Services in kubernetes is a logical unit through which the external or internal applications access the targeted application functionalities.

Analogy

Consider you own a Notebook of XYZ company which has 10 service unit in your state. You know only one of them which is near to your area. One day you went to this service unit to fix something with the notebook but found the unit is closed for 1 week. What do you do now? At this point one of your friends told you that there is a customer service unit nearby and they will send the laptop to a service unit and get it fixed. You give the laptop to the customer service unit and they will send it to a functioning service unit and get it fixed.

In Kubernetes

Kubernetes service (similar to the customer service unit which keeps a record of the functional service units) keeps a record, Endpoint, of its targeted Pods. The client calls the service which in turn forward the result to one of the actual pods to get the request processed.

In the below example we have a Deployment object of nginx server and it is connected to a service nginx.

• nginx-server.yaml

   #// kubectl run nginx-server --image=nginx
  apiVersion: apps/v1
  kind: Deployment
  metadata:
    annotations:
      deployment.kubernetes.io/revision: "1"
    creationTimestamp: null
    generation: 1
    labels:
      tier: frontend
    name: nginx
    selfLink: /apis/apps/v1/namespaces/default/deployments/nginx
  spec:
    progressDeadlineSeconds: 600
    replicas: 1
    revisionHistoryLimit: 10
    selector:
      matchLabels:
        tier: frontend
    strategy:
      rollingUpdate:
        maxSurge: 25%
        maxUnavailable: 25%
      type: RollingUpdate
    template:
      metadata:
        creationTimestamp: null
        labels:
          tier: frontend
      spec:
        containers:
        - image: nginx
          imagePullPolicy: Always
          name: nginx
          ports:
          - containerPort: 80
            protocol: TCP
          resources: {}
          terminationMessagePath: /dev/termination-log
          terminationMessagePolicy: File
        dnsPolicy: ClusterFirst
        restartPolicy: Always      

• nginx-service.yaml

  #// kubectl expose nginx-server --name=nginx-service --port=80 
  apiVersion: v1
  kind: Service
  metadata:
    creationTimestamp: null
    labels:
      tier: frontend
    name: nginx
    selfLink: /api/v1/namespaces/default/services/nginx
  spec:
    ports:
    - port: 80
      protocol: TCP
      targetPort: 80
    selector:
      tier: frontend
    sessionAffinity: None
    type: ClusterIP

How to check if the service is working

The access to the service can be checked by passing the name of the service or using the ip address of the pod to the nslookup utility .

• Using the service name

nslookup nginx.default.svc.cluster.local 10.244.0.92

The result is as given below

Server:		10.244.0.92
Address:	10.244.0.92#53

Name:	nginx.default.svc.cluster.local
Address: 10.96.50.250

• Using the pod ip address

Find the ip address of the pods in the deployment. Replace the dot in the ip address with dash and pass it to the nslookup.

nslookup 10-244-0-91.nginx.default.svc.cluster.local 10.244.0.92 The result is as given below

Server:		10.244.0.92
Address:	10.244.0.92#53

Name:	10-244-0-91.nginx.default.svc.cluster.local
Address: 10.244.0.91

Conclusion

There are three ways to expose services in kubernetes, in this blog post we have taken the example of ClusterIP to explain the concept of service and access it.