OpenShift - Docker and Kubernetes

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OpenShift is built on top of Docker and Kubernetes. All the containers are built on top of Docker cluster, which is basically Kubernetes service on top of Linux machines, using Kubernetes orchestrations feature.

In this process, we build Kubernetes master which controls all the nodes and deploys the containers to all the nodes. The main function of Kubernetes is to control OpenShift cluster and deployment flow using a different kind of configuration file. As in Kubernetes, we use kubctl in the same way we use OC command line utility to build and deploy containers on cluster nodes.

Following are the different kinds of config files used for creation of different kind of objects in the cluster.

• Images
• POD
• Service
• Replication Controller
• Replica set
• Deployment

Images

Kubernetes (Docker) images are the key building blocks of Containerized Infrastructure. As of now, Kubernetes only support Docker images. Each container in a pod has its Docker image running inside it.

apiVersion: v1
kind: pod
metadata:
   name: Tesing_for_Image_pull -----------> 1
   spec:
   containers:
- name: neo4j-server ------------------------> 2
image: <Name of the Docker image>----------> 3
imagePullPolicy: Always ------------->4
command: [“echo”, “SUCCESS”] -------------------> 5

POD

A pod is collection of containers and its storage inside a node of a Kubernetes cluster. It is possible to create a pod with multiple containers inside it. Following is an example of keeping a database container and web interface container in the same pod.

apiVersion: v1
kind: Pod
metadata:
   name: Tomcat
spec:
   containers:
   - name: Tomcat
      image: tomcat: 8.0
      ports:
- containerPort: 7500
imagePullPolicy: Always

Service

A service can be defined as a logical set of pods. It can be defined as an abstraction on top of the pod that provides a single IP address and DNS name by which pods can be accessed. With Service, it is very easy to manage load balancing configuration. It helps PODs to scale very easily.

apiVersion: v1
kind: Service
metadata:
   name: Tutorial_point_service
spec:
   ports:
   - port: 8080
      targetPort: 31999

Replication Controller

Replication Controller is one of the key features of Kubernetes, which is responsible for managing the pod lifecycle. It is responsible for making sure that specified numbers of pod replicas are running at any point of time.

apiVersion: v1
kind: ReplicationController
metadata:
   name: Tomcat-ReplicationController
spec:
   replicas: 3
   template:
   metadata:
      name: Tomcat-ReplicationController
   labels:
      app: App
      component: neo4j
   spec:
      containers:
      - name: Tomcat
      image: tomcat: 8.0
      ports:
      - containerPort: 7474

Replica Set

The replica set ensures how many replica of pod should be running. It can be considered as a replacement of the replication controller.

apiVersion: extensions/v1beta1
kind: ReplicaSet
metadata:
   name: Tomcat-ReplicaSet
spec:
   replicas: 3
   selector:
      matchLables:
      tier: Backend
   matchExpression:
      - { key: tier, operation: In, values: [Backend]}
   
   app: App
   component: neo4j
spec:
   containers:
   - name: Tomcat-
image: tomcat: 8.0
   ports:
containerPort: 7474

Deployment

Deployments are upgraded and higher versions of the replication controller. They manage the deployment of replica sets, which is also an upgraded version of the replication controller. They have the capability to update the replica set and they are also capable of rolling back to the previous version.

apiVersion: extensions/v1beta1 --------------------->1
kind: Deployment --------------------------> 2
metadata:
   name: Tomcat-ReplicaSet
spec:
   replicas: 3
   template:
      metadata:
lables:
   app: Tomcat-ReplicaSet
   tier: Backend
spec:
   containers:
name: Tomcat-
   image: tomcat: 8.0
   ports:
   - containerPort: 7474

All config files can be used to create their respective Kubernetes objects.

$ Kubectl create –f <file name>.yaml

Following commands can be used to know the details and description of the Kubernetes objects.

For POD

$ Kubectl get pod <pod name>
$ kubectl delete pod <pod name>
$ kubectl describe pod <pod name>

For Replication Controller

$ Kubectl get rc <rc name>
$ kubectl delete rc <rc name>
$ kubectl describe rc <rc name>

For Service

$ Kubectl get svc <svc name>
$ kubectl delete svc <svc name>
$ kubectl describe svc <svc name>

For more details on how to work with Docker and Kubernetes, please visit our Kubernetes tutorial using the following link kubernetes.