Understand the Kubernetes architecture

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$ als gewone gebruiker# als root (of met sudo)

Know the control plane and the nodes

The control plane decides; the worker nodes run the containers.

Component	Runs on	Job
kube-apiserver	Control plane	The front door: every request goes through its REST API
etcd	Control plane	The database of the whole cluster state
kube-scheduler	Control plane	Chooses a node for every new pod
kube-controller-manager	Control plane	Loops that fix differences (replicas, nodes, endpoints)
cloud-controller-manager	Control plane	Talks to the cloud (load balancers, disks)
kubelet	Every node	Starts the pods the API server assigned to this node
kube-proxy	Every node	Service networking rules (or replaced by the CNI)
Container runtime	Every node	containerd or CRI-O, starts containers
CNI plugin	Every node	Pod networking (Calico, Cilium, Flannel …)

$kubectl get nodes -o wide
$kubectl get pods -n kube-system
$kubectl cluster-info
$ls /etc/kubernetes/manifests/
etcd.yaml  kube-apiserver.yaml  kube-controller-manager.yaml  kube-scheduler.yaml

Onthoud

In a kubeadm cluster the control-plane parts run as static pods from /etc/kubernetes/manifests/. The kubelet starts them; change the file and the kubelet restarts the pod.

1.2

Know the main objects

Everything in Kubernetes is an object with a kind, metadata (name, namespace, labels) and a spec.

Object	Meaning
Pod	One or more containers that share network and storage — the smallest unit
Deployment	Keeps N pods of a stateless app, with rolling updates
StatefulSet	Pods with fixed names and own storage (databases)
DaemonSet	One pod on every node (log agents, CNI)
Job / CronJob	Run to completion / on a schedule
Service	A stable name and IP in front of pods
ConfigMap / Secret	Configuration and sensitive data
PersistentVolumeClaim	A request for storage
Namespace	A folder for objects, for teams and limits

1.3

Talk to the API with kubectl

kubectl sends requests to the API server, using a kubeconfig file (~/.kube/config) with clusters, users and contexts.

$kubectl config get-contexts
$kubectl config use-context lab
$kubectl config set-context --current --namespace=shop
$kubectl api-resources | head
$kubectl explain deployment.spec.strategy
$kubectl get pods -A -o wide
$kubectl get deploy web -o yaml

1.4

Write objects in YAML

Every object has the same four top fields.

pod.yaml

apiVersion: v1
kind: Pod
metadata:
  name: web
  namespace: shop
  labels:
    app: web
spec:
  containers:
    - name: nginx
      image: nginx:1.27
      ports:
        - containerPort: 80

$kubectl apply -f pod.yaml
$kubectl run web --image=nginx:1.27 --dry-run=client -o yaml > pod.yaml
$kubectl delete -f pod.yaml

Examentip

Never write YAML from zero in the exam. Generate it with --dry-run=client -o yaml, then edit.

1.5

Understand labels and selectors

Labels are key/value tags on objects. Selectors find objects by labels — that is how a Service finds its pods and a Deployment its ReplicaSets.

$kubectl get pods -l app=web
$kubectl get pods -l 'env in (prod,staging)'
$kubectl label pod web tier=frontend
$kubectl get pods --show-labels

Alle commando's in dit hoofdstuk