Injecting multiple Kubernetes volumes to the same directory in a pod

We can inject configuration into containers using Kubernetes config maps and secrets. These objects can be consumed by a pod as environment variables, command-line arguments, or as configuration files mounted in a volume. For the subject of this article, we will focus on mounting multiple config maps/secrets into a single directory on a pod.

Mounting a configmap or secret in a Volume is relatively straightforward (for anyone familiar with Kubernetes primitives). Take for example the following deployment definition:

kind: ConfigMap
apiVersion: v1
metadata:
  name: hello-world-config
data:
  application.properties: |
    greeting=Hello
    name=World

---

kind: Secret
apiVersion: v1
metadata:
  name: hello-world-credentials
data:
  credentials.properties: <base64 goes here>
type: Opaque

---

kind: Deployment
apiVersion: apps/v1
metadata:
  name: hello-world
spec:
  replicas: 1
  selector:
    matchLabels:
      app: hello-world
  template:
    metadata:
      labels:
        app: hello-world
    spec:
      containers:
        - name: hello-world
          image: busybox:latest
          command:
            - /bin/sh
            - -c
            - sleep 5000
          volumeMounts:
            - name: hello-world-config-volume
              mountPath: /config/application.properties
              readOnly: true
            - name: hello-world-credentials-volume
              mountPath: /config/credentials.properties
              readOnly: true
      volumes:
        - name: hello-world-config-volume
          configMap:
            name: hello-world-config
        - name: hello-world-credentials-volume
          secret:
            secretName: hello-world-credentials

When this deployment is created, we see two directories are created in this pod (one for the configMap, and one for the secret). How can we mount these as two files in the ‘config’ directory, rather than two individual directories?

The ‘VolumeMounts’ directive within the container spec of the deployment has an optional (less-known) parameter named ‘subPath’. By using this parameter, we can mount a configMap and secret in the same directory within a pod.

Let’s focus on the following deployment spec:

kind: ConfigMap
apiVersion: v1
metadata:
  name: hello-world-config
data:
  application.properties: |
    greeting=Hello
    name=World

---

kind: Secret
apiVersion: v1
metadata:
  name: hello-world-credentials
data:
  credentials.properties: <base64 goes here>
type: Opaque

---

kind: Deployment
apiVersion: apps/v1
metadata:
  name: hello-world
spec:
  replicas: 1
  selector:
    matchLabels:
      app: hello-world
  template:
    metadata:
      labels:
        app: hello-world
    spec:
      containers:
        - name: hello-world
          image: busybox:latest
          command:
            - /bin/sh
            - -c
            - sleep 5000
          volumeMounts:
            - name: hello-world-config-volume
              mountPath: /config/application.properties
              subPath: application.properties
              readOnly: true
            - name: hello-world-credentials-volume
              mountPath: /config/credentials.properties
              subPath: credentials.properties
              readOnly: true
      volumes:
        - name: hello-world-config-volume
          configMap:
            name: hello-world-config
        - name: hello-world-credentials-volume
          secret:
            secretName: hello-world-credentials

Now, if we deploy this, we can see that we have two files in the ‘config’ directory of the pod, rather than a subdirectory for each config/secret:

In this example, the key application.properties from the configuration map will be mounted to a file with the same name under /config/, and the secret value credentials.properties will be mounted to a second file under that directory. Both files will be read-only to the application.

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