Platform Engineering

Kubernetes CRDs & Operators: Extending the API, Episode 7
What Is a CRD? · CRDs You Already Use · CRD Anatomy · Write Your First CRD · CEL Validation · Controller Loop · Build an Operator · CRD Versioning · Admission Webhooks · CRDs in Production

TL;DR

• Building a Kubernetes operator means writing a Go reconciler with controller-runtime — kubebuilder scaffolds the project structure, RBAC markers, and Makefile targets so you focus on the reconcile logic
  (kubebuilder = a CLI and framework that generates the operator project scaffold; controller-runtime = the Go library that provides the informer cache, work queue, and reconciler interface)
• The reconciler for BackupPolicy in this episode creates and manages a CronJob — it is the behavior layer for the CRD built in EP03–EP05
• RBAC is expressed as Go code comments (//+kubebuilder:rbac:...) — kubebuilder generates the ClusterRole YAML from them
• Run the operator locally with make run during development; no cluster deployment needed until ready
• The same project that builds the operator also builds and installs the CRD — make install applies the CRD YAML generated from your Go types
• Testing: the operator ships with envtest — a local API server + etcd for controller testing without a real cluster

The Big Picture

  OPERATOR PROJECT STRUCTURE (kubebuilder scaffold)

  backup-operator/
  ├── api/v1alpha1/
  │   ├── backuppolicy_types.go     ← Go types that define CRD schema
  │   └── groupversion_info.go
  ├── internal/controller/
  │   └── backuppolicy_controller.go ← reconcile logic (our main focus)
  ├── config/
  │   ├── crd/                       ← generated CRD YAML
  │   ├── rbac/                      ← generated RBAC YAML
  │   └── manager/                   ← controller Deployment YAML
  ├── cmd/main.go                    ← entrypoint, sets up the manager
  └── Makefile                       ← build, test, install, deploy targets

  FLOW:
  Go types → kubebuilder generate → CRD YAML + RBAC YAML
  Reconcile function → runs in cluster → watches BackupPolicy → manages CronJobs

Building a Kubernetes operator with controller-runtime is where CRDs become living infrastructure — the BackupPolicy objects created in EP04 now get actual behavior attached to them.

Prerequisites

# Go 1.22+
go version

# kubebuilder CLI
curl -L -o kubebuilder \
  https://github.com/kubernetes-sigs/kubebuilder/releases/latest/download/kubebuilder_linux_amd64
chmod +x kubebuilder
sudo mv kubebuilder /usr/local/bin/

# A running cluster (kind works well for development)
kind create cluster --name operator-dev

# Verify kubectl works
kubectl cluster-info --context kind-operator-dev

Step 1: Scaffold the Project

mkdir backup-operator && cd backup-operator

# Initialize the Go module and project structure
kubebuilder init \
  --domain storage.example.com \
  --repo github.com/example/backup-operator

# Create the API (Go types + controller scaffold)
kubebuilder create api \
  --group storage \
  --version v1alpha1 \
  --kind BackupPolicy \
  --resource \
  --controller

When prompted:

Create Resource [y/n]: y
Create Controller [y/n]: y

The generated directory tree:

backup-operator/
├── api/
│   └── v1alpha1/
│       ├── backuppolicy_types.go
│       └── groupversion_info.go
├── internal/
│   └── controller/
│       └── backuppolicy_controller.go
├── cmd/
│   └── main.go
├── config/
│   ├── crd/bases/
│   ├── rbac/
│   └── manager/
├── go.mod
├── go.sum
└── Makefile

Step 2: Define the Go Types

Edit api/v1alpha1/backuppolicy_types.go to match the schema from EP03:

package v1alpha1

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

// BackupTarget specifies a namespace to include in the backup.
type BackupTarget struct {
    Namespace      string `json:"namespace"`
    IncludeSecrets bool   `json:"includeSecrets,omitempty"`
}

// BackupPolicySpec defines the desired state of BackupPolicy.
type BackupPolicySpec struct {
    // Schedule is a cron expression for when to run backups.
    // +kubebuilder:validation:Pattern=`^(\*|[0-9,\-\/]+) (\*|[0-9,\-\/]+) (\*|[0-9,\-\/]+) (\*|[0-9,\-\/]+) (\*|[0-9,\-\/]+)$`
    Schedule string `json:"schedule"`

    // RetentionDays is how long to keep backup snapshots.
    // +kubebuilder:validation:Minimum=1
    // +kubebuilder:validation:Maximum=365
    RetentionDays int32 `json:"retentionDays"`

