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jessfraz
Inject Information into Pods Using a PodPreset v1.6 task 60

{{< feature-state for_k8s_version="v1.6" state="alpha" >}}

This page shows how to use PodPreset objects to inject information like {{< glossary_tooltip text="Secrets" term_id="secret" >}}, volume mounts, and {{< glossary_tooltip text="environment variables" term_id="container-env-variables" >}} into Pods at creation time.

{{% heading "prerequisites" %}}

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one using Minikube. Make sure that you have enabled PodPreset in your cluster.

Use Pod presets to inject environment variables and volumes

In this step, you create a preset that has a volume mount and one environment variable. Here is the manifest for the PodPreset:

{{< codenew file="podpreset/preset.yaml" >}}

The name of a PodPreset object must be a valid DNS subdomain name.

In the manifest, you can see that the preset has an environment variable definition called DB_PORT and a volume mount definition called cache-volume which is mounted under /cache. The {{< glossary_tooltip text="selector" term_id="selector" >}} specifies that the preset will act upon any Pod that is labeled role:frontend.

Create the PodPreset:

kubectl apply -f https://k8s.io/examples/podpreset/preset.yaml

Verify that the PodPreset has been created:

kubectl get podpreset

NAME             CREATED AT
allow-database   2020-01-24T08:54:29Z

This manifest defines a Pod labelled role: frontend (matching the PodPreset's selector):

{{< codenew file="podpreset/pod.yaml" >}}

Create the Pod:

kubectl create -f https://k8s.io/examples/podpreset/pod.yaml

Verify that the Pod is running:

kubectl get pods

The output shows that the Pod is running:

NAME      READY     STATUS    RESTARTS   AGE
website   1/1       Running   0          4m

View the Pod spec altered by the admission controller in order to see the effects of the preset having been applied:

kubectl get pod website -o yaml

{{< codenew file="podpreset/merged.yaml" >}}

The DB_PORT environment variable, the volumeMount and the podpreset.admission.kubernetes.io annotation of the Pod verify that the preset has been applied.

Pod spec with ConfigMap example

This is an example to show how a Pod spec is modified by a Pod preset that references a ConfigMap containing environment variables.

Here is the manifest containing the definition of the ConfigMap:

{{< codenew file="podpreset/configmap.yaml" >}}

Create the ConfigMap:

kubectl create -f https://k8s.io/examples/podpreset/configmap.yaml

Here is a PodPreset manifest referencing that ConfigMap:

{{< codenew file="podpreset/allow-db.yaml" >}}

Create the preset that references the ConfigMap:

kubectl create -f https://k8s.io/examples/podpreset/allow-db.yaml

The following manifest defines a Pod matching the PodPreset for this example:

{{< codenew file="podpreset/pod.yaml" >}}

Create the Pod:

kubectl create -f https://k8s.io/examples/podpreset/pod.yaml

View the Pod spec altered by the admission controller in order to see the effects of the preset having been applied:

kubectl get pod website -o yaml

{{< codenew file="podpreset/allow-db-merged.yaml" >}}

The DB_PORT environment variable and the podpreset.admission.kubernetes.io annotation of the Pod verify that the preset has been applied.

ReplicaSet with Pod spec example

This is an example to show that only Pod specs are modified by Pod presets. Other workload types like ReplicaSets or Deployments are unaffected.

Here is the manifest for the PodPreset for this example:

{{< codenew file="podpreset/preset.yaml" >}}

Create the preset:

kubectl apply -f https://k8s.io/examples/podpreset/preset.yaml

This manifest defines a ReplicaSet that manages three application Pods:

{{< codenew file="podpreset/replicaset.yaml" >}}

Create the ReplicaSet:

kubectl create -f https://k8s.io/examples/podpreset/replicaset.yaml

Verify that the Pods created by the ReplicaSet are running:

kubectl get pods

The output shows that the Pods are running:

NAME             READY   STATUS    RESTARTS   AGE
frontend-2l94q   1/1     Running   0          2m18s
frontend-6vdgn   1/1     Running   0          2m18s
frontend-jzt4p   1/1     Running   0          2m18s

View the spec of the ReplicaSet:

kubectl get replicasets frontend -o yaml

{{< note >}} The ReplicaSet object's spec was not changed, nor does the ReplicaSet contain a podpreset.admission.kubernetes.io annotation. This is because a PodPreset only applies to Pod objects.

