Certified Kubernetes Application Developer (CKAD) Study Guide

Module 2: Core Concepts

Various Kubernetes Objects

Module 4: Multi-Container Pods

Example 4-1. A Pod defining an init container

apiVersion: v1
kind: Pod
metadata:
  name: business-app
spec:
  initContainers:
  - name: configurer
    image: busybox:1.32.0
    command: ['sh', '-c', 'echo Configuring application... && \\
              mkdir -p /usr/shared/app && echo -e "{\\"dbConfig\\": \\
              {\\"host\\":\\"localhost\\",\\"port\\":5432,\\"dbName\\":\\"customers\\"}}" \\
              > /usr/shared/app/config.json']
    volumeMounts:
    - name: configdir
      mountPath: "/usr/shared/app"
  containers:
  - image: bmuschko/nodejs-read-config:1.0.0
    name: web
    ports:
    - containerPort: 8080
    volumeMounts:
    - name: configdir
      mountPath: "/usr/shared/app"
  volumes:
  - name: configdir
    emptyDir: {}

kubectl create -f init.yaml
pod/business-app created
$ kubectl get pod business-app
NAME           READY   STATUS    RESTARTS   AGE
business-app   0/1     Init:0/1  0          2s
$ kubectl get pod business-app
NAME           READY   STATUS    RESTARTS   AGE
business-app   1/1     Running   0          8s

<aside> 👍🏼 While init containers are running, status shows Init:m/n.

</aside>

Example 4-2. An exemplary sidecar pattern implementation

apiVersion: v1
kind: Pod
metadata:
  name: webserver
spec:
  containers:
  - name: nginx
    image: nginx
    volumeMounts:
    - name: logs-vol
      mountPath: /var/log/nginx
  - name: sidecar
    image: busybox
    command: ["sh","-c","while true; do if [ \\"$(cat /var/log/nginx/error.log \\
              | grep 'error')\\" != \\"\\" ]; then echo 'Error discovered!'; fi; \\
              sleep 10; done"]
    volumeMounts:
    - name: logs-vol
      mountPath: /var/log/nginx
  volumes:
  - name: logs-vol
    emptyDir: {}

$ kubectl create -f sidecar.yaml
pod/webserver created
$ kubectl get pods webserver
NAME        READY   STATUS              RESTARTS   AGE
webserver   0/2     ContainerCreating   0          4s
$ kubectl get pods webserver
NAME        READY   STATUS    RESTARTS   AGE
webserver   2/2     Running   0          5s

Example 4-3. An exemplary adapter pattern implementation

apiVersion: v1
kind: Pod
metadata:
  name: adapter
spec:
  containers:
  - args:
    - /bin/sh
    - -c
    - 'while true; do echo "$(date) | $(du -sh ~)" >> /var/logs/diskspace.txt; \\
       sleep 5; done;'
    image: busybox
    name: app
    volumeMounts:
      - name: config-volume
        mountPath: /var/logs
  - image: busybox
    name: transformer
    args:
    - /bin/sh
    - -c
    - 'sleep 20; while true; do while read LINE; do echo "$LINE" | cut -f2 -d"|" \\
       >> $(date +%Y-%m-%d-%H-%M-%S)-transformed.txt; done < \\
       /var/logs/diskspace.txt; sleep 20; done;'
    volumeMounts:
    - name: config-volume
      mountPath: /var/logs
  volumes:
  - name: config-volume
    emptyDir: {}

$ kubectl create -f adapter.yaml
pod/adapter created
$ kubectl get pods adapter
NAME      READY   STATUS    RESTARTS   AGE
adapter   2/2     Running   0          10s
$ kubectl exec adapter --container=transformer -it -- /bin/sh
/ # cat /var/logs/diskspace.txt
Sun Jul 19 20:28:07 UTC 2020 | 4.0K	/root
Sun Jul 19 20:28:12 UTC 2020 | 4.0K	/root
/ # ls -l
total 40
-rw-r--r--  1  root  root  60 Jul 19 20:28 2020-07-19-20-28-28-transformed.txt
...
/ # cat 2020-07-19-20-28-28-transformed.txt
 4.0K	/root
 4.0K	/root

Example 4-5. An exemplary ambassador pattern implementation

apiVersion: v1
kind: Pod
metadata:
  name: rate-limiter
spec:
  containers:
  - name: business-app
    image: bmuschko/nodejs-business-app:1.0.0
    ports:
    - containerPort: 8080
  - name: ambassador
    image: bmuschko/nodejs-ambassador:1.0.0
    ports:
    - containerPort: 8081