Basic concepts of Pod

Pod is the smallest unit created and managed by Kubernetes. A pod consists of one container or multiple containers, which share storage and network.

Pod features

• A Pod can be understood as an application instance that provides services
• The container in the Pod is always deployed on one Node
• The containers in the Pod share network and storage resources
• Kubernetes manages pods directly, not containers

Pod meaning

Main usage of Pod:

• Running a single container: the most common usage. In this case, Pod can be regarded as an abstract encapsulation of a single container
• Run multiple containers: encapsulate multiple tightly coupled applications that need to share resources

If you have these requirements, you can run multiple containers:

• File interaction between two applications
• Two applications need to pass 127.0 0.1 or socket communication
• Two applications need frequent calls

Implementation mechanism of Pod resource sharing

Restart strategy

• Always: always restart the container after the container terminates and exits. The default policy is always restart
• OnFailure: restart the container only when the container exits abnormally (the exit status code is not 0) (abnormal exit, such as stop and kill)
• Never: never restart the container when the container terminates and exits (never restart)

[root@master manifest]# cat test.yml 
apiVersion: v1
kind: Pod
metadata: 
  name: web
spec: 
  containers:		//One pod multiple containers
  - name: nginx
    image: nginx
    imagePullPolicy: IfNotPresent
  - name: test
    image: busybox
    imagePullPolicy: IfNotPresent
    command: ["bin/sh","-c","sleep 450"]
  restartPolicy: Never		#By default, it does not need to be modified. If it is changed to never, the container will not restart

[root@master manifest]# kubectl apply -f test.yml  		// Create pod
pod/web created
[root@master manifest]# kubectl get pod 
NAME   READY   STATUS    RESTARTS   AGE
web    2/2     Running   0          92s
[root@master manifest]# kubectl get pod -o wide -w 		// Stop one of them on node2 and find that it will not restart
NAME   READY   STATUS   RESTARTS   AGE     IP            NODE                NOMINATED NODE   READINESS GATES
web    1/2     NotReady    0          4m51s   10.244.2.10   node2.example.com   <none>           <none>

health examination

• Livenessprobe (survival check): if the check fails, the container will be killed. Operate according to the - restartPolicy of Pod
• Readiness probe: if the check fails, Kubernetes will remove the Pod from the service endpoints

Supported inspection methods:

• httpGet: send an HTTP request and return the status code in the range of 200-400 as success
• exec: executing the s hell command returns a status code of 0, indicating success
• tcpSocket: TCP Socket initiated successfully
  Combined with restart strategy

Restart policy + health check

//Port detection
[root@master ~]# cat test.yml 
---
apiVersion: v1
kind: Pod
metadata:
  name: web
spec:
  containers:
  - name: nginx
    image: nginx
    imagePullPolicy: IfNotPresent
    ports:
    - containerPort: 80
      hostPort: 80
    livenessProbe:
      tcpSocket:
        port: 80
      initialDelaySeconds: 20 		#How many seconds after starting the container health check
      periodSeconds: 10 			#How many seconds will it be checked in the future
    readinessProbe:
      httpGet:
        port: 80
      initialDelaySeconds: 20
      periodSeconds: 10


[root@master ~]# kubectl apply -f test.yml 
pod/web created

//Check the pod and find that initialization is in progress
[root@master ~]# kubectl get pod
NAME   READY(Ready status)   STATUS(Survival status)    RESTARTS   AGE
web    0/1     Running   0          18s

//It will enter operation after waiting for a certain time
[root@master ~]# kubectl get pod
NAME   READY   STATUS    RESTARTS   AGE
web    1/1     Running   0          34s

environment variable

Variable values can be defined in several ways:

• Custom variable value
• The variable value is obtained from the Pod property
• Variable values from Secrt, ConfigMap
  

Custom variable value

---
apiVersion: v1
kind: Pod
metadata:
  name: test
spec:
  containers:
  - name: test
    image: busybox
    imagePullPolicy: IfNotPresent
    command: ["bin/sh","-c","sleep 45"]
    env:
    - name: HN
      value: tom

[root@master ~]# kubectl apply -f test.yml 
pod/test created
[root@master ~]# kubectl get pod
NAME   READY   STATUS    RESTARTS   AGE
test   1/1     Running   0          21s
[root@master ~]# kubectl exec -it test -- /bin/sh
/ # echo $HN
tom

The variable value is obtained from the Pod property

[root@master ~]# cat test.yml 
---
apiVersion: v1
kind: Pod
metadata:
  name: test
spec:
  containers:
  - name: test
    image: busybox
    imagePullPolicy: IfNotPresent
    command: ["bin/sh","-c","sleep 45"]
    env:
    - name: HN
      valueFrom:
        fieldRef:
          fieldPath: metadata.name
          
[root@master ~]# kubectl delete -f test.yml 
pod "test" deleted
[root@master ~]# kubectl apply -f test.yml 
pod/test created
[root@master ~]# kubectl get pod
NAME   READY   STATUS    RESTARTS   AGE
test   1/1     Running   0          21s
[root@master ~]# kubectl exec -it test -- /bin/sh
/ # echo $HN
test

