Controller Reconciliation

Reconciliation with the Operator SDK

In the Operator SDK, controllers implement the Reconciler interface.

Meaning that every controller will have a Reconcile method. 

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Typically, reconcile is triggered by a Controller in response to cluster Events (e.g. Creating, Updating,
Deleting Kubernetes objects) or external Events (GitHub Webhooks, polling external sources, etc).`

https://github.com/kubernetes-sigs/controller-runtime/blob/43331a6c8f823b497660b368deb4311ae2030206/pkg/reconcile/reconcile.go#L59

Simple example: 

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[root@kind ~]# cd $GOPATH/src 
[root@kind src]# mkdir example-operator
[root@kind src]# cd example-operator/
[root@kind example-operator]# operator-sdk init --domain=example.com --repo=github.com/andreaskaris/example-operator
Writing scaffold for you to edit...
Get controller runtime:
$ go get sigs.k8s.io/controller-runtime@v0.6.3
Update go.mod:
$ go mod tidy
Running make:
$ make
/root/go/bin/controller-gen object:headerFile="hack/boilerplate.go.txt" paths="./..."
go fmt ./...
go vet ./...
go build -o bin/manager main.go
Next: define a resource with:
$ operator-sdk create api
[root@kind example-operator]# operator-sdk create api --group example --version v1alpha1 --kind Example --resource=true --controller=true
Writing scaffold for you to edit...
api/v1alpha1/example_types.go
controllers/example_controller.go
Running make:
$ make
/root/go/bin/controller-gen object:headerFile="hack/boilerplate.go.txt" paths="./..."
go fmt ./...
go vet ./...
go build -o bin/manager main.go

The Example controller will be rendered with: 

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[root@kind example-operator]# cat controllers/example_controller.go 
/*
Copyright 2020.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package controllers

import (
    "context"

    "github.com/go-logr/logr"
    "k8s.io/apimachinery/pkg/runtime"
    ctrl "sigs.k8s.io/controller-runtime"
    "sigs.k8s.io/controller-runtime/pkg/client"

    examplev1alpha1 "github.com/andreaskaris/example-operator/api/v1alpha1"
)

// ExampleReconciler reconciles a Example object
type ExampleReconciler struct {
    client.Client
    Log    logr.Logger
    Scheme *runtime.Scheme
}

// +kubebuilder:rbac:groups=example.example.com,resources=examples,verbs=get;list;watch;create;update;patch;delete
// +kubebuilder:rbac:groups=example.example.com,resources=examples/status,verbs=get;update;patch

func (r *ExampleReconciler) Reconcile(req ctrl.Request) (ctrl.Result, error) {
    _ = context.Background()
    _ = r.Log.WithValues("example", req.NamespacedName)

    // your logic here

    return ctrl.Result{}, nil
}

func (r *ExampleReconciler) SetupWithManager(mgr ctrl.Manager) error {
    return ctrl.NewControllerManagedBy(mgr).
        For(&examplev1alpha1.Example{}).
        Complete(r)
}

Reconciliation logic goes into Reconcile. The Reconcile method is handed both a context.Context and a reconcile.Request: 

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// Request contains the information necessary to reconcile a Kubernetes object.  This includes the
// information to uniquely identify the object - its Name and Namespace.  It does NOT contain information about
// any specific Event or the object contents itself.
type Request struct {
    // NamespacedName is the name and namespace of the object to reconcile.
    types.NamespacedName
}

https://github.com/kubernetes-sigs/controller-runtime/blob/43331a6c8f823b497660b368deb4311ae2030206/pkg/reconcile/reconcile.go#L44

Excursion into some background

You can safely skip this section and continue with the next one ;-)

About the client interface

When retrieving objects from the API, we refer to the context: 

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        // retrieve sosreport CRD
        example := &examplev1alpha1.Example{}
        if err := r.Get(ctx, req.NamespacedName, example); err != nil {
                log.Error(err, "Failed to get Example custom resource")
                return ctrl.Result{}, err
        }


