Day 7/40

Pod In Kubernetes Explained | Imperative VS Declarative Way | YAML Tutorial

Video Link
@piyushsachdeva
Git Repository
My Git Repo

There are 2 ways of configuration and deploying a workload such as pod in Kubernetes:

Imperative
Declarative

Imperative configuration

Which means that to describe the configuration of the resource, you execute a command from a terminal’s command prompt. (using kubectl for instance)

Declarative configuration

Which means that you create a file that describes the configuration for the particular resource and then apply the content of the file to the Kubernetes cluster. (using yaml file and json format)

Note
If you need a playground you can Play with Kubernetes

At first you have to create your environment with the instruction as they mention:

This is a sandbox environment. Using personal credentials
 is HIGHLY! discouraged. Any consequences of doing so, are
 completely the user's responsibilites.

 You can bootstrap a cluster as follows:

 1. Initializes cluster master node:

 kubeadm init --apiserver-advertise-address $(hostname -i) --pod-network-cidr 10.5.0.0/16


 2. Initialize cluster networking:

 kubectl apply -f https://raw.githubusercontent.com/cloudnativelabs/kube-router/master/daemonset/kubeadm-kuberouter.yaml


 3. (Optional) Create an nginx deployment:

 kubectl apply -f https://raw.githubusercontent.com/kubernetes/website/master/content/en/examples/application/nginx-app.yaml


                          The PWK team.

[node1 ~]$ kubectl get nodes
NAME    STATUS   ROLES           AGE   VERSION
node1   Ready    control-plane   32s   v1.27.2
[node1 ~]$ kubectl config get-contexts 
CURRENT   NAME                          CLUSTER      AUTHINFO           NAMESPACE
*         kubernetes-admin@kubernetes   kubernetes   kubernetes-admin

1. Create `pod` with imperative way

root@localhost:~# kubectl get nodes
NAME                       STATUS   ROLES           AGE   VERSION
lucky-luke-control-plane   Ready    control-plane   89m   v1.30.0
lucky-luke-worker          Ready    <none>          88m   v1.30.0
lucky-luke-worker2         Ready    <none>          88m   v1.30.0
root@localhost:~# kubectl run nginx-pod --image=nginx:latest
pod/nginx-pod created
root@localhost:~# kubectl get pods
NAME        READY   STATUS              RESTARTS   AGE
nginx-pod   0/1     ContainerCreating   0          8s
root@localhost:~# kubectl get pods
NAME        READY   STATUS    RESTARTS   AGE
nginx-pod   1/1     Running   0          21s
root@localhost:~# kubectl get pods -o wide
NAME        READY   STATUS    RESTARTS   AGE   IP           NODE                 NOMINATED NODE   READINESS GATES
nginx-pod   1/1     Running   0          43s   10.244.2.2   lucky-luke-worker2   <none>           <none>

As you can see, our nginx-pod is running on worker2 node of our cluster, and it's containing one container running out of one.

2. Create `pod` with declarative way

Firstly, we need to create a yaml file with our desired options.
Read more about yaml

root@localhost:~# touch day07-yaml.yaml

The starting content:

# this is a sample file
#
apiVersion:

kind:

metadata:

spec:

for more information about it you can run the below command:

root@localhost:~# kubectl explain pod
KIND:       Pod
VERSION:    v1

DESCRIPTION:
    Pod is a collection of containers that can run on a host. This resource is
    created by clients and scheduled onto hosts.

FIELDS:
  apiVersion    <string>
    APIVersion defines the versioned schema of this representation of an object.
    Servers should convert recognized schemas to the latest internal value, and
    may reject unrecognized values. More info:
    https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#resources

  kind  <string>
    Kind is a string value representing the REST resource this object
    represents. Servers may infer this from the endpoint the client submits
    requests to. Cannot be updated. In CamelCase. More info:
    https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#types-kinds

  metadata      <ObjectMeta>
    Standard object's metadata. More info:
    https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#metadata

  spec  <PodSpec>
    Specification of the desired behavior of the pod. More info:
    https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#spec-and-status

  status        <PodStatus>
    Most recently observed status of the pod. This data may not be up to date.
    Populated by the system. Read-only. More info:
    https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#spec-and-status

final version of our yaml file:

# this is a sample file
#
apiVersion: v1

kind: Pod

metadata:
  name: nginx-pod-2
  labels:
    env: demo
    type: frontend

spec:
  containers:
    - name: nginx-container
      image: nginx
      ports:
        - containerPort: 80

Create with kubectl

root@localhost:~# kubectl create -f day07-yaml.yaml
pod/nginx-pod-2 created
root@localhost:~# kubectl get pods
NAME          READY   STATUS              RESTARTS   AGE
nginx-pod     1/1     Running             0          28m
nginx-pod-2   0/1     ContainerCreating   0          8s
root@localhost:~# kubectl get pods
NAME          READY   STATUS    RESTARTS   AGE
nginx-pod     1/1     Running   0          28m
nginx-pod-2   1/1     Running   0          16s
root@localhost:~# kubectl get pods -o wide
NAME          READY   STATUS    RESTARTS   AGE   IP           NODE                 NOMINATED NODE   READINESS GATES
nginx-pod     1/1     Running   0          28m   10.244.2.2   lucky-luke-worker2   <none>           <none>
nginx-pod-2   1/1     Running   0          32s   10.244.1.2   lucky-luke-worker    <none>           <none>

As you can see the second pod is running on worker1 node. Also instead of using create with kubectl, we can use apply with kubectl.

