Add Helm charts proposal

Signed-off-by: Atanas Dinov <atanas.dinov@suse.com>

Author: atanasdinov

design-proposals/helm-chart.md

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+# Overview
+
+Provide and support Helm charts as an alternative installation method for KubeVirt and CDI.
+
+## Motivation
+
+Helm is one of the most popular choices when it comes to managing Kubernetes applications and their release lifecycle.
+Many software platforms prefer Helm charts for their configuration, automation and distribution capabilities.
+
+## Goals
+
+- Provide Helm charts for all upcoming versions of KubeVirt and CDI
+
+## Non Goals
+
+- Removing manifests installation method is out of scope.
+  Users will continue to have the option to deploy KubeVirt and CDI using the manifests provided at build time.
+
+## Definition of Users
+
+This feature is intended for administrators of Kubernetes clusters who are managing KubeVirt's (and CDI's) release lifecycles.
+
+## User Stories
+
+- As a cluster administrator, I'd want to install, upgrade and uninstall KubeVirt using Helm chart
+- As a cluster administrator, I'd want to install, upgrade and uninstall CDI using Helm chart
+
+## Repos
+
+* [kubevirt/kubevirt](https://github.com/kubevirt/kubevirt)
+* [kubevirt/containerized-data-importer](https://github.com/kubevirt/containerized-data-importer)
+
+# Design
+
+In order to establish a solution design we need to answer the following questions:
+* How are the Helm charts going to look like?
+* How will the Helm charts be managed?
+
+## Chart Design
+
+Both KubeVirt and CDI installations depend on CRD, CR and Operator Deployment resources.
+
+The current installation procedure involves creating the CRD and Operator Deployment first closely followed by creating the CR.
+
+Helm, however, has limitations when it comes to [handling CRDs](https://helm.sh/docs/chart_best_practices/custom_resource_definitions/).
+Upgrading and deleting is not currently supported (unless templated) and CRDs must be present before creating a CR.
+The CR is required in order for the operator to deploy the rest of KubeVirt's dependencies and applications.
+
+This leaves us with the following Helm chart layouts:
+
+### Approach 1: Single chart
+
+```text
+├── Chart.yaml
+├── crds
+│   └── kubevirt-crd.yaml
+├── templates
+│    ├── kubevirt-cr.yaml
+│    ├── kubevirt-hooks.yaml
+│    └── kubevirt-operator.yaml
+└── values.yaml
+```
+
+This method allows us to have all resources in the same chart.
+
+The `CustomResourceDefinition` is extracted out of `kubevirt-operator.yaml` under the special `/crds` directory which Helm will pick up and install first.
+Note that `kubevirt-crd.yaml` cannot be templated.
+
+The installation process for this approach is simple (`$ helm install kubevirt <repo-url>`), however,
+there are some obstacles when it comes to upgrade and uninstall procedures.
+
+Upgrading a CRD is not currently possible in Helm. Once installed, further upgrades will only detect that the CRD already exists and will ignore it.
+
+Uninstalling depends on the CR being removed before the Operator. The CRD removal is also not taken care of by Helm automatically.
+
+In order to resolve these issues, we can leverage [Helm's hooks mechanism](https://helm.sh/docs/topics/charts_hooks/).
+It allows us to intervene at certain points of the release lifecycle and perform custom operations.
+These operations will be defined in `kubevirt-hooks.yaml` and will consist of the following:
+- `pre-upgrade` hook manually applying the CRD manifest in order to perform modifications to it
+- `pre-delete` hook removing the CR first in order for the operator to gracefully uninstall KubeVirt's components before Helm removes it
+- `post-delete` hook removing the leftover CRD
+
+This approach was used in [SUSE Edge's initial Helm chart](https://github.com/suse-edge/charts/tree/main/charts/kubevirt/0.1.0).
+[PR](https://github.com/suse-edge/charts/pull/13) for additional context.
+
+### Approach 2: One CRD chart and one CR + Operator chart
+
+Perhaps the most common solution to tackle the CRD problem is to extract the CRD in a separate Helm chart.
+
+```text
+├── Chart.yaml
+├── templates
+│   └── kubevirt-crd.yaml
+```
+
+```text
+├── Chart.yaml
+├── templates
+│    ├── kubevirt-cr.yaml
+│    ├── kubevirt-hooks.yaml
+│    └── kubevirt-operator.yaml
+└── values.yaml
+```
+
+Here we will install the CRD chart first and the CR / Operator chart after that.
+Upcoming releases of KubeVirt can ship newer versions of both charts in order to avoid discrepancies between chart versions.
+
+The benefits are that we don't need to worry about manually upgrading or deleting the CRD. We'd still need a `pre-delete` hook to remove the CR first though.
+
+The negatives are that we'd maintain two charts instead of a single one.
+
+### Approach 3: One CR chart and one CRD + Operator chart
+
+The showstopper is that the CRD must exist before deploying the CR.
+We could also achieve this with the following split:
+
+```text
+├── Chart.yaml
+├── templates
+│    ├── kubevirt-crd.yaml
+│    └── kubevirt-operator.yaml
+└── values.yaml
+```
+
+```text
+├── Chart.yaml
+├── templates
+│   └── kubevirt-cr.yaml
+└── values.yaml
+```
+