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Package context defines the Context type, which carries deadlines, cancellation signals, and other request-scoped values across API boundaries and between processes.

https://golang.org/pkg/context

The controller's reconciler struct inherits its methods from the Client interface: 

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// ExampleReconciler reconciles a Example object
type ExampleReconciler struct {
        client.Client
        Log    logr.Logger
        Scheme *runtime.Scheme
}


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type Client interface {
    Reader
    Writer
    StatusClient

    // Scheme returns the scheme this client is using.
    Scheme() *runtime.Scheme
    // RESTMapper returns the rest this client is using.
    RESTMapper() meta.RESTMapper
}

https://godoc.org/sigs.k8s.io/controller-runtime/pkg/client#Client

For example, the Get and List methods are inherited from Reader: 

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type Reader interface {
    // Get retrieves an obj for the given object key from the Kubernetes Cluster.
    // obj must be a struct pointer so that obj can be updated with the response
    // returned by the Server.
    Get(ctx context.Context, key ObjectKey, obj Object) error

    // List retrieves list of objects for a given namespace and list options. On a
    // successful call, Items field in the list will be populated with the
    // result returned from the server.
    List(ctx context.Context, list ObjectList, opts ...ListOption) error
}

https://godoc.org/sigs.k8s.io/controller-runtime/pkg/client#Reader

The Get method for the client is implemented here: 

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func (c *client) Get(ctx context.Context, key ObjectKey, obj Object) error {
    switch obj.(type) {
    case *unstructured.Unstructured:
        return c.unstructuredClient.Get(ctx, key, obj)
    case *metav1.PartialObjectMetadata:
        return c.metadataClient.Get(ctx, key, obj)
    default:
        return c.typedClient.Get(ctx, key, obj)
    }
}

https://github.com/kubernetes-sigs/controller-runtime/blob/43331a6c8f823b497660b368deb4311ae2030206/pkg/client/client.go#L204

typeClient then reads as follows: 

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// Get implements client.Client
func (c *typedClient) Get(ctx context.Context, key ObjectKey, obj Object) error {
    r, err := c.cache.getResource(obj)
    if err != nil {
        return err
    }
    return r.Get().
        NamespaceIfScoped(key.Namespace, r.isNamespaced()).
        Resource(r.resource()).
        Name(key.Name).Do(ctx).Into(obj)
}

https://github.com/kubernetes-sigs/controller-runtime/blob/4462fd4ebb88171af826622c1d6b73383aaf2cdf/pkg/client/typed_client.go#L135

We could go further down the rabbit hole, but: 

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// getResource returns the resource meta information for the given type of object.
// If the object is a list, the resource represents the item's type instead.
func (c *clientCache) getResource(obj runtime.Object) (*resourceMeta, error) {

https://github.com/kubernetes-sigs/controller-runtime/blob/e2261d96d733bbd8b26e1c61b138c12a2408026c/pkg/client/client_cache.go#L73

resourceMeta implements rest.Interface from "k8s.io/client-go/rest": 

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// resourceMeta caches state for a Kubernetes type.
type resourceMeta struct {
    // client is the rest client used to talk to the apiserver
    rest.Interface
    // gvk is the GroupVersionKind of the resourceMeta
    gvk schema.GroupVersionKind
    // mapping is the rest mapping
    mapping *meta.RESTMapping
}

https://github.com/kubernetes-sigs/controller-runtime/blob/e2261d96d733bbd8b26e1c61b138c12a2408026c/pkg/client/client_cache.go#L123

