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website/docs/tasks/inject-data-application/podpreset.md

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---
assignees:
- jessfraz
title: Injecting Information into Pods Using a PodPreset
redirect_from:
- "/docs/user-guide/pod-preset/index/"
- "/docs/user-guide/pod-preset/index.html"
- "/docs/tasks/run-application/podpreset/"
- "/docs/tasks/run-application/podpreset.html"
---
You can use a `podpreset` object to inject certain information into pods at creation
time. This information can include secrets, volumes, volume mounts, and environment
variables.
See [PodPreset proposal](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/pod-preset.md) for more information.
* TOC
{:toc}
## What is a Pod Preset?
A _Pod Preset_ is an API resource that you can use to inject additional runtime
requirements into a Pod at creation time. You use label selectors to specify
the Pods to which a given Pod Preset applies. Check out more information on [label
selectors](https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#label-selectors).
Using a Pod Preset allows pod template authors to not have to explicitly set
information for every pod. This way, authors of pod templates consuming a
specific service do not need to know all the details about that service.
## Admission Control
_Admission control_ is how Kubernetes applies Pod Presets to incoming pod
creation requests. When a pod creation request occurs, the system does the
following:
1. Retrieve all `PodPresets` available for use.
1. Match the label selector of the `PodPreset` to the pod being created.
1. Attempt to merge the various defined resources for the `PodPreset` into the
Pod being created.
1. On error, throw an event documenting the merge error on the pod, and create
the pod _without_ any injected resources from the `PodPreset`.
### Behavior
When a `PodPreset` is applied to one or more Pods, Kubernetes modifies the pod
spec. For changes to `Env`, `EnvFrom`, and `VolumeMounts`, Kubernetes modifies
the container spec for all containers in the Pod; for changes to Volume,
Kubernetes modifies the Pod Spec.
Kubernetes annotates the resulting modified pod spec to show that it was
modified by a `PodPreset`. The annotation is of the form
`podpreset.admission.kubernetes.io/<pod-preset name>": "<resource version>"`.
## Enabling Pod Preset
In order to use Pod Presets in your cluster you must ensure the
following
1. You have enabled the api type `settings.k8s.io/v1alpha1/podpreset`
1. You have enabled the admission controller `PodPreset`
1. You have defined your pod presets
## Creating a Pod Preset
### Simple Pod Spec Example
This is a simple example to show how a Pod spec is modified by the Pod
Injection Policy.
**User submitted pod spec:**
```yaml
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
spec:
containers:
- name: website
image: ecorp/website
ports:
- containerPort: 80
```
**Example Pod Preset:**
```yaml
kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
name: allow-database
namespace: myns
spec:
selector:
matchLabels:
role: frontend
env:
- name: DB_PORT
value: "6379"
volumeMounts:
- mountPath: /cache
name: cache-volume
volumes:
- name: cache-volume
emptyDir: {}
```
**Pod spec after admission controller:**
```yaml
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
annotations:
podpreset.admission.kubernetes.io/allow-database: "resource version"
spec:
containers:
- name: website
image: ecorp/website
volumeMounts:
- mountPath: /cache
name: cache-volume
ports:
- containerPort: 80
env:
- name: DB_PORT
value: "6379"
volumes:
- name: cache-volume
emptyDir: {}
```
### Pod Spec with `ConfigMap` Example
This is an example to show how a Pod spec is modified by the Pod Injection
Policy that defines a `ConfigMap` for Environment Variables.
**User submitted pod spec:**
```yaml
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
spec:
containers:
- name: website
image: ecorp/website
ports:
- containerPort: 80
```
**User submitted `ConfigMap`:**
```yaml
apiVersion: v1
kind: ConfigMap
metadata:
name: etcd-env-config
data:
number_of_members: "1"
initial_cluster_state: new
initial_cluster_token: DUMMY_ETCD_INITIAL_CLUSTER_TOKEN
discovery_token: DUMMY_ETCD_DISCOVERY_TOKEN
discovery_url: http://etcd_discovery:2379
etcdctl_peers: http://etcd:2379
duplicate_key: FROM_CONFIG_MAP
REPLACE_ME: "a value"
```
**Example Pod Preset:**
```yaml
kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
name: allow-database
namespace: myns
spec:
selector:
matchLabels:
role: frontend
env:
- name: DB_PORT
value: 6379
- name: duplicate_key
value: FROM_ENV
- name: expansion
value: $(REPLACE_ME)
envFrom:
- configMapRef:
name: etcd-env-config
volumeMounts:
- mountPath: /cache
name: cache-volume
- mountPath: /etc/app/config.json
readOnly: true
name: secret-volume
volumes:
- name: cache-volume
emptyDir: {}
- name: secret-volume
secretName: config-details
```
**Pod spec after admission controller:**
```yaml
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
annotations:
podpreset.admission.kubernetes.io/allow-database: "resource version"
spec:
containers:
- name: website
image: ecorp/website
volumeMounts:
- mountPath: /cache
name: cache-volume
- mountPath: /etc/app/config.json
readOnly: true
name: secret-volume
ports:
- containerPort: 80
env:
- name: DB_PORT
value: "6379"
- name: duplicate_key
value: FROM_ENV
- name: expansion
value: $(REPLACE_ME)
envFrom:
- configMapRef:
name: etcd-env-config
volumes:
- name: cache-volume
emptyDir: {}
- name: secret-volume
secretName: config-details
```
### ReplicaSet with Pod Spec Example
The following example shows that only the pod spec is modified by the Pod
Injection Policy.
