Bite-Sized Kubernetes Part 2 - Pods

#kubernetes #devops #containers

Arr!☸️

In Part 1 we were introduced to the basics of kubernetes and took a brief look at the k8s "objects".
Onward now with our expedition with Pods, the smallest object in k8s.

OBJECT	DESCRIPTION
👉 Pods	The lowest level, single instance of an application
ReplicaSets	The next level, manages a set of pods
Deployments	The next level, manages a set of ReplicaSets
Services	The next level, manages a set of deployments
Ingress	The next level, manages a set of services
Volumes	The next level, manages a set of ingress
Namespaces	The next level, manages a set of volumes
Cluster	The highest level, manages a set of namespaces

Pods:

K8s does not deploy the containers directly on the worker nodes.
The containers are encapsulated into a kubernetes object, known as 'pod'.
A pod is a single instance of an application.
Usually a pod contains a single container, but it can contain multiple containers, and these containers are not the same containers.
For example, a pod can contain a container running the application, and another container running a sidecar process or helper application.
Containers inside the pod share the same network namespace, and can communicate with each other using localhost, and can share the same storage space.

Commands:

  # deploy a container by creating a pod
  $ kubectl run <pod name> --image=<image name>

  # list all pods
  $ kubectl get pods

  # list detailed info about all pods
  $ kubectl get pods -o wide

  # describe a pod
  $ kubectl describe pod <pod name>  

  # delete a pod
  $ kubectl delete pod <pod name>

Creating pods:
- The .yml file must contain 4 top-level fields/properties i.e. apiVersion, kind, metadata and spec.
- apiVersion: the version of the k8s API that is being used e.g. v1, apps/v1
- kind: the type of object that is being created e.g. Pod, Service, Deployment, ReplicaSet
- metadata: data that helps to uniquely identify the object e.g. name, labels
  - name can only have a string value
  - labels can have any kind of key-value pairs
- spec: the specification of the object
  - containers: the list of containers that will be launched in the pod
  - name: the name of the container
  - image: the image that will be used to launch the container
- Example pod-definition.yml:
```
apiVersion: v1
kind: Pod
metadata:
  name: my-pod
  labels:
    app: my-app
    type: my-type
spec:
  containers:
  - name: my-container
    image: my-image-name:my-image-tag
```
- Once the .yml file is created, it can be used to create a pod using the kubectl create command:
```
$ kubectl create -f <file name>.yml
```
- View detailed info about the pod:
```
$ kubectl describe pod <pod name>
```
- Delete the pod:
```
$ kubectl delete pod <pod name>
```
- Update the pod with an updated .yml file e.g. with a new image, etc.:
```
$ kubectl apply -f <file name>.yml
```

Great job sailor!
You now know what pods are, hopefully you found this post an interesting and useful read.
Onwards now to "Controllers and ReplicaSets" in Part 3 which will be out soon!

Haul Wind!!!⛵