Kubernetes Operators — Writing a Custom Operator

Cloud Expert

Kubernetes Operators — Writing a Custom Operator¶

KubernetesOperatorsGoController 6 min read

Developing a Kubernetes operator with the Operator SDK. Reconciliation loop, finalizers and status management.

Operator Pattern¶

Operator = CRD + Controller. Codifies operational knowledge into software.

• CRD — defines desired state
• Controller — reconciles actual to desired state
• Reconciliation loop — observe → diff → act

Scaffolding¶

operator-sdk init --domain example.com --repo github.com/org/db-operator
operator-sdk create api --group database --version v1 --kind Database --resource --controller

# Struktura
├── api/v1/
│   └── database_types.go
├── controllers/
│   └── database_controller.go
├── config/
│   ├── crd/
│   ├── rbac/
│   └── manager/
└── main.go

Reconciliation Loop¶

func (r *DatabaseReconciler) Reconcile(ctx context.Context,
    req ctrl.Request) (ctrl.Result, error) {

    db := &databasev1.Database{}
    if err := r.Get(ctx, req.NamespacedName, db); err != nil {
        return ctrl.Result{}, client.IgnoreNotFound(err)
    }

    // Create StatefulSet if not exists
    sts := &appsv1.StatefulSet{}
    err := r.Get(ctx, types.NamespacedName{
        Name: db.Name, Namespace: db.Namespace,
    }, sts)
    if errors.IsNotFound(err) {
        sts = r.constructStatefulSet(db)
        if err := r.Create(ctx, sts); err != nil {
            return ctrl.Result{}, err
        }
    }

    // Update status
    db.Status.Ready = sts.Status.ReadyReplicas == *sts.Spec.Replicas
    r.Status().Update(ctx, db)

    return ctrl.Result{RequeueAfter: 30 * time.Second}, nil
}

Finalizers¶

const dbFinalizer = "database.example.com/finalizer"

if !db.DeletionTimestamp.IsZero() {
    if controllerutil.ContainsFinalizer(db, dbFinalizer) {
        // Cleanup
        r.dropDatabase(ctx, db)
        controllerutil.RemoveFinalizer(db, dbFinalizer)
        r.Update(ctx, db)
    }
    return ctrl.Result{}, nil
}

Summary¶

Kubernetes operators codify operational knowledge into code. Reconciliation loop, finalizers and status management are the fundamental building blocks.

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