Storage controller (syncer)

[davidfestal]

Controller, co-located in the Syncer, that would check when a KCP-related downstream PVC is bound to a downstream PV, and would prepare the downstream PV to be upsynced by the UpSync controller. This includes:

• taking care of the bidirectional PVC <-> PV link through transformations,
• as well as doing some synchronizing, so that the "Bound" status of the PVC would be synced back to KCP only after the transformed PV has itself already been upsynced to KCP.

[davidfestal]

Brain-dump of the low-level implementation bits that come to my mind:

• Bootstrap the StorageController, co-located in the Syncer, dedicated to PVCs and PVs
• Give the StorageController access to the Syncer virtual workspace upstream informer, to potentially get upstream resources (PVCs in our case)
• Setup a downstream informer for the StorageController for PVs and PVCs that will;
  • select PVCs whose value of label workloadv1alpha1.InternalDownstreamClusterLabel is the current SyncTarget key
  • select all PVs.
• Have the StorageController react to downstream PVC changes:
  • when a PVC is created (initially synced from KCP), then:
  • set a internal.workload.kcp.dev/delaystatussyncing annotation to true, which will instruct delaying the update of the PVC status back to KCP
  • when a PVC is bound to a PV, then:
  • Get the upstream PVC corresponding to this downstream PVC (from the upstream informer)
  • Copy the NamespaceLocator annotation (found on the downstream PVC namespace) to the downstream PV (after removing the namespace since PV are cluster-wide-resources)
  • Create a Json patch for the PV resource whose content would update the PVC reference to match the upstream PVC reference (namespace, uid, resource version, etc ...)
  • Add this JsonPatch as an annotation on the downstream PV (internal.workload.kcp.dev/upsyncdiff)
  • Update the PV with the Upsync requestState label for the current SyncTarget to trigger the Upsyncing of the PV to the upstream workspace
• Have the StorageController react to upstream PV changes:
  • when a PV is created upstream and has the Upsync label for the current SyncTarget, then find the corresponding downstream PVC, and remove the internal.workload.kcp.dev/delaystatussyncing annotation
• Modify the StatusSyncer to also react to changes of the internal.workload.kcp.dev/delaystatussyncing annotation, and not react to status changes when this annotation is true

[mjudeikis]

/transfer-issue contrib-tmc