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Nodes

Nodes​

Nodes are physical or VM servers where the cluster is deployed. Nodes have roles, either master or worker. A Node is an API object created outside the cluster representing an instance.

We can create a master node with:

kubeadm init

and worker nodes:

kubeadm join

To remove nodes:

kubectl delete node $nodename

# remove cluster-specific information
kubeadm reset

To view current node status:

kubectl describe node $nodename

All nodes run the following components:

  • kubelet: Ensures that containers are running in a Pod.
    • Receives resource specifications (PodSpec) and ensures that the node meets the desired specs working with container-runtime
    • Ensures that a Pod has access to specified storage, Secrets or ConfigMaps.
    • Reports status of Pods to cluster.
  • kube-proxy: Network proxy implemented as a Service. It maintains network rules on the node based on IP tables so that internal and external clients can communicate with the Pods.
  • container-runtime: Works with kubelet to ensure specified containers are running and healthy. It's an interface between Kubernetes and the container/Docker Engine.

Master Node​

The Master node runs various management services for the whole cluster, mainly:

  • kube-apiserver: All calls (internal and external traffic) are handled with this agent. It is the only connection to the etcd database. Acts as a master process for the entire cluster.
  • kube-scheduler: Responsible for assigning Pods to Nodes by provided specification (labels, taints, toletations, number of replicas, state)
  • etcd: key-value database where the state of the cluster information, as well as other persistent data and networking is kept. Requests to update DB is load-balanced by kube-apiserver and distributed to database in series. etcdctl can be used to interact with db.
  • kube-controller-manager: Determines the state of the cluster by interaction with kube-apiserver. If any action is needed, it alerts other controllers (endpoints, namespaces, replicasets) of need to add/remove resources.

Worker Node​

The Worker Node hosts the applications workload.