Kubernetes – Service object – simplifying the concepts

Hi All,

Greetings for the day!!!

We are continuing Kubernetes series. In this article we will discuss Kubernetes Service object in depth.

Take away from this article

• What is Kubernetes Service object
• Naming convention for Service object
• YAML specification of Service object
• kubectl CMLDLET for service object

What is Kubernetes Service Object

• Service object act as internal load balancer in Kubernetes cluster
• Service object forward the request to appropriate POD based on the labels and labels selector
• A Service object allow network access to set of PODs
• When Service object receives network request it selects all the PODs in cluster matching the service selector. Choose one of the POD and forwards the network requests
• Default protocol for service is TCP.
• We can use any supported protocols – UDP, SCTP, HTTP, PROXY protocol

Name of Service object

• The name of a Service object must be a valid RFC 1035 label name.
• RFC 1035 Label Names – Some resource types require their names to follow the DNS label standard as defined in RFC 1035. This means the name must:
  • contain at most 63 characters
  • contain only lowercase alphanumeric characters or ‘-‘
  • start with an alphabetic character
  • end with an alphanumeric character

YAML specification for Kubernetes Service object

kind: Service 
apiVersion: v1 
metadata:
  name: <Service - name> 
spec:
  # Expose the service on a static port on each node
  # so that we can access the service from outside the cluster 
  type: NodePort

  # When the node receives a request on the static port (30163)
  # "select pods with the label 'app' set to 'echo-hostname'"
  # and forward the request to one of them
  selector:
    app: <pod app name> 

  ports:
    # Three types of ports for a service
    # nodePort - a static port assigned on each the node
    # port - port exposed internally in the cluster
    # targetPort - the container port to send requests to
    - nodePort: 30163

      protocol: TCP 
      port: 8080 
      targetPort: 80

Here,

• type property :
  • There are 4 types of Kubernetes services
  • Tells us how the Service is exposed to Network
  • Possible values are
    • NodePort
      • Using NodePort we make accessible Service on a static port on each node in cluster
      • Means Service is able to handle request from outside cluster
      • We could connect to NodePort service from outside the cluster by <NodeIP> : <Node Port>
      • NodePort number must be range from 30000-32767
      • If we didnt specify NodePort, kubernetes will automatically assign
      • If we specify NodePort explicitly we need to make sure respective NodePort is assigned to another Service
      • Usages
        • When we want to connect our application outside Kubernetes cluster
    • ClusterIP
      • This is default value
      • In this type we can not make request to PODs from outside the cluster
      • The service is accessible only within the Kubernetes cluster
      • This type can be used while inter service communication within the cluster like communication between front-end and back-end of our application
    • LoadBalancer
      • The Service is accessible through externally using cloud providers load balancer functionality
      • Cloud providers like Azure, AWS, GCP, OpenStack – provides this kind of functionality
    • ExternalName
      • This service type is used when our POD needs to access an application outside the Kubernetes cluster like external database

Example – Creating service specification for – private registry in Service

apiVersion: v1
kind: Service
metadata:
  name: registry-service-knoweldgejunction
  namespace: sharepoint-knowledgejunction
spec:
  selector:
    app: registry
  ports:
    - port: 8800

Once we have specification ready we can use kubectl CMDLET to create Service object

kubectl create -f <service object YAML specification path>

Example : 

kubectl create -f service.yaml

What happened when Service object created

• Kubernetes assigns IP address to Service
• Controller for Service selector continuously scans for PODs that match its selector

Multi-Port services

• For Service object Kubernetes support multiple port definitions
• For allowing multiple ports, we specify port name to avoid unambiguous

apiVersion: v1
kind: Service
metadata:
  name:  registry-service-knoweldgejunction
spec:
  selector:
    app: registry
  ports:
    - name: http
      protocol: TCP
      port: 80
      targetPort: 9376
    - name: https
      protocol: TCP
      port: 443
      targetPort: 9377

Thanks for reading !!! Please feel free to discuss / suggestions / share thoughts !!!

HAVE A GREAT TIME AHEAD !!! LIFE IS BEAUTIFUL 🙂