[RFC] Flux Bootstrap for OCI-compliant Container Registries

rfcs/000X-flux-bootstrap-oci/README.md

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+# [RFC] Flux Bootstrap for OCI-compliant Container Registries
+
+**Status:** provisional
+
+**Creation date:** 2024-04-27
+
+**Last update:** 2024-04-27
+
+## Summary
+
+Flux should allow a Git-less bootstrap procedure where the cluster desired state is stored in OCI artifacts.
+
+On the client-side, the Flux CLI should offer a command for packaging its own Kubernetes manifests into
+an OCI artifact and pushing the artifact to a container registry.
+
+On the server-side, the Flux controllers should be configured to self-update from the registry
+and reconcile the cluster state from OCI artifacts stored in the same or a different registry.
+
+## Motivation
+
+Given that OCI registries are evolving into a generic artifact storage solution,
+we should allow Flux users who don't want to run a Git server as part of their
+production infrastructure to bootstrap and manage their Kubernetes clusters using OCI artifacts.
+
+To decouple the clusters reconciliation from the Git repositories, Flux allows packaging and publishing
+the Kubernetes manifests stored in Git to an OCI registry by running the `flux push artifact`
+command in CI pipelines.
+
+### Goals
+
+- Add support to the Flux CLI for bootstrapping with a container registry as the source of truth.
+- Make it easy for users to switch from Git repositories to OCI repositories.
+
+### Non-Goals
+
+- Automate the migration of Flux manifests from a Git bootstrap repository to OCI.
+
+## Proposal
+
+Implement the `flux bootstrap oci` command with the following specifications:
+
+```shell
+flux bootstrap oci \
+--url=<registry-url>/<flux-manifests>:<tag> \
+--username=<registry-username> \
+--password=<registry-password> \
+--kustomization=<local/path/to/kustomization.yaml> \
+--cluster-url=<registry-url>/<fleet-manifests>:<tag> \
+--cluster-path=<path/inside/oci/artifact>
+```
+
+The Terraform/OpenTofu counterpart is the `flux_bootstrap_oci` provider that exposes
+the same configuration options as the CLI.
+
+The bootstrap operations are split into two phases:
+
+- Install and self-update configuration for the Flux components.
+- Cluster state reconciliation configuration.
+
+### Install and self-update configuration
+
+The command performs the following steps based on the `url`, `username`,
+`password` and `kustomization` arguments:
+
+1. Logs in to the OCI registry using the provided credentials.
+2. Generates an OCI artifact from the Flux components manifests and the `kustomization.yaml` file.
+3. Applies the Flux components manifests along with their customisations to the cluster.
+4. Pushes the OCI artifact to the container registry using the specified tag.
+5. Generates an image pull secret, an OCIRepository that points to the OCI artifact and
+   a Flux Kustomization object that reconciles the OCI artifact contents.
+6. Applies the image pull secret, OCIRepository and Flux Kustomization to the cluster.
+
+Artifacts pushed to the registry:
+- `<registry-url>/<flux-manifests>:<checksum>` (immutable artifact)
+- `<registry-url>/<flux-manifests>:<tag>` (tag pointing to the immutable artifact)
+
+Objects created by the command in the `flux-system` namespace:
+- `flux-components` Secret
+- `flux-components` OCIRepository
+- `flux-components` Kustomization
+
+### Cluster state reconciliation configuration
+
+After the OCIRepository and Flux Kustomization called `flux` become ready, the command
+continues with the following steps:
+
+1. Logs in to the OCI registry where the cluster artifacts are stored using the provided credentials.
+2. If the cluster OCI artifact is not found, an empty artifact is created
+   and pushed to the registry using the provided tag.
+3. Generates an image pull secret, an OCIRepository and a Flux Kustomization object
+   that reconciles the cluster OCI artifact contents.
+4. Applies the image pull secret, OCIRepository and Flux Kustomization to the cluster.
+
+Objects created by the command in the `flux-system` namespace:
+- `flux-system` Secret
+- `flux-system` OCIRepository
+- `flux-system` Kustomization
+
+If the cluster registry is the same as the Flux components registry, the command could reuse the
+`flux-components` image pull secret.
+
+### Registry authentication
+
+The `flux bootstrap oci` command supports the following authentication methods:
+
+- Basic authentication with `--username` and `--password`. The credentials are stored in a Kubernetes Secret.
+- OIDC authentication with `--provider=<aws|azure|gcp>`. No credentials are stored in the cluster, source-controller
+  will use Kubernetes Workload Identity to authenticate to the registry.
+
+To avoid passing the credentials as CLI flags, the password can be read from the standard input, e.g.:
+`echo <password> | flux bootstrap oci` or using an environment variable `OCI_PASSWORD`.
+
+If the registry is self-hosted and uses a self-signed TLS certificate,
+the root CA certificate can be provided with the `--ca-file` flag.
+
+If the registry is exposed on HTTP and not HTTPS, the `--allow-insecure-http`
+flag can be used to force non-TLS connections.
+
+### Signing and verification
+
+The `flux bootstrap oci` command supports the following signing and verification methods:
+
+- Cosign
+- Notation
+
+TODO: Add more details about the signing and verification methods, flags and options.
+
+### User Stories
+
+#### Story 1
+
+> As a platform operator I want to bootstrap a Kubernetes cluster with Flux
+> using OCI artifacts stored in a container registry.
+
+The following example demonstrates how to bootstrap a Flux instance using GitHub Container Registry
+as the OCI registry for Flux components and the cluster state.
