# sample-controller

This repository implements a simple controller for watching Foo resources as
defined with a CustomResourceDefinition (CRD).

This particular example demonstrates how to perform basic operations such as:

* How to register a new custom resource (custom resource type) of type `Foo` using a CustomResourceDefinition.
* How to create/get/list instances of your new resource type `Foo`.
* How to setup a controller on resource handling create/update/delete events.

It makes use of the generators in [k8s.io/code-generator](https://github.com/kubernetes/code-generator)
to generate a typed client, informers, listers and deep-copy functions. You can
do this yourself using the `./hack/update-codegen.sh` script.

The `update-codegen` script will automatically generate the following files &
directories:

* `pkg/apis/samplecontroller/v1alpha1/zz_generated.deepcopy.go`
* `pkg/client/`

Changes should not be made to these files manually, and when creating your own
controller based off of this implementation you should not copy these files and
instead run the `update-codegen` script to generate your own.

## Purpose

This is an example of how to build a kube-like controller with a single type.

## Running

**Prerequisite**: Since the sample-controller uses `apps/v1` deployments, the Kubernetes cluster version should be greater than 1.9.

```sh
# assumes you have a working kubeconfig, not required if operating in-cluster
$ go run *.go -kubeconfig=$HOME/.kube/config

# create a CustomResourceDefinition
$ kubectl create -f artifacts/examples/crd.yaml

# create a custom resource of type Foo
$ kubectl create -f artifacts/examples/example-foo.yaml

# check deployments created through the custom resource
$ kubectl get deployments
```

## Use Cases

CustomResourceDefinitions can be used to implement custom resource types for your Kubernetes cluster.
These act like most other Resources in Kubernetes, and may be `kubectl apply`'d, etc.

Some example use cases:

* Provisioning/Management of external datastores/databases (eg. CloudSQL/RDS instances)
* Higher level abstractions around Kubernetes primitives (eg. a single Resource to define an etcd cluster, backed by a Service and a ReplicationController)

## Defining types

Each instance of your custom resource has an attached Spec, which should be defined via a `struct{}` to provide data format validation.
In practice, this Spec is arbitrary key-value data that specifies the configuration/behavior of your Resource.

For example, if you were implementing a custom resource for a Database, you might provide a DatabaseSpec like the following:

``` go
type DatabaseSpec struct {
	Databases []string `json:"databases"`
	Users     []User   `json:"users"`
	Version   string   `json:"version"`
}

type User struct {
	Name     string `json:"name"`
	Password string `json:"password"`
}
```

## Validation

To validate custom resources, use the [`CustomResourceValidation`](https://kubernetes.io/docs/tasks/access-kubernetes-api/extend-api-custom-resource-definitions/#validation) feature.

This feature is beta and enabled by default in v1.9. If you are using v1.8, enable the feature using
the `CustomResourceValidation` feature gate on the [kube-apiserver](https://kubernetes.io/docs/admin/kube-apiserver):

```sh
--feature-gates=CustomResourceValidation=true
```

### Example

The schema in the [example CRD](./artifacts/examples/crd.yaml) applies the following validation on the custom resource:
`spec.replicas` must be an integer and must have a minimum value of 1 and a maximum value of 10.

## Cleanup

You can clean up the created CustomResourceDefinition with:

    $ kubectl delete crd foos.samplecontroller.k8s.io

## Compatibility

HEAD of this repository will match HEAD of k8s.io/apimachinery and
k8s.io/client-go.

## Where does it come from?

`sample-controller` is synced from
https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/sample-controller.
Code changes are made in that location, merged into k8s.io/kubernetes and
later synced here.