So, r.Get() will return a Request of type HTTP GET: https://godoc.org/k8s.io/client-go/rest#Interface

r.Get().NamespaceIfScoped(key.Namespace, r.isNamespaced()) will set the namespace if the object's kind is namespaced (GET /[ns//]): https://godoc.org/k8s.io/client-go/rest#Request.NamespaceIfScoped

r.Get().NamespaceIfScoped(key.Namespace, r.isNamespaced()).Resource(r.resource()) sets the resource to access (GET /[ns//]): https://godoc.org/k8s.io/client-go/rest#Request.Resource

r.Get().NamespaceIfScoped(key.Namespace, r.isNamespaced()).Resource(r.resource()).Name(key.Name) sets the name of a resource to access (GET /[ns//]): https://godoc.org/k8s.io/client-go/rest#Request.Name

r.Get().NamespaceIfScoped(key.Namespace, r.isNamespaced()).Resource(r.resource()).Name(key.Name).Do(ctx) formats and executes the request. Returns a Result object for easy response processing: https://godoc.org/k8s.io/client-go/rest#Request.Do

r.Get().NamespaceIfScoped(key.Namespace, r.isNamespaced()).Resource(r.resource()).Name(key.Name).Do(ctx).Into(obj) converts the Result object by writing the result into the object. Once this is successfully done, obj will now contain what was retrieved by running the query.

context, context.TODO(), context.Background()

I have to do some further research, but:

Working with Reconcile

What triggers Reconcile and what information is passed to the method?

Let's modify the Reconcile method to log whenever it is triggered and to capture the contents of the req object. 

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// +kubebuilder:rbac:groups=example.example.com,resources=examples,verbs=get;list;watch;create;update;patch;delete
// +kubebuilder:rbac:groups=example.example.com,resources=examples/status,verbs=get;update;patch

func (r *ExampleReconciler) Reconcile(req ctrl.Request) (ctrl.Result, error) {
        _ = context.Background()
        log := r.Log.WithValues("example", req) // these variables will be shown whenever log prints
        log.Info("Reconciler triggered with req:")

        // your logic here

        return ctrl.Result{}, nil
}

Compile, install: 

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[root@kind example-operator]# make generate
/root/go/bin/controller-gen object:headerFile="hack/boilerplate.go.txt" paths="./..."
[root@kind example-operator]# make manifests
/root/go/bin/controller-gen "crd:trivialVersions=true" rbac:roleName=manager-role webhook paths="./..." output:crd:artifacts:config=config/crd/bases
[root@kind example-operator]# make manifests
/root/go/bin/controller-gen "crd:trivialVersions=true" rbac:roleName=manager-role webhook paths="./..." output:crd:artifacts:config=config/crd/bases
[root@kind example-operator]# make install
/root/go/bin/controller-gen "crd:trivialVersions=true" rbac:roleName=manager-role webhook paths="./..." output:crd:artifacts:config=config/crd/bases
/root/go/bin/kustomize build config/crd | kubectl apply -f -
Warning: apiextensions.k8s.io/v1beta1 CustomResourceDefinition is deprecated in v1.16+, unavailable in v1.22+; use apiextensions.k8s.io/v1 CustomResourceDefinition
customresourcedefinition.apiextensions.k8s.io/examples.example.example.com created

And run locally: 

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[root@kind example-operator]# make run ENABLE_WEBHOOKS=false
/root/go/bin/controller-gen object:headerFile="hack/boilerplate.go.txt" paths="./..."
go fmt ./...
go vet ./...
/root/go/bin/controller-gen "crd:trivialVersions=true" rbac:roleName=manager-role webhook paths="./..." output:crd:artifacts:config=config/crd/bases
go run ./main.go
2020-11-30T14:52:01.520-0500    INFO    controller-runtime.metrics  metrics server is starting to listen    {"addr": ":8080"}
2020-11-30T14:52:01.522-0500    INFO    setup   starting manager
2020-11-30T14:52:01.524-0500    INFO    controller-runtime.manager  starting metrics server {"path": "/metrics"}
2020-11-30T14:52:01.524-0500    INFO    controller  Starting EventSource    {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "source": "kind source: /, Kind="}
2020-11-30T14:52:01.625-0500    INFO    controller  Starting Controller {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example"}
2020-11-30T14:52:01.625-0500    INFO    controller  Starting workers    {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "worker count": 1}

