Kubernetes

Kubernetes

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Introduction:

• Kubernetes is Container Orchestration Software/Engine.
• Responsibilities include Container Deployment,
•                                         Scaling & descaling of container
•                                         Container load balancing.
• Kubernetes is not a replacement for docker.
• Open Source Software.
• Kubernetes V1.0 was released on July-2015.
• Current version of Kubernetes is V1.24.

Features:

• Automated Scheduling 
• Self-Healing Capabilities
• Automated rollout and rollback
• Horizantal and Load balancing
• Service Discovery & Load balancing
• Storage Orchestration.

Kubernetes Architecture:

Cluster ---  It's a group of a machine   [ Master machine + worker node ]

API Server:-    

• kubectl Api server interact with APIs, its a frentend of the kuberntes control plane. 
• like ......If i want to deploy, remove & update.   

ETCD:-          

• It's key value data store (Database).
• Kubernetes will maintain all the cluster information like....
•                                                                                             Node information,
  
                                                                                              Pods information, 
                                                                                              Services information, 
                                                                                              Volume information 
• It persists the information ETCD (Save the information in ETCD).           

Scheduler:     

• Scheduler will schedule un-Scheduled PODS in nodes with help of kubelet.
• Based on the resource availability.

kubelet:         

• kubelet is the primary node agent.
• Runs on each nodes and ensure that containers are running healthy.
• kubelet interact with container runtime to  create the containers.

Controll Manager: 

• In the backend it will manage the pods that are running healthy.
• Some of the Control are,

1. Node Controller 
2. Replication Controller
3. End Point Controller

                                

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NameSpaces:

• NameSpace is like a virtual cluster inside your k8's cluster. 
•  Cluster inside the cluster.
• We have multiple NameSpace inside our K8's cluster.
•  They are logically isolated from each other.
•  Using this Namespace concept we can place our application into that namespace. 
• If we delete ns under the object will also delete. 

1. default       
2. kube-public     [ Related to k8's system objects..!]
3. kube-system

kubectl get namespaces

kubectl get ns

kubectl get all  [ it will display all the Objects in the ns.]

kubectl get pods -n  kube-system

kubectl get pods -n  kube-public

kubectl get pods -n  public

kubectl get all -n  kube-system

kubectl create namespaces <NS_Name>

kubectl create ns <NS_Name>

kubectl create namespaces <Flipkartapp_ns>

Kubernetes Objects / Workloads :

• Pod
• Service
•  

 

POD:

• A pod always runs on a NODE.
• A pod is the smallest building block or basic unit of scheduling in k8's.
• In a k8 cluster, a pod represents a running process.
• Inside a pod, you can have one or more containers.
• Those containers all share Unique Network IP,
•                                            Storage,
•                                            Network & any other specification applied to the pod.
• Each pod has its unique IP Address within the cluster.

How to create Objects/ workloads:

1. Interactive way      [ Through Commands ]
2. Declarative way.    [ yml file ]

javawebapp.yml

apiversion: v1

kind: Pod

metadata: 

     name:

     labels:

       <Key>: <Value>

     namespaces:  <Enter_ns>

spec:

    containers:

    -   name: <Container_Name>

         image: < Image_Name>

         ports:

         -  containerport:  <Contanerport_number>

NOTE:  The Scheduler will try to identify the node wherever we have enough resources.

kubectl get pods

kubectl apply -f <yml_fn> 

kubectl apply -f javawebapp.yml

kubectl get events

kubectl describe pod <Pod_name>

kubectl get pods -o wide

Service:

• Service makes pod accessible / discoverable within the network (Cluster) 0r Outside the Network (Internet).
• When we create a Service we will get one Virtual IP  address (Cluster IP).
• This IP will be registered in Kubernetes DNS with its name (Service).
• So Other applications can communicate using the Service name.
• Cluster IP Service                          [One app can communicate other apps within the cluster] 
• Node Port Service                         [ If you want to communicate outside of the application]
• Load Balance Service
• 
  

kubectl get pods --show-labels

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Kubernetes will create PODs, PODs will have one or more containers.

How Containers can communicate one container to another container within the same PODs? 

[ Interview Question ]

 

Can we have two containers within POD  with same PortNumber? [ Interview Question ]

• Since they have used the same network.
• Within the pod container port number should not be the same.

POD LifeCycle :-

Types of  POD :-

• One-Container-Per-POD
• Multi-Container-POD     /  SideCar Container

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