Understanding kubernetes Architecture

How Kubernetes Works | Internal of Kubernetes

#Kubernetes Overview: 

• Kubernetes is a container management Platform
• Created by Google
• Written in Go/GoLang
• Also known as K8s

#Master Node

Master is the control-plane or the brain of k8s cluster. A Master comprises of few components:

• api-server - Exposes REST API to talk to k8s cluster, consumes json, only api-server talks to Cluster Store.
• Cluster Store (KV) - Cluster state and config management.
• Scheduler - Watches api-server for new pods  and assign node to work
• Controller -  A daemon that watches the state of the cluster to maintain desired state. Example are replication-controller, namespace-controller etc. Other than this it performs garbage collection of pods, nodes, events etc.

#Node

• Kubelet - k8s agent which register nodes with cluster, watches api-server, instantiate pods, report back to the api-server. If pod fails, it reports to master and master decides what to do. Exposes port 10255 on node
• Container Engine -  It does container management like pulling images, starting/stopping containers. Usually Docker is used for container runtime.
• kube-proxy - Responsible for networking, Provide unique IP to Pods, All container in a pod share same IP, Load balances across all pods in a service 

#Pods

• An environment to run containers
• It have network stack, kernel namespaces and one or more container running
• Container always runs inside a pod
• Pod can have multiple containers
• It is unit of scaling in k8s

#Services

Pods comes and go with different IPs. To distribute load and act as a single source of interaction to all pods of an application, service play the role.

• Has single IP and DNS
• Created with a manifest JSON file
• All new pods gets added/registered to the service
• Which pod should be assigned to which services is decided by labels
• service and pods have labels on the basis of which service identifies its pods
• only sends traffic to healthy pods
• service can point things outside the cluster
• uses tcp by default (udp is also supported)

#Deployments

It is a k8s object whose task is to manage identical pods running and upgrading them in controlled way.

• Deployed using YAML/JSON manifest
• Deployed via api-server
• Provide update of pods
• Provide rollbacks 

#Detailed Architecture

#Overall Flow

• kubectl writes to the API Server
• API Server validates the request and persists it to Cluster store(etcd)
• Cluster store (etcd) notifies back the API Server
• API Server invokes the Scheduler
• Scheduler decides where to run the pod on and return that to the API Server
• API Server persists it to etcd
• etcd notifies back the API Server. 
• API Server invokes the Kubelet in the corresponding node
• Kubelet talks to the Docker daemon using the API over the Docker socket to create the container
• Kubelet updates the pod status to the API Server
• API Server persists the new state in etcd

#Kubernetes Course

• Introduction To Kubernetes
• Setup Kubernetes locally for development environment
• Setup Kubernetes on AWS using KOPS
• Setup Kubernetes on Google cloud
• Setup Kubernetes cluster using kubeadm | production ready
• How to Create cron jobs in Kubernetes