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Enforce CRD Immutability with CEL Transition Rules

By Alexander Zielenski (Google) | Thursday, September 29, 2022

Immutable fields can be found in a few places in the built-in Kubernetes types. For example, you can't change the .metadata.name of an object. Specific objects have fields where changes to existing objects are constrained; for example, the .spec.selector of a Deployment.

Aside from simple immutability, there are other common design patterns such as lists which are append-only, or a map with mutable values and immutable keys.

Until recently the best way to restrict field mutability for CustomResourceDefinitions has been to create a validating admission webhook: this means a lot of complexity for the common case of making a field immutable.

Beta since Kubernetes 1.25, CEL Validation Rules allow CRD authors to express validation constraints on their fields using a rich expression language, CEL. This article explores how you can use validation rules to implement a few common immutability patterns directly in the manifest for a CRD.

Basics of validation rules

The new support for CEL validation rules in Kubernetes allows CRD authors to add complicated admission logic for their resources without writing any code!

For example, A CEL rule to constrain a field maximumSize to be greater than a minimumSize for a CRD might look like the following:

rule: |
    self.maximumSize > self.minimumSize    
message: 'Maximum size must be greater than minimum size.'

The rule field contains an expression written in CEL. self is a special keyword in CEL which refers to the object whose type contains the rule.

The message field is an error message which will be sent to Kubernetes clients whenever this particular rule is not satisfied.

For more details about the capabilities and limitations of Validation Rules using CEL, please refer to validation rules. The CEL specification is also a good reference for information specifically related to the language.

Immutability patterns with CEL validation rules

This section implements several common use cases for immutability in Kubernetes CustomResourceDefinitions, using validation rules expressed as kubebuilder marker comments. Resultant OpenAPI generated by the kubebuilder marker comments will also be included so that if you are writing your CRD manifests by hand you can still follow along.

Project setup

To use CEL rules with kubebuilder comments, you first need to set up a Golang project structure with the CRD defined in Go.

You may skip this step if you are not using kubebuilder or are only interested in the resultant OpenAPI extensions.

Begin with a folder structure of a Go module set up like the following. If you have your own project already set up feel free to adapt this tutorial to your liking:

graph LR . --> generate.go . --> pkg --> apis --> stable.example.com --> v1 v1 --> doc.go v1 --> types.go . --> tools.go

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This is the typical folder structure used by Kubernetes projects for defining new API resources.

doc.go contains package-level metadata such as the group and the version:

// +groupName=stable.example.com
// +versionName=v1
package v1

types.go contains all type definitions in stable.example.com/v1

package v1

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

// An empty CRD as an example of defining a type using controller tools
// +kubebuilder:storageversion
// +kubebuilder:subresource:status
type TestCRD struct {
   metav1.TypeMeta   `json:",inline"`
   metav1.ObjectMeta `json:"metadata,omitempty"`

   Spec   TestCRDSpec   `json:"spec,omitempty"`
   Status TestCRDStatus `json:"status,omitempty"`
}

type TestCRDStatus struct {}
type TestCRDSpec struct {
   // You will fill this in as you go along
}

tools.go contains a dependency on controller-gen which will be used to generate the CRD definition:

//go:build tools

package celimmutabilitytutorial

// Force direct dependency on code-generator so that it may be executed with go run
import (
   _ "sigs.k8s.io/controller-tools/cmd/controller-gen"
)

Finally, generate.gocontains a go:generate directive to make use of controller-gen. controller-gen parses our types.go and creates generates CRD yaml files into a crd folder:

package celimmutabilitytutorial

//go:generate go run sigs.k8s.io/controller-tools/cmd/controller-gen crd paths=./pkg/apis/... output:dir=./crds

You may now want to add dependencies for our definitions and test the code generation:

cd cel-immutability-tutorial
go mod init <your-org>/<your-module-name>
go mod tidy
go generate ./...

