Creating a reference configuration - Comparing <strong>cluster</strong> configurations | Scalability and performance

Structure of the metadata.yaml file
Configuring template relationships
Configuring expected variation in a template
- Reference template functions
Configuring the metadata.yaml file to exclude template fields
Configuring inline validation for template fields
- Configuring inline validation with the regex function
- Configuring inline validation with the capturegroups function
Configuring descriptions for the output

Configure a reference configuration to validate configuration resources from a cluster.

Structure of the metadata.yaml file

The metadata.yaml file provides a central configuration point to define and configure the templates in a reference configuration. The file features a hierarchy of parts and components. parts are groups of components and components are groups of templates. Under each component, you can configure template dependencies, validation rules, and add descriptive metadata.

Example metadata.yaml file

apiVersion: v2
parts: (1)
  - name: Part1 (2)
    components:
      - name: Component1 (3)
        <component1_configuration> (4)
  - name: Part2
      - name: Component2
        <component2_configuration>

1	Every `part` typically describes a workload or a set of workloads.
2	Specify a `part` name.
3	Specify a `component` name.
4	Specify the configuration for a template. For example, define template relationships or configure what fields to use in a comparison.

Configuring template relationships

By defining relationships between templates in your reference configuration, you can support use-cases with complex dependencies. For example, you can configure a component to require specific templates, require one template from a group, or allow any template from a group, and so on.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:

Example metadata.yaml file

apiVersion: v2
parts:
  - name: Part1
    components:
      - name: Component1
        allOf: (1)
          - path: RequiredTemplate1.yaml
          - path: RequiredTemplate2.yaml
      - name: Component2
        allOrNoneOf: (2)
          - path: OptionalBlockTemplate1.yaml
          - path: OptionalBlockTemplate2.yaml
      - name: Component3
        anyOf: (3)
          - path: OptionalTemplate1.yaml
          - path: OptionalTemplate2.yaml
      - name: Component4
        noneOf: (4)
          - path: BannedTemplate1.yaml
          - path: BannedTemplate2.yaml
      - name: Component5
        oneOf: (5)
          - path: RequiredExclusiveTemplate1.yaml
          - path: RequiredExclusiveTemplate2.yaml
      - name: Component6
        anyOneOf: (6)
          - path: OptionalExclusiveTemplate1.yaml
          - path: OptionalExclusiveTemplate2.yaml
#...

1	Specifies required templates.
2	Specifies a group of templates that are either all required or all optional. If one corresponding custom resource (CR) is present in the cluster, then all corresponding CRs must be present in the cluster.
3	Specifies optional templates.
4	Specifies templates to exclude. If a corresponding CR is present in the cluster, the plugin returns a validation error.
5	Specifies templates where only one can be present. If none, or more than one of the corresponding CRs are present in the cluster, the plugin returns a validation error .
6	Specifies templates where only one can be present in the cluster. If more than one of the corresponding CRs are present in the cluster, the plugin returns a validation error.

Configuring expected variation in a template

You can handle variable content within a template by using Golang templating syntax. Using this syntax, you can configure validation logic that handles optional, required, and conditional content within the template.

The cluster-compare plugin requires all templates to render as valid YAML. To avoid parsing errors for missing fields, use conditional templating syntax such as {{- if .spec.<optional_field> }} when implementing templating syntax. This conditional logic ensures templates process missing fields gracefully and maintains valid YAML formatting.
You can use the Golang templating syntax with custom and built-in functions for complex use cases. All Golang built-in functions are supported including the functions in the Sprig library.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:

apiVersion: v2
kind: Service
metadata:
  name: frontend (1)
  namespace: {{ .metadata.namespace }}  (2)
  labels:
    app: guestbook
    tier: frontend
spec:
  {{- if and .spec.type (eq (.spec.type) "NodePort" "LoadBalancer") }}
  type: {{.spec.type }} (3)
  {{- else }}
  type: should be NodePort or LoadBalancer
  {{- end }}
  ports:
  - port: 80
  selector:
    app: guestbook
    {{- if .spec.selector.tier }} (4)
    tier: frontend
    {{- end }}

1	Configures a required field that must match the specified value.
2	Configures a required field that can have any value.
3	Configures validation for the `.spec.type` field.
4	Configures an optional field.

