Example Kubernetes YAML for the multi-container adapter design pattern

pod.yaml

# Example YAML configuration for the adapter pattern.

# It defines a main application container which writes
# the current date and system usage information to a log file 
# every five seconds.

# The adapter container reads what the application has written and
# reformats it into a structure that a hypothetical monitoring 
# service requires. 

# To run:
#   kubectl apply -f pod.yaml

# Once the pod is running:
#   
#   (Connect to the application pod)
#   kubectl exec pod-with-adapter -c app-container -it sh
# 
#   (Take a look at what the application is writing.)
#   cat /var/log/top.txt
#   
#   (Take a look at what the adapter has reformatted it to.)
#   cat /var/log/status.txt


apiVersion: v1
kind: Pod
metadata:
  name: pod-with-adapter
spec:
  # Create a volume called 'shared-logs' that the
  # app and adapter share.
  volumes:
  - name: shared-logs 
    emptyDir: {}

  containers:

  # Main application container
  - name: app-container
    # This application writes system usage information (`top`) to a status 
    # file every five seconds.
    image: alpine
    command: ["/bin/sh"]
    args: ["-c", "while true; do date > /var/log/top.txt && top -n 1 -b >> /var/log/top.txt; sleep 5;done"]

    # Mount the pod's shared log file into the app 
    # container. The app writes logs here.
    volumeMounts:
    - name: shared-logs
      mountPath: /var/log

  # Adapter container
  - name: adapter-container
    # This sidecar container takes the output format of the application
    # (the current date and system usage information), simplifies
    # and reformats it for the monitoring service to come and collect.
    
    # In this example, our monitoring service requires status files
    # to have the date, then memory usage, then CPU percentage each 
    # on a new line.
    
    # Our adapter container will inspect the contents of the app's top file,
    # reformat it, and write the correctly formatted output to the status file.
    image: alpine
    command: ["/bin/sh"]

    # A long command doing a simple thing: read the `top.txt` file that the
    # application wrote to and adapt it to fit the status file format.
    # Get the date from the first line, write to `status.txt` output file.
    # Get the first memory usage number, write to `status.txt`.
    # Get the first CPU usage percentage, write to `status.txt`.

    args: ["-c", "while true; do (cat /var/log/top.txt | head -1 > /var/log/status.txt) && (cat /var/log/top.txt | head -2 | tail -1 | grep
 -o -E '\\d+\\w' | head -1 >> /var/log/status.txt) && (cat /var/log/top.txt | head -3 | tail -1 | grep
-o -E '\\d+%' | head -1 >> /var/log/status.txt); sleep 5; done"]


    # Mount the pod's shared log file into the adapter
    # container.
    volumeMounts:
    - name: shared-logs
      mountPath: /var/log