Projecting Files into Existing Directories in Kubernetes Containers

Introduction

One standard method of "injecting" ConfigMap items is to mount the ConfigMap as a volume. This either creates a brand new directory with ConfigMap items as files in this directory, or overwrites an existing directory leaving only the ConfigMap items.

But what if you want the directory to contain BOTH the existing files as well as the ConfigMap files?

We will use the alpine Docker container to illustrate a couple of ways to mount ConfigMap items as files into a directory that already exists in the container.

Note: we use the abbreviation k in place of kubectl.

Note the contents of the existing directory /etc/ssl/misc before adding any volumeMounts into it:

# file-mapping-pod.yml
apiVersion: v1
kind: Pod
metadata:
  name: file-mapping
spec:
  containers:
    - name: test-container
      image: alpine
      command: [ "sleep", "10000" ]

$ k apply -f file-mapping-pod.yml
$ k exec file-mapping -- ls -l /etc/ssl/misc
total 16
-rwxr-xr-x    1 root     root          7598 Sep 13 05:48 CA.pl
lrwxrwxrwx    1 root     root             8 Oct 21 13:39 tsget -> tsget.pl
-rwxr-xr-x    1 root     root          6579 Sep 13 05:48 tsget.pl

Map a ConfigMap onto a file in an existing directory: only ConfigMap items remain

# file-mapping-pod.yml
apiVersion: v1
kind: Pod
metadata:
  name: file-mapping
spec:
  containers:
    - name: test-container
      image: alpine
      command: [ "sleep", "10000" ]
      volumeMounts:
        - name: file-mapping-volume
          mountPath: /etc/ssl/misc
  volumes:
    - name: file-mapping-volume
      configMap:
        name: file-mapping-cm

$ k delete pod file-mapping
$ k apply -f file-mapping-pod.yml
$ k exec file-mapping -- ls -l /etc/ssl/misc
lrwxrwxrwx    1 root     root            10 Nov 28 20:58 cm1 -> ..data/cm1
lrwxrwxrwx    1 root     root            10 Nov 28 20:58 cm2 -> ..data/cm2

Note that all the existing files that were in the alpine Docker image are gone and the only files that remain are the ConfigMap files.

If you describe the pod you will see how the mapping works:

$ k describe pod file-mapping
...
Mounts:
  /etc/ssl/misc from file-mapping-volume (rw)
...

Map a ConfigMap onto a file in an existing directory: original directory items remain

Use the subPath attribute.

# file-mapping-pod.yml
apiVersion: v1
kind: Pod
metadata:
  name: file-mapping
spec:
  containers:
    - name: test-container
      image: alpine
      command: [ "sleep", "10000" ]
      volumeMounts:
        - name: file-mapping-volume
          mountPath: /etc/ssl/misc/cm1.txt
          subPath: cm1
        - name: file-mapping-volume
          mountPath: /etc/ssl/misc/cm2.txt
          subPath: cm2
  volumes:
    - name: file-mapping-volume
      configMap:
        name: file-mapping-cm

$ k delete pod file-mapping
$ k apply -f file-mapping-pod.yml
$ k exec file-mapping -- ls -l /etc/ssl/misc
total 36
-rwxr-xr-x    1 root     root          7598 Sep 13 05:48 CA.pl
-rw-r--r--    1 root     root          2114 Dec  2 17:35 cm1.txt
-rw-r--r--    1 root     root         15118 Dec  2 17:35 cm2.txt
lrwxrwxrwx    1 root     root             8 Oct 21 13:39 tsget -> tsget.pl
-rwxr-xr-x    1 root     root          6579 Sep 13 05:48 tsget.pl

Note that the original files from the alpine Docker container remain and the additional files cm1.txt and cm2.txt from the ConfigMap are added.

If you describe the pod you will see how the mapping works:

$ k describe pod file-mapping
...
Mounts:
  /etc/ssl/misc/cm1.txt from file-mapping-volume (rw,path="cm1")
  /etc/ssl/misc/cm2.txt from file-mapping-volume (rw,path="cm2")
...

How to reproduce the above results

1. Install jq (JSON query utility).

2. Create a file-mapping Kubernetes namespace and set it as the default:

    $ k apply -f namespace.yml
    $ k describe ns file-mapping
    $ CURRENT_CONTEXT=`kubectl config -o=json view | jq -r '."current-context"'`
    $ k config set-context $CURRENT_CONTEXT --namespace=file-mapping
    $ k config -o=json view | jq ".contexts[]|select(.name==\"$CURRENT_CONTEXT\")"
   

3. Create some configmap items:

    $ ls /etc > /tmp/cm1.txt
    $ ls /bin > /tmp/cm2.txt
    $ k create cm file-mapping-cm --from-file=cm1=/tmp/cm1.txt --from-file=cm2=/tmp/cm2.txt
    $ k get cm file-mapping-cm -o=json | jq '.data|keys'