    // StorageClass is the storage class to use for backup volumes.
    // +kubebuilder:default=standard
    // +kubebuilder:validation:Enum=standard;premium;encrypted;archive
    StorageClass string `json:"storageClass,omitempty"`

    // Targets lists the namespaces and resources to include.
    // +kubebuilder:validation:MaxItems=20
    Targets []BackupTarget `json:"targets,omitempty"`

    // Suspended pauses backup execution when true.
    // +kubebuilder:default=false
    Suspended bool `json:"suspended,omitempty"`
}

// BackupPolicyStatus defines the observed state of BackupPolicy.
type BackupPolicyStatus struct {
    // Conditions reflect the current state of the BackupPolicy.
    Conditions []metav1.Condition `json:"conditions,omitempty"`

    // LastBackupTime is when the most recent backup completed.
    LastBackupTime *metav1.Time `json:"lastBackupTime,omitempty"`

    // CronJobName is the name of the managed CronJob.
    CronJobName string `json:"cronJobName,omitempty"`
}

// +kubebuilder:object:root=true
// +kubebuilder:subresource:status
// +kubebuilder:printcolumn:name="Schedule",type=string,JSONPath=`.spec.schedule`
// +kubebuilder:printcolumn:name="Retention",type=integer,JSONPath=`.spec.retentionDays`
// +kubebuilder:printcolumn:name="Suspended",type=boolean,JSONPath=`.spec.suspended`
// +kubebuilder:printcolumn:name="Ready",type=string,JSONPath=`.status.conditions[?(@.type=='Ready')].status`
// +kubebuilder:printcolumn:name="Age",type=date,JSONPath=`.metadata.creationTimestamp`

// BackupPolicy is the Schema for the backuppolicies API.
type BackupPolicy struct {
    metav1.TypeMeta   `json:",inline"`
    metav1.ObjectMeta `json:"metadata,omitempty"`

    Spec   BackupPolicySpec   `json:"spec,omitempty"`
    Status BackupPolicyStatus `json:"status,omitempty"`
}

// +kubebuilder:object:root=true

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

func init() {
    SchemeBuilder.Register(&BackupPolicy{}, &BackupPolicyList{})
}

Regenerate the CRD YAML and DeepCopy methods:

make generate   # regenerates zz_generated.deepcopy.go
make manifests  # regenerates CRD YAML under config/crd/bases/

Step 3: Write the Reconciler

Edit internal/controller/backuppolicy_controller.go:

package controller

import (
    "context"
    "fmt"

    batchv1 "k8s.io/api/batch/v1"
    corev1 "k8s.io/api/core/v1"
    apierrors "k8s.io/apimachinery/pkg/api/errors"
    "k8s.io/apimachinery/pkg/api/meta"
    metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/apimachinery/pkg/runtime"
    "k8s.io/apimachinery/pkg/types"
    ctrl "sigs.k8s.io/controller-runtime"
    "sigs.k8s.io/controller-runtime/pkg/client"
    "sigs.k8s.io/controller-runtime/pkg/log"

    storagev1alpha1 "github.com/example/backup-operator/api/v1alpha1"
)

// BackupPolicyReconciler reconciles BackupPolicy objects.
type BackupPolicyReconciler struct {
    client.Client
    Scheme *runtime.Scheme
}

// RBAC markers — kubebuilder generates ClusterRole YAML from these comments.
//+kubebuilder:rbac:groups=storage.example.com,resources=backuppolicies,verbs=get;list;watch;create;update;patch;delete
//+kubebuilder:rbac:groups=storage.example.com,resources=backuppolicies/status,verbs=get;update;patch
//+kubebuilder:rbac:groups=storage.example.com,resources=backuppolicies/finalizers,verbs=update
//+kubebuilder:rbac:groups=batch,resources=cronjobs,verbs=get;list;watch;create;update;patch;delete

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

    // Step 1: Fetch the BackupPolicy
    bp := &storagev1alpha1.BackupPolicy{}
    if err := r.Get(ctx, req.NamespacedName, bp); err != nil {
        if apierrors.IsNotFound(err) {
            // Object deleted before we could reconcile — nothing to do.
            return ctrl.Result{}, nil
        }
        return ctrl.Result{}, fmt.Errorf("fetching BackupPolicy: %w", err)
    }

