add design doc about restore processes

Signed-off-by: Yuji Ito <llamerada.jp@gmail.com>

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, only 1 is implemented. Other goals will be achieved later.
@@ -61,7 +61,7 @@ To create/delete a backup, mantle works as follows:
 4. The controller gets the RBD image name and pool name from the PV.
 5. The controller creates/deletes an RBD snapshot corresponding to the backup in the target RBD image.
 
-### Definitions
+#### The manifest to get a backup
 
 The definition `MantleBackup` is as follows:
 
@@ -70,6 +70,7 @@ 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>
@@ -79,3 +80,39 @@ status:
     - type: "ReadyToUse"
       status: "True"
 ```
+
+### Restore flow
+
+Precondition: Process will not start until conditions are met.
+- The target MantleBackup must exist and be ready to use.
+
+1. Users crate 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 snap image name from the MantleBackup.
+5. The controller creates a new RBD clone from the RBD snap image.
+6. The controller creates a new PV/PVC using the new RBD clone.
+
+### 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 PV/PVC is deleted. If the PV/PVC is used by some pod.
+3. The controller removes the RBD clone created by the `MantleRestore` resource. The controller should not remove RBD clone volume specified by the PV.
+
+#### 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"
+```