Add RFC 0007 - Custom Event Metadata from Annotations

Signed-off-by: Matheus Pimenta <[email protected]>

rfcs/0007-custom-event-metadata-from-annotations/README.md

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+# RFC-0007 Custom Event Metadata from Annotations
+
+**Status:** provisional
+
+<!--
+Status represents the current state of the RFC.
+Must be one of `provisional`, `implementable`, `implemented`, `deferred`, `rejected`, `withdrawn`, or `replaced`.
+-->
+
+**Creation date:** 2024-05-23
+
+**Last update:** 2024-05-23
+
+## Summary
+
+Flux users often run into situations where they wish to send custom, static metadata fields defined
+in Flux objects on the events dispatched by the respective controller to notification-controller.
+This proposal offers a solution for supporting those use cases uniformly across all Flux controllers
+by sending annotation keys that are prefixed with a well-defined API Group.
+
+## Motivation
+
+The need for sending static metadata in Flux objects events can benefit users in a few ways:
+
+* When sending events about identical objects living in different clusters to the same notification channel,
+users often wish to differentiate those objects by the cluster where they live in. Sending custom metadata
+fields like `cluster`, `region` and `availabilityZone` would solve this problem.
+* When implementing integrations with notification-controller, users often wish to embed pieces of information
+in Flux objects that would help them identify entities or resources that are part of their integrations, e.g.
+the ID of a Deployment in the GitHub API.
+
+### Goals
+
+Provide a method for Flux users to embed custom/static fields in their Flux objects
+and have those fields sent to notification-controller as metadata on the event payload.
+
+### Non-Goals
+
+In this proposal we **do not** aim to provide a method for Flux users to send etcd-indexed custom metadata
+fields from Flux objects in events to notification-controller, most specifically labels. By design an event
+already contains enough identification information to locate the associated Flux object inside the cluster,
+which covers the use case of labels. Flux does not wish to incentive practices that are impactful to clusters
+without a strong reason or benefit.
+
+## Proposal
+
+When sending events about Flux objects, we propose sending annotation keys prefixed with a well-defined
+API Group in addition to those already sent by the controller to implement Flux-specific functionality.
+
+### User Stories
+
+* https://github.com/fluxcd/helm-controller/issues/680
+* https://github.com/fluxcd/helm-controller/pull/682#issuecomment-2079547110
+
+### Alternatives
+
+An alternative for specifying custom metadata fields in Flux objects for sending on events
+is defining `.spec` APIs for such, like `.spec.eventMetadata` available in the Alert API.
+This alternative is not great because:
+
+* Such APIs would be fairly redundant with the well-known Kubernetes annotations.
+* Technically speaking, it is much easier to implement an alternative where the
+field storing the custom metadata is the same and is already available across all the
+Flux objects rather than introducing a new API.
+
+In the specific case of the Alert API this field was introduced because the Alert API
+is obviously a special one in the context of events and alerting. In particular, the
+Alert objects do not generate events themselves, but rather serve as an aggregation
+configuration for matching and propagating events from other Flux objects.
+
+## Design Details
+
+For the API Group used to extract the custom metadata fields from object annotations we have a
+couple of alternatives:
+
+* `notification.toolkit.fluxcd.io`. This alternative emphasizes the close relationship of the
+feature with notification-controller and does not introduce a new Flux API group.
+* `event.toolkit.fluxcd.io`. This alternative decouples the feature from notification-controller
+and brings the concept closer to Kubernetes Events. In fact, our implementation builds on the
+`EventRecorder` interface from the Go package `k8s.io/client-go/tools/record`, hijacking the
+`AnnotatedEventf()` logic to send events not only to Kubernetes but also notification-controller.
+This would, however, introduce a new Flux API Group.
+
+Chosen alternative: ?
+
+### How can this feature be enabled / disabled?
+
+To enable the feature, use the well-defined API Group as a prefix in Flux object annotations,
+for example:
+
+* `?/cluster: prod`
+* `?/region: us-west-2`
+* `?/deploymentID: 1`
+
+It's important to notice that not all Flux objects emit events, e.g. Alert and Provider objects.
+For a list of the Flux objects that emit events, see the kinds allowed on the
+`.spec.eventSources[].kind` field of the Alert API.
+
+To disable the feature, do not use the well-defined API Group as a prefix in Flux object annotations.
+
+## Implementation History
+
+<!--
+Major milestones in the lifecycle of the RFC such as:
+- The first Flux release where an initial version of the RFC was available.
+- The version of Flux where the RFC graduated to general availability.
+- The version of Flux where the RFC was retired or superseded.
+-->