This article gives a brief overview of how to use Operator SDK (Go) to create Custom Resource Definition and Controller to extend Kubernetes API and manage Custom Resources.

This is a hands-on article and requires a good understanding in Kubernetes and how operators work. The operator code can be downloaded from GitHub

Extending Kubernetes API with Operator-SDK

In this blog, have explained how to use operator-sdk to create additional resource in Kubernetes cluster. To start with you we first scaffold the project and create api using the operator-sdk cli, this gives us a starting point to develop the CRD's and Controllers. Note, not all points are explained in detail, only focus on necessary aspect required in developing operators.

I used Windows machine for development. The operator-sdk CLI was installed in WSL2. The scaffolded operator-sdk project can be opened in Visual Studio code for development to update code.

Pre-requisites:

• Basic understanding of Kubernetes and Operators
• Docker Desktop
• KIND CLI (we will use KinD cluster, install in Windows)
• kubectl Installed in WSL2 and Windows
• Go installed in WSL2
• GNU make installed and updated in WSL2
• Operator-SDK CLI installed in WSL2

About Operator framework

• Operator framework provides tools to extend the Kubernetes API, kubectl api-resource will list the existing resources from the cluster API with operator framework we can create a new resource and deploy it to the cluster.

• Operator-SDK CLI provides option to scaffold the project and updates the project with the resource coordinates like Kind, Version and Group used to identify once the new resource is deployed to cluster. Few of the files will be edited, api/<version>/<kind-name>_type.go file contains the go struct where we can define the properties this will be used to generate CRD. The reconciler logic goes into controllers/<kind-name>_controller.go, the entry point will be the main.go file. The operator-sdk project utilizes kubebuilder underneath. The details are explained later. The operator-sdk already has the code to create new manager in main.go and provides the basic code structure for reconciler logic under controllers/ folder.

• Once the CRD is deployed to the cluster, we need a controller logic which will monitor for any changes to the resources. For example, when the resource is created the controller will know of this event and in operator will invoke the defined function.

Install GoLang in WSL2

• To install GoLang, refer this Go documentation

wget https://dl.google.com/go/go1.20.4.linux-amd64.tar.gz
sudo tar -xvf go1.20.4.linux-amd64.tar.gz
sudo mv go /usr/local

• Open the ~/.bashrc file and add the below variables so it will update as environment variables

export GOROOT=/usr/local/go
export GOPATH=$HOME/go
export PATH=$GOPATH/bin:$GOROOT/bin:$PATH

• To find the Go version, issuing go version command we should see the response like below.

$ go version
go version go1.20.4 linux/amd64

Install KinD CLI in Windows

Download and install KinD CLI, this can be installed via Chocolatey, refer the documentation

The KinD cluster Kubernetes cluster configuration details.

~$ kubectl version -o yaml
clientVersion:
  buildDate: "2023-05-17T14:20:07Z"
  compiler: gc
  gitCommit: 7f6f68fdabc4df88cfea2dcf9a19b2b830f1e647
  gitTreeState: clean
  gitVersion: v1.27.2
  goVersion: go1.20.4
  major: "1"
  minor: "27"
  platform: linux/amd64
kustomizeVersion: v5.0.1
serverVersion:
  buildDate: "2023-06-15T00:36:28Z"
  compiler: gc
  gitCommit: 25b4e43193bcda6c7328a6d147b1fb73a33f1598
  gitTreeState: clean
  gitVersion: v1.27.3
  goVersion: go1.20.5
  major: "1"
  minor: "27"
  platform: linux/amd64

Install GNU make in WSL2

• To install GNU make version, simply use sudo apt install make

$ make -version
GNU Make 4.3
Built for x86_64-pc-linux-gnu
Copyright (C) 1988-2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

Install Operator-SDK CLI in WSL2

• To install operator-sdk refer the Operator SDK documentation, it is self explainotary.

• Once installed, we can view the version refer below snippet for command and output response.

