Running a pod in Kubernetes

In this article we will look at pod concept in Kubernetes

What is pod in kubernetes?

The pod is the smallest execution unit in Kubernetes. It’s a single container or group of containers that serve a running process in the K8s cluster. Read what is container? if you are not familiar with containerization.

Each pod has a single IP address that is shared by all the containers within. Also, the port space is shared by all the containers inside.

You can view running pods in K8s by using below command –

$ kubectl get pods
NAME        READY   STATUS    RESTARTS   AGE
webserver   1/1     Running   0          10s

View pod details in K8s

To get more detailed information on each pod, you can run below command by supplying its pod name as argument –

$ kubectl describe pods webserver
Name:         webserver
Namespace:    default
Priority:     0
Node:         node01/172.17.0.9
Start Time:   Sun, 05 Jul 2020 13:50:41 +0000
Labels:       <none>
Annotations:  <none>
Status:       Running
IP:           10.244.1.3
IPs:
  IP:  10.244.1.3
Containers:
  webserver:
    Container ID:   docker://8b260effa4ada1ff80e106fb12cf6e2da90eb955321bbe3b9e302fdd33b6c0d8
    Image:          nginx
    Image ID:       docker-pullable://nginx@sha256:21f32f6c08406306d822a0e6e8b7dc81f53f336570e852e25fbe1e3e3d0d0133
    Port:           80/TCP
    Host Port:      0/TCP
    State:          Running
      Started:      Sun, 05 Jul 2020 13:50:50 +0000
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-bjcwg (ro)
Conditions:
  Type              Status
  Initialized       True
  Ready             True
  ContainersReady   True
  PodScheduled      True
Volumes:
  default-token-bjcwg:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-bjcwg
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type    Reason     Age   From               Message
  ----    ------     ----  ----               -------
  Normal  Scheduled  25s   default-scheduler  Successfully assigned default/webserver to node01
  Normal  Pulling    23s   kubelet, node01    Pulling image "nginx"
  Normal  Pulled     17s   kubelet, node01    Successfully pulled image "nginx"
  Normal  Created    16s   kubelet, node01    Created container webserver
  Normal  Started    16s   kubelet, node01    Started container webserver

pod configuration file

One can create a pod configuration file i.e. yml file which has all the details to start a pod. K8s can read this file and spin up your pod according to specifications. Sample file below –

$ cat my_webserver.yml
echo "apiVersion: v1
kind: Pod
metadata:
  name: webserver
spec:
  containers:
    - name: webserver
      image: nginx
      ports:
        - containerPort: 80" >my_webserver.yml

Its a single container pod file since we specified specs for only one kind of container in it.

Single container pod

Single container pod can be run without using a yml file. Like using simple command –

$ kubectl run single-c-pod --image=nginx
pod/single-c-pod created
$ kubectl get pods
NAME           READY   STATUS    RESTARTS   AGE
single-c-pod   1/1     Running   0          35s
webserver      1/1     Running   0          2m52s

You can spin the single container pod using simple yml file stated above.

Multiple container pod

For multiple container pods, let’s edit the above yml file to add another container specs as well.

$ cat << EOF >web-bash.yml
apiVersion: v1
kind: Pod
metadata:
  name: web-bash
spec:
  containers:
    - name: apache
      image: httpd
      ports:
        - containerPort: 80
    - name: linux
      image: ubuntu
      ports:
      command: ["/bin/bash", "-ec", "while true; do echo '.'; sleep 1 ; done"]
EOF

In the above file, we are spinning up a pod that has 1 webserver container and another is Ubuntu Linux container.

$ kubectl create -f web-bash.yml
pod/web-bash created
$ kubectl get pods
NAME       READY   STATUS    RESTARTS   AGE
web-bash   2/2     Running   0          12s

How to delete pod

Its a simple delete pod command

$ kubectl delete pods web-bash
pod "web-bash" deleted

How to view pod logs in Kubernetes

I am running a single container pod of Nginx. We will then check pod logs to confirm this messages.

$ kubectl run single-c-pod --image=nginx
pod/single-c-pod created
$ kubectl logs single-c-pod
/docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration
/docker-entrypoint.sh: Looking for shell scripts in /docker-entrypoint.d/
/docker-entrypoint.sh: Launching /docker-entrypoint.d/10-listen-on-ipv6-by-default.sh
10-listen-on-ipv6-by-default.sh: Getting the checksum of /etc/nginx/conf.d/default.conf
10-listen-on-ipv6-by-default.sh: Enabled listen on IPv6 in /etc/nginx/conf.d/default.conf
/docker-entrypoint.sh: Launching /docker-entrypoint.d/20-envsubst-on-templates.sh
/docker-entrypoint.sh: Configuration complete; ready for start up