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Docs changes to PR #4092 #4224

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Docs changes to PR #4092
chenopis Jun 29, 2017
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chenopis Jun 29, 2017
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92 changes: 45 additions & 47 deletions docs/admin/extensible-admission-controllers.md
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Expand Up @@ -12,13 +12,12 @@ title: Dynamic Admission Control

## Overview

The [admission controllers documentation](/doc/admin/admission-controllers.md)
The [admission controllers documentation](/docs/admin/admission-controllers/)
introduces how to use standard, plugin-style admission controllers. However,
plugin admission controllers are not flexible enough for all use cases, due to
the following:

* They need to be compiled into kube-apiserver.

* They are only configurable when the apiserver starts up.

1.7 introduces two alpha features, *Initializers* and *External Admission
Expand All @@ -31,34 +30,34 @@ This page describes how to use Initializers and External Admission Webhooks.

### What are initializers?

Two meanings:
*Initializer* has two meanings:

* A list of pending pre-initialization tasks, stored in every object's metadata
(e.g., "AddMyCorporatePolicySidecar").

* The controllers which actually perform those tasks. The name of the task
* A controller, which actually perform those tasks. The name of the task
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Users are supposed to write these controllers. Is this message clear? Or shall we s/A controller/a user costomized controller/?

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Good point, I'll make the change.

corresponds to the controller which performs the task. For clarity, we call
them "initializer controllers" in this page.
them *initializer controllers* in this page.

Once the controller has performed its assigned task, it removes its name from
the list. For example, it may send a PATCH that inserts a container in a pod and
also removes its name from `metadata.initalizers`. Initializers may make
also removes its name from `metadata.initializers`. Initializers may make
mutations to objects.

Objects which have a non-empty initializer list are considered uninitialized,
and are not visible in the API unless specifically requested
(`?includeUninitialized=true`).
and are not visible in the API unless specifically requested by using the query parameter,
`?includeUninitialized=true`.

### When to use initializers?

Initializers are useful for admins to force policies (e.g., the
[AlwaysPullImages](https://kubernetes.io/docs/admin/admission-controllers/#alwayspullimages)
[AlwaysPullImages](/docs/admin/admission-controllers/#alwayspullimages)
admission controller), or to inject defaults (e.g., the
[DefaultStorageClass](https://kubernetes.io/docs/admin/admission-controllers/#defaultstorageclass)
[DefaultStorageClass](/docs/admin/admission-controllers/#defaultstorageclass)
admission controller), etc.

Note that if your use case does not involve mutating objects, consider using
external admission webhooks as they have better performance.
**Note:** If your use case does not involve mutating objects, consider using
external admission webhooks, as they have better performance.

### How are initializers triggered?

Expand All @@ -69,17 +68,17 @@ all `spec.initializers[].name`s are appended to the new object's

An initializer controller should list and watch for uninitialized objects, by
using the query parameter `?includeUninitialized=true`. If using client-go, just
set the
set
[listOptions.includeUninitialized](https://github.com/kubernetes/kubernetes/blob/v1.7.0-rc.1/staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/types.go#L315)
to true.

For the observed uninitialized objects, an initializer controller should first
check if its name matches `metadata.initializers[0]`, if so, it should then
check if its name matches `metadata.initializers[0]`. If so, it should then
perform its assigned task and remove its name from the list.

### Enable initializers alpha feature

Initializers are an alpha feature, which is disabled by default. To turn it on,
*Initializers* is an alpha feature, so it is disabled by default. To turn it on,
you need to:

* Include "Initializer" in the `--admission-control` flag when starting
Expand All @@ -94,16 +93,16 @@ you need to:

### Deploy an initializer controller

We suggest that deploying an initializer controller via the [deployment
API](https://kubernetes.io/docs/api-reference/v1.6/#deployment-v1beta1-apps).
You should deploy an initializer controller via the [deployment
API](/docs/api-reference/v1.6/#deployment-v1beta1-apps).

### Configure initializers on the fly

You can configure what initializers are enabled and what resources are subject
to the initializers by creating `initializerconfigurations`.

We suggest that you first deploy the initializer controller and make sure it is
working properly before creating the `initializerconfigurations`, otherwise any
You should first deploy the initializer controller and make sure that it is
working properly before creating the `initializerconfigurations`. Otherwise, any
newly created resources will be stuck in an uninitialized state.