So far, the reconciler has not been triggered. So how can we trigger it? In comes the SetupWithManager method: 

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func (r *ExampleReconciler) SetupWithManager(mgr ctrl.Manager) error {
        return ctrl.NewControllerManagedBy(mgr).
                For(&examplev1alpha1.Example{}).
                Complete(r)
}

We'll go into more details a bit later, but this triggers execution of Reconcile of this controller whenever a Create, Update, Delete happens to an Example Custom Resource: 

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[root@kind example-operator]# cat config/samples/example_v1alpha1_example_namespace_test1.yaml
apiVersion: example.example.com/v1alpha1
kind: Example
metadata:
  name: example-sample
  namespace: test1
spec:
  # Add fields here
  foo: bar
[root@kind example-operator]# cat config/samples/example_v1alpha1_example_namespace_test2.yaml
apiVersion: example.example.com/v1alpha1
kind: Example
metadata:
  name: example-sample2
  namespace: test2
spec:
  # Add fields here
  foo: bar


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[root@kind example-operator]# oc apply -f config/samples/example_v1alpha1_example_namespace_test1.yaml
example.example.example.com/example-sample created

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2020-11-30T15:07:29.201-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-11-30T15:07:29.201-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}

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[root@kind example-operator]# oc apply -f config/samples/example_v1alpha1_example_namespace_test2.yaml
example.example.example.com/example-sample created

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2020-11-30T15:08:36.354-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test2/example-sample2"}
2020-11-30T15:08:36.354-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample2", "namespace": "test2"}

Whenever something happens to an object of type &examplev1alpha1.Example{}, this controller's Reconcile will be triggered. The only information that it will be passed is a controllerruntime.Request which is an alias to reconcile.Request and inherits from types.NamespacedName: https://godoc.org/sigs.k8s.io/controller-runtime/pkg/reconcile#Request and https://github.com/kubernetes-sigs/controller-runtime/blob/43331a6c8f823b497660b368deb4311ae2030206/alias.go#L39 and https://github.com/kubernetes/apimachinery/blob/master/pkg/types/namespacedname.go#L37

Triggering the reconciler

The SetupWithManager method determines when the reconciler is triggered: 

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import (
(...)
        (...)ctrl "sigs.k8s.io/controller-runtime"
(...)
)
(...)
func (r *ExampleReconciler) SetupWithManager(mgr ctrl.Manager) error {
        return ctrl.NewControllerManagedBy(mgr).
                For(&examplev1alpha1.Example{}).
                Complete(r)
}
(...)

NewControllerManagedBy is an alias to builder.ControllerManagedBy: https://godoc.org/sigs.k8s.io/controller-runtime and https://godoc.org/sigs.k8s.io/controller-runtime/pkg/builder#ControllerManagedBy

For

The Builder's For method tells the controller to watch all objects of this specifi custom resourse. It will then pass a NamespacedName via a Request to the reconciler. The For method is documented in: https://godoc.org/sigs.k8s.io/controller-runtime/pkg/builder#Builder.For

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func (*Builder) For

func (blder *Builder) For(object client.Object, opts ...ForOption) *Builder

For defines the type of Object being *reconciled*, and configures the ControllerManagedBy to respond to create / delete / update events by *reconciling the object*. This is the equivalent of calling Watches(&source.Kind{Type: apiType}, &handler.EnqueueRequestForObject{})

This means that one could rewrite the SetupWithManager method as: 

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func (r *ExampleReconciler) SetupWithManager(mgr ctrl.Manager) error {
        return ctrl.NewControllerManagedBy(mgr).
                Watches(&source.Kind{Type: &examplev1alpha1.Example{}}, &handler.EnqueueRequestForObject{}).
                Complete(r)
}

This doesn't quite work. something is missing here [TBD]

Owns

If we want to watch objects of other types that were created by an instance of our CRD, then we use Owns().