+This approach is also similar to the current installation / uninstallation flows:
+
+```shell
+$ helm install kubevirt-operator <repo-url>
+$ helm install kubevirt-cr <repo-url>
+...
+$ helm uninstall kubevirt-cr
+$ sleep 45 # wait for the operator to uninstall the different components
+$ helm uninstall kubevirt-operator
+```
+
+Note that the CRD is under `/templates` for this example.
+This means that we can execute upgrades / deletes without custom hooks.
+This isn't possible if the CR is also a template in the same chart.
+
+### Approach N: Exotic options
+
+As seen above, Helm provides a lot of customization possibilities.
+I'd only mention some more and move on.
+Let me know if you're interested in either of the following:
+- One CRD + Operator chart, post-install hook creating the CR
+- One CRD, one CR and one Operator chart
+
+### Overview
+
+Many Helm chart layouts can work. Choosing one depends on how much we'd want to rely on chart hooks.
+
+[Approach 1](#approach-1-single-chart) was chosen for the initial Helm chart linked above in order to:
+- stay as close as possible to the original manifest setup
+- demonstrate chart hooks in action
+
+As mentioned earlier, I believe [Approach 2](#approach-2-one-crd-chart-and-one-cr--operator-chart) is the most popular so far.
+
+[Approach 3](#approach-3-one-cr-chart-and-one-crd--operator-chart) is most similar to the current setup.
+
+The CDI project has the same `CRD-Operator-CR` setup so all of the above applies to it as well.
+
+Note that the file structures above are purely for illustrative purposes.
+Different Kubernetes resources (e.g. `rbac`) could be extracted as separate templates (files) in order to keep the charts cleaner and easier to navigate.
+
+## Chart Management
+
+The second problem which needs to be addressed is how Helm charts are going to be created and modified.
+
+Currently, the manifests in KubeVirt are generated in two steps:
+1. `resource-generator` generates semi-populated configurations under `/manifests/generated`
+2. `manifest-templator` produces the final version of the manifests which are then shipped as part of each GitHub release
+
+### Approach 1: Generate Helm chart(s)
+
+Helm charts are in general not that different from plain Kubernetes manifests.
+It all depends on how much customization we'd allow the user but for the SUSE Edge initial chart we kept as little modifications as possible.
+
+We should be able to plug in the functionality to generate a `Chart.yaml`, `values.yaml` and possibly hooks and organize those in the necessary file structure.
+The charts can then be generated, packaged and pushed to the configured Helm repository at build time.
+
+### Approach 2: Manually curate a Helm chart
+
+We can depend on the Kubernetes manifests provided as part of GitHub releases and create a Helm chart based on those.
+This is the approach I've seen most people attempt when trying to come up with a custom KubeVirt installation method.
+
+It does not involve any changes to the manifest generation flow, however, some degree of automation would still be necessary in order to patch, package and publish the charts.
+The end result probably needs to be kept outside the main KubeVirt repository.
+
+### Overview
+
+In theory, both approaches should work.
+
+[Approach 1](#approach-1-generate-helm-charts) will probably require more effort when it comes to the implementation,
+but it should be stable and both manifests and charts would be released simultaneously.
+
+[Approach 2](#approach-2-manually-curate-a-helm-chart) is the more popular choice,
+however, we need to keep in mind that the complete manifests are not version controlled.
+This would also mean that updated Helm charts are released separately from the manifests.
+
+Unfortunately, I'm still very new to KubeVirt so feedback in this section would be more than welcome.
+
+I'm not familiar with CDI's current manifest creation flow yet, but I *believe* it is similar.
+
+## API Examples
+
+Working with Helm charts is fairly straightforward. Users should be able to manage the applications by:
+
+```shell
+$ helm repo add kubevirt <repo-url>
+$ helm install kubevirt kubevirt/kubevirt --version <version> \
+      # additional configuration
+$ helm install cdi kubevirt/cdi --version <version> \
+      # additional configuration
+$ helm uninstall kubevirt
+$ helm uninstall cdi
+```
+
+## Scalability
+
+N/A
+
+## Update/Rollback Compatibility
+
+The feature will not impact older versions of KubeVirt or CDI.
+Helm charts will, however, be a viable way of updating & rolling back future releases.
+
+## Functional Testing Approach
+
+- Deploy KubeVirt from Helm chart
+- Ensure all components are created and work as expected
+- Slightly modify the configuration of the CRD, CR and operator's Deployment and upgrade KubeVirt
+- Ensure all components are working as expected and the configurations have been properly updated
+- Uninstall KubeVirt chart
+- Ensure all components have been removed
+
+The same tests should be performed for CDI.
+
+# Implementation Phases
+
+- Create a KubeVirt Helm chart
+- Configure a Helm repository (using GitHub pages is probably the best option)
+- Create a CDI Helm chart and add it to the same Helm repository