The following is an example `initiallizerConfiguration`.
Expand All @@ -129,21 +128,21 @@ spec:
```

Make sure that all expansions of the `<apiGroup, apiVersions, resources>` tuple
in a `rule` are valid; if they are not, separate them in different `rules`.
in a `rule` are valid. If they are not, separate them in different `rules`.

After you create the `initializerConfiguration`, the system will take a few
After you create the `initializerConfiguration`, the system will take a few
seconds to honor the new configuration.

## External Admission Webhooks

### What are external admission webhooks?

External admission webhooks are HTTP callbacks that are intended to receive
admission requests and do something with them. What the external admission
webhook does is up to you but there is an
admission requests and do something with them. What an external admission
webhook does is up to you, but there is an
[interface](https://github.com/kubernetes/kubernetes/blob/v1.7.0-rc.1/pkg/apis/admission/v1alpha1/types.go)
the external admission webhook must follow and that is to respond back with
whether or not the admission request should be allowed.
that it must adhere to so that it responds with whether or not the
admission request should be allowed.

Unlike initializers or the plugin-style admission controllers, external
admission webhooks are not allowed to mutate the admission request in any way.
Expand All @@ -153,12 +152,11 @@ must support TLS.

### When to use admission webhooks?

A good example use of an external admission webhook is to do semantic validation
of Kubernetes resources. Imagine your infrastructure requires that all `Pod`
resources have a common set of labels and you do not want any `Pod` to be
A simple example use case for an external admission webhook is to do semantic validation
of Kubernetes resources. Suppose that your infrastructure requires that all `Pod`
resources have a common set of labels, and you do not want any `Pod` to be
persisted to Kubernetes if those needs are not met. You could write your
external admission webhook to do this validation and respond accordingly. Of
course this is a very simple case but you get the idea.
external admission webhook to do this validation and respond accordingly.

### How are external admission webhooks triggered?

Expand All @@ -167,11 +165,11 @@ get the list of interested external admission webhooks from
`externalAdmissionHookConfiguration` objects (explained below) and call them in
parallel. If **all** of the external admission webhooks approve the admission
request, the admission chain continues. If **any** of the external admission
webhooks deny the admission request, the admission request will be denied and
webhooks deny the admission request, the admission request will be denied, and
the reason for doing so will be based on the _first_ external admission webhook
denial reason. _(This means if there is more than one external admission webhook
denial reason. _This means if there is more than one external admission webhook
that denied the admission request, only the first will be returned to the
user.)_ If there is an error encountered when calling an external admission
user._ If there is an error encountered when calling an external admission
webhook, that request is ignored and will not be used to approve/deny the
admission request.

Expand All @@ -180,7 +178,7 @@ admission request.

### Enable external admission webhooks

External Admission Webhooks is an alpha feature, which is disabled by default.
*External Admission Webhooks* is an alpha feature, so it is disabled by default.
To turn it on, you need to

* Include "GenericAdmissionWebhook" in the `--admission-control` flag when
Expand All @@ -195,7 +193,7 @@ To turn it on, you need to

### Write a webhook admission controller

See [here](https://github.com/caesarxuchao/example-webhook-admission-controller)
See [caesarxuchao/example-webhook-admission-controller](https://github.com/caesarxuchao/example-webhook-admission-controller)
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@lavalamp you might want to take a look at my example. I'll consolidate it a little bit more tonight.

for an example webhook admission controller.

The communication between the webhook admission controller and the apiserver, or
Expand All @@ -215,24 +213,24 @@ gathers information like `object`, `oldobject`, and `userInfo`, from

### Deploy the webhook admission controller

See [here](https://github.com/caesarxuchao/example-webhook-admission-controller/tree/master/deployment)
See [caesarxuchao/example-webhook-admission-controller deployment](https://github.com/caesarxuchao/example-webhook-admission-controller/tree/master/deployment)
for an example deployment.

We suggest that deploying the webhook admission controller via the [deployment
API](https://kubernetes.io/docs/api-reference/v1.6/#deployment-v1beta1-apps).
The webhook admission controller should be deployed via the
[deployment API](/docs/api-reference/v1.6/#deployment-v1beta1-apps).
You also need to create a
[service](https://kubernetes.io/docs/api-reference/v1.6/#service-v1-core) as the
[service](/docs/api-reference/v1.6/#service-v1-core) as the
front-end of the deployment.

### Configure webhook admission controller on the fly

You can configure what webhook admission controller are enabled and what
You can configure what webhook admission controllers are enabled and what
resources are subject to the admission controller via creating
externaladmissionhookconfigurations.

We suggest that you first deploy the webhook admission controller and make sure
it is working properly before creating the externaladmissionhookconfigurations,
otherwise depending whether the webhook is configured as fail open or fail
it is working properly before creating the externaladmissionhookconfigurations.
Otherwise, depending whether the webhook is configured as fail open or fail
closed, operations will be unconditionally accepted or rejected.

The following is an example externaladmissionhookconfiguration.
Expand Down Expand Up @@ -275,12 +273,12 @@ differences:
* The `resources` field accepts subresources in the form or resource/subresource.

Make sure that all expansions of the `<apiGroup, apiVersions,resources>` tuple
in a `rule` are valid; if they are not, separate them to different `rules`.
in a `rule` are valid. If they are not, separate them to different `rules`.

You can also specify the `failurePolicy`. In 1.7, the system supports `Ignore`
and `Fail` policies, meaning upon an communication error with the webhook
admission controller, if the `GenericAdmissionWebhook` will admit or reject the
operation.
and `Fail` policies, meaning that upon a communication error with the webhook
admission controller, the `GenericAdmissionWebhook` can admit or reject the
operation based on the configured policy.

After you create the `initializerConfiguration`, the system will take a few
After you create the `initializerConfiguration`, the system will take a few
seconds to honor the new configuration.