bhoopesh123 / k8s-concepts

Kubernetes Components

When you deploy Kubernetes, you get a cluster.

A Kubernetes cluster consists of a set of worker machines, called nodes, that run containerized applications. Every cluster has at least one worker node. In production environments, the control plane usually runs across multiple computers and a cluster usually runs multiple nodes, providing fault-tolerance and high availability.

Control Plane Components :

The control plane's components make global decisions about the cluster Detecting and responding to cluster events Scheduling starting up a new pod when a deployment's replicas field is unsatisfied

1. kube-apiserver: (Front End of Control Plane)
  The API server is a component of the Kubernetes control plane that exposes the Kubernetes API. The API server is the front end for the Kubernetes control plane
  kube-apiserver is designed to scale horizontally
  You can run several instances of kube-apiserver and balance traffic between those instances

2. etcd:
  Consistent and highly-available key value store used as Kubernetes' backing store for all cluster data.

3. kube-scheduler:
  Control plane component that watches for newly created Pods with no assigned node, and selects a node for them to run on.

4. kube-controller-manager:
  Each controller is a separate process, but to reduce complexity, they are all compiled into a single binary and run in a single process.
  Some types of these controllers are:

    a. Node controller: Responsible for noticing and responding when nodes go down.
    b. Job controller: Watches for Job objects that represent one-off tasks, then creates Pods to run those tasks to completion.
    c. EndpointSlice controller: Populates EndpointSlice objects (to provide a link between Services and Pods).
    d. ServiceAccount controller: Create default ServiceAccounts for new namespaces

5. cloud-controller-manager: (Absent for Minikube since it is Local)
  The cloud controller manager lets you link your cluster into your cloud provider's API.
  The cloud-controller-manager only runs controllers that are specific to your cloud provider

Reference Diagram:

Node Components:

Node components run on every node, maintaining running pods and providing the Kubernetes runtime environment.

1. kubelet: An agent that runs on each node in the cluster. It makes sure that containers are running in a Pod.
The kubelet takes a set of PodSpecs that are provided through various mechanisms and ensures that the containers described in those PodSpecs are running and healthy

2. kube-proxy: kube-proxy is a network proxy that runs on each node in your cluster, implementing part of the Kubernetes Service concept.
kube-proxy maintains network rules on nodes. These network rules allow network communication to your Pods from network sessions inside or outside of your cluster.

Container runtime:

The container runtime is the software that is responsible for running containers.
Kubernetes supports container runtimes such as containerd, CRI-O, and any other implementation of the Kubernetes CRI (Container Runtime Interface).

Container Run Time in Nutshell:
https://kubernetes.io/docs/setup/production-environment/container-runtimes/