kyverno/documentation/writing-policies-validate.md

<small>*[documentation](/README.md#documentation) / [Writing Policies](/documentation/writing-policies.md) / Validate*</small>


# Validate Configurations 

A validation rule is expressed as an overlay pattern that expresses the desired configuration. Resource configurations must match fields and expressions defined in the pattern to pass the validation rule. The following rules are followed when processing the overlay pattern:

1. Validation will fail if a field is defined in the pattern and if the field does not exist in the configuration. 
2. Undefined fields are treated as wildcards. 
3. A validation pattern field with the wildcard value '*' will match zero or more alphanumeric characters. Empty values are matched. Missing fields are not matched.
4. A validation pattern field with the wildcard value '?' will match any single alphanumeric character. Empty or missing fields are not matched. 
5. A validation pattern field with the wildcard value '?*' will match any alphanumeric characters and requires the field to be present with non-empty values.
6. A validation pattern field with the value `null` or "" (empty string) requires that the field not be defined or has no value.
7. The validation of siblings is performed only when one of the field values matches the value defined in the pattern. You can use the parenthesis operator to explictly specify a field value that must be matched. This allows writing rules like 'if fieldA equals X, then fieldB must equal Y'.
8. Validation of child values is only performed if the parent matches the pattern.

## Patterns

### Wildcards
1. `*` - matches zero or more alphanumeric characters
2. `?` - matches a single alphanumeric character

### Operators

| Operator   | Meaning                   |
|------------|---------------------------| 
| `>`        | greater than              | 
| `<`        | less than                 | 
| `>=`       | greater than or equals to |
| `<=`       | less than or equals to    | 
| `!`        | not equals                |
|  \|        | logical or                |

There is no operator for `equals` as providing a field value in the pattern requires equality to the value.

## Example
The next rule prevents the creation of Deployment, StatefuleSet and DaemonSet resources without label 'app' in selector:
````yaml

apiVersion : kyverno.io/v1alpha1
kind : Policy
metadata :
  name : validation-example
spec :
  rules:
    - name: check-label
      match:
        resources:
          # Kind specifies one or more resource types to match
          kinds:
            - Deployment
            - StatefuleSet
            - DaemonSet
          # Name is optional and can use wildcards
          name: "*"
          # Selector is optional
          selector:
      validate:
        # Message is optional
        message: "The label app is required"    
        pattern:
          spec:
            template:
              metadata:
                labels:
                  app: "?*"

````

### Check if one exist
A variation of an anchor, is to check existance of one element. This is done by using the ^(...) notation for the field.

For example, this pattern will check the existance of "name" field in the list:

````yaml
apiVersion : kyverno.io/v1alpha1
kind : Policy
metadata :
  name : validation-example2
spec :
  rules:
    - name: check-memory_requests_link_in_yaml_relative
      match:
        resources:
          # Kind specifies one or more resource types to match
          kinds:
            - Deployment
          # Name is optional and can use wildcards
          name: "*"
          # Selector is optional
          selector:
      validate:
        pattern:
          spec:
            containers:
              - ^(name): "*"
                resources:
                  requests:
                    memory: "$(<=./../../lim(its/mem)ory)"
                  lim(its:
                    mem)ory: "2048Mi"
````

Additional examples are available in [examples](/examples/)


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<small>*Read Next >> [Generate](/documentation/writing-policies-mutate.md)*</small>