$ operator-sdk version
operator-sdk version: "v1.30.0", commit: "b794fe909abc1affa1f28cfb75ceaf3bf79187e6", kubernetes version: "1.26.0", go version: "go1.19.10", GOOS: "linux", GOARCH: "amd64"

Creating new Operator-SDK project for development

• Create a new folder, using the operator-sdk CLI we can initalize the project. The operator-sdk init command will scaffold the necessary files. The command looks like below, the switch options explained below.

operator-sdk init --domain greetapp.com --repo github.com/thirumurthis/app-operator

Output of operator-sdk init command

Writing kustomize manifests for you to edit...
Writing scaffold for you to edit...
Get controller runtime:
$ go get sigs.k8s.io/controller-runtime@v0.14.1
Update dependencies:
$ go mod tidy
Next: define a resource with:
$ operator-sdk create api

Info:

• The switch option in the operator-sdk command above used to scaffold a project.
  • --domain in the command is required which will identify the resource when using kubectl api-resource

    $ kubectl api-resources | grep -e greet -e NAME
    NAME        SHORTNAMES   APIVERSION                             NAMESPACED   KIND
    greets                   greet.greetapp.com/v1alpha1            true         Greet
    

  • --repo is used to specify that this project is outside go path.

Update the scaffolded project with the API with operator-sdk CLI

• As mentioned above the API is identified by Kubernetes cluster using Kind, Version and Group, we are using operator-sdk with these coordinates as switch options to update the already created operator-sdk project.
• This will update the project structure by updating generated _type.go file and other files.
• The output of operator-sdk init command also directs this step to be next.
• The kind and version will be used in the CRD manifest.
• Command to use operator-sdk CLI to create API on the scaffolded project.

$ operator-sdk create api --group greet --version v1alpha1 --kind Greet --resource --controller

Info:

• --resource and --controller switch will update the resources and creates the controller. If not provided in the command, the CLI will prompt from user whether resources and controller required to be created for the project.

Output of operator-sdk create api command

Writing kustomize manifests for you to edit...
Writing scaffold for you to edit...
api/v1alpha1/deploymentscaler_types.go
controllers/deploymentscaler_controller.go
Update dependencies:
$ go mod tidy
Running make:
$ make generate
mkdir -p /mnt/c/goOperator/deploymentscaler/bin
test -s /mnt/c/goOperator/deploymentscaler/bin/controller-gen && /mnt/c/goOperator/deploymentscaler/bin/controller-gen --version | grep -q v0.11.1 || \
GOBIN=/mnt/c/goOperator/deploymentscaler/bin go install sigs.k8s.io/controller-tools/cmd/controller-gen@v0.11.1
/mnt/c/goOperator/deploymentscaler/bin/controller-gen object:headerFile="hack/boilerplate.go.txt" paths="./..."
Next: implement your new API and generate the manifests (e.g. CRDs,CRs) with:
$ make manifests

Note:

• The operator-sdk project includes a Makefile with bunch of useful commands that is useful during development.
• In WSL2 terminal, if we are already navigated to the project folder issue make manifests which will generate CRD file. This yaml file will be under config/crd/bases/, in this case the file name will start with greet.

Update the go mod with the latest library version

• This step is optional, the go.mod file is updated in the project structure so the latest Kubernetes libraries will be used. These are the latest version at the time of writing this article.

• The go.mod file with the updated version looks like below. Once the file is updated, issue go mod tidy to download the latest libraries. go.mod file is more like dependency manager for Go, like maven for Java or pacakge.json in node.js.

go 1.19

require (
        github.com/onsi/ginkgo/v2 v2.9.5
        github.com/onsi/gomega v1.27.7
        k8s.io/apimachinery v0.27.4
        k8s.io/client-go v0.27.4
        sigs.k8s.io/controller-runtime v0.15.0
)

Adding new property to the CRD

• Let's add new property to the CRD called Name, latter we will see how we can read this value from the reconciler code and print this in log

• The Name property should be updated in the api/v1alpha1/*_types.go, where v1alpha1 is the version used in the operator-sdk create api command.

