doc: add helper function details to the migration design doc

This commit add some more details like helper or utility functions
which will be introduced as part of the effort and also add some more
details about the CSI operations a particular identified change touches.

Signed-off-by: Humble Chirammal <hchiramm@redhat.com>

docs/design/proposals/intree-migrate.md

@@ -4,7 +4,13 @@ Prior to CSI, Kubernetes provided a powerful volume plugin system. These volume
 plugins were “in-tree” meaning their code was part of the core Kubernetes code
 and shipped with the core Kubernetes binaries. The CSI migration effort enables
 the replacement of existing in-tree storage plugins such as kubernetes.io/rbd
-with a corresponding CSI driver. For more information on CSI migration effort
+with a corresponding CSI driver. This will be an opt-in feature turned on at
+cluster creation time that will redirect in-tree plugin operations to a
+corresponding CSI Driver. The main mechanism we will use to migrate plugins is
+redirecting in-tree operation calls to the CSI Driver instead of the in-tree
+driver, the external components will pick up these in-tree PV's and use a
+translation library to translate to CSI Source. For more information on CSI
+migration effort
 refer [design doc](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/storage/csi-migration.md)
 
 ## RBD in-tree plugin to CSI migration
@@ -18,13 +24,17 @@ migration for RBD plugin:
 
 Considering the `clusterID` field is a required one for CSI, but in-tree
 StorageClass has `monitors` field as a required thing the clusterID will be sent
-from the migration library based on the monitors field, Kubernetes storage admin
-supposed to create a clusterID based on the monitors hash ( ex: `#md5sum <<<
+from the migration library based on the monitors field as like any other CSI
+request, Kubernetes storage admin supposed to create a clusterID based on the
+monitors hash ( ex: `#md5sum <<<
 "<monaddress:port>"`) in the CSI config map and keep the monitors under this
 configuration before enabling the migration. While CSI driver receive the volume
-ID it will look at the configmap and figure out the monitors to do the
-operations. Thus, CSI operations are unchanged or untouched wrt to the clusterID
-field.
+ID, it will look at the configmap and figure out the monitors to do the
+operations like create volume. Thus, CSI operations are unchanged or untouched
+wrt to the clusterID field. In other words, for Ceph CSI this is yet another
+request coming in from a client with required parameters. This is an opaque/NOOP
+change from the Ceph CSI side for its operations, however mentioning it here as
+a reference.
 
 ### New Volume ID for existing volume operations
 
@@ -45,13 +55,19 @@ Details on the hash values:
 
 * PoolHash: this is an encoded string of pool name.
 
-The existing in-tree volume's node operations should work without any issues as
-those will be tracked as static volumes for the CSI driver. From above volume
-ID, the CSI driver can also connect to the backend cluster and clean/delete the
-image. The migration volume ID carry the information like monitors, pool and
-image name which is good enough for the driver to identify and connect to the
-backend cluster for its operations. With above volume ID in place, we will be
-able to support mount, unmount, delete and resize operations.
+This volume handle is introduced as the existing in-tree volume does not have
+the volume ID format (
+ex: `0001-0020-b7f67366bb43f32e07d8a261a7840da9 -0000000000000002-c867ff35-3f04-11ec-b823-0242ac110003`)
+which Ceph CSI use by default. However, Ceph CSI need certain information like
+clusterID, pool, Image name to perform deletion and expansion of the volumes.
+The existing in-tree
+volume's `NodeStageVolume, NodePublishVolume, NodeUnstageVolume, NodeUnpublishVolume`
+operations should work without any issues as those will be tracked as static
+volumes for the CSI driver. For `DeleteVolume and ControllerExpandVolume`
+operations, from above migration specific volume ID, the CSI driver can derive
+the required details and connect to the backend cluster ( as it carry
+information like monitors, pool and image name) then perform deletion of the
+image or perform resize operation.
 
 ### Migration secret for CSI operations
 
@@ -68,8 +84,36 @@ field.
 field
 
 if `adminId` field is nil or not set, `UserID` field will be filled with default
-value ie `admin`.The above logic get activated only when the secret is a
+value ie `admin`. The above logic get activated only when the secret is a
 migration secret, otherwise skipped to the normal workflow as we have today.
+CreateVolume, DeleteVolume, NodeStage and ExpandVolume has to validate the
+secret and use for its operations:
+
+## Helper functions
+
+Few helper/utility functions will be introduced as part of this effort to handle
+migration specific logic to it.
+
+internal/rbd/migration.go
+
+* isMigrationVolID() - Identify the volume ID is of above-mentioned form of
+  migration volume ID.
+* parseMigrationVolID() - Parse the migration volume ID and populate the
+  required information like `pool`, `image`, `clusterID`..etc. into
+  `migrationVolID` structure.
+* deleteMigratedVolume() - Deletes the volume from the cluster by making use of
+  the `migrationVolID` structure fields like `pool, image, clusterID`.
+
+internal/util/credentials.go
+
+* isMigrationSecret() - Identify the passed in secret is a migration secret
+  based on the map values (ex: `key` field) of request secret.
+* NewUserCredentialsWithMigration() - This take request secret map as input and
+  validate whether it is migration secret with the help of isMigrationSecret(),
+  and return credentials by calling NewUserCredentials().
+* ParseAndSetSecretMapFromMigSecret() - Parse the secret map from the migration
+  request and return a map with adjusted secret fields for the CSI driver to
+  continue performing secret specific operations.
 
 ## CephFS in-tree plugin to CSI migration