS   AGE
myapp-pod   0/1     Init:0/2   0          12m

Inspect Pods. Get two errors:

nslookup: can't resolve 'myservice.dev.svc.cluster.local'
container "init-mydb" in pod "myapp-pod" is waiting to start: PodInitializing

kubectl logs myapp-pod -c init-myservice # Inspect the first init container
kubectl logs myapp-pod -c init-mydb      # Inspect the second init container

At this point, those init containers will be waiting to discover Services named mydb and myservice.

Create the mydb and myservice services:

kubectl apply -f - << EOF
apiVersion: v1
kind: Service
metadata:
  name: myservice
spec:
  ports:
  - protocol: TCP
    port: 80
    targetPort: 9376
---
apiVersion: v1
kind: Service
metadata:
  name: mydb
spec:
  ports:
  - protocol: TCP
    port: 80
    targetPort: 9377
EOF

Get current status of Services.

kubectl get service

Both of Services are up.

NAME        TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)   AGE
mydb        ClusterIP   11.244.239.149   <none>        80/TCP    6s
myservice   ClusterIP   11.244.116.126   <none>        80/TCP    6s

Get current Pod status.

kubectl get pod myapp-pod -o wide

The Pod is up.

NAME        READY   STATUS     RESTARTS   AGE     IP             NODE     NOMINATED NODE   READINESS GATES
myapp-pod   0/1     Init:0/2   0          2m40s   10.244.112.2   cka002   <none>           <none>

We now see that those init containers complete, and that the myapp-pod Pod moves into the Running state.

Clean up.

kubectl delete service mydb myservice 
kubectl delete pod myapp-pod

References: