Services

The base of networking in Kuberneties

• Service Types
  • NodePort
    • Makes a pod availble by mapping the port on the node to a port on the pod
    • The service is accessible from outside the cluster via <NodeIP>:<NodePort>.
  • ClusterIP
    • An internal setup for communication between pods
    • Creates a virtual IP address accessible only from within the cluster.
    • For example, Pod-A wants to communicate with Pod-B. You set up a ClusterIP service named “service-pod-b”. Pod-A can then use the DNS name “service-pod-b” to send traffic to Pod-B.

• Port types
  • NodePort: the port on the node that someone might connect to.
  • Port: The port on the service that receives traffic and forwards it to the TargetPor
  • Target Port: The port on the node that is listening on / The port on the Pod that the service forwards traffic to.
  • Example:
    • NodePort: 40000 (external port on the Node)
    • Port: 80 (service port accessible within the cluster)
    • TargetPort: 3000 (port on the Pod where the application is listening)
    • Explination
      • External access: A client outside the cluster accesses the Pod by connecting to <NodeIP>:40000.
      • Internal access: A client inside the cluster (such as a pod) uses the service’s DNS name (<service-name>) and port 80.
      • Pod: The Pod’s application listens on port 3000.

So with this Pod-B accessable from inside and outside the cluster, so that should be a ClusterIP

apiVersion: v1
kind: Service
metadata:
  name: service-pod-b
spec:
  selector:
    app: pod-b. #These are the labels from your pod/deployment files so it knows what pod this should be attached to.
  ports:  # Please note ports is a list, so you can setup multiple ports here
  - port: 80       # The port used inside the cluster
    targetPort: 3000 # The port the Pod listens on
    nodePort: 40000 # The externally accessible port
  type: NodePort

If it was just going to be accessable from inside the cluster it would be

apiVersion: v1
kind: Service
metadata:
  name: service-pod-b
spec:
  selector:
    app: pod-b
  ports:
  - port: 80
    targetPort: 3000
  type: ClusterIP

If you have multiple pods the service will load-balance between them randomly.