Import data to config map from kubernetes secret - kubernetes

Question

I'm using a kubernetes ConfigMap that contains database configurations for an app and there is a secret that has the database password.
I need to use this secret in the ConfigMap so when I try to add environment variable in the ConfigMap and specify the value in the pod deployment from the secret I'm not able to connect to mysql with password as the values in the ConfigMap took the exact string of the variable.
apiVersion: v1
kind: ConfigMap
metadata:
name: config
data:
APP_CONFIG: |
port: 8080
databases:
default:
connector: mysql
host: "mysql"
port: "3306"
user: "root"
password: "$DB_PASSWORD"
and the deployment.yaml
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: app
labels:
app: backend
spec:
replicas: 1
template:
metadata:
labels:
app: backend
spec:
containers:
- name: app
image: simple-app-image
ports:
- name: "8080"
containerPort: 8080
env:
- name: APP_CONFIG
valueFrom:
configMapKeyRef:
name: config
key: APP_CONFIG
- name: DB_PASSWORD
valueFrom:
secretKeyRef:
name: "mysql-secret"
key: "mysql-root-password"
Note: the secret exist and I'm able to get "mysql-root-password" value and use to login to the database

Answer 1

Kubernetes can't make that substitution for you, you should do it with shell in the entrypoint of the container.
This is a working example. I modify the default entrypoint to create a new variable with that substitution. After this command you should add the desired entrypoint.
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
name: app
labels:
app: backend
spec:
replicas: 1
template:
metadata:
labels:
app: backend
spec:
containers:
- name: app
image: simple-app-image
command:
- /bin/bash
- -c
args:
- "NEW_APP_CONFIG=$(echo $APP_CONFIG | envsubst) && echo $NEW_APP_CONFIG && <INSERT IMAGE ENTRYPOINT HERE>"
ports:
- name: "app"
containerPort: 8080
env:
- name: APP_CONFIG
valueFrom:
configMapKeyRef:
name: config
key: APP_CONFIG
- name: DB_PASSWORD
valueFrom:
secretKeyRef:
name: "mysql-secret"
key: "mysql-root-password"

Answer 2

You could do something like this in HELM:
{{- define "getValueFromSecret" }}
{{- $len := (default 16 .Length) | int -}}
{{- $obj := (lookup "v1" "Secret" .Namespace .Name).data -}}
{{- if $obj }}
{{- index $obj .Key | b64dec -}}
{{- else -}}
{{- randAlphaNum $len -}}
{{- end -}}
{{- end }}
Then you could do something like this in configmap:
{{- include "getValueFromSecret" (dict "Namespace" .Release.Namespace "Name" "<secret_name>" "Length" 10 "Key" "<key>") -}}
The secret should be already present while deploying; or you can control the order of deployment using https://github.com/vmware-tanzu/carvel-kapp-controller

Answer 3

I would transform the whole configMap into a secret and deploy the database password directly in there.
Then you can mount the secret as a file to a volume and use it like a regular config file in the container.