KEP 1645: add labels and annotations export

Signed-off-by: Arthur Outhenin-Chalandre <[email protected]>

Author: MrFreezeex

keps/sig-multicluster/1645-multi-cluster-services-api/README.md

@@ -107,6 +107,7 @@ tags, and then generate with `hack/update-toc.sh`.
     - [Service Port](#service-port)
     - [Headlessness](#headlessness)
     - [Session Affinity](#session-affinity)
+    - [Labels and Annotations](#labels-and-annotations)
   - [Test Plan](#test-plan)
   - [Graduation Criteria](#graduation-criteria)
     - [Alpha -> Beta Graduation](#alpha---beta-graduation)
@@ -119,6 +120,7 @@ tags, and then generate with `hack/update-toc.sh`.
   - [Export services via label selector](#export-services-via-label-selector)
   - [Export via annotation](#export-via-annotation)
   - [Other conflict resolution algorithms](#other-conflict-resolution-algorithms)
+  - [Exporting labels/annotations from the Service/ServiceExport objects](#exporting-labelsannotations-from-the-serviceserviceexport-objects)
 - [Infrastructure Needed](#infrastructure-needed)
 <!-- /toc -->
 
@@ -413,9 +415,19 @@ type ServiceExport struct {
         // +optional
         metav1.ObjectMeta `json:"metadata,omitempty"`
         // +optional
+        Spec ServiceExportSpec `json:"spec,omitempty"`
+        // +optional
         Status ServiceExportStatus `json:"status,omitempty"`
 }
 
+// ServiceExportSpec describes an exported service and extra exported information
+type ServiceExportSpec struct {
+        // +optional
+        ExportedLabels map[string]string `json:"exportedLabels"`
+        // +optional
+        ExportedAnnotations map[string]string `json:"exportedAnnotations"`
+}
+
 // ServiceExportStatus contains the current status of an export.
 type ServiceExportStatus struct {
         // +optional
@@ -497,9 +509,13 @@ single authority across all clusters. It is that authority’s responsibility to
 ensure that a name is shared by multiple services within the namespace if and
 only if they are instances of the same service.
 
-All information about the service, including ports, backends and topology, will
-continue to be stored in the `Service` objects, which are each name mapped to a
-`ServiceExport`.
+Most information about the service, including ports, backends, topology and
+session affinity, will continue to be stored in the `Service` objects, which
+are each name mapped to a `ServiceExport`. This does not apply for labels and
+annotations which are stored in `ServiceExport` directly in `spec.exportedLabels`
+and `spec.exportedAnnotations`. Exporting labels and annotations is optionally
+supported by MCS-API implementations. If supported, annotations or labels must
+not be exported from the `metadata` of the `Service` or `ServiceExport` resources.
 
 Deleting a `ServiceExport` will stop exporting the name-mapped `Service`.
 
@@ -1013,6 +1029,13 @@ Session affinity affects a service as a whole for a given consumer. The derived
 service's session affinity will be decided according to the conflict resolution
 policy.
 
+#### Labels and Annotations
+
+If supported, exporting labels and annotations would affect a `Service` as a whole
+for a given consumer. The derived service's labels and annotations will be decided
+according to the conflict resolution if the set of name/value pairs are not identical
+between the constituent clusters.
+
 ### Test Plan
 
 E2E tests can use [kind](https://kind.sigs.k8s.io/) to create multiple clusters
@@ -1229,7 +1252,7 @@ retain the flexibility of selectors.
 
 ### Export via annotation
 
-`ServiceExport` as described has no spec and seems like it could just be
+`ServiceExport` initially had no spec and seemed like it could just be
 replaced with an annotation, e.g. `multicluster.kubernetes.io/export`. When a
 service is found with the annotation, it would be considered marked for export
 to the clusterset. The controller would then create `EndpointSlices` and an
@@ -1258,6 +1281,31 @@ more confusing for users. Having just one simple deciding factor based on
 ServiceExport oldness makes resolving conflicts straightforward, and this
 alternative conflict resolution algorithm could hinder this ease of use.
 
+### Exporting labels/annotations from the Service/ServiceExport objects
+
+`Service` and `ServiceExport` have labels and annotations which could be used during
+export and propagated to the `ServiceImport`. However various tools such as kubectl or
+ArgoCD add some labels and annotations which would then need to be actively
+filtered to avoid any conflict. Filtering those labels and annotations is not
+something easy and we chose to avoid this problem entirely by not using the metadata
+object and adding dedicated fields in the spec of the `ServiceExport` resource.
+
+Also if we were using the labels and annotations from the metadata of either the
+`ServiceExport` or `Service` resources, it may be more confusing for users as it
+would be the only fields present in both resources. For instance, should an
+implementation merge the labels/annotations from both objects? Should it favor one?
+Should it takes only from the `Service` object? With dedicated fields for labels
+and annotations in the spec of the `ServiceExport` resource, it may becomes more
+straightforward that each resource have their own labels and annotations in their
+metadata and that the exported labels and annotations are from the dedicated
+fields in the `ServiceExport` spec.
+
+We also favored dedicated fields on the `ServiceExport` resource to allow for better
+flexibility, as it will allow to export labels and annotations fully decorrelated
+from the `Service` and `ServiceExport` metadata. More flexibility could also be
+achieved with CEL expression on the `ServiceExport` at the cost of greater
+complexity (managing CEL expressions on potentially many `ServiceExport` across clusters).
+
 ## Infrastructure Needed
 <!--
 Use this section if you need things from the project/SIG.  Examples include a