sample-controller
This repository implements a simple controller for watching Foo resources as
defined with a CustomResourceDefinition (CRD).
Note: go-get or vendor this package as k8s.io/sample-controller
.
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
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/generated/
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.
Details
The sample controller uses client-go library extensively.
The details of interaction points of the sample controller with various mechanisms from this library are
explained here.
Fetch sample-controller and its dependencies
Like the rest of Kubernetes, sample-controller has used
godep and $GOPATH
for years and is
now adopting go 1.11 modules. There are thus two alternative ways to
go about fetching this demo and its dependencies.
Fetch with godep
When NOT using go 1.11 modules, you can use the following commands.
go get -d k8s.io/sample-controller
cd $GOPATH/src/k8s.io/sample-controller
godep restore
When using go 1.11 modules
When using go 1.11 modules (GO111MODULE=on
), issue the following
commands --- starting in whatever working directory you like.
git clone https://github.com/kubernetes/sample-controller.git
cd sample-controller
Note, however, that if you intend to
generate code then you will also need the
code-generator repo to exist in an old-style location. One easy way
to do this is to use the command go mod vendor
to create and
populate the vendor
directory.
A Note on kubernetes/kubernetes
If you are developing Kubernetes according to
https://github.com/kubernetes/community/blob/master/contributors/guide/github-workflow.md
then you already have a copy of this demo in
kubernetes/staging/src/k8s.io/sample-controller
and its dependencies
--- including the code generator --- are in usable locations
(valid for all Go versions).
Purpose
This is an example of how to build a kube-like controller with a single type.
Running
Prerequisite: Since the sample-controller uses apps/v1
deployments, the Kubernetes cluster version should be greater than 1.9.
# assumes you have a working kubeconfig, not required if operating in-cluster
go build -o sample-controller .
./sample-controller -kubeconfig=$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:
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
feature.
This feature is beta and enabled by default in v1.9.
Example
The schema in crd-validation.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.
In the above steps, use crd-validation.yaml
to create the CRD:
# create a CustomResourceDefinition supporting validation
kubectl create -f artifacts/examples/crd-validation.yaml
Subresources
Custom Resources support /status
and /scale
subresources as a beta feature in v1.11 and is enabled by default.
This feature is alpha in v1.10 and to enable it you need to set the CustomResourceSubresources
feature gate on the kube-apiserver:
--feature-gates=CustomResourceSubresources=true
Example
The CRD in crd-status-subresource.yaml
enables the /status
subresource
for custom resources.
This means that UpdateStatus
can be used by the controller to update only the status part of the custom resource.
To understand why only the status part of the custom resource should be updated, please refer to the Kubernetes API conventions.
In the above steps, use crd-status-subresource.yaml
to create the CRD:
# create a CustomResourceDefinition supporting the status subresource
kubectl create -f artifacts/examples/crd-status-subresource.yaml
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.