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Kubernetes: 19. Configure Application

Configuring application consists of
  1. Configuring commands and arguments on applications
  2. Configuring environment variables
  3. Configuring secrets

Docker Commands
docker run ubuntu 
-> Runs ubuntu container and exit, container CMD is set to [bash], so the container quitely exits

docker run ubuntu echo "Hello World"
-> Runs ubuntu container, prints "Hello World" exits quitely.

To update the default settings, create your own image from the base image
lets call this ubuntu-sleeper image

FROM ubuntu
CMD sleep 5

CMD can also be mentioned in the JSON format like CMD ["sleep", "5"]
Note that with JSON format the first element should always be the command to execute, 
for eg, it CANNOT be ["sleep 5"]

Run build the new ubuntu-sleeper image and run the new image
docker build -t ubuntu-sleeper . 
-> Build the image

docker run ubuntu-sleeper
-> Run the new image


So the new image will launch ubuntu container, sleep for 5 seconds and quitely exits.
What if we want to overwrite the inputs sent to the container, we can do as below


docker run ubuntu-sleeper sleep 10
-> Looks ugly, but does the job, CMD is overwritten with the input values sent to container


Now to send just the input for the sleep command and not overwrite the command itself, 
we can build the image as

FROM ubuntu
ENTRYPOINT ["sleep"]

docker run ubuntu-sleeper 10
-> Entrypoint appends sleep and the input 10 is appended to sleep command

docker run ubuntu-sleeper
-> Error operand is missing!

Update the image with default sleep
FROM ubuntu
ENTRYPOINT ["sleep"]
CMD ["5"]

docker run ubuntu-sleeper
-> This now gets executed with default 5 seconds sleep

docker run --entrypoint sleep2.0 ubuntu-sleeper 10
-> Overwrite the entrypoint command

docker run -e APP_COLOR=pink simple-webapp-color
-> Create a environment variable in the container

Kubernetes Commands
  • The same container can be transferred into a Kubernetes pod
  • Entrypoint in docker will correspond to command in kubernetes
  • Command in docker will correspond to args in kubernetes
  • Note that args is an array

pod-definition.yaml
apiVersion: v1
kind: Pod
metadata:
    name: ubuntu-sleeper-pod

spec:
    containers:
    - image: ubuntu-sleeper
      name: ubuntu-sleeper-pod
      command: ["sleep"]
      args: ["5"]

Environment Variables
They can be set in different ways
  • key: value pair
  • ConfigMap
  • Secrets

pod-definition.yaml
apiVersion: v1
kind: Pod
metadata:
    name: ubuntu-sleeper-pod

spec:
    containers:
    - image: ubuntu-sleeper
      name: ubuntu-sleeper-pod
      env:
      - name: APP_COLOR
        value: pink
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