After running these commands you now have completed the basic project structure. Your folder tree should look like the following:

graph LR . --> crds --> stable.example.com_testcrds.yaml . --> generate.go . --> go.mod . --> go.sum . --> pkg --> apis --> stable.example.com --> v1 v1 --> doc.go v1 --> types.go . --> tools.go

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The manifest for the example CRD is now available in crds/stable.example.com_testcrds.yaml.

Immutablility after first modification

A common immutability design pattern is to make the field immutable once it has been first set. This example will throw a validation error if the field after changes after being first initialized.

// +kubebuilder:validation:XValidation:rule="!has(oldSelf.value) || has(self.value)", message="Value is required once set"
type ImmutableSinceFirstWrite struct {
   metav1.TypeMeta   `json:",inline"`
   metav1.ObjectMeta `json:"metadata,omitempty"`

   // +kubebuilder:validation:Optional
   // +kubebuilder:validation:XValidation:rule="self == oldSelf",message="Value is immutable"
   // +kubebuilder:validation:MaxLength=512
   Value string `json:"value"`
}

The +kubebuilder directives in the comments inform controller-gen how to annotate the generated OpenAPI. The XValidation rule causes the rule to appear among the x-kubernetes-validations OpenAPI extension. Kubernetes then respects the OpenAPI spec to enforce our constraints.

To enforce a field's immutability after its first write, you need to apply the following constraints:

Field must be allowed to be initially unset +kubebuilder:validation:Optional
Once set, field must not be allowed to be removed: !has(oldSelf.value) | has(self.value) (type-scoped rule)
Once set, field must not be allowed to change value self == oldSelf (field-scoped rule)

Also note the additional directive +kubebuilder:validation:MaxLength. CEL requires that all strings have attached max length so that it may estimate the computation cost of the rule. Rules that are too expensive will be rejected. For more information on CEL cost budgeting, check out the other tutorial.

Example usage

Generating and installing the CRD should succeed:

# Ensure the CRD yaml is generated by controller-gen
go generate ./...
kubectl apply -f crds/stable.example.com_immutablesincefirstwrites.yaml

customresourcedefinition.apiextensions.k8s.io/immutablesincefirstwrites.stable.example.com created

Creating initial empty object with no value is permitted since value is optional:

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: ImmutableSinceFirstWrite
metadata:
  name: test1
EOF

immutablesincefirstwrite.stable.example.com/test1 created

The initial modification of value succeeds:

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: ImmutableSinceFirstWrite
metadata:
  name: test1
value: Hello, world!
EOF

immutablesincefirstwrite.stable.example.com/test1 configured

An attempt to change value is blocked by the field-level validation rule. Note the error message shown to the user comes from the validation rule.

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: ImmutableSinceFirstWrite
metadata:
  name: test1
value: Hello, new world!
EOF

The ImmutableSinceFirstWrite "test1" is invalid: value: Invalid value: "string": Value is immutable

An attempt to remove the value field altogether is blocked by the other validation rule on the type. The error message also comes from the rule.

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: ImmutableSinceFirstWrite
metadata:
  name: test1
EOF

The ImmutableSinceFirstWrite "test1" is invalid: <nil>: Invalid value: "object": Value is required once set

Generated schema

Note that in the generated schema there are two separate rule locations. One is directly attached to the property immutable_since_first_write. The other rule is associated with the crd type itself.

openAPIV3Schema:
  properties:
    value:
      maxLength: 512
      type: string
      x-kubernetes-validations:
      - message: Value is immutable
        rule: self == oldSelf
  type: object
  x-kubernetes-validations:
  - message: Value is required once set
    rule: '!has(oldSelf.value) || has(self.value)'

Immutability upon object creation

A field which is immutable upon creation time is implemented similarly to the earlier example. The difference is that that field is marked required, and the type-scoped rule is no longer necessary.

