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Sina Tavakkol
Sina Tavakkol

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40 Days Of Kubernetes (13/40)

Day 13/40

Static Pods, Manual Scheduling, Labels, and Selectors in Kubernetes

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In this part, node selector, label and selector, static pod and manual scheduling will be covered.

There's one component named scheduler decides which workload run on which node.

root@localhost:~# kubectl get po -n kube-system | grep scheduler
kube-scheduler-lucky-luke-control-plane            1/1     Running             1 (6d10h ago)   7d
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In Kubernetes, a static pod is a concept wherein you can deploy a pod that is not managed by the API-server.

Static pods are directly managed by the Kubelet component. The Kubelet service is deployed with the configuration path where we can add the pod manifest for the Kubelet to deploy.source, read more

Note we provisioned our cluster with kind and it's actually kubernetes in docker which means every node is a container so we get help with docker exec command.

root@localhost:~# docker ps
CONTAINER ID   IMAGE                  COMMAND                  CREATED        STATUS      PORTS                                                                                          NAMES
c0266722d131   kindest/node:v1.30.0   "/usr/local/bin/entr…"   7 days ago     Up 6 days                                                                                                  lucky-luke-worker
f791fa85c269   kindest/node:v1.30.0   "/usr/local/bin/entr…"   7 days ago     Up 6 days   0.0.0.0:30001->30001/tcp, 127.0.0.1:39283->6443/tcp                                            lucky-luke-control-plane
9c2d43b4f977   kindest/node:v1.30.0   "/usr/local/bin/entr…"   7 days ago     Up 6 days                                                                                                  lucky-luke-worker2
c9d85c72c573   weejewel/wg-easy       "docker-entrypoint.s…"   5 months ago   Up 6 days   0.0.0.0:51820->51820/udp, :::51820->51820/udp, 0.0.0.0:51821->51821/tcp, :::51821->51821/tcp   wg-easy
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1. Static pods and manual scheduling:

  • Run bash on control-plane node
root@localhost:~# docker exec -it lucky-luke-control-plane bash
root@lucky-luke-control-plane:/# pwd
/
root@lucky-luke-control-plane:/# ps ef | grep kubelet
  94749 pts/1    S+     0:00  \_ grep kubelet HOSTNAME=lucky-luke-control-plane PWD=/ container=docker HOME=/root TERM=xterm NO_PROXY= SHLVL=1 HTTPS_PROXY= HTTP_PROXY= KUBECONFIG=/etc/kubernetes/admin.conf PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin _=/usr/bin/grep

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  • Directory of manifests kubelet is monitoring this directory and as soon as you remove something in the directory, the container will be removed from cluster
root@lucky-luke-control-plane:/# cd /etc/kubernetes/manifests/
root@lucky-luke-control-plane:/etc/kubernetes/manifests# ls -lh
total 16K
-rw------- 1 root root 2.4K Jul  1 16:16 etcd.yaml
-rw------- 1 root root 3.9K Jul  1 16:16 kube-apiserver.yaml
-rw------- 1 root root 3.4K Jul  1 16:16 kube-controller-manager.yaml
-rw------- 1 root root 1.5K Jul  1 16:16 kube-scheduler.yaml

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  • Manual scheduling The scheduler looks for nodeName if it's not specified in the manifest of a workload or yaml file to schedule and provision on a node. And if it's specified, it's not its responsible to scheduling it.
root@lucky-luke-control-plane:~# kubectl run nginx --image=nginx -o yaml > nginx-pod.yaml

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Define nodeName

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: "2024-07-08T16:55:26Z"
  labels:
    run: nginx
  name: nginx
  namespace: default
  resourceVersion: "915219"
  uid: 17ebe9ec-78db-4886-95ba-3882dd141e5f
spec:
  containers:
  - image: nginx
    imagePullPolicy: Always
    name: nginx
  nodeName: lucky-luke-worker


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First, we need to delete the pod which is created

root@localhost:~# kubectl get pods
NAME             READY   STATUS    RESTARTS   AGE
nginx            1/1     Running   0          9m45s
nginx-ds-946m4   1/1     Running   0          4d
nginx-ds-rslrm   1/1     Running   0          4d
root@localhost:~# kubectl delete pods/nginx
pod "nginx" deleted
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Then runing the pod:

root@lucky-luke-control-plane:~# kubectl apply -f nginx-pod.yaml
pod/nginx created
root@lucky-luke-control-plane:~# kubectl get pod -o wide
NAME             READY   STATUS    RESTARTS   AGE    IP            NODE                 NOMINATED NODE   READINESS GATES
nginx            1/1     Running   0          70s    10.244.1.13   lucky-luke-worker    <none>           <none>
nginx-ds-946m4   1/1     Running   0          4d     10.244.2.10   lucky-luke-worker2   <none>           <none>
nginx-ds-rslrm   1/1     Running   0          4d1h   10.244.1.12   lucky-luke-worker    <none>           <none>

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2. Labels and Selectors

The labels are metadata which help to filtering the resources. We also have labels in spec for pods and selector section that is matching the labels of pods with deployments.

root@lucky-luke-control-plane:~# kubectl get pods --show-labels
NAME             READY   STATUS    RESTARTS   AGE    LABELS
nginx            1/1     Running   0          11m    run=nginx
nginx-ds-946m4   1/1     Running   0          4d1h   controller-revision-hash=76c9ffb96,env=demo,pod-template-generation=1
nginx-ds-rslrm   1/1     Running   0          4d1h   controller-revision-hash=76c9ffb96,env=demo,pod-template-generation=1
root@lucky-luke-control-plane:~# kubectl get pods --selector run=nginx
NAME    READY   STATUS    RESTARTS   AGE
nginx   1/1     Running   0          11m
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Also, we can see an additional metadata annotations in a workload, which is similar to labels and storing additional details and information related to that object.

root@localhost:~# kubectl edit pod nginx
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# Please edit the object below. Lines beginning with a '#' will be ignored,
# and an empty file will abort the edit. If an error occurs while saving this file will be
# reopened with the relevant failures.
#
apiVersion: v1
kind: Pod
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"v1","kind":"Pod","metadata":{"annotations":{},"creationTimestamp":"2024-07-08T16:55:26Z","labels":{"run":"nginx"},"name":"nginx","namespace":"default","resourceVersion":"915219","uid":"17ebe9ec-78db-4886-95ba-3882dd141e5f"},"spec":{"containers":[{"image":"nginx","imagePullPolicy":"Always","name":"nginx"}],"nodeName":"lucky-luke-worker"}}
  creationTimestamp: "2024-07-08T17:09:50Z"
  labels:
    run: nginx
...
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