Multi-Container Pods (10%)

Implementing the Adapter Pattern

The adapter pattern helps with providing a simplified, homogenized view of an application running within a container. For example, we could stand up another container that unifies the log output of the application container. As a result, other monitoring tools can rely on a standardized view of the log output without having to transform it into an expected format.

1. Create a new Pod in a YAML file named adapter.yaml. The Pod declares two containers. The container app uses the image busybox and runs the command while true; do echo "$(date) | $(du -sh ~)" >> /var/logs/diskspace.txt; sleep 5; done;. The adapter container transformer uses the image busybox and runs the command sleep 20; while true; do while read LINE; do echo "$LINE" | cut -f2 -d"|" >> $(date +%Y-%m-%d-%H-%M-%S)-transformed.txt; done < /var/logs/diskspace.txt; sleep 20; done; to strip the log output off the date for later consumption my a monitoring tool. Be aware that the logic does not handle corner cases (e.g. automatically deleting old entries) and would look different in production systems.
2. Before creating the Pod, define an emptyDir volume. Mount the volume in both containers with the path /var/logs.
3. Create the Pod, log into the container transformer. The current directory should continuously write a new file every 20 seconds.

Solution:

kubectl run adapter --image=busybox --restart=Never -o yaml --dry-run -- /bin/sh -c 'while true; do echo "$(date) | $(du -sh ~)" >> /var/logs/diskspace.txt; sleep 5; done;' > adapter.yaml

The final Pod YAML file should look something like this:

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  name: adapter
spec:
  volumes:
    - name: config-volume
      emptyDir: {}
  containers:
  - args:
    - /bin/sh
    - -c
    - 'while true; do echo "$(date) | $(du -sh ~)" >> /var/logs/diskspace.txt; sleep 5; done;'
    image: busybox
    name: app
    volumeMounts:
      - name: config-volume
        mountPath: /var/logs
    resources: {}
  - image: busybox
    name: transformer
    args:
    - /bin/sh
    - -c
    - 'sleep 20; while true; do while read LINE; do echo "$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
  dnsPolicy: ClusterFirst
  restartPolicy: Never
status: {}

kubectl exec adapter --container=transformer -it -- /bin/sh
/ # ls -l
-rw-r--r--    1 root     root           205 May 12 20:43 2019-05-12-20-43-32-transformed.txt
-rw-r--r--    1 root     root           369 May 12 20:43 2019-05-12-20-43-52-transformed.txt
...
/ # cat 2019-05-12-20-43-52-transformed.txt
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
 4.0K    /root
/ # exit