Reference template functions

The cluster-compare plugin supports all sprig library functions, except for the env and expandenv functions. For the full list of sprig library functions, see "Sprig Function Documentation".

The following table describes the additional template functions for the cluster-compare plugin:

Table 1. Additional **cluster**-compare template functions
Function	Description	Example
`fromJson`	Parses the incoming string as a structured JSON object.	`value: {{ obj := spec.jsontext \| fromJson }}{{ obj.field }}`
`fromJsonArray`	Parses the incoming string as a structured JSON array.	`value: {{ obj := spec.jsontext \| fromJson}}{{ index $obj 0 }}`
`fromYaml`	Parses the incoming string as a structured YAML object.	`value: {{ obj := spec.yamltext \| fromYaml }}{{ obj.field }}`
`fromYamlArray`	Parses the incoming string as a structured YAML array.	`value: {{ obj := spec.yamltext \| fromYaml}}{{ index $obj 0 }`
`toJson`	Renders incoming data as JSON while preserving object types.	`jsonstring: {{ $variable \| toJson }}`
`toToml`	Renders the incoming string as structured TOML data.	`tomlstring: {{ $variable \| toToml }}`
`toYaml`	Renders incoming data as YAML while preserving object types.	For simple scalar values: `value: {{ $data \| toYaml }}` For lists or dictionaries: `value: {{ $dict \| toYaml \| nindent 2 }}`
`doNotMatch`	Prevents a template from matching a cluster resource, even if it would normally match. You can use this function inside a template to conditionally exclude certain resources from correlation. The specified reason is logged when running with the `--verbose` flag. Templates excluded due to `doNotMatch` are not considered comparison failures. This function is especially useful when your template does not specify a fixed name or namespace. In these cases, you can use the `doNotMatch` function to exclude specific resources based on other fields, such as `labels` or `annotations`.	`{{ if $condition }}{{ doNotMatch $reason }}{{ end }}`
`lookupCRs`	Returns an array of objects that match the specified parameters. For example: `lookupCRs $apiVersion $kind $namespace $name`. If the `$namespace` parameter is an empty string (`""`) or ``, the function matches all namespaces. For cluster-scoped objects, the function matches objects with no namespace. If the `$name` is an empty string or ``, the function matches any named object.	-
`lookupCR`	Returns a single object that matches the parameters. If multiple objects match, the function returns nothing. This function takes the same arguments as the `lookupCRs` function.	-

The following example shows how to use the lookupCRs function to retrieve and render values from multiple matching resources:

Config map example using lookupCRs

kind: ConfigMap
apiVersion: v1
metadata:
  labels:
    k8s-app: kubernetes-dashboard
  name: kubernetes-dashboard-settings
  namespace: kubernetes-dashboard
data:
  dashboard: {{ index (lookupCR "apps/v1" "Deployment" "kubernetes-dashboard" "kubernetes-dashboard") "metadata" "name" \| toYaml }}
  metrics: {{ (lookupCR "apps/v1" "Deployment" "kubernetes-dashboard" "dashboard-metrics-scraper").metadata.name \| toYaml }}

The following example shows how to use the lookupCR function to retrieve and use specific values from a single matching resource:

