Task 1 - Creating and Managing Pods

Objective: Learn Pod, container and init container

What is Pod

In Kubernetes, a Pod is the smallest and most basic deployable unit. It represents a single instance of a running process in your cluster. Pods are the building blocks of Kubernetes applications and encapsulate one or more containers, storage resources, a unique network IP, and options that govern how the containers should run. More info about Pods:

The smallest, most basic unit in Kubernetes.
Represents a single instance of a running process in your cluster.
Can contain one or more containers that are tightly coupled and share resources, such as storage and networking.
Often represents a single microservice, but can also encapsulate multiple closely related containers.
Pods are ephemeral by nature, meaning they can be created, destroyed, and replaced dynamically based on the cluster’s needs.
Managed by Kubernetes controllers like Deployments, ReplicaSets, or StatefulSets to ensure desired state and high availability.(We will learn about these in next chapters)
Provides a unique IP address within the cluster, allowing communication between Pods.
Can have one or more volumes attached to it for persistent storage.
Logs and metrics of individual containers within a Pod can be accessed using kubectl.

Pods in a Kubernetes cluster are used in two main ways:

Pods that run a single container: The “one-container-per-Pod” model is the most common Kubernetes use case; in this case, you can think of a Pod as a wrapper around a single container; Kubernetes manages Pods rather than managing the containers directly.

Pods that run multiple containers: that need to work together. A Pod can encapsulate an application composed of multiple co-located containers that are tightly coupled and need to share resources. These co-located containers form a single cohesive unit.

Copy the below manifest file to create a Pod.

cat << EOF | kubectl apply -f -
apiVersion: v1
kind: Pod
metadata:
  name: nginx
spec:
  containers:
  - name: nginx
    image: nginx:latest
    ports:
    - containerPort: 80
EOF

Init container

Init containers in Kubernetes are specialized containers that run before the main containers in a Pod, serving to prepare the environment or perform initialization tasks. They execute sequentially, ensuring that each init container completes successfully before the next one starts and before the main containers begin execution. Init containers share the same volume mounts as the main containers, facilitating the sharing of resources such as configuration files or secrets. They are transient in nature, running to completion once and then terminating, and are not restarted if they fail.

To create Pod with init containers:

cat << EOF |  kubectl apply -f -
apiVersion: v1
kind: Pod
metadata:
  name: nginx-pod-manual
spec:
  volumes:
  - name: shared-data
    emptyDir: {}
  containers:
  - name: nginx
    image: nginx:latest
    ports:
    - containerPort: 80
    volumeMounts:
    - name: shared-data
      mountPath: /usr/share/nginx/html
  initContainers:
  - name: init-nginx
    image: busybox
    command: ['sh', '-c', 'echo "Hello, NGINX at $(date)!" >> /usr/share/nginx/html/index.html']
    volumeMounts:
    - name: shared-data
      mountPath: /usr/share/nginx/html
EOF

To check Pods, run kubectl get pods
To check more detail of Pod, use kubectl describe pod

check the output difference between kubectl describe pod nginx and kubectl describe pod nginx-pod-manual.

To access nginx Pod from contaier itself, use

kubectl exec -it po/nginx-pod-manual -- curl http://127.0.0.1

expected outcome

Hello, NGINX at Tue Feb 27 01:18:50 AM UTC 2024!

Review Questions

What is the purpose of spec.containers.volumeMounts in a Pod definition? Click for Answer…
Specify where volumes should be mounted within containers.
Which lines in the Pod nginx-pod-manual define a volume?
Click for Answer…
```
volumeMounts:
- name: shared-data
  mountPath: /usr/share/nginx/html
```

Why can the init container access the shared-data folder?

Click for Answer…

Both the init container and the main nginx container mount this same volume

Can you restart a Pod created with kubectl create pod or a YAML file specifying kind: Pod?
Click for Answer…
```
You cannot directly restart a Pod created with kind: Pod
```
If you delete the Pod nginx-pod-manual and recreate it using the same YAML, will the previous data in the shared-data folder persist?"
Click for Answer…
```
No, The volume is defined as an emptyDir, emptyDir volumes are ephemeral and tied to the Pod's lifecycle.
```

Clean up

kubectl delete pod nginx-pod-manual