My take on Kubernetes

04.04.2023 — kubernetes, docker — 3 min read

My learnings along the way to Kubernetes heaven.

Masters and Nodes

Machines in a Kubernetes cluster are referred to as Nodes. A Kubernetes cluster contains two types of Nodes:

Master Nodes
Worker Nodes

Usually, Master Nodes are referred to as “Masters” and Worker Nodes are called “Nodes”. Maters host the control plane and Nodes are where you run user applications.

Hosted Kubernetes

Workflow

Breaking down the Dockerfile

# use the node:current slim image as the base
FROM node:current-slim

# copy the application and dependencies from the current directory 
# into the /src directory
COPY . /src
RUN cd /src; npm install

# informs Docker that the container listens on the specified network ports 
# at runtime
EXPOSE 8080

CMD cd /src && node ./app.js

Containerisation

The process of building an application into a container image is called containerisation.

docker image build -t andrewchaa/qsk-course:1.0 .
docker image ls

Host the image on a registry

docker login --username andrewchaa
docker image push andrewchaa/qsk-course:1.0

Deploying the first application

# nginx.yaml
apiVersion: v1
kind: Pod 
metadata:
    name: nginx 
spec:
    containers:
    - name: nginx-container
      image: registry.hub.docker.com/library/nginx

kubectl apply -f nginx.yaml

# port forward to open the pod
kubectl port-forward --address 0.0.0.0 nginx 3000:80
kubectl get pods

# stream logs
kubectl logs --follow nginx

The workers

Where our applications actually run.

The master

In the reconciliation loop, the master compares the current state with the desired state and makes the necessary changes

Difference between pods and containers

Multiple containers can run inside a single pod. A way to group containers

Executing commands in a container running in the pod

kubectl exec nginx -- ls

kubectl exec -it nginx -- bash
kubectl exec -it apache -- sh
root@nginx:/#

Killing pods

kubectl delete pod nginx
kubectl delete -f nginx.yaml

kubectl get pods

Deployment

Scale the applications by increasing or decreasing the number of replicas
Roll out new versions of the application without downtime
Easily rollback bad releases

apiVersion: apps/v1
kind: Deployment
metadata:
  name: hellok8s # unique name for the deployment
spec:
  replicas: 1
  selector:      # manage all the pods tha have a label called app with
    matchLabels: # the value hellok8s
      app: hellok8s
  template:
    metadata:
      labels:
        app: hellok8s
    spec:
      containers:
      - image: brianstorti/hellok8s:v1
        name: hellok8s-container

kubectl apply -f deployment.yaml
# deployment.apps/hellok8s created

kubectl get deployments
# NAME       DESIRED   CURRENT   UP-TO-DATE   AVAILABLE
# hellok8s   1         1         1            1

kubectl get pods
# NAME                        READY   STATUS    RESTARTS
# hellok8s-6678f66cb8-42jtr   1/1     Running   0

kubectl port-forward hellok8s-78997b6f8f-zwv62 --address 0.0.0.0 3004567

Controlling the Rollout Rate

maxSurge: define how many pods we can have exceeding our desired replica count. The default value is 25%
maxUnavailable: defines how many pods we can have below the desired count

spec:
  strategy:
     rollingUpdate:
      maxSurge: 1
      maxUnavailable: 1

Probing bad releases

readinessProbe: make the container ready to start receiving requests only after it has received a specified number of successful responses

spec:
    spec:
      containers:
      - image: brianstorti/hellok8s:buggy # Adding a buggy version
        name: hellok8s-container
        readinessProbe:
          periodSeconds: 1
          successThreshold: 5
          httpGet:
            path: /
            port: 4567

livenessProbe: Kubernetes will keep calling this probe periodically to make sure the pod is healthy.

spec:
    spec:
      containers:
      - image: brianstorti/hellok8s:v2
        name: hellok8s-container
        readinessProbe:
          periodSeconds: 1
          successThreshold: 5
          httpGet:
            path: /
            port: 4567
        livenessProbe:
          httpGet:
            path: /
            port: 4567