Config map example using lookupCR

kind: ConfigMap
apiVersion: v1
metadata:
  labels:
    k8s-app: kubernetes-dashboard
  name: kubernetes-dashboard-settings
  namespace: kubernetes-dashboard
data:
  {{- $objlist := lookupCRs "apps/v1" "Deployment" "kubernetes-dashboard" "*" }}
  {{- $dashboardName := "unknown" }}
  {{- $metricsName := "unknown" }}
  {{- range $obj := $objlist }}
    {{- $appname := index $obj "metadata" "labels" "k8s-app" }}
    {{- if contains "metrics" $appname }}
      {{- $metricsName = $obj.metadata.name }}
    {{- end }}
    {{- if eq "kubernetes-dashboard" $appname }}
      {{- $dashboardName = $obj.metadata.name }}
    {{- end }}
  {{- end }}
  dashboard: {{ $dashboardName }}
  metrics: {{ $metricsName }}

Additional resources

Sprig Function Documentation

Configuring the metadata.yaml file to exclude template fields

You can configure the metadata.yaml file to exclude fields from a comparison. Exclude fields that are irrelevant to a comparison, for example annotations or labels that are inconsequential to a cluster configuration.

You can configure exclusions in the metadata.yaml file in the following ways:

Exclude all fields in a custom resource not specified in a template.
Exclude specific fields that you define using the pathToKey field.

pathToKey is a dot separated path. Use quotes to escape key values featuring a period.

Excluding all fields not specified in a template

During the comparison process, the cluster-compare plugin renders a template by merging fields from the corresponding custom resource (CR). If you configure the ignore-unspecified-fields to true, all fields that are present in the CR, but not in the template, are excluded from the merge. Use this approach when you want to focus the comparison on the fields specified in the template only.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:

apiVersion: v2
parts:
  - name: Part1
    components:
      - name: Namespace
        allOf:
          - path: namespace.yaml
            config:
              ignore-unspecified-fields: true (1)
#...

1	Specify `true` to exclude from the comparison all fields in a CR that are not explicitly configured in the corresponding `namespace.yaml` template.

Excluding specific fields by setting default exclusion fields

You can exclude fields by defining a default value for fieldsToOmitRefs in the defaultOmitRef field. This default exclusion applies to all templates, unless overridden by the config.fieldsToOmitRefs field for a specific template.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:
Example metadata.yaml file
```
apiVersion: v2
parts:

#...

fieldsToOmit:
   defaultOmitRef: default (1)
   items:
      default:
         - pathToKey: a.custom.default."k8s.io" (2)
```
1 Sets the default exclusion for all templates, unless overridden by the config.fieldsToOmitRefs field for a specific template.

2 The value is excluded for all templates.

Excluding specific fields

You can specify fields to exclude by defining the path to the field, and then referencing the definition in the config section for a template.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:

Example metadata.yaml file

apiVersion: v2
parts:
  - name: Part1
    components:
      - name: Component1
        - path: deployment.yaml
          config:
            fieldsToOmitRefs:
              - deployments (1)

#...

fieldsToOmit:
   items:
      deployments:
         - pathToKey: spec.selector.matchLabels.k8s-app (2)

1	References the `fieldsToOmit.items.deployments` item for the `deployment.yaml` template.
2	Excludes the `spec.selector.matchLabels.k8s-app` field from the comparison.

Setting fieldsToOmitRefs replaces the default value.

Excluding specific fields by setting default exclusion groups

You can create default groups of fields to exclude. A group of exclusions can reference another group to avoid duplication when defining exclusions.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:

Example metadata.yaml file

apiVersion: v2
parts:

#...

fieldsToOmit:
   defaultOmitRef: default
   items:
    common:
      - pathToKey: metadata.annotations."kubernetes.io/metadata.name"
      - pathToKey: metadata.annotations."kubernetes.io/metadata.name"
      - pathToKey: metadata.annotations."kubectl.kubernetes.io/last-applied-configuration"
      - pathToKey: metadata.creationTimestamp
      - pathToKey: metadata.generation
      - pathToKey: spec.ownerReferences
      - pathToKey: metadata.ownerReferences
    default:
      - include: common (1)
      - pathToKey: status

1	The `common` group is included in the default group.

Configuring inline validation for template fields

You can enable inline regular expressions to validate template fields, especially in scenarios where Golang templating syntax is difficult to maintain or overly complex. Using inline regular expressions simplifies templates, improves readability, and allows for more advanced validation logic.

