k8s Pods

• PODs
  • smallest object in k8s, contains containers, can contain one or more containers
  • scale service by creating more pod instances.
  • additional container could be helper container.
  • network space is shared between containers in the same pod, they can communicate on localhost.
• Static Pods

  • On standalone VM, the kubelet can be configured to

    • read pod definitions from

      1. (Option:--pod-manifest-path=/etc/kubernetes/manifests).

         •  ps ax | grep kubeconfig 

      2. kubeconfig.yaml staticPodPath: /etc/kubernetes/manifest.
    • It also assures they stay alive.
    • Only POD's , nothing else.

    • view with  docker ps 

    • the kubeadm k8s setup uses static pods to run the k8s management software as pods on nodes.

  • Static Pods and DaemonSets are ignored by the Kube-Scheduler

• Pod, eq in Docker ENTRYPOINT->command & CMD->args

  apiVersion: v1
  kind: Pod
  metadata:
    name: ubuntu-sleeper-pod
  spec:
    containers:
      - name: ubuntu-sleeper
        image: ubuntu-sleeper
        command: ["sleep2.0"]
        args: ["10",]
        ...
        envFrom:
          - configMapRef:
               name: app-conf-map
          - secretRef:
               name: app-secret
        ...
        env:
          - name: APP_COLOR1
            value: pink
  
          - name: App_COLOR2
            valueFrom:
                 configMapKeyRef:
  
          - name: App_PWD
            valueFrom:
                 secretKeyRef:
                    name: app-secret
                    key: DB_Password    
  
        ...
    volumes:
      - name: app-secret-volume
        secret:
           secretName: app-secret  

• Multi-container pods
  • Types

    1. SideCar

    2. Adapter
    3. Ambassador
  • All containers are expected to stay alive in POD. If any fail, the POD restarts.

    • InitContainers: separate section in line with containers:

      • Runs one at a time, before Containers: and must all complete before real container starts.

        • if an InitContainer fails, Pod restarts