add design doc about restore processes

docs/design.md

@@ -8,7 +8,7 @@ We want to backup and restore RBD PVCs managed by a Rook/Ceph cluster, either by
 
 1. Backup arbitrary RBD PVCs.
 2. Restore RBD PVCs from backups.
-3. Backup arbitary RBD PVCs periodically.
+3. Backup arbitrary RBD PVCs periodically.
 4. Copy backup data to another cluster in another data center.
 
 Currently, the goal 1 and 3 are implemented. Other goals will be achieved later.
@@ -21,39 +21,71 @@ Currently, the goal 1 and 3 are implemented. Other goals will be achieved later.
 flowchart LR
 
     style Architecture fill:#FFFFFF
+
+    USER([User])
+
     subgraph Architecture
 
-      USER([User])
-      RBSC[mantle-controller]
-      RPB[MantleBackup]
-      PVC[PersistentVolumeClaim]
-      PV[PersistentVolume]
-      RI[RBD Image]
-      RS[RBD Snapshot]
-      MBC[MantleBackupConfig]
-      MBCCronJob[CronJob]
+      %% restore
+      MR -- point --> MB
+      MRR -- watch --> MR
+      MRR -- create/delete --> RC
+      MRR -- create/delete --> RES_PVC
+      MRR -- create/delete --> RES_PV
+      USER -- create/delete --> MR
+      RES_PVC -- consume --> RES_PV
+      MR -.-|related| RC
+      RES_PV -- point --> RC
+      RC -- point --> RS
+
+      %% backup config
+      MBCCronJob -- create/delete --> MB
+      MBCR -- watch --> MBC
+      MBC -- point --> SRC_PVC
+      MBCR -- create --> MBCCronJob
+      MBCCronJob -.-|related| MBC
+
+      %% backup
+      MB -.-|related| RS
+      USER -- create/delete --> MB
+      MBR -- watch --> MB
+      MB -- point --> SRC_PVC
+      SRC_PVC -- consume --> SRC_PV
+      USER -- create/delete --> MBC
+      MBR -- create/delete --> RS
+      SRC_PV -- point --> RI
+      RS -- point --> RI
 
-      subgraph Kubernetes Layer
-        USER -- create/delete --> RPB
-        RBSC -- watch --> RPB
-        RPB -- point --> PVC
-        PVC -- consume --> PV
-        USER -- create/delete --> MBC
-        RBSC -- watch --> MBC
-        RBSC -- create --> MBCCronJob
-        MBCCronJob -- create/delete --> RPB
-        MBCCronJob -.-|related| MBC
-        MBC -- point --> PVC
-      end
 
       subgraph Ceph Layer
-        RBSC -- create/delete --> RS
-        PV -- point --> RI
-        RS -- point --> RI
-
+        RI[RBD Image]
+        RS[RBD Snapshot]
+        RC[RBD cloned Image]
       end
 
-
+      subgraph Kubernetes Layer
+
+        SRC_PVC[source PersistentVolumeClaim]
+        SRC_PV[source PersistentVolume]
+
+        subgraph Mantle controller
+          MBCR[MantleBackupConfigReconciler]
+          MBR[MantleBackupReconciler]
+          MRR[MantleRestoreReconciler]
+        end
+
+        subgraph Backup related manifests
+          MBC[MantleBackupConfig]
+          MBCCronJob[CronJob]
+          MB[MantleBackup]
+        end
+
+        subgraph Restore related manifests
+          MR[MantleRestore]
+          RES_PVC[restored PersistentVolumeClaim]
+          RES_PV[restored PersistentVolume]
+        end
+      end
     end
 ```
 
@@ -88,6 +120,7 @@ apiVersion: mantle.cybozu.io/v1
 kind: MantleBackup
 metadata:
   name: <MantleBackup resource name>
+  namespace: <should be the same as the target PVC>
 spec:
   # The name of the backup target PVC
   pvc: <target PVC name>
@@ -111,3 +144,39 @@ spec:
   expire: 2w # when the MantleBackups generated by this MantleBackupConfig should expire.
   suspend: false # whether the periodic backup is active or not.
 ```
+
+### Restore flow
+
+Precondition: Process will not start until conditions are met.
+- The target MantleBackup must exist and be ready to use.
+
+1. Users create a `MantleRestore` resource.
+2. The controller gets the target MantleBackup from the `MantleRestore` resource.
+3. The controller stores the pool name for the `status.pool` field and cluster ID for the `status.clusterID` field. This value is used to remove the restored PV/PVC when the MantleRestore resource is deleted.
+4. The controller gets backup target RBD snapshot name from the MantleBackup.
+5. The controller creates a new RBD clone image from the RBD snapshot.
+6. The controller creates a new PV/PVC using the above-mentioned RBD clone image.
+
+### Cleanup restore flow
+
+1. Users delete the `MantleRestore` resource.
+2. The controller tries to delete the PV/PVC created by the `MantleRestore` resource and wait until the Pod consuming the PV/PVC are stopped and deleted.
+3. The controller removes the RBD clone image created by the `MantleRestore` resource. However, the controller should not remove the RBD clone image if the previous step is not completed and a PV/PVC exists.
+
+#### The manifest to get restore PV/PVC from a backup
+
+```yaml
+apiVersion: mantle.cybozu.io/v1
+kind: MantleRestore
+metadata:
+  name: <MantleRestore resource name>
+  namespace: <should be the same as the target MantleBackup>
+spec:
+  # The name of the restore target backup
+  backup: <MantleBackup resource name>
+status:
+  conditions:
+    # The corresponding restore PV/PVC is ready to use if `status` is "True"
+    - type: "ReadyToUse"
+      status: "True"
+```