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K8S Architecture

Kubernetes is a container orchestration tool, i.e. using Kubernetes you can automate the management of containerized applications. kubernetes cluster follows master-node architecture. instead of managing containers' kubernets actually manages pods which can consist of one or more container.

Benifits​

  • High availability : If pod goes down, kubernetes replaces with new pod in the same node or any other node in the cluster based on the resource availability.
  • scalability : Can run multiple pods with the same defination and can scaled using policies.
  • Robust CD : Kubernetes intelligently manages the upgrades and rollbacks.

Control PLane Components​

kube-apiserver​

  • Responsible for providing API endpoint for kubernetes components
  • Gateway for all communications with K8s cluster
  • Only component whcih connects to ectd
  • Responsible for authentication & autherization
  • Inform the kubelet to manage the pod.
  • Any CLI command or GUI commands reaches kube-apiserver and gets the responce
  • Cluster endpoint in the browser will list all the apis available
  • Documentation 
kubectl explain pod
kubectl explain pod.spec
kubectl explain pod.spec.container

etcd​

Distributed key-value store used by k8s to store entire cluster data (state) and keep update the changes.

kube-scheduler​

  • It watches newly creates pods that have node node assigned and selects a node fot them to run on
  • Factors considered while scheduling the pod
    • Resource requirements of pod
    • Hardware/Software policy constrains
    • Affinity & Anti-Affinity
    • Data Locality

kube-control-manager​

-Every resource/object in k8s is controlled by a controller (control loop) with spec fields as values for desired state. -Responsible for controlling various aspects of the clusters, like

- Node controller
- Replication controller
kubectl api-resources

Cloud-controller-manager​

Runs controller that interact with the underlying cloud provider. It uses cloud connector to interact with cloud.

Node Components​

kubelet​

  • Is an agent in the each node which communicates with APIserver
  • Interact with container-runtime to manage the pod.

kube-proxy​

Acts as network proxy which maintains network rules on the host and perform connection forwarding

container-runtime​

like Docker engine, contrainerd