    // Step 2: Define the desired CronJob name
    cronJobName := fmt.Sprintf("%s-backup", bp.Name)

    // Step 3: Fetch the existing CronJob (if any)
    existing := &batchv1.CronJob{}
    err := r.Get(ctx, types.NamespacedName{Name: cronJobName, Namespace: bp.Namespace}, existing)
    notFound := apierrors.IsNotFound(err)
    if err != nil && !notFound {
        return ctrl.Result{}, fmt.Errorf("fetching CronJob: %w", err)
    }

    // Step 4: Build the desired CronJob
    desired := r.buildCronJob(bp, cronJobName)

    // Step 5: Create or update
    if notFound {
        logger.Info("Creating CronJob", "name", cronJobName)
        if err := r.Create(ctx, desired); err != nil {
            return ctrl.Result{}, fmt.Errorf("creating CronJob: %w", err)
        }
    } else {
        // Update schedule and suspend state if they differ
        if existing.Spec.Schedule != desired.Spec.Schedule ||
            existing.Spec.Suspend != desired.Spec.Suspend {
            existing.Spec.Schedule = desired.Spec.Schedule
            existing.Spec.Suspend = desired.Spec.Suspend
            logger.Info("Updating CronJob", "name", cronJobName)
            if err := r.Update(ctx, existing); err != nil {
                return ctrl.Result{}, fmt.Errorf("updating CronJob: %w", err)
            }
        }
    }

    // Step 6: Update status
    bpCopy := bp.DeepCopy()
    meta.SetStatusCondition(&bpCopy.Status.Conditions, metav1.Condition{
        Type:               "Ready",
        Status:             metav1.ConditionTrue,
        Reason:             "CronJobReady",
        Message:            fmt.Sprintf("CronJob %s is configured", cronJobName),
        ObservedGeneration: bp.Generation,
    })
    bpCopy.Status.CronJobName = cronJobName

    if err := r.Status().Update(ctx, bpCopy); err != nil {
        return ctrl.Result{}, fmt.Errorf("updating status: %w", err)
    }

    return ctrl.Result{}, nil
}

func (r *BackupPolicyReconciler) buildCronJob(bp *storagev1alpha1.BackupPolicy, name string) *batchv1.CronJob {
    suspend := bp.Spec.Suspended
    retentionArg := fmt.Sprintf("--retention-days=%d", bp.Spec.RetentionDays)

    cj := &batchv1.CronJob{
        ObjectMeta: metav1.ObjectMeta{
            Name:      name,
            Namespace: bp.Namespace,
            Labels: map[string]string{
                "app.kubernetes.io/managed-by": "backup-operator",
                "backuppolicy":                 bp.Name,
            },
        },
        Spec: batchv1.CronJobSpec{
            Schedule: bp.Spec.Schedule,
            Suspend:  &suspend,
            JobTemplate: batchv1.JobTemplateSpec{
                Spec: batchv1.JobSpec{
                    Template: corev1.PodTemplateSpec{
                        Spec: corev1.PodSpec{
                            RestartPolicy: corev1.RestartPolicyOnFailure,
                            Containers: []corev1.Container{
                                {
                                    Name:    "backup",
                                    Image:   "backup-tool:latest",
                                    Args:    []string{retentionArg},
                                },
                            },
                        },
                    },
                },
            },
        },
    }

    // Set owner reference — CronJob is garbage-collected when BackupPolicy is deleted
    _ = ctrl.SetControllerReference(bp, cj, r.Scheme)
    return cj
}

// SetupWithManager registers the controller with the manager and declares what to watch.
func (r *BackupPolicyReconciler) SetupWithManager(mgr ctrl.Manager) error {
    return ctrl.NewControllerManagedBy(mgr).
        For(&storagev1alpha1.BackupPolicy{}).
        Owns(&batchv1.CronJob{}).    // reconcile BackupPolicy when owned CronJob changes
        Complete(r)
}

Step 4: Install the CRD and Run Locally

# Install the CRD into the cluster
make install

customresourcedefinition.apiextensions.k8s.io/backuppolicies.storage.example.com created

# Run the controller locally (outside the cluster)
make run

2026-04-25T08:00:00Z  INFO  Starting manager
2026-04-25T08:00:00Z  INFO  Starting workers  {"controller": "backuppolicy", "worker count": 1}

In a separate terminal:

kubectl apply -f - <<'EOF'
apiVersion: storage.example.com/v1alpha1
kind: BackupPolicy
metadata:
  name: nightly
  namespace: default
spec:
  schedule: "0 2 * * *"
  retentionDays: 30
EOF

