Mutate Configurations

The mutate rule contains actions that will be applied to matching resource before their creation. A mutate rule can be written as a JSON Patch or as an overlay. By using a patch in the (JSONPatch - RFC 6902)[http://jsonpatch.com/] format, you can make precise changes to the resource being created. Using an overlay is convenient for describing the desired state of the resource.

Resource mutation occurs before validation, so the validation rules should not contradict the changes performed by the mutation section.

Patches

This patch adds an init container to all deployments.

apiVersion : kyverno.io/v1alpha1
kind : Policy
metadata :
  name : policy-v1
spec :
  rules:
    - name: "add-init-secrets"
      match:
        resources:
          kinds:
          - Deployment
      mutate:
        patches:
        - path: "/spec/template/spec/initContainers/0/"
          op: add
          value:
            - image: "nirmata.io/kube-vault-client:v2"
              name: "init-secrets"

JSONPatch supports the following operations (in the 'op' field):

add
replace
remove

With Kyverno, the add and replace have the same behavior i.e. both operations will add or replace the target element.

Here is the example of a patch that removes a label from the secret:

apiVersion : kyverno.io/v1alpha1
kind : Policy
metadata :
  name : policy-remove-label
spec :
  rules:
    - name: "Remove unwanted label"
      match:
        resources:
          kinds:
            - Secret
      mutate:
        patches:
        - path: "/metadata/labels/purpose"
          op: remove

Note, that if remove operation cannot be applied, then this remove operation will be skipped with no error.

Overlay

A mutation overlay describes the desired form of resource. The existing resource values are replaced with the values specified in the overlay. If a value is specified in the overlay but not present in the target resource, then it will be added to the resource. The overlay cannot be used to delete values in a resource: use patches for this purpose.

The following mutation overlay will add (or replace) the memory request and limit to 10Gi for every Pod with a label memory: high:

apiVersion : kyverno.io/v1alpha1
kind : Policy
metadata :
  name : policy-change-memory-limit
spec :
  rules:
    - name: "Set hard memory limit to 2Gi"
      match:
        resources:
          kinds:
            - Pod
          selector:
            matchLabels:
              memory: high
      mutate:
        overlay:
          spec:
            containers:
            # the wildcard * will match all containers in the list
            - (name): "*"
              resources:
                requests:
                  memory: "10Gi"
                limits:
                  memory: "10Gi"

Working with lists

Applying overlays to a list type is fairly straightforward: new items will be added to the list, unless they already exist. For example, the next overlay will add IP "192.168.10.172" to all addresses in all Endpoints:

apiVersion: kyverno.io/v1alpha1
kind: Policy
metadata:
  name: policy-endpoints
spec:
  rules:
  - name: "Add IP to subsets"
    match:
      resources:
        kinds :
          - Endpoints
    mutate:
      overlay:
        subsets:
        - addresses:
          - ip: 192.168.42.172

Conditional logic using anchors

An anchor field, marked by parentheses, allows conditional processing of configurations. Processing stops when the anchor value does not match. Once processing stops, any child elements or any remaining siblings in a list, will not be processed.

For example, this overlay will add or replace the value 6443 for the port field, for all ports with a name value that starts with "secure":

apiVersion: kyverno.io/v1alpha1
kind : Policy
metadata :
  name : policy-set-port
spec :
  rules:
  - name: "Set port"
    match:
      resources:
        kinds :
          - Endpoints
    mutate:
      overlay:
        subsets:
        - ports:
          - (name): "secure*"
            port: 6443

The anchors values support wildcards:

* - matches zero or more alphanumeric characters
? - matches a single alphanumeric character

Add if not present

A variation of an anchor, is to add a field value if it is not already defined. This is done by using the +(...) notation for the field.

For example, this overlay will set the port to 6443, if a port is not already defined:

apiVersion: kyverno.io/v1alpha1
kind : Policy
metadata :
  name : policy-set-port
spec :
  rules:
  - name: "Set port"
    match:
      resources:
        kinds :
          - Endpoints
    mutate:
      overlay:
        subsets:
        - (ports):
            +(port): 6443

Additional Details

Additional details on mutation overlay behaviors are available on the wiki: Mutation Overlay

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