The cluster-compare plugin provides two functions for inline validation:

regex: Validates content in a field using a regular expression.
capturegroups: Enhances multi-line text comparisons by processing non-capture group text as exact matches, applying regular expression matching only within named capture groups, and ensuring consistency for repeated capture groups.

When you use either the regex or capturegroups function for inline validation, the cluster-compare plugin enforces that identically named capture groups have the same values across multiple fields within a template. This means that if a named capture group, such as (?<username>[a-z0-9]+), appears in multiple fields, the values for that group must be consistent throughout the template.

Configuring inline validation with the regex function

Use the regex inline function to validate fields using regular expressions.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:

apiVersion: v2
parts:
- name: Part1
  components:
  - name: Example
    allOf:
    - path: example.yaml
      config:
        perField:
        - pathToKey: spec.bigTextBlock (1)
          inlineDiffFunc: regex (2)

1	Specifies the field for inline validation.
2	Enables inline validation using regular expressions.

Use a regular expression to validate the field in the associated template:

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: dashboard
data:
  username: "(?<username>[a-z0-9]+)"
  bigTextBlock: |-
    This is a big text block with some static content, like this line.
    It also has a place where (?<username>[a-z0-9]+) would put in their own name. (?<username>[a-z0-9]+) would put in their own name.

Configuring inline validation with the capturegroups function

Use the capturegroups inline function for more precise validation of fields featuring multi-line strings. This function also ensures that identically named capture groups have the same values across multiple fields.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:

apiVersion: v2
parts:
- name: Part1
  components:
  - name: Example
    allOf:
    - path: example.yaml
      config:
        perField:
        - pathToKey: data.username (1)
          inlineDiffFunc: regex (2)
        - pathToKey: spec.bigTextBlock (3)
          inlineDiffFunc: capturegroups (4)

1	Specifies the field for inline validation.
2	Enables inline validation using capture groups.
3	Specifies the multi-line field for capture-group validation.
4	Enables inline validation using capture groups.

Use a regular expression to validate the field in the associated template:

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: dashboard
data:
  username: "(?<username>[a-z0-9]+)" (1)
  bigTextBlock: |-
    This static content outside of a capture group should match exactly.
    Here is a username capture group: (?<username>[a-z0-9]+).
    It should match this capture group: (?<username>[a-z0-9]+).

1	If the username value in the `data.username` field and the value captured in `bigTextBlock` do not match, the `cluster-compare` plugin warns you about the inconsistent matching.

Example output with warning about the inconsistent matching:

WARNING: Capturegroup (?<username>…) matched multiple values: « mismatchuser | exampleuser »

Configuring descriptions for the output

Each part, component, or template can include descriptions to provide additional context, instructions, or documentation links. These descriptions are helpful to convey why a specific template or structure is required.

Procedure

Create a metadata.yaml file to match your use case. Use the following structure as an example:

apiVersion: v2
parts:
  - name: Part1
    description: |-
      General text for every template under this part, unless overridden.
    components:
      - name: Component1
        # With no description set, this inherits the description from the part above.
        OneOf:
          - path: Template1.yaml
            # This inherits the component description, if set.
          - path: Template2.yaml
          - path: Template3.yaml
            description: |-
              This template has special instructions that don't apply to the others.
      - name: Component2
        description: |-
          This overrides the part text with something more specific.
          Multi-line text is supported, at all levels.
        allOf:
          - path: RequiredTemplate1.yaml
          - path: RequiredTemplate2.yaml
            description: |-
              Required for important reasons.
          - path: RequiredTemplate3.yaml

1	Sets the default exclusion for all templates, unless overridden by the `config.fieldsToOmitRefs` field for a specific template.
2	The value is excluded for all templates.