KodeCloud Kubernetes Services

Links: 111 KodeCloud Index

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Services

• Different service types
  • 
  • NodePort: Pods are made accessible on a port on the node.

TargetPort is the port of the pod to which the service forwards the request.

In short TargetPort is the target for the service. All the terms are from the view point of the service.

• In a service definition file the only mandatory field is port.

  • If you don't provide NodePort it is assigned by k8s
  • If you don't provide TargetPort it is assumed to be same as Port.
  • Ports is an array and we can have multiple such port mappings.

• Sample service file

  apiVersion: vl
  kind: Service
  metadata:
      name: myapp-service
      # we can also provide labels but they are not necessary
  spec:
      type: NodePort
      ports:
       - targetPort: 80
         port: 80
         nodePort: 30008
      # no matchLabels key inside selector like we had in ReplicaSet
      selector:
          app: myapp
  

• Whenever we create a NodePort a ClusterIP is automatically created for us.

  • 

The selector labels in the service should match the labels of the pod.

If we are using a deployment then the pod definition and its labels will be under the spec section of the of deployment manifest.

• Service acts as a load balancer with a random algorithm with a SessionAffinity set to Yes.

• What happens when pods are distributed across nodes?

  • We can use IP of any of the nodes.
  • 

• Use of ClusterIP

  • 
  • Each layer can scale and move without impacting communication between various services.

• Each service gets an IP and name assigned to it and that is the name that should be used by other pods to access the service.

  • Application code which needs to communicate with other pods in a different service should also use this name.
  • 
  • Instead of hardcoding we should be using environment variables.

• ClusterIP is the default type.

• Disadvantage of NodePort

  • Even if we have pods on only 2 out of 4 nodes they are accessible from the IP addresses of all the nodes.
  • 

We can bootstrap a k8s cluster from scratch using kubeadm

References

• Might read: Kubernetes - Services Deep Dive

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Last updated: 2023-05-16