**User submitted ReplicaSet:**
```yaml
apiVersion: settings.k8s.io/v1alpha1
kind: ReplicaSet
metadata:
name: frontend
spec:
replicas: 3
selector:
matchLabels:
tier: frontend
matchExpressions:
- {key: tier, operator: In, values: [frontend]}
template:
metadata:
labels:
app: guestbook
tier: frontend
spec:
containers:
- name: php-redis
image: gcr.io/google_samples/gb-frontend:v3
resources:
requests:
cpu: 100m
memory: 100Mi
env:
- name: GET_HOSTS_FROM
value: dns
ports:
- containerPort: 80
```
**Example Pod Preset:**
```yaml
kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
name: allow-database
namespace: myns
spec:
selector:
matchLabels:
tier: frontend
env:
- name: DB_PORT
value: "6379"
volumeMounts:
- mountPath: /cache
name: cache-volume
volumes:
- name: cache-volume
emptyDir: {}
```
**Pod spec after admission controller:**
```yaml
kind: Pod
metadata:
labels:
app: guestbook
tier: frontend
annotations:
podpreset.admission.kubernetes.io/allow-database: "resource version"
spec:
containers:
- name: php-redis
image: gcr.io/google_samples/gb-frontend:v3
resources:
requests:
cpu: 100m
memory: 100Mi
volumeMounts:
- mountPath: /cache
name: cache-volume
env:
- name: GET_HOSTS_FROM
value: dns
- name: DB_PORT
value: "6379"
ports:
- containerPort: 80
volumes:
- name: cache-volume
emptyDir: {}
```
### Multiple PodPreset Example
This is an example to show how a Pod spec is modified by multiple Pod
Injection Policies.
**User submitted pod spec:**
```yaml
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
spec:
containers:
- name: website
image: ecorp/website
ports:
- containerPort: 80
```
**Example Pod Preset:**
```yaml
kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
name: allow-database
namespace: myns
spec:
selector:
matchLabels:
role: frontend
env:
- name: DB_PORT
value: "6379"
volumeMounts:
- mountPath: /cache
name: cache-volume
volumes:
- name: cache-volume
emptyDir: {}
```
**Another Pod Preset:**
```yaml
kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
name: proxy
namespace: myns
spec:
selector:
matchLabels:
role: frontend
volumeMounts:
- mountPath: /etc/proxy/configs
name: proxy-volume
volumes:
- name: proxy-volume
emptyDir: {}
```
**Pod spec after admission controller:**
```yaml
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
annotations:
podpreset.admission.kubernetes.io/allow-database: "resource version"
podpreset.admission.kubernetes.io/proxy: "resource version"
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: {}
```
### Conflict Example
This is a example to show how a Pod spec is not modified by the Pod Injection
Policy when there is a conflict.
**User submitted pod spec:**
```yaml
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
spec:
containers:
- name: website
image: ecorp/website
volumeMounts:
- mountPath: /cache
name: cache-volume
ports:
volumes:
- name: cache-volume
emptyDir: {}
- containerPort: 80
```
**Example Pod Preset:**
```yaml
kind: PodPreset
apiVersion: settings.k8s.io/v1alpha1
metadata:
name: allow-database
namespace: myns
spec:
selector:
matchLabels:
role: frontend
env:
- name: DB_PORT
value: "6379"
volumeMounts:
- mountPath: /cache
name: other-volume
volumes:
- name: other-volume
emptyDir: {}
```
**Pod spec after admission controller will not change because of the conflict:**
```yaml
apiVersion: v1
kind: Pod
metadata:
name: website
labels:
app: website
role: frontend
spec:
containers:
- name: website
image: ecorp/website
volumeMounts:
- mountPath: /cache
name: cache-volume
ports:
volumes:
- name: cache-volume
emptyDir: {}
- containerPort: 80
```
**If we run `kubectl describe...` we can see the event:**
```
$ kubectl describe ...
....
Events:
FirstSeen LastSeen Count From SubobjectPath Reason Message
Tue, 07 Feb 2017 16:56:12 -0700 Tue, 07 Feb 2017 16:56:12 -0700 1 {podpreset.admission.kubernetes.io/allow-database } conflict Conflict on pod preset. Duplicate mountPath /cache.
```
## Deleting a Pod Preset
Once you don't need a pod preset anymore, you can delete it with `kubectl`:
```shell
$ kubectl delete podpreset allow-database
podpreset "allow-database" deleted
```
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