Kubernetes Services

Services

• Kubernetes Services enables communication between various components within and outside of the application. A service can help us group the pods together and provide a single interface to access the pod in a group.

External Communication

• How do we as an external user access the web page?

• From the node (Able to reach the application as expected)

• From outside world (This should be our expectation, without something in the middle it will not reach the application)

Service Types

There are 4 types of service types in kubernetes.

1. NodePort

• Where the service makes an internal port accessible on a port on the NODE.

apiVersion: v1
kind: Service
metadata:
name: myapp-service
spec:
types: NodePort
ports:
- targetPort: 80
    port: 80
    nodePort: 30008

To connect the service to the pod

apiVersion: v1
kind: Service
metadata:
name: myapp-service
spec:
type: NodePort
ports:
- targetPort: 80
    port: 80
    nodePort: 30008
selector:
    app: myapp
    type: front-end

To create the service

$ kubectl create -f service-definition.yaml

To list the services

$ kubectl get services

To access the application from CLI instead of web browser

$ curl http://<ip-address>:<port>

2. ClusterIP

• In this case the service creates a Virtual IP inside the cluster to enable communication between different services such as a set of frontend servers to a set of backend servers.

To create a service of type ClusterIP

apiVersion: v1
kind: Service
metadata:
 name: back-end
spec:
 types: ClusterIP
 ports:
 - targetPort: 80
   port: 80
 selector:
   app: myapp
   type: back-end

$ kubectl create -f service-definition.yaml

To list the services

$ kubectl get services

3. LoadBalancer

• Where the service provisions a loadbalancer for our application in supported cloud providers. With a simple YAML file declaring your service name, port, and label selector, the cloud controller will provision a load balancer for you automatically.

apiVersion: v1
kind: Service
metadata:
  name: search-svc # the name of our service
spec:
  type: loadBalancer
  selector:
    app: SearchApp # pods are deployed with the label app=SearchApp
  ports:
    - port: 80

This is great because of how simple it is to put a load balancer in front of your application. The service spec has been extended over the years with annotations and additional configuration. A second option is to use an ingress rule and an ingress controller to route external traffic into Kubernetes pods.

4. ExternalName

An ExternalName Service is a special case of Service that does not have selectors and uses DNS names instead, e.g.

apiversion: v1
kind: Service
metadata:
  name: my-database-svc
  namespace: prod
spec:
  type: ExternalName
  externalName: my.database.example.com

When looking up the service my-database-svc.prod.svc.cluster.local, the cluster DNS Service returns a CNAME record for my.database.example.com.