CEL Conditionals

This guide covers how objects can be changed or excluded based on conditions on the configuration or environment.

Here are a few use cases that can be accomplished using this feature:

  • Create different objects depending on the version of Kubernetes.
  • Support different installation modes.
  • Support multiple deployment topologies.
  • Support differences across Kubernetes distributions (e.g. Red Hat OpenShift).

Examples are available in the package-operator/examples repository within the 4_conditionals folder.

Common Expression Language - CEL

The Common Expression Language (CEL) is used in Package Operator to declare inclusion rules and other constraints or conditions. The CEL feature set is limited to expressions with a Boolean return type (to ensure type safety in Go).

Common Expression Language in Kubernetes

1. Go Templates

Go templates can used to exclude objects.
Package Operator tolerates templates evaluating to empty strings and just skips these objects. This also works in multi-document YAML files.
CEL expressions can be used in templates as well, see CEL Expressions.

Example:

{{ if eq .config.banana "bread" -}}
apiVersion: v1
kind: ConfigMap
metadata:
  name: filter-via-template
  annotations:
    package-operator.run/phase: deploy
{{ end }}
---
# other YAML document

2. CEL via Annotation

Custom CEL rules can be attached to any object with the package-operator.run/condition annotation.
The object will only be included if the expression evaluates to true.

Example:

apiVersion: v1
kind: ConfigMap
metadata:
  name: filter-via-annotation
  annotations:
    package-operator.run/phase: deploy
    package-operator.run/condition: .config.banana == "bread"

3. CEL via Glob

CEL rules can also be attached to folder globs to exclude whole parts of the file system tree. These rules can be specified as part of the PackageManifest.

Example:

apiVersion: manifests.package-operator.run/v1alpha1
kind: PackageManifest
spec:
  # [...]
  filter:
    paths:
    - expression: '.config.banana == "bread"'
      glob: 'subfolder/**'

4. Reuse Expressions

To make refactoring and the creation of more complex packages easier, CEL expressions can be setup once in the PackageManifest and referenced from either Globs or annotations.

Example:

apiVersion: manifests.package-operator.run/v1alpha1
kind: PackageManifest
spec:
  # [...]
  filter:
    # reusable conditions:
    conditions:
    - name: config_banana_bread
      expression: '.config.banana == "bread"'
    paths:
    - expression: 'cond.config_banana_bread'
      glob: 'subfolder/**'

Example use in Annotation:

apiVersion: v1
kind: ConfigMap
metadata:
  name: filter-via-annotation
  annotations:
    package-operator.run/phase: deploy
    # This annotation can contain a CEL expression controlling if this object should be included or not.
    # In this example we are referencing the named condition "config_banana_bread" from the PackageManifest.
    package-operator.run/condition: cond.config_banana_bread
← Templates from in-cluster Sources