Kubernetes API Objects

• Everything containered in K8S is represented by RESTful resource, we refer to those resources as kubernetes objects.
• The most basic command of viewing k8s objects is get.

kubectl get <resource-name> => all resources 
kubectl get <resource-name> <object-name> => to view the specific resource

• To get all the resources of the k8s we can execute kubectl api-resources
  
  
  
• We can also view the k8s object in the json or yaml format kubectl get <resource-name> <object-name> -o json or kubectl get <resource-name> <object-name> -o yaml
  

Pods

• Lets assume we have an application which has webserver and database
  
• One Option is to create two containers in a Pod
  
• The above option is considered to be bad practice or antipattern
• Generally we will have one container in a Pod to make scaling effective.
• In some cases we might have two containers in a pod
• maincar
• sidecar
  

Representing an K8s object in yaml file

• Refer Here for understaning API and objects
• In Kubernetes since resources are exposed as REST APIs, We have to understand API Versioining
• Kubernetes groups resources in API Group
• core: Referred as legacy group . In REST API => api/v1 and in yaml apiVersion: v1
• named groups: In REST API apis/$GROUP_NAME/$VERSION and in yaml => apiVersion: $GROUPNAME/$VERSION
• To describe a k8s object we have the following nested fields
• Object Spec:
  • Here we describe the desired state of the object
• Object Status
  • Actual state of object
• When we try to write a desired state / k8s manifest file in yaml

apiVersion: <version>
kind: <kind of the resource>
metadata:
  name: <name of the object>
spec: 
  <object-spec>

• Now lets look at the example which we have used earlier

apiVersion: v1
kind: Pod
metadata:
  name: httpd
spec:
  containers:
    - name: httpd
      image: httpd:latest
      ports:
        - containerPort: 80

• When we execute the following command kubectl get pods httpd -o yaml

apiVersion: v1
items:
- apiVersion: v1
  kind: Pod
  metadata:
    annotations:
      kubectl.kubernetes.io/last-applied-configuration: |
        {"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"name":"httpd","namespace":"default"},"spec":{"containers":[{"image":"httpd:latest","name":"httpd","ports":[{"containerPort":80}]}]}}
    creationTimestamp: "2022-02-12T14:08:29Z"
    name: httpd
    namespace: default
    resourceVersion: "10096"
    uid: cac0c00f-f6e7-4684-9eb4-cf99e1ada436
  spec:
    containers:
    - image: httpd:latest
      imagePullPolicy: Always
      name: httpd
      ports:
      - containerPort: 80
        protocol: TCP
      resources: {}
      terminationMessagePath: /dev/termination-log
      terminationMessagePolicy: File
      volumeMounts:
      - mountPath: /var/run/secrets/kubernetes.io/serviceaccount
        name: kube-api-access-zw8x6
        readOnly: true
    dnsPolicy: ClusterFirst
    enableServiceLinks: true
    nodeName: ip-172-31-33-127
    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: kube-api-access-zw8x6
      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: "2022-02-12T14:08:29Z"
      status: "True"
      type: Initialized
    - lastProbeTime: null
      lastTransitionTime: "2022-02-12T14:08:31Z"
      status: "True"
      type: Ready
    - lastProbeTime: null
      lastTransitionTime: "2022-02-12T14:08:31Z"
      status: "True"
      type: ContainersReady
    - lastProbeTime: null
      lastTransitionTime: "2022-02-12T14:08:29Z"
      status: "True"
      type: PodScheduled
    containerStatuses:
    - containerID: docker://4cf2b8387d82cd5ddd280b4be549496c663bb0dfb05d950d7d86c1e58f05e66e
      image: httpd:latest
      imageID: docker-pullable://httpd@sha256:5cc947a200524a822883dc6ce6456d852d7c5629ab177dfbf7e38c1b4a647705
      lastState: {}
      name: httpd
      ready: true
      restartCount: 0
      started: true
      state:
        running:
          startedAt: "2022-02-12T14:08:31Z"
    hostIP: 172.31.33.127
    phase: Running
    podIP: 192.168.2.6
    podIPs:
    - ip: 192.168.2.6
    qosClass: BestEffort
    startTime: "2022-02-12T14:08:29Z"
kind: List
metadata:
  resourceVersion: ""
  selfLink: ""

• To write manifests lets use the reference page
• Refer Here for the reference links
• Since we are using 1.23 we use Refer Here
• Since we are writing Pod Manifest Refer Here
• Activity 1: Create a Pod to run nginx container in it
• Create a yaml and add apiVersion => v1 & kind => Pod
• Now we need to add metadata Refer Here
• To write Pod Spec Refer Here
• Since we decided to use nginx container in the pod we use containers section
• To describe container Refer Here
• Refer Here for the pod manifest created
  
• Now delete the pod
  
• Activity 2: Lets create a Pod with mysql container and set the following environmental variables
• MYSQL_ROOT_PASSWORD => root123
• MYSQL_DATABASE => qtecommerce
• MYSQL_USER => qtdevops
• MYSQL_PASSWORD => qtdevops123
• solution: Refer Here
  
• Lets login into the container