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Kubernetes - Storage: Pod Volume

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Kubernetes - Storage: Pod Volume
- Container storage
- Pod Volume
  - Lab: without volume vs with volume
    - no volume
    - Pod volume

Container storage

each container has its own isolated filesystem provided by the container image.
Mounting
- the act of attaching the filesystem of the storage device or volume into a specific location in the operating system’s file tree
Note: When any volume is mounted to a directory in the container’s filesystem, the files that are in the container image in that directory can no longer be accessed.
- That is why subPath can be used.

Container Storage Interface

Container Runtime interface(CRI)
- used to handle the communication between the Kubernetes and container runtimes.
Container Network Interface(CNI)
- used to coordinate between different network solution and kubernetes.
Container Storage Interface(CSI)
- used to coordinate between different storage solution and kubernetes.
- define a set of remote procedure calls(rpc)

Container Volume

Volume
- a directory accessible by containers along with the pod’s lifecycle, allowing data to persist beyond a container’s lifecycle
- Lifecycle:
  - Tied to the pod’s existence
- Data Persistence:
  - Data persisted when container restarts.
  - Data is lost when the pod is deleted
- Scope:
  - Defined within the pod spec
- no an API object
Use case:
- e.g.,
  - main container: web server, read only, show content, /var/html
  - sidecar container: agen app, read/write, edit content, /var/data
- e.g.,
  - init container: create config file, read/write
  - main container: use config file, read only

A container can mount zero or more of these volumes in different locations
A pod can have multiple volumes
A volume can be mounted in more than one container to share files.

Inject data as file using volume

inject data from:
- configMap object: spec.volumes.configMap
- secret object: spec.volumes.secret
- downwardAPI object: spec.volumes.downwardAPI

Pod Volume

Without the volume mounted in the container, the entire filesystem of the container is ephemeral.
Volumes can persist data across container restarts.
- All volumes in a pod are created when the pod is set up
  - before any of its containers are started.
- Volumes are torn down when the pod is shut down.
- The lifecycle of a volume is tied to the lifecycle of the entire pod
  - independent of the lifecycle of the container in which it is mounted.
- the new restrart container can access the same data in the mounted volume.

Lab: without volume vs with volume

no volume

# demo-non-volume.yaml
apiVersion: v1
kind: Pod
metadata:
  name: demo-non-volume
spec:
  containers:
    - name: nginx
      image: nginx
    - name: mongo
      image: mongo

# create app
kubectl apply -f demo-non-volume.yaml
# pod/demo-non-volume created

# confirm
kubectl get pod
# NAME              READY   STATUS    RESTARTS   AGE
# demo-non-volume   2/2     Running   0          27s

# insert data
kubectl exec -it demo-non-volume -c mongo -- mongosh
db.telemetry.insertOne({device_id: 1, x: 1, y: 2});
# {
#   acknowledged: true,
#   insertedId: ObjectId('694b4e86dae1c244908de666')
# }

# get data
kubectl exec -it demo-non-volume -c mongo -- mongosh
db.telemetry.find()
# [
#   {
#     _id: ObjectId('694b4e86dae1c244908de666'),
#     device_id: 1,
#     x: 1,
#     y: 2
#   }
# ]

# shutdown db
kubectl exec -it demo-non-volume -c mongo -- mongosh admin --eval "db.shutdownServer()"
# command terminated with exit code 137

# monitor terminal
kubectl get ev -w
# 0s          Normal    Pulling     pod/demo-non-volume   Pulling image "mongo"
# 0s          Normal    Pulled      pod/demo-non-volume   Successfully pulled image "mongo" in 1.263s (1.263s including waiting). Image size: 327004046 bytes.
# 0s          Normal    Created     pod/demo-non-volume   Created container: mongo
# 0s          Normal    Started     pod/demo-non-volume   Started container mongo

kubectl get pod -w
# NAME              READY   STATUS    RESTARTS   AGE
# demo-non-volume   2/2     Running   0          3m32s
# demo-non-volume   1/2     NotReady   0          4m
# demo-non-volume   2/2     Running    1 (3s ago)   4m3s