• The new property is updated in the GreetSpec struct.

  • On top of the new property, we add two markers to perform length validation (comments marked as 1 and 2).
  • Other section of the greet_type.go file remains unchanged

Note:-

• The comment //Name of the resource above new property will be displayed in the CRD as description.
• To create a new CRD from the project use make manifests command in WSL2 the file will be created under 'config/crd/bases/'. Once deployed to the cluster the same can be viewed using kubectl explain <crd-resource-name> command.

package v1alpha1

import (
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
)

type GreetSpec struct {

    //Name of the resource 
    // +kubebuilder:validation:MaxLength=15   // --->  1 maker for validation
    // +kubebuilder:validation:MinLength=1    // --->  2 maker for validation
    Name string `json:"name"`    //------------------> 3 new property for the CRD
}

// GreetStatus defines the observed state of Greet
type GreetStatus struct {
}

//+kubebuilder:object:root=true
//+kubebuilder:subresource:status
type Greet struct {
    metav1.TypeMeta   `json:",inline"`
    metav1.ObjectMeta `json:"metadata,omitempty"`

    Spec   GreetSpec   `json:"spec,omitempty"`
    Status GreetStatus `json:"status,omitempty"`
}

//+kubebuilder:object:root=true

// GreetList contains a list of Greet
type GreetList struct {
    metav1.TypeMeta `json:",inline"`
    metav1.ListMeta `json:"metadata,omitempty"`
    Items           []Greet `json:"items"`
}

func init() {
    SchemeBuilder.Register(&Greet{}, &GreetList{})
}

• With the above code change, if we issue make manifests command, the CRD YAML file gets created under config/crd/bases/. The newly added properties will look like below.

            properties:
              name:
                description: Name of the resource
                maxLength: 15
                minLength: 1
                type: string
            required:
            - name
            type: object

Update the controller code to read the new property

• Whenever the api/v1alpha1/greet_types1file is updated, we need to use make generate command. This command will perform updates the project with the newly included properties.
• Below is the reconciler Go code created by operator-sdk under controllers.
• In the Reconcile function, the log.Log is used to log simple message
• We initialize an instance of the Greet CRD object which has Name property
• The r.Get() will fetch the CRD instance from the Cluster, in case of error when obtaining the object simply return.
• If the CRD instance is available, we get the name and log it.
• At this point there are NO changes to other files in the project.

func (r *GreetReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
    _ = log.FromContext(ctx)

    log.Log.Info("Reconciler invoked..") // -------------> Logs the message passed
    instance := &greetv1alpha1.Greet{}   //--------------> initalize an instance of the CRD 

    err := r.Get(ctx, req.NamespacedName, instance)

    if err != nil {
        return ctrl.Result{}, nil
    }

    appName := instance.Spec.Name        // Get the name from the CR deployed 

    log.Log.Info(fmt.Sprintf("app Name for CRD is - %s ",appName))
    if instance.Spec.Name != "" {
      log.Log.Info(fmt.Sprintf("appName for CRD is - %s ",instance.Spec.Name))
    } else {
      log.Log.Info("instance.Spec.Name - NOT FOUND")
    }

    return ctrl.Result{}, nil
}

Create the Custom resource for the Greet CRD

• The skeleton of the Custom Resource file will be created when we issue make manifests, where the newly added property is used here name: first-app

apiVersion: greet.greetapp.com/v1alpha1
kind: Greet
metadata:
  labels:
    app.kubernetes.io/name: greet
    app.kubernetes.io/instance: greet-sample
    app.kubernetes.io/part-of: app-op
    app.kubernetes.io/managed-by: kustomize
    app.kubernetes.io/created-by: app-op
  name: greet-sample
spec:
  # TODO(user): Add fields here
  name: first-app

Deploy operator to local KinD cluster

• The operator-sdk includes additional make targets, to help in development. Issuing below command will deploy the controller changes to local kind cluster. The output looks like below.

