Basic terminologies in Kubernetes

https://youtu.be/IKDrwiAOiyI

https://kubernetes.io/docs/concepts/overview/components/

Kubernetes:?It is an open-source container orchestration platform designed to automate the deployment, scaling, and management of containerized applications.

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.

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.

Control Plane Components

The control plane's components make global decisions about the cluster (for example, scheduling), as well as detecting and responding to cluster events (for example, starting up a new?pod?when a Deployment's?replicas?field is unsatisfied).

Control plane components can be run on any machine in the cluster. However, for simplicity, setup scripts typically start all control plane components on the same machine, and do not run user containers on this machine.?

kube-apiserver

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. You can run several instances of kube-apiserver and balance traffic between those instances.

etcd

Consistent and highly-available key value store used as Kubernetes' backing store for all cluster data. If your Kubernetes cluster uses etcd as its backing store, make sure you have a?back up?plan for the data.

kube-scheduler

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.

kube-controller-manager

Control plane component that runs?controller?processes.

Logically, each?controller?is a separate process, but to reduce complexity, they are all compiled into a single binary and run in a single process.

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.

The above is not an exhaustive list.

cloud-controller-manager

A Kubernetes?control plane?component that embeds cloud-specific control logic. The?cloud controller manager?lets you link your cluster into your cloud provider's API, and separates out the components that interact with that cloud platform from components that only interact with your cluster.

Node Components

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

kubelet

An agent that runs on each?node?in the cluster. It makes sure that?containers?are running in a?Pod.

The kubelet works in terms of a PodSpec. A PodSpec is a YAML or JSON object that describes a pod. The kubelet takes a set of PodSpecs that are provided through various mechanisms (primarily through the apiserver) 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

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

A fundamental component that empowers Kubernetes to run containers effectively. It is responsible for managing the execution and lifecycle of containers within the Kubernetes environment.

Kubernetes supports container runtimes such as?containerd,?CRI-O, and any other implementation of the?Kubernetes CRI (Container Runtime Interface).

kubeadm

Kubeadm is a tool built to provide?kubeadm init?and?kubeadm join?as best-practice "fast paths" for creating Kubernetes clusters.

kubeadm performs the actions necessary to get a minimum viable cluster up and running. By design, it cares only about bootstrapping, not about provisioning machines. Likewise, installing various nice-to-have addons, like the Kubernetes Dashboard, monitoring solutions, and cloud-specific addons, is not in scope.

kubectl - command line tool

Kubernetes provides a command line tool for communicating with a Kubernetes cluster's?control plane, using the Kubernetes API. This tool is named?kubectl. For configuration,?kubectl?looks for a file named?config?in the?$HOME/.kube?directory.