To see the effects of the preset having been applied, you need to look at individual Pods. {{< /note >}}

The command to view the specs of the affected Pods is:

kubectl get pod --selector=role=frontend -o yaml

{{< codenew file="podpreset/replicaset-merged.yaml" >}}

Again the podpreset.admission.kubernetes.io annotation of the Pods verifies that the preset has been applied.

Multiple Pod presets example

This is an example to show how a Pod spec is modified by multiple Pod presets.

Here is the manifest for the first PodPreset:

{{< codenew file="podpreset/preset.yaml" >}}

Create the first PodPreset for this example:

kubectl apply -f https://k8s.io/examples/podpreset/preset.yaml

Here is the manifest for the second PodPreset:

{{< codenew file="podpreset/proxy.yaml" >}}

Create the second preset:

kubectl apply -f https://k8s.io/examples/podpreset/proxy.yaml

Here's a manifest containing the definition of an applicable Pod (matched by two PodPresets):

{{< codenew file="podpreset/pod.yaml" >}}

Create the Pod:

kubectl create -f https://k8s.io/examples/podpreset/pod.yaml

View the Pod spec altered by the admission controller in order to see the effects of both presets having been applied:

kubectl get pod website -o yaml

{{< codenew file="podpreset/multi-merged.yaml" >}}

The DB_PORT environment variable, the proxy-volume VolumeMount and the two podpreset.admission.kubernetes.io annotations of the Pod verify that both presets have been applied.

Conflict example

This is an example to show how a Pod spec is not modified by a Pod preset when there is a conflict. The conflict in this example consists of a VolumeMount in the PodPreset conflicting with a Pod that defines the same mountPath.

Here is the manifest for the PodPreset:

{{< codenew file="podpreset/conflict-preset.yaml" >}}

Note the mountPath value of /cache.

Create the preset:

kubectl apply -f https://k8s.io/examples/podpreset/conflict-preset.yaml

Here is the manifest for the Pod:

{{< codenew file="podpreset/conflict-pod.yaml" >}}

Note the volumeMount element with the same path as in the PodPreset.

Create the Pod:

kubectl create -f https://k8s.io/examples/podpreset/conflict-pod.yaml

View the Pod spec:

kubectl get pod website -o yaml

{{< codenew file="podpreset/conflict-pod.yaml" >}}

You can see there is no preset annotation (podpreset.admission.kubernetes.io). Seeing no annotation tells you that no preset has not been applied to the Pod.

However, the PodPreset admission controller logs a warning containing details of the conflict. You can view the warning using kubectl:

kubectl -n kube-system logs -l=component=kube-apiserver

The output should look similar to:

W1214 13:00:12.987884       1 admission.go:147] conflict occurred while applying podpresets: allow-database on pod:  err: merging volume mounts for allow-database has a conflict on mount path /cache:
v1.VolumeMount{Name:"other-volume", ReadOnly:false, MountPath:"/cache", SubPath:"", MountPropagation:(*v1.MountPropagationMode)(nil), SubPathExpr:""}
does not match
core.VolumeMount{Name:"cache-volume", ReadOnly:false, MountPath:"/cache", SubPath:"", MountPropagation:(*core.MountPropagationMode)(nil), SubPathExpr:""}
 in container

Note the conflict message on the path for the VolumeMount.

Deleting a PodPreset

Once you don't need a PodPreset anymore, you can delete it with kubectl:

kubectl delete podpreset allow-database

The output shows that the PodPreset was deleted:

podpreset "allow-database" deleted