Kubernetes object lock

When building complex workflows in Kubernetes, prohibiting users from editing or deleting existing objects becomes neccessary. For instance, when a Pod runs a long running task it is important that the pod survises until the task is finished. Particular example of such long running task is a virtual machine snapshot started within the KubeVirt project to save the virtual machine state. However the concept is generic and can be used for locking any object in Kubernetes.

Technically, the Lock does only one thing. When update or delete is issued on existing object, the Lock checks whether that object is locked. If object is locked, request is rejected.

Although simple, currently there is no direct way of locking objects stored in Kubernetes. However, by analysing how data gets into the store, a solution can be found.

Admission controll is the exact place, where the Lock can check whether the object is locked and rejects the request. The remaining question is, how to mark object locked. There are three options:

1. Annotation/label on an object, this is first and most obvious solution. When the objects has an annotation/label bearing the lock information, request is rejected. However there is big drawback. The annotation/label is placed on an object comming from the user. Therefore it is user, who decides the object is locked. This is not what the Lock is supposed to do.
2. Annotation/label on an object in a store, second solution taking the simmilar approach. However, this time the annotation/label is placed carefully on the existing object in store, achieved preferably by higher level controller and not by direct user interaction. However, this solution would not work. The issue is every edit/delete request have to go through the same pipeline, as illustrated above. This applies even for controller api calls. Thus, the result is permanently locked object. Once the annotation/label is in place, the Lock will prohibit every edit/delete.
3. Lock object introduced as CRD. The lock object is placed in the same namespace with the same name as the object being locked. The Lock simply checks whether lock object exists. If so, the request if rejected. Unlocking object is a matter of deleting the lock object from the namespace.

The best approach is the third option. Introduce the lock object and the Lock will check upon its existence.

Implementation

The implementation is fairly straightforward and follows pretty much the same path as any Kubernetes extension. Write the controller. In this case the controller is simple https server providing the validating endpoint. Register the controller within the Kubernetes. Introduce custom resource. Profit.

Components

Controller

The "brain" component of the Lock is the controller. It is a simple https server providing single endpoint: /validate. To make it work with the Kubernetes, endpoint has to accept and return a json payload, with AdmissionReview object. AdmissionReview contains both AdmissionRequest and AdmissionResponse, containing data accordingly to context.

When asked for validation the AdmissionRequest is filled with the data belonging to the object being validated. The information contained in the request comprises of the Name and the Namespace of the object, name of the operation being performed, object Kind, which subresource is demanded, object resource and, of course, object itself.

For the purpose of a simple lock, the implementation will make use only of the Name and the Namespace, following the flow shown in following diagram.

The controller checks whether there is lock object with the same name in the same namespace as the requested object. If lock exists, the request fails with reponse "Object is locked".

There is one tricky part in the implementation. It is related to Name passed to the request. The name is only present when it is not expected that it will be generated by Kubernetes. Name generation is commonly used for Pods created by deployements, but it applies also to other higher level controllers. Luckily this is not an issue for the Lock, because locking only applies to already existing objects. Name generation applies only to new objects. Therefore to solve the missing name, simply skipping the "CREATE" operation is sufficient.

Configuration

When the controller is done, it is time to register it in the Kubernetes. The registration is straightforward. It follows the same pattern as everything else. First the controller has to be deployed to the cluster. Second the controller have to be registered in the Kubernetes API.

The overall process is fully described in the official documentation. I will focus only on the murky parts of the configuration.