Note:

• In below output first deployed operator changes using make generate install run, then deployed the Custom Resource manifest.
• Once the operator sees the CR deployment a create event is received by reconciler and the based on the logic to print the log the log info will be printed.

Info:

• If make generate install run from the WSL2 terminal was not able to find the Kubernetes cluster, you can copy the kube config from the windows .kube to WSL2 path.
• Form WSL2 terminal, you can use below command to copy,

  cp /mnt/c/Users/<username>/.kube/config ~/.kube/config
  

$ make generate install run
....
go run ./main.go
2023-07-23T13:28:13-07:00       INFO    controller-runtime.metrics      Metrics server is starting to listen    {"addr": ":8080"}
2023-07-23T13:28:13-07:00       INFO    setup   starting manager
2023-07-23T13:28:13-07:00       INFO    Starting server {"kind": "health probe", "addr": "[::]:8081"}
2023-07-23T13:28:13-07:00       INFO    starting server {"path": "/metrics", "kind": "metrics", "addr": "[::]:8080"}
2023-07-23T13:28:13-07:00       INFO    Starting EventSource    {"controller": "greet", "controllerGroup": "greet.greetapp.com", "controllerKind": "Greet", "source": "kind source: *v1alpha1.Greet"}
2023-07-23T13:28:13-07:00       INFO    Starting Controller     {"controller": "greet", "controllerGroup": "greet.greetapp.com", "controllerKind": "Greet"}
2023-07-23T13:28:13-07:00       INFO    Starting workers        {"controller": "greet", "controllerGroup": "greet.greetapp.com", "controllerKind": "Greet", "worker count": 1}
2023-07-23T13:28:28-07:00       INFO    Reconciler invoked..
2023-07-23T13:28:28-07:00       INFO    app Name for CRD is - first-app
2023-07-23T13:28:28-07:00       INFO    appName for CRD is - first-app
2023-07-23T13:28:28-07:00       INFO    GET invoked reconcile for resource name - greet-sample

Output once operator deployed

• kubectl describe to describe the CRD and looks like below, note there are no event info we can add it later in next section to controller code.

Tracking Event in the Controller

• To record the events, first we need to add recorder to the GreetReconciler struct in the controllers/greet_controller.go file and import the library for record.

 import(
  ....
    "k8s.io/client-go/tools/record"    // Import the library for record
  ....
)

type GreetReconciler struct {
    client.Client
    Scheme *runtime.Scheme
    Recorder record.EventRecorder      // New Recorder event
}

• We will use the Recorder event object to record events and the code looks like below

func (r *GreetReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
    _ = log.FromContext(ctx)

    log.Log.Info("Reconciler invoked..")
        instance := &greetv1alpha1.Greet{}
    err := r.Get(ctx, req.NamespacedName, instance)

    if err != nil {
        // If there are errors we record an event.
        r.Recorder.Event(instance, corev1.EventTypeWarning, "Object", "Failed to read Object")
        return ctrl.Result{}, nil
    }

       appName := instance.Spec.Name
       // Recorder event added
       r.Recorder.Event(instance, corev1.EventTypeWarning, "Object", fmt.Sprintf("Created - %s ",appName))

       //..... other code
    return ctrl.Result{}, nil
}

• Since the GreetReconciler struct is updated to include a recorder object, the main.go needs to fetch the recorder object from the manager and set to the struct.

   if err = (&controllers.GreetReconciler{
               Client: mgr.GetClient(),
               Scheme: mgr.GetScheme(),
               Recorder: mgr.GetEventRecorderFor("greet-controller"), //Obtain the recorder
   }).SetupWithManager(mgr); err != nil {
       setupLog.Error(err, "unable to create controller", "controller", "Greet")
       os.Exit(1)
   }

Output events are displayed when we describe the CRD

• If the operator is running in local cluster stop it. make generate install run.
• Delete the CR if it already deployed using kubectl delete.
• Once deployed the operator will display the log message.