Variable value from Secrt

[root@master ~]# cat test.yml 
---
apiVersion: v1
kind: Pod
metadata:
  name: test
spec:
  containers:
  - name: test
    image: busybox
    imagePullPolicy: IfNotPresent
    command: ["bin/sh","-c","sleep 45"]
    env:
    - name: HN
      valueFrom:
        fieldRef:
          fieldPath: spec.nodeName

[root@master ~]# kubectl apply -f test.yml 
pod/test created

[root@master ~]# kubectl get pod
NAME   READY   STATUS    RESTARTS   AGE
test   1/1     Running   0          17s
[root@master ~]# kubectl exec -it test -- /bin/sh
/ #  echo $HN
node1

Init container

Initialize container

• Init Container: used to initialize work and end after execution (one-time task)
• Most application container configurations are supported, but health checks are not supported
• Priority application container execution

Application scenario:

• Environment check: for example, ensure that the service that the application container depends on is started before starting the application container
• Initialize configuration: for example, prepare a configuration file for the application container

[root@master manifest]# cat test.yml 
---
apiVersion: v1
kind: Pod
metadata:
  name: web
  namespace: default
spec:
  initContainers:
  - name: download
    image: busybox
    imagePullPolicy: IfNotPresent
    volumeMounts:
    - name: data
      mountPath: /tmp
  containers:
  - name: nginx
    image: nginx
    imagePullPolicy: IfNotPresent
    ports:
    - containerPort: 80
      hostPort: 80
    volumeMounts:
    - name: data
      mountPath: /usr/share/nginx/html
  volumes:
  - name: data
    hostPath:
      path: /var/www/html

//Create storage volume mapping directory
[root@node1 ~]# mkdir /var/www/html/ -p
[root@node1 ~]# cd /var/www/html/
[root@node1 html]# echo "hello world" > index.html

[root@node2 ~]# mkdir /var/www/html/ -p
[root@node2 ~]# cd /var/www/html/
[root@node2 html]# echo "123" > index.html

[root@master manifest]# kubectl apply -f test.yml 
pod/web created
[root@master manifest]# kubectl get pod -o wide
NAME   READY   STATUS    RESTARTS   AGE   IP            NODE                NOMINATED NODE   READINESS GATES
web    1/1     Running   0          21s   10.244.2.11   node2.example.com   <none>           <none>
[root@master manifest]# curl 10.244.2.11
123

//pod details
[root@master manifest]# kubectl describe pod web
Name:         web
Namespace:    default
Priority:     0
Node:         node2.example.com/192.168.237.141
Start Time:   Thu, 23 Dec 2021 11:15:45 +0800
Labels:       <none>
Annotations:  <none>
Status:       Running
IP:           10.244.2.11
IPs:
  IP:  10.244.2.11
Init Containers:
  download:
    Container ID:   docker://ee2cad1f98d7cc104a0a9e8463d5dc3d4790693de38df6c5b0454bb08d76338a
    Image:          busybox
    Image ID:       docker-pullable://busybox@sha256:b5cfd4befc119a590ca1a81d6bb0fa1fb19f1fbebd0397f25fae164abe1e8a6a
    Port:           <none>
    Host Port:      <none>
    State:          Terminated
      Reason:       Completed
      Exit Code:    0
      Started:      Thu, 23 Dec 2021 11:15:46 +0800
      Finished:     Thu, 23 Dec 2021 11:15:46 +0800
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /tmp from data (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-r4pr2 (ro)
Containers:
  nginx:
    Container ID:   docker://3ccb5aaf458a5b7e6afe4c60a24ea8314690261e342525d0be6f92341723df90
    Image:          nginx
    Image ID:       docker-pullable://nginx@sha256:1f105601bfded0fa298d8c5efd5569f4ed3bf53dc7f4c41c691c29999550f6a3
    Port:           80/TCP
    Host Port:      80/TCP
    State:          Running
      Started:      Thu, 23 Dec 2021 11:15:47 +0800
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /usr/share/nginx/html from data (rw)
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-r4pr2 (ro)
Conditions:
  Type              Status
  Initialized       True 
  Ready             True 
  ContainersReady   True 
  PodScheduled      True 
Volumes:
  data:
    Type:          HostPath (bare host directory volume)
    Path:          /var/www/html
    HostPathType:  
  default-token-r4pr2:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-r4pr2
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                 node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  74s   default-scheduler  Successfully assigned default/web to node2.example.com
  Normal  Pulled     73s   kubelet            Container image "busybox" already present on machine
  Normal  Created    73s   kubelet            Created container download
  Normal  Started    73s   kubelet            Started container download
  Normal  Pulled     73s   kubelet            Container image "nginx" already present on machine
  Normal  Created    72s   kubelet            Created container nginx
  Normal  Started    72s   kubelet            Started container nginx

These types of containers are available in the Pod

• Infrastructure Container: base container
  Maintain the entire Pod cyberspace
• lnitContainers: initialize the container
  Execute before business container
• Containers: Business Container
  Parallel start