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