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

   // +kubebuilder:validation:Required
   // +kubebuilder:validation:XValidation:rule="self == oldSelf",message="Value is immutable"
   // +kubebuilder:validation:MaxLength=512
   Value string `json:"value"`
}

This field will be required when the object is created, and after that point will not be allowed to be modified. Our CEL Validation Rule self == oldSelf

Usage example

Generating and installing the CRD should succeed:

# Ensure the CRD yaml is generated by controller-gen
go generate ./...
kubectl apply -f crds/stable.example.com_immutablesincecreations.yaml

customresourcedefinition.apiextensions.k8s.io/immutablesincecreations.stable.example.com created

Applying an object without the required field should fail:

kubectl apply -f - <<EOF
apiVersion: stable.example.com/v1
kind: ImmutableSinceCreation
metadata:
  name: test1
EOF

The ImmutableSinceCreation "test1" is invalid:
* value: Required value
* <nil>: Invalid value: "null": some validation rules were not checked because the object was invalid; correct the existing errors to complete validation

Now that the field has been added, the operation is permitted:

kubectl apply -f - <<EOF
apiVersion: stable.example.com/v1
kind: ImmutableSinceCreation
metadata:
  name: test1
value: Hello, world!
EOF

immutablesincecreation.stable.example.com/test1 created

If you attempt to change the value, the operation is blocked due to the validation rules in the CRD. Note that the error message is as it was defined in the validation rule.

kubectl apply -f - <<EOF
apiVersion: stable.example.com/v1
kind: ImmutableSinceCreation
metadata:
  name: test1
value: Hello, new world!
EOF

The ImmutableSinceCreation "test1" is invalid: value: Invalid value: "string": Value is immutable

Also if you attempted to remove value altogether after adding it, you will see an error as expected:

kubectl apply -f - <<EOF
apiVersion: stable.example.com/v1
kind: ImmutableSinceCreation
metadata:
  name: test1
EOF

The ImmutableSinceCreation "test1" is invalid:
* value: Required value
* <nil>: Invalid value: "null": some validation rules were not checked because the object was invalid; correct the existing errors to complete validation

Generated schema

openAPIV3Schema:
  properties:
    value:
      maxLength: 512
      type: string
      x-kubernetes-validations:
      - message: Value is immutable
        rule: self == oldSelf
  required:
  - value
  type: object

Append-only list of containers

In the case of ephemeral containers on Pods, Kubernetes enforces that the elements in the list are immutable, and can’t be removed. The following example shows how you could use CEL to achieve the same behavior.

// +kubebuilder:validation:XValidation:rule="!has(oldSelf.value) || has(self.value)", message="Value is required once set"
type AppendOnlyList struct {
   metav1.TypeMeta   `json:",inline"`
   metav1.ObjectMeta `json:"metadata,omitempty"`

   // +kubebuilder:validation:Optional
   // +kubebuilder:validation:MaxItems=100
   // +kubebuilder:validation:XValidation:rule="oldSelf.all(x, x in self)",message="Values may only be added"
   Values []v1.EphemeralContainer `json:"value"`
}

Once set, field must not be deleted: !has(oldSelf.value) || has(self.value) (type-scoped)
Once a value is added it is not removed: oldSelf.all(x, x in self) (field-scoped)
Value may be initially unset: +kubebuilder:validation:Optional

Note that for cost-budgeting purposes, MaxItems is also required to be specified.

Example usage

Generating and installing the CRD should succeed:

# Ensure the CRD yaml is generated by controller-gen
go generate ./...
kubectl apply -f crds/stable.example.com_appendonlylists.yaml

customresourcedefinition.apiextensions.k8s.io/appendonlylists.stable.example.com created

Creating an initial list with one element inside should succeed without problem:

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: AppendOnlyList
metadata:
  name: testlist
value:
  - name: container1
    image: nginx/nginx
EOF

appendonlylist.stable.example.com/testlist created

Adding an element to the list should also proceed without issue as expected:

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: AppendOnlyList
metadata:
  name: testlist
value:
  - name: container1
    image: nginx/nginx
  - name: container2
    image: mongodb/mongodb
EOF

appendonlylist.stable.example.com/testlist configured

But if you now attempt to remove an element, the error from the validation rule is triggered:

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: AppendOnlyList
metadata:
  name: testlist
value:
  - name: container1
    image: nginx/nginx
EOF

The AppendOnlyList "testlist" is invalid: value: Invalid value: "array": Values may only be added

Additionally, to attempt to remove the field once it has been set is also disallowed by the type-scoped validation rule.

