diff --git a/apps/emqx_durable_storage/src/emqx_ds_storage_layer.erl b/apps/emqx_durable_storage/src/emqx_ds_storage_layer.erl
index d6d36bf97..47fe047fc 100644
--- a/apps/emqx_durable_storage/src/emqx_ds_storage_layer.erl
+++ b/apps/emqx_durable_storage/src/emqx_ds_storage_layer.erl
@@ -168,11 +168,14 @@
     until := emqx_ds:time() | undefined
 }.
 
+%% Module-specific runtime data, as instantiated by `Mod:open/5` callback function.
+-type generation_data() :: term().
+
 %% Schema for a generation. Persistent term.
 -type generation_schema() :: generation(term()).
 
 %% Runtime view of generation:
--type generation() :: generation(term()).
+-type generation() :: generation(generation_data()).
 
 %%%% Shard:
 
@@ -204,21 +207,21 @@
     rocksdb:db_handle(),
     gen_id(),
     Options :: map(),
-    PrevRuntimeData :: term()
+    generation_data() | undefined
 ) ->
     {_Schema, cf_refs()}.
 
 %% Open the existing schema
 -callback open(shard_id(), rocksdb:db_handle(), gen_id(), cf_refs(), _Schema) ->
-    _Data.
+    generation_data().
 
 %% Delete the schema and data
--callback drop(shard_id(), rocksdb:db_handle(), gen_id(), cf_refs(), _RuntimeData) ->
+-callback drop(shard_id(), rocksdb:db_handle(), gen_id(), cf_refs(), generation_data()) ->
     ok | {error, _Reason}.
 
 -callback prepare_batch(
     shard_id(),
-    _Data,
+    generation_data(),
     [{emqx_ds:time(), emqx_types:message()}, ...],
     emqx_ds:message_store_opts()
 ) ->
@@ -226,30 +229,42 @@
 
 -callback commit_batch(
     shard_id(),
-    _Data,
+    generation_data(),
     _CookedBatch
 ) -> ok | emqx_ds:error(_).
 
--callback get_streams(shard_id(), _Data, emqx_ds:topic_filter(), emqx_ds:time()) ->
+-callback get_streams(
+    shard_id(), generation_data(), emqx_ds:topic_filter(), emqx_ds:time()
+) ->
     [_Stream].
 
--callback make_iterator(shard_id(), _Data, _Stream, emqx_ds:topic_filter(), emqx_ds:time()) ->
+-callback make_iterator(
+    shard_id(), generation_data(), _Stream, emqx_ds:topic_filter(), emqx_ds:time()
+) ->
     emqx_ds:make_iterator_result(_Iterator).
 
 -callback make_delete_iterator(
-    shard_id(), _Data, _DeleteStream, emqx_ds:topic_filter(), emqx_ds:time()
+    shard_id(), generation_data(), _DeleteStream, emqx_ds:topic_filter(), emqx_ds:time()
 ) ->
     emqx_ds:make_delete_iterator_result(_Iterator).
 
--callback next(shard_id(), _Data, Iter, pos_integer(), emqx_ds:time(), _IsCurrent :: boolean()) ->
+-callback next(
+    shard_id(), generation_data(), Iter, pos_integer(), emqx_ds:time(), _IsCurrent :: boolean()
+) ->
     {ok, Iter, [emqx_types:message()]} | {ok, end_of_stream} | {error, _}.
 
 -callback delete_next(
-    shard_id(), _Data, DeleteIterator, emqx_ds:delete_selector(), pos_integer(), emqx_ds:time()
+    shard_id(),
+    generation_data(),
+    DeleteIterator,
+    emqx_ds:delete_selector(),
+    pos_integer(),
+    emqx_ds:time()
 ) ->
     {ok, DeleteIterator, _NDeleted :: non_neg_integer(), _IteratedOver :: non_neg_integer()}.
 
--callback handle_event(shard_id(), _Data, emqx_ds:time(), CustomEvent | tick) -> [CustomEvent].
+-callback handle_event(shard_id(), generation_data(), emqx_ds:time(), CustomEvent | tick) ->
+    [CustomEvent].
 
 -optional_callbacks([handle_event/4]).