Kubernetes Pods & Namespaces

Creating a Pod

apiVersion: v1
kind: Pod
metadata:
  name: hazelcast
  labels:
    app: hazelcast
    env: prod
spec:
  containers:
  - env:
    - name: DNS_DOMAIN
      value: cluster
    image: hazelcast/hazelcast
    name: hazelcast
    ports:
    - containerPort: 5701
  restartPolicy: Never

kubectl create -f pod.yaml

# list pods
kubectl get pods

# get pod 
kubectl get pods hazelcast

# rendering pod details
kubectl describe pods hazelcast

# accessing logs of a pod
kubectl logs hazelcast

# executin command in a container
kubectl exec -it hazelcast -- /bin/sh
kubectl exec hazelcast -- env

Defining a Command for a Pod

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - args:
    - /bin/sh
    - -c
    - while true; do date; sleep 10; done
    image: busybox
    name: mypod
  restartPolicy: Never

Same result with:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
  - command: ["/bin/sh"]
    args: ["-c", "while true; do date; sleep 10; done"]
    image: busybox
    name: mypod
  restartPolicy: Never

Understanding Namespaces

Namespaces are an API construct to avoid naming collisions and represent a scope for object names. A good use case for namespaces is to isolate the objects by team or responsibility. Most questions in the CKAD exam will ask you to execute the command in a specific namespace which has been set up for you. The following sections briefly touch on the basic operations needed to deal with a namespace.

The default namespace hosts object that haven’t been assigned to an explicit namespace. Namespaces starting with the prefix kube- are not considered end user-namespaces. They have been created by the Kubernetes system. You will not have to interact with them as an application developer.

Listing namespaces:

kubectl get namespaces

Creating and Using a Namespace

To create a new namespace, use the create namespace command. The following command uses the name code-red:

kubectl create namespace code-red
kubectl get namespace code-red

The corresponding representation as a YAML manifest would look as follows:

apiVersion: v1
kind: Namespace
metadata:
  name: code-red

Once the namespace is in place, you can create objects within it. You can do so with the command line option --namespace or its short-form -n. The following commands create a new Pod in the namespace code-red and then lists the available Pods in the namespace

kubectl run pod --image=nginx --restart=Never -n code-red
kubectl get pods -n code-red

Deleting a Namespace

Deleting a namespace has a cascading effect on the object existing in it. Deleting a namespace will automatically delete its objects:

kubectl delete namespace code-red
kubectl get pods -n code-red