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Injecting Information into Pods Using Pod Presets

Overview

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

Pod presets is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs), might not be functionally complete, and Red Hat does not recommend to use them for production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information on Red Hat Technology Preview features support scope, see https://access.redhat.com/support/offerings/techpreview/.

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:

  • An administrator must enable the pod preset admission controller plug-in through the /etc/origin/master/master-config.yaml;

  • The pod preset author must enable the API type settings.k8s.io/v1alpha1/podpreset through the pod preset and add injectable information to the pod preset.

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

The Pod Preset feature is available only if the Service Catalog has been installed.
Sample pod preset object
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" (4)
  envFrom:
    - configMapRef: (5)
      name: etcd-env-config
    - secretKeyRef: (6)
      name: test-secret
  volumeMounts: (7)
    - mountPath: /cache
      name: cache-volume
  volumes: (8)
    - name: cache-volume
      emptyDir: {}
1 Specify the settings.k8s.io/v1alpha1 API.
2 Name of the pod preset. This name is used in the pod annotation.
3 A label selector that matches the label in the pod specification.
4 Creates an environment variable to pass to the container.
5 Adds a ConfigMap to the pod specification.
6 Adds a secrets object to the pod specification.
7 Specifies where external storage volumes should be mounted within the container.
8 Defines storage volumes that are available to the container(s).
Sample pod specification
apiVersion: v1
kind: Pod
metadata:
  name: website
  labels:
    app: website
    role: frontend (1)
spec:
  containers:
    - name: website
      image: ecorp/website
      ports:
        - containerPort: 80
1 A label to match the label selector in the pod preset.
Sample pod specification after a pod preset
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: {}
1 The annotation added to show a pod preset was injected.
2 The volume mount is added to the pod.
3 The environment variable is added to the pod.
4 The ConfigMap and secrets object added to the pod.
5 The volume mount is added to the pod.

Creating Pod Presets

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

Add the Admission Controller

An administrator can check the /etc/origin/master/master-config.yaml file to make sure the pod preset admission controller plug-in is present. If the admission controller is not present, add the plug-in using the following:

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
Create the Pod Preset

An administrator or developer creates the pod preset with the settings.k8s.io/v1alpha1 API, the information to inject, and a label selector to match with the pods:

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: {}
Create the Pod

The developer creates the pod with a label that matches the label selector in the pod preset:

  1. Create a standard pod specification with a label that matches the label selector in the pod preset:

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

  2. Create the pod:

    $ oc create -f pod.yaml

  3. Check the pod spec after creation:

    $ oc get pod website -o yaml

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: (1)
        - mountPath: /cache
          name: cache-volume
      ports:
        - containerPort: 80
      env: (1)
        - name: DB_PORT
          value: "6379"
  volumes:
    - name: cache-volume
      emptyDir: {}
    1 The annotation is present and the container storage and environment variables are injected.

Using Multiple Pod Presets

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

  • Make sure the pod preset admission controller plug-in is enabled.

  • Create a pod preset, similar to the following, with environment variables, mount points, and/or storage volumes:

    kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
  name: allow-database
spec:
  selector:
    matchLabels:
      role: frontend (1)
  env:
    - name: DB_PORT
      value: "6379"
  volumeMounts:
    - mountPath: /cache
      name: cache-volume
  volumes:
    - name: cache-volume
      emptyDir: {}
    1 Label selector to match the pod labels.

  • Create a second pod preset, similar to the following:

    kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
  name: proxy
spec:
  selector:
    matchLabels:
      role: frontend (1)
  volumeMounts:
    - mountPath: /etc/proxy/configs
      name: proxy-volume
  volumes:
    - name: proxy-volume
      emptyDir: {}
    1 Label selector to match the pod labels.

  • Create a standard pod specification:

    apiVersion: v1
kind: Pod
metadata:
  name: website
  labels:
    app: website
    role: frontend (1)
spec:
  containers:
    - name: website
      image: ecorp/website
      ports:
        - containerPort: 80
    1 Label to match both pod preset label selectors.

  • Create the pod:

    $ oc create -f pod.yaml

  • Check the pod spec after creation:

    apiVersion: v1
kind: Pod
metadata:
  name: website
  labels:
    app: website
    role: frontend
  annotations:
    podpreset.admission.kubernetes.io/allow-database: "resource version" (1)
    podpreset.admission.kubernetes.io/proxy: "resource version" (1)
spec:
  containers:
    - name: website
      image: ecorp/website
      volumeMounts:
        - mountPath: /cache
          name: cache-volume
        - mountPath: /etc/proxy/configs
          name: proxy-volume
      ports:
        - containerPort: 80
      env:
        - name: DB_PORT
          value: "6379"
  volumes:
    - name: cache-volume
      emptyDir: {}
    - name: proxy-volume
      emptyDir: {}
    1 Annotation indicating that multiple pod presets were injected.

Deleting Pod Presets

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