kubernetes Pod,Cluster,Deployment,Replica Set Light Dive

by Anish

Posted on Tuesday July 17, 2018

Introduction

In this section we will learn the core concept of kubernetes like Pod,cluster,Deployment,Replica Set. The idea is to keep it simple and making more intuitive learning

At the beginning we have setup one master node and minion node

1. kube-master
2. kube-minion

kubectl - Main CLI tool for running commands and managing Kubernetes clusters. so what is the cluster

root@kube-master:$ kubectl cluster-info
Kubernetes master is running at https://172.16.9.12:6443
KubeDNS is running at https://172.16.9.12:6443/api/v1/namespaces/kube-system/services/kube-dns:dns/proxy

cluster consists of at least one cluster master and multiple worker machines called nodes

Verify the nodes which has join the cluster

root@kube-master:$ kubectl get nodes 
NAME          STATUS    ROLES     AGE       VERSION
kube-master   Ready     master    18h       v1.11.0
kube-minion   Ready     <none>    18h       v1.11.0

Kubernetes API Resources

1. po: Pod : Kubernetes pod is a group of containers that are deployed together on the same host
2. svc: Service : A Service in Kubernetes is a REST object, similar to a Pod
3. deploy: Deployment : A Deployment controller provides declarative updates for Pods and ReplicaSets
4. rs: Replica Set : A ReplicaSet ensures that a specified number of pod replicas are running at any given time

Example : List all resources in the name space

kubectl get all

To List all supported resource types along with their shortnames, API group, whether they are namespaced, and Kind: run the kubectl api-resources

root@kube-master:$ kubectl api-resources
NAME                              SHORTNAMES   APIGROUP                       NAMESPACED   KIND
bindings                                                                      true         Binding
pods                              po                                          true         Pod
services                          svc                                         true         Service
deployments                       deploy       apps                           true         Deployment
replicasets                       rs           extensions                     true         ReplicaSet
clusterroles                                   rbac.authorization.k8s.io      false        ClusterRole
rolebindings                                   rbac.authorization.k8s.io      true         RoleBinding
.......
.......

Dig lighter to kubernetes flow

Start a single instance of nginx with one replicaset

root@kube-master:$ kubectl run nginx --image=nginx --replicas=1

The output will be deployment.apps/nginx created

Ok, so lets see if we actually have a Kubernetes pods up and running:

root@kube-master:$ kubectl get pods
NAME                     READY     STATUS              RESTARTS   AGE
nginx-64f497f8fd-sqmjk   0/1       ContainerCreating   0          12s

The container is creating on nginx pod , again hitting the same command, this time shows pods are up and running

root@kube-master:$ kubectl get pods 
NAME                     READY     STATUS    RESTARTS   AGE
nginx-64f497f8fd-sqmjk   1/1       Running   0          46s

We can retrieve a lot more information about each of these pods using kubectl describe pod. For example:

root@kube-master:$ kubectl describe pod nginx-64f497f8fd-sqmjk

There will be big YAML output

......
 ......
    Containers:
      nginx:
        Container ID:   docker://a7bc2921ca62187778c5f65da4e139516f2701caf32e325cbeef2a1ee082da0b
        Image:          nginx
        Image ID:       docker-pullable://nginx@sha256:a65beb8c90a08b22a9ff6a219c2f363e16c477b6d610da28fe9cba37c2c3a2ac
        Port:           <none>
        Host Port:      <none>
        State:          Running
          Started:      Mon, 16 Jul 2018 18:48:53 +0530
        Ready:          True
        Restart Count:  0
        Environment:    <none>
        Mounts:
          /var/run/secrets/kubernetes.io/serviceaccount from default-token-8wxrj (ro)

Here you can see configuration information about the container(s) and Pod (labels, resource requirements, etc.), as well as status information about the container(s) and Pod (state, readiness, restart count, events, etc.).

The container state is one of Waiting, Running, or Terminated. here you can see that for a container in Running state, the system tells you when the container started.

Ready tells you whether the container passed its last readiness probe.

