Kubernetes Deep Dive into its components.

To understand the different components of the Kubernetes cluster we need to see what components are current running on the system. I have a minikube instance running and I can ssh into the instance to see all the processes that are running. For this I can run the following command.

    docker@minikube:~$ minikube ssh
    docker@minikube:~$ pgrep -l kube
    docker@minikube:~$ pgrep -l etcd

components

Control Plane (Brain of the cluster)

As you can see that there are 6 main components. The kube-controller, kube-scheduler, kube-apiserver are part of the Control Plane which is the BRAIN of the Cluster.

kube-controller

Kube-Controller maintains the desired state of your cluster. It runs in a loop and follows the 3 steps.

Observe
Checks the current state by calling the api server.
Compares Compares it wil the desired state
Acts on the difference
It makes the current state back inline with the desired state.

kube-apiserver

Front End API Exposes different endpoints which are accessed by various components. It also does validation, authentication of the requests and schema matching.

kube-scheduler

Assigns the POD to the node It check for any new request by calling the kube-apiserver using the watch=true to get the new updates from the last time it called the endpoint. It then assigns the Node to the Pod. kubectl will then create the pod.

yash@YashDevBox:~$ TOKEN=$(kubectl create token default)
yash@YashDevBox:~$ echo $TOKEN
yash@YashDevBox:~$ APISERVER = $(kubectl config view --minify -o jsonpath='.clusters[0].cluster.server}')
yash@YashDevBox:~$ curl -N -s -K -H "Authorization: Bearer $TOKEN" "$APISERVER/api/v1/pods?watch=true"

etcd (Distributed Database of the Cluster)

Along with this another component thats very critical is the etcd which is the Key-Value Pair database of the Cluster. This contains all the resource informations like the pods, nodes, configMaps, Secrets, services, CRD Custom Resource Definition.

    yash@YashDevBox ~> kubectl get pods -n kube-system

    yash@YashDevBox ~> kubectl describe pod etcd-minikube -n kube-system

    yash@YashDevBox ~> kubectl exec -it etcd-minikube -n kube-system -- \
                       etcdctl --endpoints=https://127.0.0.1:2379 \
                       --cacert=/var/lib/minikube/certs/etcd/ca.crt \
                       --cert=/var/lib/minikube/certs/etcd/server.crt \
                       --key=/var/lib/minikube/certs/etcd/server.key \
                       get / --prefix --keys-only

You can view all the database keys/resources with the above commands.

etcd

kubectl (Systemd service)

kubectl is the systemd service and this is the first components that starts when the cluster is started. It looks at /etc/kubernetes/manifest folder and reads the yaml files and creates static pods for each of the component mentioned above.

    docker@minikube:~$ sudo ls -laht /etc/kubernetes/manifests

If we add yaml files to manifests folder, the kubectl will create the resource for us bypassing the api-server. We can see the kube-controller yaml file and see that is part of the control-plane and that its a pod specifically static pods.

Kubernetes_controller

To recap, the below image makes all the concepts clear.

Control_Plane