3. Update and edit directly an object on the cluster

For instance we need to change the version of nginx which is running with the latest version to 1.18. We run the command and change the version in front of image key and then save it.

# 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:
  creationTimestamp: "2024-06-27T16:48:31Z"
  labels:
    run: nginx-pod
  name: nginx-pod
  namespace: default
  resourceVersion: "9983"
  uid: 6d0bd9ee-9f92-45b4-9416-87e02568a435
spec:
  containers:
  - image: nginx:1.18
    imagePullPolicy: Always
...

root@localhost:~# kubectl edit pod nginx-pod
pod/nginx-pod edited

for confirmation and see what happened, you can run the below command and see the logs and container version that is already is running.

root@localhost:~# kubectl describe pod nginx-pod
Name:             nginx-pod
Namespace:        default
Priority:         0
Service Account:  default
Node:             lucky-luke-worker2/172.19.0.2
Start Time:       Thu, 27 Jun 2024 16:48:31 +0000
Labels:           run=nginx-pod
Annotations:      <none>
Status:           Running
IP:               10.244.2.2
IPs:
  IP:  10.244.2.2
Containers:
  nginx-pod:
    Container ID:   containerd://b9ff29ff39e7446d9ad8ab9a60fe8f29d8c4ed4ceb35c9628bc692d228bd096c
    Image:          nginx:1.18
    Image ID:       docker.io/library/nginx@sha256:e90ac5331fe095cea01b121a3627174b2e33e06e83720e9a934c7b8ccc9c55a0
    Port:           <none>
    Host Port:      <none>
    State:          Running
      Started:      Thu, 27 Jun 2024 17:40:58 +0000
    Last State:     Terminated
      Reason:       Completed
      Exit Code:    0
      Started:      Thu, 27 Jun 2024 16:48:41 +0000
      Finished:     Thu, 27 Jun 2024 17:40:51 +0000
    Ready:          True
    Restart Count:  1
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-j4sf7 (ro)
Conditions:
  Type                        Status
  PodReadyToStartContainers   True
  Initialized                 True
  Ready                       True
  ContainersReady             True
  PodScheduled                True
Volumes:
  kube-api-access-j4sf7:
    Type:                    Projected (a volume that contains injected data from multiple sources)
    TokenExpirationSeconds:  3607
    ConfigMapName:           kube-root-ca.crt
    ConfigMapOptional:       <nil>
    DownwardAPI:             true
QoS Class:                   BestEffort
Node-Selectors:              <none>
Tolerations:                 node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                             node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
  Type    Reason     Age                From               Message
  ----    ------     ----               ----               -------
  Normal  Scheduled  52m                default-scheduler  Successfully assigned default/nginx-pod to lucky-luke-worker2
  Normal  Pulling    52m                kubelet            Pulling image "nginx:latest"
  Normal  Pulled     52m                kubelet            Successfully pulled image "nginx:latest" in 8.319s (8.319s including waiting). Image size: 71010466 bytes.
  Normal  Killing    17s                kubelet            Container nginx-pod definition changed, will be restarted
  Normal  Pulling    17s                kubelet            Pulling image "nginx:1.18"
  Normal  Created    10s (x2 over 52m)  kubelet            Created container nginx-pod
  Normal  Started    10s (x2 over 52m)  kubelet            Started container nginx-pod
  Normal  Pulled     10s                kubelet            Successfully pulled image "nginx:1.18" in 6.882s (6.882s including waiting). Image size: 53630764 bytes.

4. Interaction with the container

root@localhost:~# kubectl exec -it nginx-pod -- sh
# pwd
/
command terminated with exit code 127
root@localhost:~# kubectl get pods
NAME          READY   STATUS    RESTARTS        AGE
nginx-pod     1/1     Running   1 (8m19s ago)   60m
nginx-pod-2   1/1     Running   0

Terminating with Ctrl+D

5. Create `yaml` file of a `pod` in `dry-run` mode

root@localhost:~# kubectl run nginx-pod-3 --image=nginx:latest --dry-run=client -o yaml
apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: nginx-pod3
  name: nginx-pod3
spec:
  containers:
  - image: nginx:latest
    name: nginx-pod3
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Always
status: {}

You can direct the result into a new yaml file and make your own change.

root@localhost:~# kubectl run nginx-pod-3 --image=nginx:latest --dry-run=client -o yaml > nginx-pod-3.yaml

Note in dry-run step nothing is creates and you can use the dry-run step to check your manifests before deployment.