Watch the controller output:

2026-04-25T08:01:00Z  INFO  Creating CronJob  {"name": "nightly-backup"}

Check the result:

kubectl get bp nightly

NAME      SCHEDULE    RETENTION   SUSPENDED   READY   AGE
nightly   0 2 * * *   30          false       True    10s

kubectl get cronjob nightly-backup

NAME             SCHEDULE    SUSPEND   ACTIVE   LAST SCHEDULE   AGE
nightly-backup   0 2 * * *   False     0        <none>          10s

Test self-healing — delete the CronJob and watch the controller recreate it:

kubectl delete cronjob nightly-backup
# Controller output:
# 2026-04-25T08:02:00Z  INFO  Creating CronJob  {"name": "nightly-backup"}

kubectl get cronjob nightly-backup
# Back within seconds

Test suspend:

kubectl patch bp nightly --type=merge -p '{"spec":{"suspended":true}}'
kubectl get cronjob nightly-backup -o jsonpath='{.spec.suspend}'
# true

Step 5: Deploy to Cluster

When ready for in-cluster deployment:

# Build and push the controller image
make docker-build docker-push IMG=your-registry/backup-operator:v0.1.0

# Deploy to cluster (creates Deployment, RBAC, CRD)
make deploy IMG=your-registry/backup-operator:v0.1.0

kubectl get pods -n backup-operator-system

NAME                                          READY   STATUS    RESTARTS   AGE
backup-operator-controller-manager-abc123     2/2     Running   0          30s

Understanding the RBAC Markers

The //+kubebuilder:rbac:... comments in the controller generate the ClusterRole YAML when you run make manifests:

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

Generated YAML under config/rbac/role.yaml:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: manager-role
rules:
  - apiGroups: ["storage.example.com"]
    resources: ["backuppolicies"]
    verbs: ["get", "list", "watch", "create", "update", "patch", "delete"]
  - apiGroups: ["storage.example.com"]
    resources: ["backuppolicies/status"]
    verbs: ["get", "update", "patch"]
  - apiGroups: ["batch"]
    resources: ["cronjobs"]
    verbs: ["get", "list", "watch", "create", "update", "patch", "delete"]

This approach keeps RBAC co-located with the code that needs it — if you add a new resource access in the controller, you add the marker next to it.

⚠ Common Mistakes

Not setting an owner reference on child resources. Without ctrl.SetControllerReference(parent, child, scheme), deleting the BackupPolicy leaves orphaned CronJobs. Owner references enable automatic garbage collection of child resources.

Updating the object after r.Get() without handling conflicts. If two reconciles run concurrently (possible after a controller restart), both may try to update the same resource. The API server uses resource version for optimistic concurrency — you will get a conflict error. Retry the reconcile on conflict errors rather than failing.

Writing to bp directly instead of bp.DeepCopy() for status updates. If the status update fails and you retry, the original bp object now has the modified status in memory. Always update a deep copy when writing status so the in-memory state stays consistent with what was actually persisted.

Not watching owned resources. If you forget .Owns(&batchv1.CronJob{}) in SetupWithManager, the controller will not reconcile when a CronJob is deleted. Self-healing requires watching the resources you manage.

Quick Reference

# Scaffold a new API + controller
kubebuilder create api --group mygroup --version v1alpha1 --kind MyKind

# Regenerate deep copy methods after changing types
make generate

# Regenerate CRD YAML + RBAC from markers
make manifests

# Install CRD into current cluster
make install

# Run controller locally (outside cluster)
make run

# Build + push image, then deploy to cluster
make docker-build docker-push IMG=registry/operator:tag
make deploy IMG=registry/operator:tag

# Uninstall CRD (WARNING: deletes all instances)
make uninstall

Key Takeaways

• kubebuilder scaffolds the project; you write the types and the reconcile function
• Go struct markers (//+kubebuilder:...) generate the CRD YAML and RBAC — keep them close to the code they describe
• ctrl.SetControllerReference enables automatic garbage collection of child resources
• Always deep-copy the object before writing status; retry on conflict errors
• make run runs the controller locally — no Docker build needed during development

What’s Next

EP08: Kubernetes CRD Versioning covers how to evolve the BackupPolicy schema from v1alpha1 to v1 without breaking existing clients — storage versions, conversion webhooks, and the hub-and-spoke model for safe API evolution in production clusters.

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