Starting to Learn Kubernetes a Step Behind - 16. Components -

Story

Last Time

In Starting to Learn Kubernetes a Step Behind - 15. Security -, we learned about permissions through RBAC. This time, we will finally learn about the components of Kubernetes.

Components

In Kubernetes, the configuration is as follows.

※ https://kubernetes.io/docs/concepts/architecture/cloud-controller/

We will learn about each component.

Current Status

pi@raspi001:~/tmp $ k get nodes
NAME       STATUS   ROLES    AGE   VERSION
raspi001   Ready    master   42d   v1.14.1
raspi002   Ready    worker   42d   v1.14.1
raspi003   Ready    worker   42d   v1.14.1
pi@raspi001:~/tmp $ k get pods -n kube-system -o=wide
NAME                               READY   STATUS    RESTARTS   AGE   IP             NODE       NOMINATED NODE   READINESS GATES
coredns-fb8b8dccf-mtzvd            1/1     Running   34         37d   10.244.0.26    raspi001   <none>           <none>
coredns-fb8b8dccf-nv6dj            1/1     Running   81         37d   10.244.2.151   raspi003   <none>           <none>
etcd-raspi001                      1/1     Running   31         42d   192.168.3.32   raspi001   <none>           <none>
kube-apiserver-raspi001            1/1     Running   95         42d   192.168.3.32   raspi001   <none>           <none>
kube-controller-manager-raspi001   1/1     Running   89         42d   192.168.3.32   raspi001   <none>           <none>
kube-flannel-ds-arm-4s22p          1/1     Running   73         38d   192.168.3.34   raspi003   <none>           <none>
kube-flannel-ds-arm-7nnbj          1/1     Running   88         38d   192.168.3.33   raspi002   <none>           <none>
kube-flannel-ds-arm-ckwq5          1/1     Running   86         38d   192.168.3.32   raspi001   <none>           <none>
kube-proxy-6fwl5                   1/1     Running   31         42d   192.168.3.32   raspi001   <none>           <none>
kube-proxy-wgjdq                   1/1     Running   28         42d   192.168.3.33   raspi002   <none>           <none>
kube-proxy-zvmqf                   1/1     Running   28         42d   192.168.3.34   raspi003   <none>           <none>
kube-scheduler-raspi001            1/1     Running   87         42d   192.168.3.32   raspi001   <none>           <none>

The following are running on the MasterNode.

etcd-raspi001
kube-apiserver-raspi001
kube-controller-manager-raspi001
kube-scheduler-raspi001

The following are running on all Nodes.

kube-flannel-ds
kube-proxy

coredns is running on one Master and one Worker.

※ We set this up at this time.

etcd

This is a component that exists on the MasterNode. The distributed Key-ValueStore, etcd, stores all the information in the Kubernetes cluster. Therefore, it seems to be recommended to form a cluster so that it does not become a single point of failure. Access to the data here must be via the kube-apiserver. If you want to check directly, it would be good to use etcdctl.

kube-apiserver

This is a component that exists on the MasterNode. This is a component that provides the KubernetesAPI. It is called from kube-scheduler, kube-controller-manager, and kubelet. It only manages resources for etcd and does not start Pods.

kube-scheduler

This is a component that exists on the MasterNode. It detects unassigned Pods from Node information and sends a request to assign a Node to the Pod to the kube-apiserver. It only assigns, it does not start the Pod. When assigning a Node, it takes into account NodeAffinity and Taints.

kubelet

This is a component that operates on each Node. It detects that an unassigned Node has been assigned, and actually starts the Pod.

kube-controller-manager

This is a component that exists on the MasterNode. This is a component that runs various controllers. In Deployment or ControllerReplicaSetController, it monitors the state and looks at the expected number of Pods and the current number of Pods. It requests the kube-apiserver to adjust the excess or deficiency of Pods. After that, it becomes the flow of kube-scheduler and kubelet mentioned earlier.

kube-proxy

This is a component that operates on each Node. It forwards traffic to NodePort or ClusterIP.

kube-dns

This is a DNS server used for name resolution and service discovery within the kubernetes cluster. In my environment, I was using CoreDNS.

Others

CustomResourceDefinition(CRD) and Operator

CRD is a resource that allows you to define your own resources. This extensibility allows various developments to proceed. Since CRD is just a Kubernetes object, you need to create a custom controller called Operator as a set. It seems that it can be easily created with something called the Operator Framework.

Finally

Finally, we have been able to finish reading the contents of the Complete Guide to Kubernetes. Initially, I didn't intend to continue outputting articles to this extent. When I actually ran Kubernetes on a Raspberry Pi, I made various discoveries and got hooked.

However, from around Starting to learn Kubernetes a step behind - 12. Resource Limits -, for various reasons, I ended up using the content of the book almost as it was. (laughs)

From now on, I'm thinking of trying to develop applications using GKE.

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Related tags

Kubernetes