Kubernetes Tutorial – Advanced Overview of K8s

What is Kubernetes?

Kubernetes is an open source orchestrator for deploying containerized applications. Kubernetes also known as K8s, is an open-source Container Management tool. It provides container runtime, container orchestration, container centric infrastructure orchestration, self healing mechanisms, service discovery, load balancing and container (de)scaling. Initially It was developed by Google for managing containerized applications in clustered environments but later donated to CNCF (Cloud Native Computing Foundation, a child entity of Linux Foundation).

Kubernetes is Written in Go programming language. It is a platform designed to completely manage the life cycle of containerized applications and services using methods that provide predictability, scalability, and high availability.

What is Container orchestration?

Container orchestration automates the deployment, management, scaling, and networking of containers across the cluster. It is focused on managing the life cycle of containers. Enterprises that usually deploy and manage hundreds or thousands of Linux containers and hosts can take benefits from container orchestration.

Container orchestration is used to automate the following tasks at scale:

• Configuring and Scheduling of containers
• Provisioning and Deployment of containers
• Redundancy and Availability of containers
• Health monitoring of containers and hosts
• Scaling up or removing containers to spread application load evenly across host infrastructure
• Movement of containers from one host to another if there is a shortage of resources in a host, or if a host dies
• Allocation of resources between containers
• External exposure of services running in a container with the outside world
• Load balancing of service discovery between containers

Docker Swarm vs Kubernetes

Kubernetes and Docker Swarm are Container Orchestration tools. Both are used to deploy containers inside a cluster but there are few differences between them.

Features	Docker Swarm	Kubernetes
Installation & Cluster Configuration	It’s installation is very simple; but cluster is not very strong	It’s installation is complicated but once setup done, the cluster is very strong
Scalability	It is highly scalable & scales 5x faster than Kubernetes	Highly scalable & scales fast
Auto Scaling	Docker Swarm cannot do auto scaling	Kubernetes can do auto scaling
Rolling Updates & Rollbacks	It can deploy Rolling updates, but not automatic Rollbacks	It can deploy Rolling updates & does automatic Rollbacks
GUI	There is no GUI in Docker swarm	GUI is the Kubernetes Dashboard
Data Volumes	It can share storage volumes with any other container	Can share storage volumes only with other containers in same Pod
Logging & Monitoring	3rd party tools like ELK can be used for logging & monitoring	In built tools for logging & monitoring

Certified Kubernetes Distributions

• Cloud Managed : EKS by AWS, AKS by Azure and GKE by GCP
• Self Managed : OpenShift by Redhat and Docker Enterprise
• Local dev/test : Micro K8s by Canonical, Minikube
• Vanilla Kubernetes : The core Kubernetes project( baremetal ), Kubeadm
• Special builds : K3s by Rancher, a light weight K8s distribution for Edge devices

Online Emulator: https://labs.play-with-k8s.com/

Kubernetes Cluster

A Kubernetes cluster is a collection of physical or virtual machines and other infrastructure resources that are required to run your containerized applications. Each machine in a Kubernetes cluster is called a Node.    

There are two types of Nodes in each Kubernetes cluster.

• Master Node : Hosts the Kubernetes control plane components and manages the Kubernetes Cluster
• Worker Node : Runs your containerized applications

Control Plane or Master Node Components

Kubernetes Master is responsible for managing the complete cluster. You can access master node via the CLI, GUI, or API. The master watches over the nodes in the cluster and is responsible for the actual orchestration of containers on the worker nodes For achieving fault tolerance. There can be more than one master node in a cluster. It is the access point from which administrators and other users interact with a cluster to manage the scheduling and deployment of containers.

There are four main components of Master Node or Control Plane in Kubernetes.

• kube-apiserver
• etcd
• kube-controller-manager
• kube-scheduler

Kube-API Server Manager

Masters communicate with the rest of the cluster through the kube apiserver , the main access point to the control plane. It validates and executes user’s REST commands. Kube apiserver also makes sure that configurations in etcd match with configurations of containers deployed in the cluster.

ETCD

ETCD is a distributed reliable key value store used by Kubernetes to store all data used to manage the cluster. When you have multiple nodes and multiple masters in your cluster, etcd stores all that information on all the nodes in the cluster in a distributed manner. ETCD is responsible for implementing locks within the cluster to ensure there are no conflicts between the Masters.

Kube-Controller-Manager

The controllers are the brain behind orchestration. They are responsible for noticing and responding when nodes, containers or endpoints goes down. The controllers makes decisions to bring up new containers in such cases. The kube controller manager runs control loops that manage the state of the cluster by checking if the required deployments, replicas, and nodes are running in the cluster.

Kube-Schedular

The scheduler is responsible for distributing work or containers across multiple nodes. It looks for newly created containers and assigns them to Nodes. 

Kubernetes Worker

Kubernetes worker node contains below services.

• Kubelet
• Kube-Proxy
• Container Runtime Engine

Kubelet

Worker nodes have the kubelet agent which interacts with master to provide health information of the worker node. To carry out actions requested by the master on the worker nodes. Kubelet acts as the node agent responsible for managing the lifecycle of every worker node.

Kube-Proxy

Kube Proxy is responsible for ensuring network traffic is routed properly to internal and external services as required and is based on the rules defined by network policies in kube controller manager and other custom controllers. It manages the network rules on each node. It also performs connection forwarding or load balancing for Kubernetes cluster services.

Container Runtime Engine

A container runtime is a CRI (Container Runtime Interface) compatible application that executes and manages containers.

Conclusion

Hence you got better understanding of Kubernetes. In our upcoming tutorials, we will install and setup kubernetes clusters on different platforms. Stay tuned for other DevOps tutorials.

Read Also : An Advanced Overview of Docker