Add the following to the imports list: 

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        appsv1 "k8s.io/api/apps/v1"
        batchv1 "k8s.io/api/batch/v1"

Then, change the SetupWithManager method to: 

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func (r *ExampleReconciler) SetupWithManager(mgr ctrl.Manager) error {
        return ctrl.NewControllerManagedBy(mgr).
                For(&examplev1alpha1.Example{}).
                Owns(&appsv1.Deployment{}).
                Owns(&batchv1.Job{}).
                Complete(r)
}

Now, spawn a new Example object: 

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[root@kind example-operator]# cat config/samples/example_v1alpha1_example_namespace_test1.yaml
apiVersion: example.example.com/v1alpha1
kind: Example
metadata:
  name: example-sample
  namespace: test1
spec:
  # Add fields here
  foo: bar
oc apply -f config/samples/example_v1alpha1_example_namespace_test1.yaml

As expected, the reconcile loop gets triggered for this object.

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uid=$(oc get example -n test1 -o json example-sample | jq -r '.metadata.uid')
apiversion=$(oc get example -n test1 -o json example-sample | jq -r '.apiVersion')
kind=$(oc get example -n test1 -o json example-sample | jq -r '.kind')
namespace=$(oc get example -n test1 -o json example-sample | jq -r '.metadata.namespace')
name=$(oc get example -n test1 -o json example-sample | jq -r '.metadata.name')

Then, spawn a deployment and set the test1/example-sample CR as the owner: 

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cat << EOF | oc apply -f -
apiVersion: apps/v1
kind: Deployment
metadata:
  name: fedora-deployment
  namespace: $namespace
  labels:
    app: fedora-deployment
  ownerReferences:
  - kind: $kind
    apiVersion: $apiversion
    name: $name
    uid: $uid
    controller: true
spec:
  replicas: 1
  selector:
    matchLabels:
      app: fedora-pod
  template:
    metadata:
      labels:
        app: fedora-pod
    spec:
      containers:
      - name: fedora
        image: fedora
        command:
          - sleep
          - infinity
        imagePullPolicy: IfNotPresent
EOF

This will trigger a run of the Reconciler: 

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2020-12-03T03:23:14.431-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:23:14.431-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
2020-12-03T03:23:14.440-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:23:14.440-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
2020-12-03T03:23:14.457-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:23:14.457-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
2020-12-03T03:23:14.479-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:23:14.479-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
2020-12-03T03:23:15.621-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:23:15.621-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}

And if we scale the deployment, for example: 

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[root@kind example-operator]# oc scale --replicas=2 deployment fedora-deployment -n test1
deployment.apps/fedora-deployment scaled


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2020-12-03T03:26:47.860-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:26:47.860-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
2020-12-03T03:26:47.876-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:26:47.877-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
2020-12-03T03:26:47.888-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:26:47.888-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
2020-12-03T03:26:47.909-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:26:47.909-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
2020-12-03T03:26:49.645-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T03:26:49.646-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}

The Owns method can be rewritten with the Watches method (see the next section for further details). The following example behaves exactly the same as the earlier code: 

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import (
        (...)
        appsv1 "k8s.io/api/apps/v1"
        batchv1 "k8s.io/api/batch/v1"
    (...)
)
(...)
func (r *ExampleReconciler) SetupWithManager(mgr ctrl.Manager) error {
        return ctrl.NewControllerManagedBy(mgr).
                For(&examplev1alpha1.Example{}).
                Watches(&source.Kind{
                        Type: &appsv1.Deployment{}},
                        &handler.EnqueueRequestForOwner{
                                OwnerType: &examplev1alpha1.Example{},
                                IsController: true,
                        }).
                Watches(&source.Kind{
                        Type: &batchv1.Job{}},
                        &handler.EnqueueRequestForOwner{
                                OwnerType: &examplev1alpha1.Example{},
                                IsController: true,
                        }).
                Complete(r)
}

Watches for independent resources

If you want to monitor other custom resources which are not owned by this Controller, that's also possible. For some theory on this, see https://godoc.org/sigs.k8s.io/controller-runtime/pkg/handler.