kubectl describe pod demo-non-volume
# Events:
#   Type    Reason     Age                 From               Message
#   ----    ------     ----                ----               -------
#   Normal  Scheduled  5m11s               default-scheduler  Successfully assigned default/demo-non-volume to docker-desktop
#   Normal  Pulling    5m11s               kubelet            Pulling image "nginx"
#   Normal  Pulled     5m10s               kubelet            Successfully pulled image "nginx" in 1.256s (1.256s including waiting). Image size: 59795293 bytes.
#   Normal  Created    5m9s                kubelet            Created container: nginx
#   Normal  Started    5m9s                kubelet            Started container nginx
#   Normal  Pulled     5m8s                kubelet            Successfully pulled image "mongo" in 797ms (797ms including waiting). Image size: 327004046 bytes.
#   Normal  Pulling    71s (x2 over 5m9s)  kubelet            Pulling image "mongo"
#   Normal  Pulled     70s                 kubelet            Successfully pulled image "mongo" in 1.263s (1.263s including waiting). Image size: 327004046 bytes.
#   Normal  Created    69s (x2 over 5m8s)  kubelet            Created container: mongo
#   Normal  Started    69s (x2 over 5m8s)  kubelet            Started container mongo

# try to read data
kubectl exec -it demo-non-volume -c mongo -- mongosh --eval "db.telemetry.find()"
# none

Pod has 2 containers: nginx + mongo container mongo restart, but nginx is still running, therefore the pod still runs. No data persists after mongo restart -> container without volume cannot persists data.

Pod volume

# demo-volume.yaml
apiVersion: v1
kind: Pod
metadata:
  name: demo-volume
spec:
  # pod vol
  volumes:
    - name: mongo-vol
      emptyDir: {}
  containers:
    - name: nginx
      image: nginx
    - name: mongo
      image: mongo
      volumeMounts:
        - name: mongo-vol # reference vol
          mountPath: /data/db # path in container

kubectl apply -f demo-volume.yaml
# pod/demo-volume created

kubectl get pod demo-volume
# NAME          READY   STATUS    RESTARTS   AGE
# demo-volume   2/2     Running   0          29s

# insert data
kubectl exec -it demo-volume -c mongo -- mongosh
db.telemetry.insertOne({device_id: 1, x: 1, y: 2});
# {
#   acknowledged: true,
#   insertedId: ObjectId('694b56d088e4cd1e7c8de666')
# }

# get data
kubectl exec -it demo-volume -c mongo -- mongosh
db.telemetry.find()
# [
#   {
#     _id: ObjectId('694b56d088e4cd1e7c8de666'),
#     device_id: 1,
#     x: 1,
#     y: 2
#   }
# ]

# shutdown db
kubectl exec -it demo-volume -c mongo -- mongosh admin --eval "db.shutdownServer()"
# command terminated with exit code 137

# monitor terminal
kubectl get ev -w
# 0s          Normal    Pulling     pod/demo-volume       Pulling image "mongo"
# 0s          Normal    Pulled      pod/demo-volume       Successfully pulled image "mongo" in 1.144s (1.144s including waiting). Image size: 327004046 bytes.
# 0s          Normal    Created     pod/demo-volume       Created container: mongo
# 0s          Normal    Started     pod/demo-volume       Started container mongo

kubectl get pod -w
# NAME          READY   STATUS    RESTARTS   AGE
# demo-volume   2/2     Running   0          3m12s
# demo-volume   1/2     NotReady   0          3m58s
# demo-volume   2/2     Running    1 (3s ago)   4m1s

kubectl describe pod demo-volume
# Events:
#   Type    Reason     Age                  From               Message
#   ----    ------     ----                 ----               -------
#   Normal  Scheduled  4m34s                default-scheduler  Successfully assigned default/demo-volume to docker-desktop
#   Normal  Pulling    4m34s                kubelet            Pulling image "nginx"
#   Normal  Pulled     4m33s                kubelet            Successfully pulled image "nginx" in 1.214s (1.215s including waiting). Image size: 59795293 bytes.
#   Normal  Created    4m32s                kubelet            Created container: nginx
#   Normal  Started    4m32s                kubelet            Started container nginx
#   Normal  Pulled     4m32s                kubelet            Successfully pulled image "mongo" in 764ms (764ms including waiting). Image size: 327004046 bytes.
#   Normal  Pulling    36s (x2 over 4m32s)  kubelet            Pulling image "mongo"
#   Normal  Created    35s (x2 over 4m31s)  kubelet            Created container: mongo
#   Normal  Pulled     35s                  kubelet            Successfully pulled image "mongo" in 1.144s (1.144s including waiting). Image size: 327004046 bytes.
#   Normal  Started    34s (x2 over 4m31s)  kubelet            Started container mongo

# try to read data
kubectl exec -it demo-volume -c mongo -- mongosh --eval "db.telemetry.find()"
# [
#   {
#     _id: ObjectId('694b56d088e4cd1e7c8de666'),
#     device_id: 1,
#     x: 1,
#     y: 2
#   }
# ]