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Notes on Different UX with rkt Container Runtime

Notes on Different UX with rkt container runtime

Doesn’t support ENTRYPOINT + CMD feature

To run a Docker image, rkt will convert it into App Container Image (ACI) format first. However, during the conversion, the ENTRYPOINT and CMD are concatentated to construct ACI’s Exec field. This means after the conversion, we are not able to replace only ENTRYPOINT or CMD without touching the other part. So for now, users are recommended to specify the executable path in Command and arguments in Args. (This has the same effect if users specify the executable path + arguments in Command or Args alone).

For example:

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  labels:
    name: nginx
spec:
  containers:
  - name: nginx
    image: nginx
    ports:
    - containerPort: 80

The above pod yaml file is valid as it’s not specifying Command or Args, so the default ENTRYPOINT and CMD of the image will be used.

apiVersion: v1
kind: Pod
metadata:
  name: busybox
  labels:
    name: busybox
spec:
  containers:
  - name: busybox
    image: busybox
    command:
    - /bin/sleep
    - 1000

apiVersion: v1
kind: Pod
metadata:
  name: busybox
  labels:
    name: busybox
spec:
  containers:
  - name: busybox
    image: busybox
    command:
    - /bin/sleep
    args:
    - 1000

apiVersion: v1
kind: Pod
metadata:
  name: busybox
  labels:
    name: busybox
spec:
  containers:
  - name: busybox
    image: busybox
    args:
    - /bin/sleep
    - 1000

All the three examples above are valid as they contain both the executable path and the arguments.

apiVersion: v1
kind: Pod
metadata:
  name: busybox
  labels:
    name: busybox
spec:
  containers:
  - name: busybox
    image: busybox
    args:
    - 1000

The last example is invalid, as we cannot override just the CMD of the image alone.