Services

▪ If a pod needs to communicate with another pod, it needs a way to know the IP address of the other pod. Kubernetes services provide a mechanism for locating other pods.

▪ According to the Kubernetes networking model, pod IPs are ephemeral; if a pod crashes or is deleted and a new pod is created in its place, it most likely receives a new IP address. Kubernetes services allows you to select a mechanism for locating other pods.

▪ Services select Pods based on their labels. When a network request is made to the service, it selects all Pods in the cluster matching the service's selector, chooses one of them, and forwards the network request to it.

What does ClusterIP, NodePort, and LoadBalancer mean?

The type property in the Service's spec determines how the service is exposed to the network. It changes where a Service is able to be accessed from. The possible types are ClusterIP, NodePort, and LoadBalancer

• ClusterIP – The default value. The service is only accessible from within the Kubernetes cluster – you can’t make requests to your Pods from outside the cluster!

• NodePort – This makes the service accessible on a static port on each Node in the cluster. This means that the service can handle requests that originate from outside the cluster.

• LoadBalancer – The service becomes accessible externally through a cloud provider's load balancer functionality. GCP, AWS, Azure, and OpenStack offer this functionality. The cloud provider will create a load balancer, which then automatically routes requests to your Kubernetes Service.

Example YAML code that shows you how to use a NodePort service in Kubernetes.

kind: Service 
apiVersion: v1 
metadata:
  name: hostname-service 
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: echo-hostname 

  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
      port: 8080 
      targetPort: 80

ClusterIP Service & its subtypes

ClusterIP is an internal service, not accessible from outside the cluster

NodePort Service

▪ Unlike internal Service, is accessible directly from outside cluster

▪ A ClusterIP Service, to which the NodePort Service routes, is automatically created

▪ Exposes the Service on each Node's IP at a port

Loadbalancer Service

Exposes the Service externally using a cloud provider's load balancer

NodePort and ClusterIP Services, to which the external load balancer routes, are automatically created