Pods with YAML

Hello, and welcome to this tutorial on creating a Pod using a YAML-based configuration file. In this guide, you will learn how to write and apply YAML files specifically tailored for Kubernetes. Kubernetes leverages YAML files to create various objects like Pods, ReplicaSets, Deployments, and Services. Each definition file adheres to a common structure containing four top-level fields: apiVersion, kind, metadata, and spec.

Understanding the Key Fields

Every Kubernetes YAML definition file must include the following properties:

1. Note

   The four main properties in every file are: apiVersion, kind, metadata, and spec. These are essential for Kubernetes to correctly parse and create objects.

1. apiVersion

This property specifies the version of the Kubernetes API you are using to create your object. For Pods, you typically specify "v1". For other objects like Deployments, you might use "apps/v1" or "extensions/v1beta1".

2. kind

The kind field defines the type of object being created. In this lesson, we will focus on creating a Pod, so you would set kind as "Pod". Other common values include ReplicaSet, Deployment, or Service.

3. metadata

The metadata section contains identifying information about the object, such as its name and labels. It is a dictionary rather than a string. Ensure all keys (like name and labels) have proper and consistent indentation. For example:

apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp

In this snippet:

• name uniquely identifies the Pod.
• labels are key-value pairs used to categorize and manage Pods based on roles such as front-end, back-end, or database.

4. spec

The spec section provides detailed configuration for the object. For a Pod, this involves specifying one or more containers that will run within it. The key containers holds a list of container definitions; each item in the list is a dictionary containing that container's configuration.

Below is a complete example of defining a Pod using YAML:

apiVersion: v1
kind: Pod
metadata:
  name: myapp-pod
  labels:
    app: myapp
    type: front-end
spec:
  containers:
    - name: nginx-container
      image: nginx

In this configuration:

• The containers key defines a list of container specifications.
• The dash before name indicates the first (and only) container in the list.
• The container is named nginx-container and it uses the Docker image nginx.

Deploying Your Pod

After creating your YAML configuration file (e.g., pod-definition.yml), you can deploy the Pod using the following command:

kubectl create -f pod-definition.yml

Kubernetes will then create the Pod as defined in your file.

Checking Pod Status

To verify that your Pod is running, use:

kubectl get pods

The output should resemble:

NAME        READY   STATUS    RESTARTS   AGE
myapp-pod   1/1     Running   0          20s

To retrieve detailed information about the Pod, execute:

kubectl describe pod myapp-pod

This command displays comprehensive details including creation time, labels, container status, and events. An example output might look like:

Name:           myapp-pod
Namespace:      default
Node:           minikube/192.168.99.100
Start Time:     Sat, 03 Mar 2018 14:26:14 +0800
Labels:         app=myapp
                name=myapp-pod
Annotations:    <none>
Status:         Running
IP:             172.17.0.24
Containers:
  nginx:
    Container ID:   docker://830bb56c8c2a86b4b70e9c1488fae1bc38663e491b6c2f5a783e7688b8c9d
    Image:          nginx
    Image ID:       docker-pullable://nginx@sha256:4771d095787c6a605299e110b3ee1e0a2529f5ea261d823e4ffe7a4cab1c5de
    Port:           <none>
    State:          Running
      Started:      Sat, 03 Mar 2018 14:26:21 +0800
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-x95w7 (ro)
Conditions:
  Type              Status
  ------            -------
  Initialized       True
  Ready             True
  PodScheduled      True
Events:
  Type     Reason                  Age                From                Message
  ----     ------                  ----               ----                -------
  Normal   Scheduled               34s                default-scheduler   Successfully assigned myapp-pod to minikube
  Normal   SuccessfulMountVolume   34s                kubelet, minikube   MountVolume.Setup succeeded for volume "default-token-x95w7"
  Normal   Pulling                 27s                kubelet, minikube   pulling image "nginx"
  Normal   Pulled                  27s                kubelet, minikube   Successfully pulled image "nginx"
  Normal   Created                 27s                kubelet, minikube   Created container
  Normal   Started                 27s                kubelet, minikube   Started container

Summary

In summary, when creating a YAML file for Kubernetes, always include the four top-level properties:

• apiVersion: Specifies the API version.
• kind: Defines the type of Kubernetes object.
• metadata: Holds essential information such as name and labels.
• spec: Details the configuration specifics, such as container settings in a Pod.

This lesson demonstrated how to create a Pod using YAML. The same principles apply when defining other Kubernetes objects such as Deployments, Services, and ReplicaSets. For more information, explore the Kubernetes Documentation.