+
+```shell
+flux bootstrap oci \
+--url=ghcr.io/stefanprodan/flux-manifests:production \
+--username=<ghcr-username> \
+--password=<ghcr-token> \
+--kustomization=flux-manifests/kustomization.yaml \
+--cluster-url=ghcr.io/stefanprodan/fleet-manifests:production \
+--cluster-username=<ghcr-username> \
+--cluster-password=<ghcr-token> \
+--cluster-path=clusters/production
+```
+
+Generated OCI artifacts:
+
+- `ghcr.io/stefanprodan/flux-manifests:88b028f`
+- `ghcr.io/stefanprodan/flux-manifests:production`
+- `ghcr.io/stefanprodan/fleet-manifests:6f7a258`
+- `ghcr.io/stefanprodan/fleet-manifests:production`
+
+Objects created in the `flux-system` namespace:
+
+Flux components reconciliation:
+
+```yaml
+apiVersion: source.toolkit.fluxcd.io/v1beta2
+kind: OCIRepository
+metadata:
+  name: flux-components
+  namespace: flux-system
+spec:
+  interval: 1m
+  url: oci://ghcr.io/stefanprodan/flux-manifests
+  ref:
+    tag: production
+  secretRef:
+    name: flux-components
+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: flux-components
+  namespace: flux-system
+spec:
+    interval: 1h
+    retryInterval: 5m
+    sourceRef:
+        kind: OCIRepository
+        name: flux-components
+    path: ./
+    prune: true
+```
+
+Cluster state reconciliation:
+
+```yaml
+apiVersion: source.toolkit.fluxcd.io/v1beta2
+kind: OCIRepository
+metadata:
+  name: flux-system
+  namespace: flux-system
+spec:
+   interval: 1m
+   url: oci://ghcr.io/stefanprodan/fleet-manifests
+   ref:
+    tag: production
+   secretRef:
+    name: flux-system
+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: flux-system
+  namespace: flux-system
+spec:
+    interval: 1h
+    retryInterval: 5m
+    sourceRef:
+        kind: OCIRepository
+        name: flux-system
+    path: clusters/production
+    prune: true
+```
+
+#### Story 2
+
+> As a platform operator I want to sync the cluster state with the fleet Git repository.
+
+Push changes from the fleet Git repository to the container registry:
+
+```shell
+# clone the fleet Git repository
+git clone https://github.com/stefanprodan/fleet.git
+cd fleet
+git switch main
+
+# push the contents the fleet OCI repository and tag it with the commit short SHA
+flux push artifact oci://ghcr.io/stefanprodan/fleet-manifests:$(git rev-parse --short HEAD) \
+--path="./" \
+--source="$(git config --get remote.origin.url)" \
+--revision="$(git branch --show-current)@sha1:$(git rev-parse HEAD)"
+
+# tag the new version for production
+flux tag artifact oci://ghcr.io/stefanprodan/fleet-manifests:$(git rev-parse --short HEAD) \
+--tag=production
+```
+
+This operation can be automated using the Flux GitHub Action.
+
+The Git repository structure would be similar to the
+[flux2-kustomize-helm-example](https://github.com/fluxcd/flux2-kustomize-helm-example) with the following changes:
+
+- The `clusters/production/flux-system` directory is no more.
+- The Flux Kustomization objects defined in the `clusters/production` directory, such as
+  `infrastructure.yaml` and `apps.yaml`, have the `.spec.sourceRef` set to
+  `kind: OCIRepository` and  `name: flux-system`.
+
+#### Story 3
+
+> As a platform operator I want to update the Flux controllers on my production cluster
+> from CI without access to the Kubernetes API.
+
+Download the latest CLI version and update Flux directly in the registry, without rerunning bootstrap:
+
+```shell
+# pull the latest manifests from the registry
+flux pull artifact oci://ghcr.io/stefanprodan/flux-manifests:production \
+--output=./flux-manifests
+
+# update the Flux components manifests
+flux install --export > ./flux-manifests/flux-system/gotk-components.yaml
+
+# calculate the checksum of the manifests
+checksum=$(grep -ar -e . ./flux-manifests/ | shasum | cut -c-16)
+
+# extract the Flux version and commit
+flux_version=$(flux version --client | awk '{print $2}')
+flux_commit=$(go version -m  $(which flux) | grep vcs.revisio | awk -F= '{print $NF}')
+
+# push the updated manifests to the registry using the checksum as tag
+flux push artifact oci://ghcr.io/stefanprodan/flux-manifests:${checksum} \
+--path="./flux-manifests" \
+--source="https://github.com/fluxcd/flux2" \
+--revision="${flux_version}@sha1:${flux_commit}"
+
+# tag the new version for production
+flux tag artifact oci://ghcr.io/stefanprodan/flux-manifests:${checksum} \
+--tag=production
+```
+
+This operation could be simplified by implementing a dedicated CLI command and/or GitHub Action.
+
+#### Story 4
+
+> As a platform operator I want to update the registry credentials on my clusters.
+
+To rotate the registry credentials, generate a new GitHub token and overwrite the image pull secret:
+
+```shell
+flux create secret oci flux-system \
+--url=ghcr.io \
+--username=<ghcr-username> \
+--password=<ghcr-token>
+```
+
+Another option is to rerun the bootstrap command with the new credentials.
+
+## Design Details
+
+The bootstrap feature will be implemented as a Go package under `fluxcd/flux2/pkg/bootstrap/oci`
+using the [fluxcd/pkg/oci](https://github.com/fluxcd/pkg/tree/main/oci)
+library for OCI operations such as auth, push, pull, tag, etc.
+
+Both the Flux CLI and the Terraform/OpenTofu provider will use the `fluxcd/flux2/pkg/bootstrap/oci` package
+and expose the same configuration options.
+
+### Enabling the feature
+
+The feature is enabled by default.
+
+## Implementation History
+
+* NONE