Enhancement Proposal (Implementable): NKG config (#930)

Update enhancement proposal with implementable details for NKG control plane dynamic configuration.

docs/proposals/control-plane-config.md

@@ -1,7 +1,7 @@
 # Enhancement Proposal-928: Control Plane Dynamic Configuration
 
 - Issue: https://github.com/nginxinc/nginx-kubernetes-gateway/issues/928
-- Status: Provisional
+- Status: Implementable
 
 ## Summary
 
@@ -17,3 +17,99 @@ option that we will support is log level.
 ## Non-Goals
 
 This proposal is *not* defining a way to dynamically configure the data plane.
+
+## Introduction
+
+The NKG control plane will evolve to have various user-configurable options. These could include, but are not
+limited to, log level, tracing, or metrics. For the best user experience, these options should be able to be
+changed at runtime, to avoid having to restart NKG. The first option that we will allow users to configure is the
+log level. The easiest and most intuitive way to implement a Kubernetes-native API is through a CRD.
+
+In this doc, the term "user" will refer to the cluster operator (the person who installs and manages NKG). The
+cluster operator owns this CRD resource.
+
+## API, Customer Driven Interfaces, and User Experience
+
+The API would be provided in a CRD. An authorized user would interact with this CRD using `kubectl` to `get`
+or `edit` the configuration.
+
+Proposed configuration CRD example:
+
+```yaml
+apiVersion: gateway.nginx.org/v1alpha1
+kind: NginxGateway
+metadata:
+    name: nginx-gateway-config
+    namespace: nginx-gateway
+spec:
+    logging:
+        level: info
+    ...
+status:
+    conditions:
+    ...
+```
+
+- The CRD would be Namespace-scoped, living in the same Namespace as the controller that it applies to.
+- CRD is initialized and created when NKG is deployed.
+- NKG references the name of this CRD via CLI arg (`--nginx-gateway-config-name`), and only watches this CRD.
+  If the resource doesn't exist, then an error is logged and event created, and default values are used.
+- If user deletes resource, NKG logs an error and creates an event. NKG will revert to default values.
+
+This resource won't be referenced in the `parametersRef` of the GatewayClass, reserving that option for a data
+plane CRD. The control plane may end up supporting multiple GatewayClasses, so linking the control CRD to a
+GatewayClass wouldn't make sense. Referencing the CRD via a CLI argument ensures we only support one instance of
+the CRD per control plane.
+
+For discussion with team:
+
+- kind name
+- default resource name
+
+## Use Cases
+
+The high level use case for dynamically changing settings in the NKG control plane is to allow users to alter
+behavior without the need for restarting NKG and experiencing downtime.
+
+For the specific log level use case, users may be experiencing problems with NKG that require more information to
+diagnose. These problems could include:
+
+- Not seeing or processing Kubernetes resources that it should be.
+- Configuring the data plane incorrectly based on the defined Kubernetes resources.
+- Crashes or errors without enough detail.
+
+Being able to dynamically change the log level can allow for a quick way to obtain more information about
+the state of the control plane without losing that state due to a required restart.
+
+## Testing
+
+Unit tests can be leveraged for verifying that NKG properly watches and acts on CRD changes. These tests would
+be similar in behavior as the current unit tests that verify Gateway API resource processing.
+
+## Security Considerations
+
+We need to ensure that any configurable fields that are exposed to a user are:
+
+- Properly validated. This means that the fields should be the correct type (integer, string, etc.), have appropriate
+length, and use regex patterns or enums to prevent any unwanted input. This will initially be done through
+OpenAPI schema validation. If necessary as the CRD evolves, CEL or webhooks could be used.
+- Have a valid use case. The more fields we expose, the more attack vectors we create. We should only be exposing
+fields that are genuinely useful for a user to change dynamically.
+
+RBAC via the Kubernetes API server will ensure that only authorized users can update the CRD containing NKG control
+plane configuration.
+
+## Alternatives
+
+- ConfigMap
+A ConfigMap is another type of resource that a user can provide configuration options within, however it lacks the
+benefits of a CRD, specifically built-in schema validation, versioning, and conversion webhooks.
+
+- Custom API server
+The NKG control plane could implement its own custom API server. However the overhead of implementing this, which
+would include auth, validation, endpoints, and so on, would not be worth it due to the fact that the Kubernetes
+API server already does all of these things for us.
+
+## References
+
+- [Kubernetes Custom Resources](https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/)