In a Kubernetes Pod, Overriding a File using ConfigMap

Many applications don’t use environment variables but instead, use one or more config files.

However, it could be challenging to override an existing file by using ConfigMap.

I will walk through replacing an existing file in a pod. For the sake of simplicity, I will replace the /etc/host.conf file.

The config file on my computer:

➜ ~ cat host-config-file.txt
multi off

You may want to create a ConfigMap entry in an imperative way:

kubectl create configmap host-config-data --from-file=host-config-file.txt

But if you create the config map, in an imperative way, the local filename is assigned as the data key.

kubectl get configmaps host-config-data -o yaml
apiVersion: v1
data:
  host-config-file.txt: |
    multi off

Declarative way:

apiVersion: v1
kind: ConfigMap
metadata:
  name: host-config-data
data:
  data_custom_key: |-
    multi off

Create the config map:

➜ ~ kubectl create -f configmap.yml
configmap/host-config-data created
➜ ~ kubectl get configmaps host-config-data -o yaml
apiVersion: v1
data:
  data_custom_key: multi off

Create the deployment file:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: memcached-server-deployment
spec:
  replicas: 2
  selector:
    matchLabels:
      cache: memcached
  template:
    metadata:
      labels:
        cache: memcached
    spec:
      containers:
        - name: memcached-container
          image: memcached:bullseye
          volumeMounts:
            - name: config-override
              mountPath: /etc/host.conf
              subPath: custom_path
      volumes:
        - name: config-override
          configMap:
            name: host-config-data
            items:
              - key: data_custom_key
                path: custom_path

I used a different word for each segment so you can distinguish which parts are related to each other.

Result:

kubectl exec -it memcached-server-deployment-67858696f7-b72vn -- /bin/bash

memcache@memcached-server-deployment-67858696f7-b72vn:/$ cat /etc/host.conf
multi off