Watching for changes to Nodes

For example, let's monitor updates to Nodes. If any node in the cluster is updated, then we are going to inform all of the Example Custom Resources.

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(...)
import (
(...)
        corev1 "k8s.io/api/core/v1"
        "k8s.io/apimachinery/pkg/types"
        "sigs.k8s.io/controller-runtime/pkg/reconcile" 
(...)
)
(...)
func (r *ExampleReconciler) SetupWithManager(mgr ctrl.Manager) error {
        mapFn := handler.ToRequestsFunc(
                func(a handler.MapObject) []reconcile.Request {
                        client := mgr.GetClient()
                        log := mgr.GetLogger()
                        log.Info("Detected update to resource",
                                "Kind", a.Object.GetObjectKind(),
                                "NameSpace", a.Meta.GetNamespace(),
                                "Name", a.Meta.GetName())

                        log.Info("Retrieving all Example CRs")
                        exampleList := &examplev1alpha1.ExampleList{}
                        err := client.List(context.Background(), exampleList)
                        if err != nil {
                                log.Info("Err, returning empty []reconcile.Request{}")
                                return []reconcile.Request{}
                        }
                        var reconcileRequests []reconcile.Request
                        for _, example := range exampleList.Items {
                                log.Info("Sending reconcile request to example custom resource",
                                        "Name",
                                        example.Name)
                                reconcileRequests = append(reconcileRequests, reconcile.Request{
                                        NamespacedName: types.NamespacedName{
                                                Name:      example.Name,
                                                Namespace: example.Namespace,
                                        },
                                })
                        }
                        return reconcileRequests
                })


        return ctrl.NewControllerManagedBy(mgr).
                For(&examplev1alpha1.Example{}).
                Watches(&source.Kind{Type: &corev1.Node{}},
                        &handler.EnqueueRequestsFromMapFunc{ToRequests: mapFn}).
                Complete(r)
}

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[root@kind example-operator]# oc annotate  node kind-control-plane foo=bar
node/kind-control-plane annotated
[root@kind example-operator]# 

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2020-12-03T04:11:37.062-0500    INFO    Detected update to resource {"Kind": "&TypeMeta{Kind:,APIVersion:,}", "NameSpace": "", "Name": "kind-control-plane"}
2020-12-03T04:11:37.062-0500    INFO    Retrieving all Example CRs
2020-12-03T04:11:37.062-0500    INFO    Sending reconcile request to example custom resource    {"Name": "example-sample"}
2020-12-03T04:11:37.062-0500    INFO    Detected update to resource {"Kind": "&TypeMeta{Kind:,APIVersion:,}", "NameSpace": "", "Name": "kind-control-plane"}
2020-12-03T04:11:37.062-0500    INFO    Retrieving all Example CRs
2020-12-03T04:11:37.062-0500    INFO    Sending reconcile request to example custom resource    {"Name": "example-sample"}
2020-12-03T04:11:37.062-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T04:11:37.062-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}
Watching for changes to other Custom Resources

Let's create a new custom resource, ExampleInstance: 

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operator-sdk create api --group example --version v1alpha1 --kind ExampleInstance --resource=true --controller=false
make generate
make manifests
make install

We need to make an important change to mapFn. Given that we are working with a namespaced Custom Resource, we need to make sure that we only trigger the reconciler when the namespace of the Example and ExampleInstance resources are the same: 

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(...)
                        for _, example := range exampleList.Items {
                                if a.Meta.GetNamespace() != example.Namespace {
                                        continue
                                }
(...)

