Lighthouse hub-controller enhancement proposal

Signed-off-by: Donald Hunter <donald.hunter@redhat.com>

lighthouse/hub-controller.md

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+# Hub Controller for Lighthouse
+
+## Summary
+
+The current Lighthouse implementation has a passive broker, it is a central apiserver that is
+accessed by each cluster in the clusterset. Clusters synchronise resources to and from the
+broker with all sync and reconciliation logic implemented in the clusters.
+
+There are feature gaps that would be much easier to address if we introduce an active broker
+that can implement reconciliation logic.
+
+- `ServiceExport` conflict resolution, per the MCS KEP [^1]
+- Merging exported `Services` into single `ServiceImport`
+
+Benefits of central reconciliation in an active broker include:
+
+- Single consistent model of intended state, rather than models computed per cluster
+- Target state computed _before_ it gets synced out to clusters
+- Synchronise fewer resources from broker to each cluster
+
+## Proposal
+
+This enhancement proposes to add an mcs-controller to the broker that will implement the
+reconciliation logic from `ServiceExport` to `ServiceImport` resources. This will result in a
+clear separation of logic for MCS resources:
+
+- Clusters will sync resources to and from the broker
+- The mcs-controller on the broker will reconcile `ServiceImport` object state from the
+  provided `ServiceExport` state.
+
+Once an mcs-controller has been introduced, the reconciliation logic will be extended to address
+the following open issues:
+
+- Mark conflicting `ServiceExport` objects with the `ServiceExportConflict` condition
+- Merge matching `ServiceExport` objects into a single `ServiceImport` object
+
+The resulting solution is expected to be less complex overall, ready to be more conformant to
+the MCS KEP and to provide opportunity for minimising the data synchronisation between clusters.
+
+[^1]: [KEP-1645: Multi-Cluster Services API](https://github.com/kubernetes/enhancements/tree/master/keps/sig-multicluster/1645-multi-cluster-services-api#constraints-and-conflict-resolution)