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: AppendOnlyList
metadata:
  name: testlist
EOF

The AppendOnlyList "testlist" is invalid: <nil>: Invalid value: "object": Value is required once set

Generated schema

openAPIV3Schema:
  properties:
    value:
      items: ...
      maxItems: 100
      type: array
      x-kubernetes-validations:
      - message: Values may only be added
        rule: oldSelf.all(x, x in self)
  type: object
  x-kubernetes-validations:
  - message: Value is required once set
    rule: '!has(oldSelf.value) || has(self.value)'

Map with append-only keys, immutable values

// A map which does not allow keys to be removed or their values changed once set. New keys may be added, however.
// +kubebuilder:validation:XValidation:rule="!has(oldSelf.values) || has(self.values)", message="Value is required once set"
type MapAppendOnlyKeys struct {
	metav1.TypeMeta   `json:",inline"`
	metav1.ObjectMeta `json:"metadata,omitempty"`

	// +kubebuilder:validation:Optional
	// +kubebuilder:validation:MaxProperties=10
	// +kubebuilder:validation:XValidation:rule="oldSelf.all(key, key in self && self[key] == oldSelf[key])",message="Keys may not be removed and their values must stay the same"
	Values map[string]string `json:"values,omitempty"`
}

Once set, field must not be deleted: !has(oldSelf.values) || has(self.values) (type-scoped)
Once a key is added it is not removed nor is its value modified: oldSelf.all(key, key in self && self[key] == oldSelf[key]) (field-scoped)
Value may be initially unset: +kubebuilder:validation:Optional

Example usage

Generating and installing the CRD should succeed:

# Ensure the CRD yaml is generated by controller-gen
go generate ./...
kubectl apply -f crds/stable.example.com_mapappendonlykeys.yaml

customresourcedefinition.apiextensions.k8s.io/mapappendonlykeys.stable.example.com created

Creating an initial object with one key within values should be permitted:

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: MapAppendOnlyKeys
metadata:
  name: testmap
values:
    key1: value1
EOF

mapappendonlykeys.stable.example.com/testmap created

Adding new keys to the map should also be permitted:

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: MapAppendOnlyKeys
metadata:
  name: testmap
values:
    key1: value1
    key2: value2
EOF

mapappendonlykeys.stable.example.com/testmap configured

But if a key is removed, the error messagr from the validation rule should be returned:

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: MapAppendOnlyKeys
metadata:
  name: testmap
values:
    key1: value1
EOF

The MapAppendOnlyKeys "testmap" is invalid: values: Invalid value: "object": Keys may not be removed and their values must stay the same

If the entire field is removed, the other validation rule is triggered and the operation is prevented. Note that the error message for the validation rule is shown to the user.

kubectl apply -f - <<EOF
---
apiVersion: stable.example.com/v1
kind: MapAppendOnlyKeys
metadata:
  name: testmap
EOF

The MapAppendOnlyKeys "testmap" is invalid: <nil>: Invalid value: "object": Value is required once set

Generated schema

openAPIV3Schema:
  description: A map which does not allow keys to be removed or their values
    changed once set. New keys may be added, however.
  properties:
    values:
      additionalProperties:
        type: string
      maxProperties: 10
      type: object
      x-kubernetes-validations:
      - message: Keys may not be removed and their values must stay the same
        rule: oldSelf.all(key, key in self && self[key] == oldSelf[key])
  type: object
  x-kubernetes-validations:
  - message: Value is required once set
    rule: '!has(oldSelf.values) || has(self.values)'

Going further

The above examples showed how CEL rules can be added to kubebuilder types. The same rules can be added directly to OpenAPI if writing a manifest for a CRD by hand.

For native types, the same behavior can be achieved using kube-openapi’s marker +validations.

Usage of CEL within Kubernetes Validation Rules is so much more powerful than what has been shown in this article. For more information please check out validation rules in the Kubernetes documentation and CRD Validation Rules Beta blog post.

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