Validating webhook configuration

The configuration is as ussual a YAML. The main parts are:

• clientConfig - defines where the request are going to be delivered. There are two options. Either set the URL, which can run everywhere, or set the service and configure it to run in the cluster. When service is configured, the accompanyning caBundle has to be set properly. It contains the server certificate to verify the communication. Only https is allowed to communicate with the Kubernetes API. The right value for the caBundle is the base64 encoded PEM server certificate. When in doubt, you can read great post about tool from CloudFlare how they handle certificates.
• rules - contains the rules used to filter which resources will be delivered for the validation. operations limits for which action the resource will be validated. For the Lock, only "UPDATE" and "DELETE" are relevant. apiGroups limits for which API group to validate, can be for instance kotas.tech group. apiVersions allows to limit validation only for a specific version, which is handy when testing new stuff. Finally resources enables limiting only for those resources of interest.
• namespaceSelector - is just a label selector and allows limiting validation only for desired namespace.

apiVersion: admissionregistration.k8s.io/v1beta1
kind: ValidatingWebhookConfiguration
metadata:
  name: lockvalidation-cfg
  labels:
    app: lockvalidation
webhooks:
  - name: lockvalidation.kotas.tech
    clientConfig:
      service:
        name: lockvalidation-svc
        namespace: default
        path: "/validate"
      caBundle: {{ CA_BUNDLE }}
    rules:
      - operations:
          - UPDATE
          - DELETE
        apiGroups:
          - ""
        apiVersions:
          - v1
        resources:
          - "pods"
    namespaceSelector:
      matchLabels:
        lockable: "true"

The caBundle is filled by the helper script, that generates Kubernetes secret to store the certificates for the server hack/gen_certs.sh.

Deployment

Deployment is exactly the same as any other Kubernetes deplyment. The one murky part is serviceAccountName, which attaches account to the deplyment. This is required, since the controller communicates with the Kubernetes API. Without proper account, the calls would be rejected.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: lockvalidation-dpl
  labels:
    app: lockvalidation
spec:
  replicas: 1
  selector:
    matchLabels:
      app: lockvalidation
  template:
    metadata:
      labels:
        app: lockvalidation
    spec:
      serviceAccountName: lockvalidation-sa
      containers:
        - name: lockvalidation
          image: pkotas/lockvalidation:v1
          imagePullPolicy: IfNotPresent
          resources:
            requests:
              memory: "128Mi"
              cpu: "250m"
            limits:
              memory: "256Mi"
              cpu: "500m"
          volumeMounts:
            - name: webhook-certs
              mountPath: /etc/lockvalidation/cert
              readOnly: true
      volumes:
        - name: webhook-certs
          secret:
            secretName: lockvalidation-crt

To enable https, proper certificate and key has to be added to the server. This is done via Kubernetes secret. It mounts the certificate to the pod. The secret is generated via helper script in hack/gen_certs.sh.

Also, the service to expose the controller to the cluster is required. It allows to connect to single stable endpoint, while the pod may change in time.

apiVersion: v1
kind: Service
metadata:
  name: lockvalidation-svc
  labels:
    app: lockvalidation
spec:
  ports:
  - port: 443
    targetPort: 443
  selector:
    app: lockvalidation

Custom Resource Definition

The custom resource introducing the lock object is not complicated. It follows the example given in the documentation. It does not have to be complicated, since it is not required to carry additional information. Only its mere presence is sufficient to lock the object.

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: locks.kotas.tech
spec:
  group: kotas.tech
  versions:
    - name: v1
      served: true
      storage: true
  scope: Namespaced
  names:
    plural: locks
    singular: lock
    kind: Lock
    shortNames:
    - l

Cluster role and service account

To enable controller to access the Kubernetes API, the cluster role has to be created. It is as simple as allowing to access cluster resources such as pods and resources belonging to the API group kotas.tech.

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: lockvalidation-cr
  labels:
    app: lockvalidation
rules:
- apiGroups:
  - kotas.tech
  resources:
  - "*"
  verbs:
  - "*"
- apiGroups:
  - ""
  resources:
  - pods
  verbs:
  - "*"

The cluster role has to be bounded to service account via cluster role binding.

Further work

Add support for lock configuration stored in the CRD

To enable even more refined locks, there has to be some sort of configuration. The configuration should be optinal and configurable for each object lock, as each lock can have diferent properties.

Possible solution to this, is store the configuration directly in the CRD. This would require registering validation controller for all operations and possibly for all Kinds. Filtering will be done in the controller via lock configuration.