6. Get more details of running pods

root@localhost:~# kubectl get pods -o wide
NAME          READY   STATUS    RESTARTS      AGE   IP           NODE                 NOMINATED NODE   READINESS GATES
nginx-pod     1/1     Running   1 (19m ago)   71m   10.244.2.2   lucky-luke-worker2   <none>           <none>
nginx-pod-2   1/1     Running   0             43m   10.244.1.2   lucky-luke-worker    <none>           <none>
root@localhost:~# kubectl get pods --show-labels
NAME          READY   STATUS    RESTARTS      AGE   LABELS
nginx-pod     1/1     Running   1 (21m ago)   73m   run=nginx-pod
nginx-pod-2   1/1     Running   0             44m   env=demo,type=frontend

Labels are good for grouping our pods.

Task 3

apiVersion: v1
kind: Pod
metadata:
  labels:
    app: test
  name: redis
spec:
  containers:
  - image: rediss
    name: redis

root@localhost:~# kubectl apply -f task3.yaml
pod/redis created
root@localhost:~# kubectl get pods
NAME          READY   STATUS         RESTARTS      AGE
nginx-pod     1/1     Running        1 (36m ago)   89m
nginx-pod-2   1/1     Running        0             60m
redis         0/1     ErrImagePull   0             12s
root@localhost:~# kubectl describe pod redis
Name:             redis
Namespace:        default
Priority:         0
Service Account:  default
Node:             lucky-luke-worker2/172.19.0.2
Start Time:       Thu, 27 Jun 2024 18:17:24 +0000
Labels:           app=test
Annotations:      <none>
Status:           Pending
IP:               10.244.2.3
IPs:
  IP:  10.244.2.3
Containers:
  redis:
    Container ID:
    Image:          rediss
    Image ID:
    Port:           <none>
    Host Port:      <none>
    State:          Waiting
      Reason:       ImagePullBackOff
    Ready:          False
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-qks6m (ro)
Conditions:
  Type                        Status
  PodReadyToStartContainers   True
  Initialized                 True
  Ready                       False
  ContainersReady             False
  PodScheduled                True
Volumes:
  kube-api-access-qks6m:
    Type:                    Projected (a volume that contains injected data from multiple sources)
    TokenExpirationSeconds:  3607
    ConfigMapName:           kube-root-ca.crt
    ConfigMapOptional:       <nil>
    DownwardAPI:             true
QoS Class:                   BestEffort
Node-Selectors:              <none>
Tolerations:                 node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
                             node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
  Type     Reason     Age                From               Message
  ----     ------     ----               ----               -------
  Normal   Scheduled  94s                default-scheduler  Successfully assigned default/redis to lucky-luke-worker2
  Normal   BackOff    17s (x5 over 92s)  kubelet            Back-off pulling image "rediss"
  Warning  Failed     17s (x5 over 92s)  kubelet            Error: ImagePullBackOff
  Normal   Pulling    3s (x4 over 93s)   kubelet            Pulling image "rediss"
  Warning  Failed     2s (x4 over 92s)   kubelet            Failed to pull image "rediss": failed to pull and unpack image "docker.io/library/rediss:latest": failed to resolve reference "docker.io/library/rediss:latest": pull access denied, repository does not exist or may require authorization: server message: insufficient_scope: authorization failed
  Warning  Failed     2s (x4 over 92s)   kubelet            Error: ErrImagePull
root@localhost:~# kubectl edit pod redis
pod/redis edited
root@localhost:~# kubectl get pods
NAME          READY   STATUS             RESTARTS      AGE
nginx-pod     1/1     Running            1 (38m ago)   90m
nginx-pod-2   1/1     Running            0             62m
redis         0/1     ImagePullBackOff   0             2m5s
root@localhost:~# kubectl get pods
NAME          READY   STATUS    RESTARTS      AGE
nginx-pod     1/1     Running   1 (38m ago)   91m
nginx-pod-2   1/1     Running   0             62m
redis         1/1     Running   0             2m10s

As we see the image is not valid and we edit to correct image name and the problem solved!

Blog

40 Days Of Kubernetes (7/40)

Sina Tavakkol

Day 7/40

Pod In Kubernetes Explained | Imperative VS Declarative Way | YAML Tutorial

Imperative configuration

Declarative configuration

1. Create `pod` with imperative way

2. Create `pod` with declarative way

3. Update and edit directly an object on the cluster

4. Interaction with the container

5. Create `yaml` file of a `pod` in `dry-run` mode

6. Get more details of running pods

Task 3

Join Our Newsletter. No Spam, Only the good stuff.

Related

40 Days Of Kubernetes (7/40)

Sina Tavakkol

Day 7/40

Pod In Kubernetes Explained | Imperative VS Declarative Way | YAML Tutorial

Imperative configuration

Declarative configuration

1. Create pod with imperative way

2. Create pod with declarative way

3. Update and edit directly an object on the cluster

4. Interaction with the container

5. Create yaml file of a pod in dry-run mode

6. Get more details of running pods

Task 3

Join Our Newsletter. No Spam, Only the good stuff.

Related

1. Create `pod` with imperative way

2. Create `pod` with declarative way

5. Create `yaml` file of a `pod` in `dry-run` mode