flux2/rfcs/0000-custom-health-checks/README.md

# RFC-0000 Custom Health Checks for Kustomization using Common Expression Language(CEL)

**Status:** provisional

**Creation date:** 2024-01-05

**Last update:** 2025-01-17

## Summary

This RFC proposes to extend the Flux `Kustomization` API with custom health checks for
custom resources using the Common Expression Language (CEL).

In order to provide flexibility, we propose to use CEL expressions for defining the
conditions that need to be met in order to determine the status of a custom resource.
We will introduce a new field called `healthCheckExprs` in the `Kustomization` CRD
which will be a list of CEL expressions for evaluating the status of a particular
Kubernetes resource kind.

## Motivation

Flux uses the `kstatus` library during the health check phase to compute owned 
resources status. This works just fine for all the Kubernetes core resources
and custom resources that comply with the `kstatus` conventions.

There are cases where the status of a custom resource does not follow the
`kstatus` conventions. For example, we might want to compute the status of a custom
resource based on a condition other than `Ready`. This is the case for resources
that do intermediate patching like `Certificate` where you should look at the `Issued`
condition to know if the certificate has been issued or not before looking at the
`Ready` condition.

In order to provide a generic solution for custom resources, that would not imply
writing a custom `kstatus` reader for each CRD, we need to provide a way for the user
to express the conditions that need to be met in order to determine the status.
It should be done in a way that is flexible enough to cover all possible use cases,
without having to change Flux source code for each new CRD.

### Goals

- Provide a generic solution for users to customise the health check evaluation of custom resources.
- Provide a space for the community to contribute custom health checks for popular custom resources.

### Non-Goals

- We do not plan to support custom health checks for Kubernetes core resources.

## Proposal

### Introduce a new field `HealthCheckExprs` in the `Kustomization` CRD

The `HealthCheckExprs` field will be a list of `CustomHealthCheck` objects.
The `CustomHealthCheck` object fields would be: `apiVersion`, `kind`, `inProgress`,
`failed` and `current`.

To give an example, here is how we would declare a custom health check for a `Certificate`
resource:

```yaml
---
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: app-certificate
spec:
  commonName: cert-manager-tls
  dnsNames:
  - app.ns.svc.cluster.local
  ipAddresses:
  - x.x.x.x
  isCA: true
  issuerRef:
    group: cert-manager.io
    kind: ClusterIssuer
    name: app-issuer
  secretName: app-tls-certs
  subject:
    organizations:
    - example.com
```

This `Certificate` resource will transition through the following `conditions`: 
`Issuing` and `Ready`.

In order to compute the status of this resource, we need to look at both the `Issuing`
and `Ready` conditions.

The Flux `Kustomization` object used to apply the `Certificate` will look like this:

```yaml
apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
  name: certs
spec:
  interval: 5m
  prune: true
  sourceRef:
    kind: GitRepository
    name: flux-system
  path: ./certs
  wait: true
  healthCheckExprs:
  - apiVersion: cert-manager.io/v1
    kind: Certificate
    inProgress: "status.conditions.filter(e, e.type == 'Issuing').all(e, e.observedGeneration == metadata.generation && e.status == 'True')"
    failed: "status.conditions.filter(e, e.type == 'Ready').all(e, e.observedGeneration == metadata.generation && e.status == 'False')"
    current: "status.conditions.filter(e, e.type == 'Ready').all(e, e.observedGeneration == metadata.generation && e.status == 'True')"
```

The `.spec.healthCheckExprs` field contains an entry for the `Certificate` kind, its `apiVersion`,
and the CEL expressions that need to be met in order to determine the health status of all custom resources
of this kind reconciled by the Flux `Kustomization`.

Note that all the Kubernetes core resources are discarded from the `healthCheckExprs` list.

### Custom Health Check Library

To help users define custom health checks, we will provide on the [fluxcd.io](https://fluxcd.io)
website a library of custom health checks for popular custom resources.

The Flux community will be able to contribute to this library by submitting pull requests
to the [fluxcd/website](https://github.com/fluxcd/website) repository. 

### User Stories

#### Configure health checks for non-standard custom resources

> As a Flux user, I want to be able to specify health checks for
> custom resources that don't have a Ready condition, so that I can be notified
> when the status of my resources transitions to a failed state based on the evaluation
> of a different condition.

Using `.spec.healthCheckExprs`, Flux users have the ability to
specify the conditions that need to be met in order to determine the status of
a custom resource. This enables Flux to query any `.status` field,
besides the standard `Ready` condition, and evaluate it using a CEL expression.

Example for `SealedSecret` which has a `Synced` condition:

```yaml
  - apiVersion: bitnami.com/v1alpha1
    kind: SealedSecret
    inProgress: "metadata.generation != status.observedGeneration"
    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')"
```

#### Use Flux dependencies for Kubernetes ClusterAPI

> As a Flux user, I want to be able to use Flux dependencies bases on the 
> readiness of ClusterAPI resources, so that I can ensure that my applications
> are deployed only when the ClusterAPI resources are ready.

The ClusterAPI resources have a `Ready` condition, but this is set in the status
after the cluster is first created. Given this behavior, at creation time, Flux
cannot find any condition to evaluate the status of the ClusterAPI resources,
thus it considers them as static resources which are always ready.

Using `.spec.healthCheckExprs`, Flux users can specify that the `Cluster`
kind is expected to have a `Ready` condition which will force Flux into waiting
for the ClusterAPI resources status to be populated.

