Kubernetes Pods

What is a Pod?

A Pod is the smallest execution unit of an application and means something like a virtual host grouped as the basic management unit in Kubernetes.
In other words, it is the name Kubernetes uses for a container or a group of containers. You can think of it simply as a container.

Because a Pod shares a virtual network interface, the same IP, and filesystem, it acts like a virtual host.
It contains information such as one or more application containers, storage, and networking.

Characteristics of Pods

Each Pod is assigned a unique private IP.
Containers inside a Pod share the Pod IP as localhost.
A Pod can practically be considered a virtual machine that contains Pods.

Controlling Pods

Let’s look at how to create these Pods and check their information.

Starting a Pod: kubectl run

First, let’s see how to start a Pod. Running the following command starts a Pod.

kubectl run –-image [image name]:[tag name] (options) [Pod name]

Start nginx as a web server with the following command.

kubectl run webserver --image=nginx:1.14 --port 80

Execution result:

% kubectl run webserver --image=nginx:1.14 --port 80
pod/webserver created

As with docker run, various options can also be specified for kubectl run. For example, you can set environment variables with the -e option. Refer to the following page for other options.

Command line tool (kubectl) | Official documentation

Listing Pods: kubectl get

Run the following command to see the Pod list.

kubectl get pods

Execution result:

% kubectl get pods
NAME        READY   STATUS              RESTARTS   AGE
webserver   0/1     ContainerCreating   0          4s

If you run it immediately after creation, you will see that the state is ContainerCreating.

% kubectl get pods
NAME        READY   STATUS    RESTARTS   AGE
webserver   1/1     Running   0          16s

After a little more time, the state becomes Running.

To see the Pod list in more detail, run the following command.

kubectl get pods -o wide

Execution result:

% kubectl get pods -o wide
NAME        READY   STATUS              RESTARTS   AGE   IP       NODE       NOMINATED NODE   READINESS GATES
webserver   0/1     Running             0          9s    <none>   minikube   <none>           <none>

Viewing Pod details: kubectl describe

To see Pod metadata, run the following command.

kubectl describe pod [Pod name]

Execution result:

% kubectl describe pod webserver
Name:             webserver
Namespace:        default
Priority:         0
Service Account:  default
Node:             minikube/192.168.49.2
Start Time:       Fri, 09 Sep 2022 08:36:18 +0900
Labels:           run=webserver
Annotations:      <none>
Status:           Running
IP:               172.17.0.3
IPs:
  IP:  172.17.0.3
Containers:
  webserver:
    Container ID:   docker://dddee2b98f8badd1a09208d99e02ba881f79091a9670e42862a7aa5f45ec4200
    Image:          nginx:1.14
    Image ID:       docker-pullable://nginx@sha256:f7988fb6c02e0ce69257d9bd9cf37ae20a60f1df7563c3a2a6abe24160306b8d
    Port:           80/TCP
    Host Port:      0/TCP
    State:          Running
      Started:      Fri, 09 Sep 2022 08:36:30 +0900
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-xmw8p (ro)
Conditions:
  Type              Status
  Initialized       True
  Ready             True
  ContainersReady   True
  PodScheduled      True
Volumes:
  kube-api-access-xmw8p:
    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  22s   default-scheduler  Successfully assigned default/webserver to minikube
  Normal  Pulling    22s   kubelet            Pulling image "nginx:1.14"
  Normal  Pulled     11s   kubelet            Successfully pulled image "nginx:1.14" in 11.180480547s
  Normal  Created    11s   kubelet            Created container webserver
  Normal  Started    10s   kubelet            Started container webserver

Checking Pod logs: kubectl logs

If you want to see Pod logs, run the following command.

kubectl logs [Pod name]

Execution result:

% kubectl logs webserver
172.17.0.4 - - [08/Sep/2022:23:38:30 +0000] "GET / HTTP/1.1" 200 612 "-" "curl/7.85.0-DEV" "-"

Checking Pod execution result: curl

Check whether it can be accessed with the curl command.

curl [IP]

Execution result:

$ curl 172.17.0.3
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
    body {
        width: 35em;
        margin: 0 auto;
        font-family: Tahoma, Verdana, Arial, sans-serif;
    }
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>

<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>

<p><em>Thank you for using nginx.</em></p>
</body>
</html>

If you are using minikube...

In minikube, attempting to connect from the actual machine does not work. ```bash % curl 172.17.0.3 curl: (7) Failed to connect to 172.17.0.3 port 80 after 4036 ms: No route to host ``` Enter the cluster and run `curl` as follows. ```bash % kubectl run --image curlimages/curl:latest -it --restart Never --rm curl sh If you don't see a command prompt, try pressing enter. / $ curl 172.17.0.3 ```

Entering a Pod: kubectl exec

To enter the created Pod, run the following command.

kubectl exec -it [Pod name] [image name]

Execution result:

Enter the Pod.

% kubectl exec webserver -it -- /bin/bash
root@webserver:/#

Move to the path where nginx’s first page file exists.

root@webserver:/# cd /usr/share/nginx/html/

Check the contents.

root@webserver:/usr/share/nginx/html# cat index.html
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
    body {
        width: 35em;
        margin: 0 auto;
        font-family: Tahoma, Verdana, Arial, sans-serif;
    }
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>

<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>

<p><em>Thank you for using nginx.</em></p>
</body>
</html>

Modify the contents.

root@webserver:/usr/share/nginx/html# echo "Hello, devkuma" > index.html

Access the server to check whether the modified contents were applied.

$ curl 172.17.0.3
Hello, devkuma

Port forwarding from a Pod: kubectl port-forward

kubectl port-forward 8080:80

Create a YAML file and start a Pod

Here, create YAML for starting a Pod and then start the Pod using that YAML.

Create the command
First, use the --dry-run flag to create a command that is not actually executed.
```
kubectl run webserver --image=nginx:1.14 --port 80 --dry-run=client
```
With the --dry-run flag, the object to be sent to the cluster can be previewed without actually executing it.

Execution result:
```
% kubectl run webserver --image=nginx:1.14 --port 80 --dry-run=client
pod/webserver created (dry run)
```

Check the YAML configuration

kubectl run webserver --image=nginx:1.14 --port 80 --dry-run=client -o yaml

Execution result:

% kubectl run webserver --image=nginx:1.14 --port 80 --dry-run=client -o yaml
apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    run: webserver
  name: webserver
spec:
  containers:
  - image: nginx:1.14
    name: webserver
    ports:
    - containerPort: 80
    resources: {}
  dnsPolicy: ClusterFirst
  restartPolicy: Always
status: {}

Create the YAML file
Create YAML with this file content under the file name webserver-pod.yaml.

% kubectl run webserver --image=nginx:1.14 --port 80 --dry-run=client -o yaml > webserver-pod.yaml

Delete the existing Pod Delete the existing webserver Pod with the same name.
```
% kubectl delete pod webserver
pod "webserver" deleted
```

Create a new Pod from YAML Create a new Pod with the YAML file.

kubectl create -f webserver-pod.yaml

Check whether it was created correctly.

% kubectl get pods
NAME        READY   STATUS    RESTARTS   AGE
webserver   1/1     Running   0          12s

Delete a Pod: kubectl delete

To delete a created Pod, run the following command.

kubectl delete pod [Pod name]

Execution result:

% kubectl delete pod webserver
pod "webserver" deleted

Deploy Pods

kubectl create deployment mainui --image=httpd: --replicas=3

kubectl get deployment.apps

kubectl describe deployment.apps mainui

kubectl get pods

kubectl get pods -o wide

kubectl edit deployment.apps maini