• In Recorder.Event the "Object" is the displayed as reason of the event.

 r.Recorder.Event(instance, corev1.EventTypeWarning, "Object", fmt.Sprintf("Created - %s ",appName))

Output where the Recorder.Event is updated with different value

Update the status when the Custom Resource deployed

• Once the Controller and Custom resources are deployed, we can track the status with kubectl get greet/greet-sample -w, in this section we will see the code changes required to include the APPNAME and STATUS so when using kubectl get <crd-resource-name> will provide the info which like in the snapshot.

• In the api/v1alpha1/greet_type.go file we need to add the marker like below over the corresponding Greet struct in this case.

//+kubebuilder:printcolumn:name="APPNAME",type="string",JSONPath=".spec.name",description="Name of the app"
//+kubebuilder:printcolumn:name="STATUS",type="string",JSONPath=".status.status",description="Status of the app"

• In the code snippet of the greet_type.go file, the GreetStatus struct includes new property Status which is of type string.
• In the controllers/greet_controller.go reconciler function we check the status and updated it dynamically based on resource status.

// GreetStatus defines the observed state of Greet
type GreetStatus struct {
        Status string `json:"status,omitempty"`   // ----------> newly added status property
}

//Don't leave any new line space between the marker // below marker used to display APPNAME and STATUS

//+kubebuilder:object:root=true
//+kubebuilder:printcolumn:name="APPNAME",type="string",JSONPath=".spec.name",description="Name of the app"
//+kubebuilder:printcolumn:name="STATUS",type="string",JSONPath=".status.status",description="Status of the app"
//+kubebuilder:subresource:status
// +operator-sdk:gen-csv:customresourcedefinitions.displayName="Greet App"
// +operator-sdk:gen-csv:customresourcedefinitions.resources="Deployment,v1,\"A Kubernetes Deployment of greet app\""

type Greet struct {
    metav1.TypeMeta   `json:",inline"`
    metav1.ObjectMeta `json:"metadata,omitempty"`

    Spec   GreetSpec   `json:"spec,omitempty"`
    Status GreetStatus `json:"status,omitempty"`
}

The reconciler logic to validate, set and update the status

func (r *GreetReconciler) Reconcile(ctx context.Context, req ctrl.Request) (ctrl.Result, error) {
    _ = log.FromContext(ctx)
    log.Log.Info("Reconciler invoked..")
    instance := &greetv1alpha1.Greet{}

    if err := r.Get(ctx, req.NamespacedName, instance); err != nil {
             r.Recorder.Event(instance, corev1.EventTypeWarning, "Object", "Failed to read Object")
             log.Log.Info("Error while reading the object")
             return ctrl.Result{},client.IgnoreNotFound(err)
    }

       appName := instance.Spec.Name

       if instance.Spec.Name != "" {
      log.Log.Info(fmt.Sprintf("appName for CRD is - %s ",instance.Spec.Name))
          r.Recorder.Event(instance, corev1.EventTypeWarning, "Greet", fmt.Sprintf("Created - %s ",appName))
       } else {
          log.Log.Info("instance.Spec.Name - NOT FOUND")
       }

      // update the ok when it is blank
      if instance.Status.Status == "" {
          instance.Status.Status = "OK"
          log.Log.Info("instance.Spec.Name - is set to OK")
      }

      // update the status with client ------------> Only status will be updated
      if err := r.Status().Update(ctx, instance); err != nil {
             log.Log.Info("Error while reading the object")
             return ctrl.Result{},client.IgnoreNotFound(err)
      }
      return ctrl.Result{}, nil
}

Output for status change

• Stop the operator if it was deployed in local cluster.
• Delete the Custom Resource if it was deployed to local cluster
• Deploy the operator to local cluster using make generate install run
• Deploy the custom resource yaml manifest to the local cluster using kubectl apply -f config/samples/.
• Withkubectl get greet/greet-sample -w command we can monitor the resource status.

Points that where not covered

• Deploying the operator in actual cluster by creating an image.
• We can package the CRDs to helm and deploy it via helm CLI