[RFC-0009] Add CEL custom healthchecks

content/en/flux/cheatsheets/cel-healthchecks.md

@@ -0,0 +1,52 @@
+---
+title: "CEL cheatsheet"
+linkTitle: "CEL Health Checks"
+description: "Common Expression Language (CEL) expressions for checking the health of custom resources."
+weight: 31
+---
+
+## About
+
+The Kustomization API supports defining custom logic for performing health
+checks on custom resources through the field
+[`.spec.healthCheckExprs`](/flux/components/kustomize/kustomizations/#health-check-exprs).
+This field accepts a set of Common Expression Language (CEL) expressions.
+
+Here you can find a set of community-maintained CEL expressions for popular
+custom resources.
+
+## Contributing
+
+For contributing to this library, open a pull request making changes to this file:
+
+https://github.com/fluxcd/website/blob/main/content/en/flux/cheatsheets/cel-healthchecks.md
+
+Please make sure to test your expressions and post evidence of their correctness
+in the pull request, i.e. configure a Kustomization with the expressions, verify
+that they work as expected and post logs or screenshots in the pull request.
+
+The [CEL Playground](https://playcel.undistro.io/) is a useful resource for
+testing your expressions. The input passed to each expression is the custom
+resource object itself.
+
+## Library
+
+The items in this library are sorted in alphabetical order.
+
+### `Cluster`
+
+```yaml
+- apiVersion: cluster.x-k8s.io/v1beta1
+  kind: Cluster
+  failed: status.conditions.filter(e, e.type == 'Ready').all(e, e.status == 'False')
+  current: status.conditions.filter(e, e.type == 'Ready').all(e, e.status == 'True')
+```
+
+### `SealedSecret`
+
+```yaml
+- apiVersion: bitnami.com/v1alpha1
+  kind: SealedSecret
+  failed: status.conditions.filter(e, e.type == 'Synced').all(e, e.status == 'False')
+  current: status.conditions.filter(e, e.type == 'Synced').all(e, e.status == 'True')
+```