Example for `Cluster`:

```yaml
  - apiVersion: cluster.x-k8s.io/v1beta1
    kind: Cluster
    inProgress: "metadata.generation != status.observedGeneration"
    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')"
```

### Alternatives

We need an expression language that is flexible enough to cover all possible use
cases, without having to change Flux source code for each new use case.

An alternative that has been considered was to use `CUE` instead of `CEL`.
`CUE` lang is a more powerful expression language, but given the fact that
Kubernetes makes use of `CEL` for CRD validation and admission control,
we have decided to also use `CEL` in Flux in order to be consistent with
the Kubernetes ecosystem.

## Design Details

### Introduce a new field `HealthCheckExprs` in the `Kustomization` CRD

The `api/v1/kustomization_types.go` file will be updated to add the `HealthCheckExprs`
field to the `KustomizationSpec` struct.

```go
type KustomizationSpec struct {
	// +optional
	HealthCheckExprs []CustomHealthCheck `json:"healthCheckExprs,omitempty"`
}

type CustomHealthCheck struct {
	// APIVersion of the custom resource under evaluation.
	// +required
	APIVersion string `json:"apiVersion"`
	// Kind of the custom resource under evaluation.
	// +required
	Kind string `json:"kind"`
	// Current is the CEL expression that determines if the status
	// of the custom resource has reached the desired state.
	// +required
	Current string `json:"current"`
	// InProgress is the CEL expression that determines if the status
	// of the custom resource has not yet reached the desired state.
	// +optional
	InProgress string `json:"inProgress,omitempty"`
	// Failed is the CEL expression that determines if the status
	// of the custom resource has failed to reach the desired state.
	// +optional
	Failed string `json:"failed,omitempty"`
}
```

If a CEL expression evaluation results in an error, for example looking for a field that does not exist,
the health check will fail. Users will be encouraged to test their expressions
in the [CEL Playground](https://playcel.undistro.io/). Here is where the community maintained
[library](#custom-health-check-library) will be super useful as some of the expressions might be complex.

The `InProgress` expression is optional, when not specified the  controller will determine
if the resource is in progress if both `Failed` and `Current` evaluate to `false`.
Moreover, if the `InProgress` expression is not specified and the custom resource has a
`.status.observedGeneration` field, the controller with compare it with the `.metadata.generation`
field to determine if the resource is in progress.

The `Failed` expression is optional, when not specified the controller will keep evaluating the
`Current` expression until it returns `true`, and will give up after the timeout is reached.
Users will be encouraged to provide a `Failed` expression to avoid staling the reconciliation
loop until the timeout is reached.

### Introduce a generic custom status reader

We'll Introduce a `StatusReader` that will be used to compute the status
of custom resources based on the `CEL` expressions provided in the `CustomHealthCheck`:

```go
import (
  "k8s.io/apimachinery/pkg/runtime/schema"
  "github.com/fluxcd/cli-utils/pkg/kstatus/polling/engine"
  "github.com/fluxcd/cli-utils/pkg/kstatus/polling/event"
  kstatusreaders "github.com/fluxcd/cli-utils/pkg/kstatus/polling/statusreaders"
)

type CELStatusReader struct {
	genericStatusReader engine.StatusReader
	gvk                 schema.GroupVersionKind
}

func NewCELStatusReader(mapper meta.RESTMapper, gvk schema.GroupVersionKind, exprs map[string]string) engine.StatusReader {
	genericStatusReader := kstatusreaders.NewGenericStatusReader(mapper, genericConditions(gvk.Kind, exprs))
	return &CELStatusReader{
		genericStatusReader: genericStatusReader,
		gvk:                 gvk,
	}
}

func (g *CELStatusReader) Supports(gk schema.GroupKind) bool {
	return gk == g.gvk.GroupKind()
}

func (g *CELStatusReader) ReadStatus(ctx context.Context, reader engine.ClusterReader, resource object.ObjMetadata) (*event.ResourceStatus, error) {
	return g.genericStatusReader.ReadStatus(ctx, reader, resource)
}

func (g *CELStatusReader) ReadStatusForObject(ctx context.Context, reader engine.ClusterReader, resource *unstructured.Unstructured) (*event.ResourceStatus, error) {
	return g.genericStatusReader.ReadStatusForObject(ctx, reader, resource)
}
```

The `genericConditions` function will take a `kind` and a map of `CEL` expressions as parameters
and returns a function that takes an `Unstructured` object and returns a `status.Result` object.

````go
import (
  "github.com/fluxcd/cli-utils/pkg/kstatus/status"
  "github.com/fluxcd/pkg/runtime/cel"
  "k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
)

func genericConditions(kind string, exprs map[string]string) func(u *unstructured.Unstructured) (*status.Result, error) {
  return func(u *unstructured.Unstructured) (*status.Result, error) {
		obj := u.UnstructuredContent()

		for statusKey, expr := range exprs {
			// Use CEL to evaluate the expression
			result, err := cel.ProcessExpr(expr, obj)
			if err != nil {
				// handle error
			}
			switch statusKey {
			case status.CurrentStatus.String():
			// If the expression evaluates to true, we return the current status
			case status.FailedStatus.String():
			// If the expression evaluates to true, we return the failed status
			case status.InProgressStatus.String():
			// If the expression evaluates to true, we return the reconciling status
			}
		}
		
		// If none of the expressions evaluate to true, we return the reconciling status
	}
}
````

The CEL status reader will be used by the `statusPoller` provided to the kustomize-controller `reconciler`
to compute the status of the resources for the registered custom resources GVKs.

We will implement a `CEL` environment that will use the Kubernetes CEL library to evaluate the `CEL` expressions.

## Implementation History