In this blog, we will cover the Introduction of Kubernetes and its Components.

What is Kubernetes

Kubernetes is an open source container management tool. You can set up a Kubernetes cluster on a Linux-based OS to deploy, manage, and scale Docker container applications on multiple hosts.

Kubernetes is made up of the following components:

• master
• nodes
• etcd
• network

These components are connected via a network, as shown in the following diagram:

• Kubernetes master: It connects to etcd via HTTP or HTTPS to store the data
• Kubernetes nodes: It connect to the Kubernetes master via HTTP or HTTPS to get a command and report the status
• Kubernetes network: It L2, L3 or overlay make a connection of their container applications

Kubernetes master

The Kubernetes master is the main component of the Kubernetes cluster. It serves several functionalities, such as the following:

• Authorization and authentication
• RESTful API entry point
• Container deployment scheduler to Kubernetes nodes
• Scaling and replicating controllers
• Reading the configuration to set up a cluster

API server (kube-apiserver)

The API server provides an HTTP- or HTTPS-based RESTful API, which is the hub between Kubernetes components, such as kubectl, the scheduler, the replication controller, the etcd data store, the kubelet and kube-proxy, which runs on Kubernetes nodes, and so on.

Scheduler (kube-scheduler)

The scheduler helps to choose which container runs on which nodes. It is a simple algorithm that defines the priority for dispatching and binding containers to nodes. For example:

• CPU
• Memory
• How many containers are running?

Controller manager (kube-controller-manager)

The controller manager performs cluster operations. For example:

• Manages Kubernetes nodes
• Creates and updates the Kubernetes internal information
• Attempts to change the current status to the desired status

Command-line interface (kubectl)

After you install the Kubernetes master, you can use the Kubernetes command-line interface, kubectl, to control the Kubernetes cluster.

//see the nodes
# kubectl get nodes
NAME          LABELS                           STATUS    AGE
kub-node1   kubernetes.io/hostname=kub-node1   Ready     26d
kub-node2   kubernetes.io/hostname=kub-node2   Ready     26d

Kubernetes node

The Kubernetes node is a slave node in the Kubernetes cluster. It is controlled by the Kubernetes master to run container applications using Docker.

The following diagram displays the role and tasks of daemon processes in the node:

The node also has two daemon processes, named kubelet and kube-proxy, to support its functionalities.

kubelet

kubelet is the main process on the Kubernetes node that communicates with the Kubernetes master to handle the following operations:

• Periodically accesses the API controller to check and report
• Performs container operations
• Runs the HTTP server to provide simple APIs

Proxy (kube-proxy)

The proxy handles the network proxy and load balancer for each container. It changes Linux iptables rules (nat table) to control TCP and UDP packets across the containers.

etcd

etcd is the distributed key-value data store. It can be accessed via the RESTful API to perform CRUD operations over the network. Kubernetes uses etcd as the main data store.

Kubernetes network

Network communication between containers is the most difficult part. Because Kubernetes manages multiple nodes (hosts) running several containers, those containers on different nodes may need to communicate with each other.

If the container’s network communication is only within a single node, you can use Docker network or Docker compose to discover the peer. However, along with multiple nodes, Kubernetes uses an overlay network or container network interface (CNI) to achieve multiple container communication.

Congratulations !! We have successfully  cover the Introduction of Kubernetes and its Components.