Chapter 15. Injecting Information into Pods Using Pod Presets

A pod preset is an object that injects user-specified information into pods as they are created.

Using pod preset objects you can inject:

Developers only need make sure the pod labels match the label selector on the PodPreset in order to add all that information to the pod. The label on a pod associates the pod with one or more pod preset objects that have a matching label selectors.

Using pod presets, a developer can provision pods without needing to know the details about the services the pod will consume. An administrator can keep configuration items of a service invisible from a developer without preventing the developer from deploying pods. For example, an administrator can create a pod preset that provides the name, user name, and password for a database through a secret and the database port through environment variables. The pod developer only needs to know the label to use to include all the information in pods. A developer can also create pod presets and perform all the same tasks. For example, the developer can create a preset that injects environment variable automatically into multiple pods.

When a pod preset is applied to a pod, OpenShift Container Platform modifies the pod specification, adding the injectable data and annotating the pod spec to show that it was modified by a pod preset. The annotation is of the form:

podpreset.admission.kubernetes.io/<pod-preset name>: `resource version`

In order to use pod presets in your cluster:

If the pod creation encounters an error, the pod is created without any injected resources from the pod preset.

You can exclude specific pods from being altered by any pod preset mutations using the podpreset.admission.kubernetes.io/exclude: "true" parameter in the pod specification. See the example pod specification below.

kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1 1
metadata:
  name: allow-database 2
spec:
  selector:
    matchLabels:
      role: frontend 3
  env:
    - name: DB_PORT 4
      value: "6379" 5
  envFrom:
    - configMapRef: 6
      name: etcd-env-config
    - secretKeyRef: 7
      name: test-secret
  volumeMounts: 8
    - mountPath: /cache
      name: cache-volume
  volumes: 9
    - name: cache-volume
      emptyDir: {}

apiVersion: v1
kind: Pod
metadata:
  name: website
  labels:
    app: website
    role: frontend 1
spec:
  containers:
    - name: website
      image: ecorp/website
      ports:
        - containerPort: 80

apiVersion: v1
kind: Pod
metadata:
  name: website
  labels:
    app: website
    role: frontend
  annotations:
    podpreset.admission.kubernetes.io/allow-database: "resource version" 1
spec:
  containers:
    - name: website
      image: ecorp/website
      volumeMounts: 2
        - mountPath: /cache
          name: cache-volume
      ports:
        - containerPort: 80
      env: 3
        - name: DB_PORT
          value: "6379"
      envFrom: 4
        - configMapRef:
          name: etcd-env-config
        - secretKeyRef:
          name: test-secret
  volumes: 5
    - name: cache-volume
      emptyDir: {}

apiVersion: v1
kind: Pod
metadata:
  name: no-podpreset
  labels:
    app: website
    role: frontend
  annotations:
    podpreset.admission.kubernetes.io/exclude: "true" 1
spec:
  containers:
    - name: hello-pod
      image: docker.io/ocpqe/hello-pod

The following example demonstrates how to create and use pod presets.

admissionConfig:
  pluginConfig:
    PodPreset:
      configuration:
        kind: DefaultAdmissionConfig
        apiVersion: v1
        disable: false

Then, restart the OpenShift Container Platform services:

# systemctl restart atomic-openshift-master-api atomic-openshift-master-controllers

kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
  name: allow-database
spec:
  selector:
    matchLabels:
      role: frontend
  env:
    - name: DB_PORT
      value: "6379"
  volumeMounts:
    - mountPath: /cache
      name: cache-volume
  volumes:
    - name: cache-volume
      emptyDir: {}

You can use multiple pod presets to inject multiple pod injection policies.

You can delete a pod preset using the following command:

$ oc delete podpreset <name>

For example:

$ oc delete podpreset allow-database

podpreset "allow-database" deleted