DevOps Classroom Series – 08/Sept/2021

Namespaces

• k8s uses namespaces to organize objects in the cluster.
• Namespace can be thought as a virtual cluster.
• By default kubectl command line interacts with default namespace
• If you want to get the objects from all namespaces

kubectl get <object> --all-namespaces

• If you want to get the object from a specific namespace other than default

kubectl get <object> --namespace <name>

K8s objects

• k8s objects are persistent entities in k8s system.
• k8s uses these objects to represent state of the cluster
  • what containerized applications are running
  • Resources available to those applications
• To work with k8s objects whether to create, modify, delete, you will need to use the Kubernetes API
• Kubernetes API is HTTP API that API-Server exposes and lets end users or different parts of k8s cluster communicate with one another
• For us we will be using Kubernetes API through the kubectl command-line
• Object Spec and Status
  • Every k8s object includes two nested object fields
    • object spec
    • object status

Pods

• Pod is the smallest deployable unit in k8s.

• A Pod is group of one or more containers with shared storage and network resources and a specification for how to run containers

• Each container in the Pod runs in its own cgroup, but they share a number of Linux Namespaces

• Applications running in the Pod share same IP address, port space (network namespace) , have the same hostname.

• However, containers running in different Pods are isolated from each other

• Lets try to write our first Pod nginx pod

• To write the Pod we need to create a manifest yaml file

• Any manifest in k8s will have the following sections

apiVersion: 
kind:
metadata:
spec: 

• To understand API version we need to know about API groups and versioning

  • API Groups make it easier to extend k8s API. API group is specified in a REST path and in the apiVersion field of the object
  • There are several api groups in k8s
    • The core group: This is legacy group.
      • if the object belongs to core group and has version v1 apiVersion: v1
    • The named groups
      • if the object belongs to apps group and has version v1 apiVersion: apps/v1
  • K8s will have multiple api versions such v1, v1beta1, v1alpha1

• To write the Pod manifest Refer Here

---
apiVersion: v1
kind: Pod
metadata:
  name: my-first-pod
spec:
  containers:
    - name: nginx-container
      image: nginx:1.21.1

• Create a file called as nginx-pod.yaml
• Execute the following commands

kubectl apply -f nginx-pod.yaml
kubectl get pods
kubectl get pods -o wide
kubectl describe pods my-first-pod
kubectl delete -f nginx-pod.yaml
#or
kubectl delete pods my-first-pod

• Now lets look some common commands
  • Viewing logs: kubectl logs <pod-name>
  • Executing commands inside a container with in a pod kubectl exec <pod-name> -- <command>
  • To login into the container in a terminal kubectl exec -it <pod-name> -- bash
• Lets write one-more manifest for jenkins

---
apiVersion: v1
kind: Pod
metadata:
  name: my-jenkins-pod
spec:
  containers:
    - name: jenkins
      image: jenkins/jenkins:jdk11

• Exercise: Create one pod which runs alpine image