Our method would look like this: 

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func (r *ExampleReconciler) SetupWithManager(mgr ctrl.Manager) error {
        mapFn := handler.ToRequestsFunc(
                func(a handler.MapObject) []reconcile.Request {
                        client := mgr.GetClient()
                        log := mgr.GetLogger()
                        log.Info("Detected update to resource",
                                "Kind", a.Object.GetObjectKind(),
                                "NameSpace", a.Meta.GetNamespace(),
                                "Name", a.Meta.GetName())

                        log.Info("Retrieving all Example CRs")
                        exampleList := &examplev1alpha1.ExampleList{}
                        err := client.List(context.Background(), exampleList)
                        if err != nil {
                                log.Info("Err, returning empty []reconcile.Request{}")
                                return []reconcile.Request{}
                        }
                        var reconcileRequests []reconcile.Request
                        for _, example := range exampleList.Items {
                                if a.Meta.GetNamespace() != example.Namespace {
                                        continue
                                }
                                log.Info("Sending reconcile request to example custom resource",
                                        "Name",
                                        example.Name)
                                reconcileRequests = append(reconcileRequests, reconcile.Request{
                                        NamespacedName: types.NamespacedName{
                                                Name:      example.Name,
                                                Namespace: example.Namespace,
                                        },
                                })
                        }
                        return reconcileRequests
                })

        return ctrl.NewControllerManagedBy(mgr).
                For(&examplev1alpha1.Example{}).
                Watches(&source.Kind{Type: &examplev1alpha1.ExampleInstance{}},
                        &handler.EnqueueRequestsFromMapFunc{ToRequests: mapFn}).
                Complete(r)
}

Now, if we test this, we can see that the reconciler is only triggered if the Namespace is correct: 

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[root@kind example-operator]# cat config/samples/example_v1alpha1_exampleinstance2.yaml
apiVersion: example.example.com/v1alpha1
kind: ExampleInstance
metadata:
  name: exampleinstance-sample
  namespace: test2
spec:
  # Add fields here
  foo: bar
[root@kind example-operator]# oc apply -f config/samples/example_v1alpha1_exampleinstance2.yaml
exampleinstance.example.example.com/exampleinstance-sample created


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2020-12-03T04:25:59.924-0500    INFO    Detected update to resource {"Kind": "&TypeMeta{Kind:,APIVersion:,}", "NameSpace": "test2", "Name": "exampleinstance-sample"}
2020-12-03T04:25:59.924-0500    INFO    Retrieving all Example CRs

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[root@kind example-operator]# cat config/samples/example_v1alpha1_exampleinstance.yaml
apiVersion: example.example.com/v1alpha1
kind: ExampleInstance
metadata:
  name: exampleinstance-sample
  namespace: test1
spec:
  # Add fields here
  foo: bar
[root@kind example-operator]# oc apply -f config/samples/example_v1alpha1_exampleinstance.yaml
exampleinstance.example.example.com/exampleinstance-sample created

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2020-12-03T04:27:01.639-0500    INFO    Detected update to resource {"Kind": "&TypeMeta{Kind:,APIVersion:,}", "NameSpace": "test1", "Name": "exampleinstance-sample"}
2020-12-03T04:27:01.639-0500    INFO    Retrieving all Example CRs
2020-12-03T04:27:01.639-0500    INFO    Sending reconcile request to example custom resource    {"Name": "example-sample"}
2020-12-03T04:27:01.639-0500    INFO    controllers.Example Reconciler triggered with req:  {"example": "test1/example-sample"}
2020-12-03T04:27:01.639-0500    DEBUG   controller  Successfully Reconciled {"reconcilerGroup": "example.example.com", "reconcilerKind": "Example", "controller": "example", "name": "example-sample", "namespace": "test1"}