Look for Events Look for the event generated to perform this action kubectl run nginx --image=nginx --replicas=1 in the kubernetes master node

Events:
  Type    Reason     Age   From                  Message
  ----    ------     ----  ----                  -------
  Normal  Scheduled  11m   default-scheduler     Successfully assigned default/nginx-64f497f8fd-7w5mn to kube-minion
  Normal  Pulling    10m   kubelet, kube-minion  pulling image "nginx"
  Normal  Pulled     10m   kubelet, kube-minion  Successfully pulled image "nginx"
  Normal  Created    10m   kubelet, kube-minion  Created container
  Normal  Started    10m   kubelet, kube-minion  Started container

In the Minion node docker images are pulled and container created and started , for docker background user can issue docker related command to see how docker & kubernetes are orchestrating the deployment

root@kube-minion:$ docker images
REPOSITORY  TAG IMAGE ID  CREATED SIZE
nginx latest  8b89e48b5f15  2 hours ago 109 MB
nginx <none>  3c5a05123222  10 days ago 109 MB

To list all events you can use kubectl get events

Get the deployment configuration of nginx by the command

root@kube-master:$ kubectl get deployment nginx
NAME      DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
nginx     1         1         1            1           6m

• DESIRED =1
• CURRENT =1

During the initializing state we have told the kubernetes to maintain one replica only by setting the flag --replicas=1

Delete the nginx pod to delete the pod specify the pod name
root@kube-master:$ kubectl delete pod nginx-64f497f8fd-sqmjk

Query on pod still shows one pod is running with diffrent id, this happen because initially we have tell the kuberntes to keep one running replica always Great Kubernetes !!! you have save my production server from accidently destroyed

root@kube-master:$ kubectl get pods 
NAME                     READY     STATUS    RESTARTS   AGE
nginx-64f497f8fd-fg9q7   1/1       Running   0          1m

Scale Up the Pods

To scale up the pods tell to kubectl how many current replica is there (current-replicas) and how many needs to be scaled (replicas)

• DESIRED =3
• CURRENT =1

root@kube-master:$ kubectl scale --current-replicas=1 --replicas=3 deployment/nginx 

In the background two new container will get created and will get deployed

root@kube-master:$ kubectl get pods
NAME                     READY     STATUS              RESTARTS   AGE
nginx-64f497f8fd-brn22   0/1       ContainerCreating   0          17s
nginx-64f497f8fd-fg9q7   1/1       Running             0          2h
nginx-64f497f8fd-z2vbb   0/1       ContainerCreating   0          17s

After creating the containers check the status of these pods

root@kube-master:$ kubectl get pods
NAME                     READY     STATUS    RESTARTS   AGE
nginx-64f497f8fd-brn22   1/1       Running   0          30s
nginx-64f497f8fd-fg9q7   1/1       Running   0          2h
nginx-64f497f8fd-z2vbb   1/1       Running   0          30s

Look at the events

kubectl get events

root@kube-master:$ kubectl scale --current-replicas=3 --replicas=1 deployment/nginx 
deployment.extensions/nginx scaled

root@kube-master:$ kubectl get pods 
NAME                     READY     STATUS        RESTARTS   AGE
nginx-64f497f8fd-fg9q7   1/1       Running       0          2h
nginx-64f497f8fd-fpjk9   0/1       Terminating   0          35s
nginx-64f497f8fd-lk2pw   0/1       Terminating   0          35s

root@kube-master:$ kubectl get pods
NAME                     READY     STATUS    RESTARTS   AGE
nginx-64f497f8fd-fg9q7   1/1       Running   0          2h

root@kube-master:$ kubeadm token create 
I0717 10:32:47.753179   22047 feature_gate.go:230] feature gates: &{map[]}
yy8zho.n3w5inti3twy7v0y

root@kube-master:$ openssl x509 -pubkey -in /etc/kubernetes/pki/ca.crt | openssl rsa -pubin -outform der 2>/dev/null | openssl dgst -sha256 -hex | sed 's/^.* //'

root@kube-minion2:$  kubeadm join 172.16.9.12:6443 --token yy8zho.n3w5inti3twy7v0y --discovery-token-ca-cert-hash sha256:30e3baf5cb4474b23d5d2500836f6b4da19fa629b64339b1301d3e04892e08aa

root@kube-master:$ kubectl get nodes
NAME           STATUS    ROLES     AGE       VERSION
kube-master    Ready     master    18h       v1.11.0
kube-minion    Ready     <none>    18h       v1.11.0
kube-minion2   Ready     <none>    2m        v1.11.0

kubectl scale --current-replicas=1 --replicas=5 deployment/nginx 

kubectl describe pods | grep kube-minion