Saturday, November 6, 2021

Kubernetes for Developers #24: Kubernetes Volume hostPath in-detail

In the previous article (Kubernetes for Developers #23: Kubernetes Volume emptyDir in-detail), we discussed about emptyDir volume for storing and sharing data among multiple/single container(s) in the pod. However, emptyDir volume and its contents get deleted automatically when the Pod is deleted from the worker node.

Kubernetes hostPath volume helps us to persist volume contents even after pod deleted from the worker node.

K8 hostPath volume mounts a file or directory from the worker node filesystem into the pod.

A pod running on the same worker node can only mount to the file/directory of that node.
  • It is useful when the container wants to access docker system files from the host (i.e., /var/lib/docker)
  • It is useful when the container needs to access kubeconfig file (or) CA certificates (or) /var/logs from the host
  • It is useful when the container needs to access host /sys files for cAdvisor
  • It is useful when the container wants to check given path exists in the host before running
Kubernetes hostPath volume supports following types while mounting

Type

Description

Directory

A directory must exist in the specified path on the host

DirectoryOrCreate

An empty directory will be created when the specified path does not exist on the host

File

A file must exist in the specified path on the host

FileOrCreate

An empty file will be created when the specified path does not exist on the host

Socket

A UNIX socket must exist in the specified path


apiVersion: v1
kind: Pod
metadata:
  name: pod-vol-hostpath
spec:
  containers:
    - name: alpine
      image: alpine
      command:
        [
          "sh",
          "-c",
          'while true; do echo "random message text `date`" >> html/index.html;sleep 10;done',
        ]
      volumeMounts:
        - name: vol-hostpath
          mountPath: /html
    - name: nginx
      image: nginx:alpine
      ports:
        - containerPort: 80
          protocol: TCP
      volumeMounts:
        - name: vol-hostpath
          mountPath: /usr/share/nginx/html
  volumes:
    - name: vol-hostpath
      hostPath:
        path: /mydoc
        type: DirectoryOrCreate


As per above yaml ,

  1. A multi-container pod gets created with volume type “hostPath” named as “vol-hostpath” and mounted on “/mydoc” directory from the host filesystem
  2. “mydoc” directory gets created automatically when a pod is assigned to a worker-node if not exists on the host filesystem as we specified volume type “DirectoryOrCreate”
  3. First “alpine” container creates random text message for every 10 seconds and appends to /html/index.html file.
  4. First “alpine” container mounted a volume at ‘/html’. So, all the new/modified files under this directory referring to “/mydoc” host filesystem
  5. Second “nginx” container mounted a same volume at ‘/usr/share/nginx/html’ (this is the default directory for nginx to serve index.html file ). As we mounted same volume which has “index.html”, nginx web server serves the file (i.e., index.html) which is created by the first container.
  6. As first container adds new random message to index.html file for every 10 seconds, we see different message each time when we request index.html from nginx webserver.
  7. Volume contents won’t be deleted on Pod termination. So, whenever the new pod is scheduled on the same node with same hostpath will see all the previous contents.

save above yaml content as "pod-vol-hostpath.yaml" and run the following kubectl command
// create pod
$ kubectl apply -f pod-vol-hostpath.yaml
pod/pod-vol-hostpath created

// display pods
$ kubectl get po
NAME                    READY   STATUS      RESTARTS   AGE
pod-vol-hostpath        2/2     Running     0          1m10s

run the following kubectl command to forward a port from local machine to the pod
// syntax
// kubectl port-forward <pod-name> <local-port>:<container-port>
$ kubectl port-forward pod-vol-hostpath 8081:80
Forwarding from 127.0.0.1:8081 -> 80
Forwarding from [::1]:8081 -> 80

run the following curl command to check random messages which are appending after every 10 seconds
$ curl http://localhost:8081
random message text Tue Nov  7 12:01:10 UTC 2021

$ curl http://localhost:8081
random message text Tue Nov  7 12:01:10 UTC 2021
random message text Tue Nov  7 12:01:20 UTC 2021
random message text Tue Nov  7 12:01:30 UTC 2021

Volume contents won’t be deleted on Pod termination. So, whenever the new pod is scheduled on the same node with same hostpath will see all the previous contents.

delete the Pod and recreate all above steps to check existing data is printing while doing curl command
// delete pod
$ kubectl delete pod/pod-vol-hostpath
pod/pod-vol-hostpath deleted

// create pod
$ kubectl apply -f pod-vol-hostpath.yaml
pod/pod-vol-hostpath created

// display pods
$ kubectl get po
NAME                    READY   STATUS      RESTARTS   AGE
pod-vol-hostpath        2/2     Running     0          1m10s

// syntax
// kubectl port-forward <pod-name> <local-port>:<container-port>
$ kubectl port-forward pod-vol-hostpath 8081:80
Forwarding from 127.0.0.1:8081 -> 80
Forwarding from [::1]:8081 -> 80


$ curl http://localhost:8081
random message text Tue Nov  7 12:01:10 UTC 2021
random message text Tue Nov  7 12:01:20 UTC 2021
random message text Tue Nov  7 12:01:30 UTC 2021
random message text Tue Nov  7 14:12:40 UTC 2021

// first 3 lines are generated by the previous pod

It is confirmed that curl command showing both previous pod generated contents and new pod contents.

Kubernetes for Developers Journey.
Happy Coding :)

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