Kubernetes env variable to containers - docker

Question

I want to pass some values from Kubernetes yaml file to the containers. These values will be read in my Java app using System.getenv("x_slave_host").
I have this dockerfile:
FROM jetty:9.4
...
ARG slave_host
ENV x_slave_host $slave_host
...
$JETTY_HOME/start.jar -Djetty.port=9090
The kubernetes yaml file contains this part where I added env section:
apiVersion: apps/v1beta1
kind: Deployment
metadata:
name: master
spec:
template:
metadata:
labels:
app: master
spec:
volumes:
- name: shared-data
emptyDir: {}
containers:
- name: master
image: xregistry.azurecr.io/Y:latest
ports:
- containerPort: 9090
volumeMounts:
- name: shared-data
mountPath: ~/.X/experiment
- env:
- name: slave_host
value: slavevalue
- name: jupyter
image: xregistry.azurecr.io/X:latest
ports:
- containerPort: 8000
- containerPort: 8888
volumeMounts:
- name: shared-data
mountPath: /var/folder/experiment
imagePullSecrets:
- name: acr-auth
Locally when I did the same thing using docker compose, it worked using args. This is a snippet:
master:
image: master
build:
context: ./master
args:
- slave_host=slavevalue
ports:
- "9090:9090"
So now I am trying to do the same thing but in Kubernetes. However, I am getting the following error (deploying it on Azure):
error: error validating "D:\\a\\r1\\a\\_X\\deployment\\kub-deploy.yaml": error validating data: field spec.template.spec.containers[1].name for v1.Container is required; if you choose to ignore these errors, turn validation off with --validate=false
In other words, how to rewrite my docker compose file to kubernetes and passing this argument.
Thanks!

Answer 1

env section should be added under containers, like this:
containers:
- name: master
env:
- name: slave_host
value: slavevalue

Answer 2

To elaborate a on #Kun Li's answer, besides adding environment variables e.g. in the Deployment manifest directly you can create a ConfigMap (or Secret depending on the data being stored) and reference these in your manifests. This is a good way of sharing the same environment variables across applications, compared to manually adding environment variables to several different applications.
Note that a ConfigMap can consist of one or more key: value pairs and it's not limited to storing environment variables, it's just one of the use cases. And as i mentioned before, consider using a Secret if the data is classified as sensitive.
Example of a ConfigMap manifest, in this case used for storing an environment variable:
apiVersion: v1
kind: ConfigMap
metadata:
name: my-env-var
data:
slave_host: slavevalue
To create a ConfigMap holding one key=value pair using kubectl create:
kubectl create configmap my-env --from-literal=slave_host=slavevalue
To get hold of all environment variables configured in a ConfigMap use the following in your manifest:
containers:
envFrom:
- configMapRef:
name: my-env-var
Or if you want to pick one specific environment variable from your ConfigMap containing several variables:
containers:
env:
- name: slave_host
valueFrom:
configMapKeyRef:
name: my-env-var
key: slave_host
See this page for more examples of using ConfigMap's in different situations.