Kubernetes – Architecture

Pre-requisite: Kubernetes

Kubernetes Cluster mainly consists of Worker Machines called Nodes and a Control Plane. In a cluster, there is at least one worker node. The Kubectl CLI communicates with the Control Plane and Control Plane manages the Worker Nodes.

Now we see the architecture in detail.

Kubernetes is composed of a number of components, each of which plays a specific role in the overall system. These components can be divided into two categories:

• Control plane 
• Worker nodes

Control Plane/ Master Node:

It is basically a collection of various components that help us in managing the overall health of a cluster.  For example, if you want to set up new pods, destroy pods, scale pods, etc. Basically, 4 services run on Control Plane:

1. API Server: The API server is a component of the Kubernetes control plane that exposes the Kubernetes API. It is like an initial gateway to the cluster that listens to updates or queries via CLI like Kubectl. Kubectl communicates with API Server to inform what needs to be done like creating pods or deleting pods etc. It also works as a gatekeeper. It generally validates requests received and then forwards them to other processes. No request can be directly passed to the cluster, it has to be passed through the API Server.
2. Scheduler: When API Server receives a request for Scheduling Pods then the request is passed on to the Scheduler. It intelligently decides on which node to schedule the pod for better efficiency of the cluster.
3. Controller Manager: The kube-controller-manager is responsible for running the controllers that handle the various aspects of the cluster’s control loop.These controllers include the replication controller, which ensures that the desired number of replicas of a given application are running, and the node controller, which ensures that nodes are correctly marked as “ready” or “not ready” based on their current state.
4. etcd: It is a key-value store of a Cluster. The Cluster State Changes get stored in the etcd. It acts as the Cluster brain because it tells the Scheduler and other processes about which resources are available and about cluster state changes.

Worker Node:

These are the nodes where the actual work happens. Each Node can have multiple pods and pods have containers running inside them. There are 3 processes in every Node that are used to Schedule and manage those pods.

1. Container runtime: A container runtime is needed to run the application containers running on pods inside a pod. Example-> Docker
2. kubelet: kubelet interacts with both the container runtime as well as the Node. It is the process responsible for starting a pod with a container inside.
3. kube-proxy: It is the process responsible for forwarding the request from Services to the pods. It has intelligent logic to forward the request to the right pod in the worker node.

Commands

Here are some common commands for interacting with a Kubernetes cluster:

Viewing the cluster state

To view a list of all the pods in the cluster, you can use the following command:

kubectl get pods

To view a list of all the nodes in the cluster, you can use the following command:

kubectl get nodes

To view a list of all the services in the cluster, you can use the following command:

kubectl get services

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