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What is Kubernetes?

Kubernetes (sometimes shortened to K8s with the 8 standing for the number of letters between the “K” and the “s”) is an open source system to deploy, scale, and manage containerized applications anywhere.It is used for automating operational tasks of container management and includes built-in commands for deploying applications, rolling out changes in container applications, scaling container applications up and down to fit changing needs, monitoring coittainer applications, and more—making it easier to manage container applications.

Kubernetes Features

  • Automated rollouts and rollbacks

  • Storage orchestration

  • Secret and configuration management

  • Batch execution

  • IPv4/IPv6 dual-stack

  • Service discovery and load balancing

  • Self-healing

  • Automatic bin packing

Kubernetes Architecture and Components

  • A Kubernetes cluster consists of a set of worker machines, called nodes, that run containerized applications. Every cluster has at least one worker node.

  • The worker node(s) host the Pods that are the components of the application workload. The control plane manages the worker nodes and the Pods in the cluster. 

  • In production environments, the control plane usually runs across multiple computers and a cluster usually runs multiple nodes, providing fault-tolerance and high availability.

   Kubernetes Components

  Control Plane Components:

  • 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.
  • The main implementation of a Kubernetes API server is kube-apiserver. kube-apiserver is designed to scale horizontally—that is, it scales by deploying more instances. 

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

  • Control plane component that watches for newly created Pods with no assigned node, and selects a node for them to run on.
Factors taken into account for scheduling decisions include: individual and collective resource requirements, hardware/software/policy constraints, affinity and anti-affinity specifications, data locality, inter-workload interference, and deadlines.

Control plane component that runs controller processes.
There are many different types of controllers. Some examples of them are:
Node controller: Responsible for noticing and responding when nodes go down.
Job controller: Watches for Job objects that represent one-off tasks, then creates Pods to run those tasks to completion.
EndpointSlice controller: Populates EndpointSlice objects (to provide a link between Services and Pods).
ServiceAccount controller: Create default ServiceAccounts for new namespaces.

    Node Components:

  • 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. The kubelet doesn't manage containers which were not created by Kubernetes.

  • 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.
  • kube-proxy uses the operating system packet filtering layer if there is one and it's available. Otherwise, kube-proxy forwards the traffic itself.

   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).
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