diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -58,7 +58,7 @@
 from swh.model.hashutil import DEFAULT_ALGORITHMS, hash_to_bytes, hash_to_hex
 
 from swh.storage.cassandra import CassandraStorage
-from swh.storage.cassandra.model import BaseRow
+from swh.storage.cassandra.model import BaseRow, ObjectCountRow
 from swh.storage.interface import (
     ListOrder,
     PagedResult,
@@ -220,12 +220,27 @@
         )
 
 
+class InMemoryCqlRunner:
+    def __init__(self):
+        self._stat_counters = defaultdict(int)
+
+    def increment_counter(self, object_type: str, nb: int):
+        self._stat_counters[object_type] += nb
+
+    def stat_counters(self) -> Iterable[ObjectCountRow]:
+        for (object_type, count) in self._stat_counters.items():
+            yield ObjectCountRow(partition_key=0, object_type=object_type, count=count)
+
+
 class InMemoryStorage(CassandraStorage):
+    _cql_runner: InMemoryCqlRunner  # type: ignore
+
     def __init__(self, journal_writer=None):
         self.reset()
         self.journal_writer = JournalWriter(journal_writer)
 
     def reset(self):
+        self._cql_runner = InMemoryCqlRunner()
         self._contents = {}
         self._content_indexes = defaultdict(lambda: defaultdict(set))
         self._skipped_contents = {}
@@ -311,6 +326,8 @@
             self._contents[key] = attr.evolve(self._contents[key], data=None)
             content_add += 1
 
+        self._cql_runner.increment_counter("content", content_add)
+
         summary = {
             "content:add": content_add,
         }
@@ -467,6 +484,8 @@
             self._skipped_contents[key] = content
             summary["skipped_content:add"] += 1
 
+        self._cql_runner.increment_counter("skipped_content", len(contents))
+
         return summary
 
     def skipped_content_add(self, content: List[SkippedContent]) -> Dict:
@@ -499,6 +518,8 @@
             self._directories[directory.id] = directory
             self._objects[directory.id].append(("directory", directory.id))
 
+        self._cql_runner.increment_counter("directory", len(directories))
+
         return {"directory:add": count}
 
     def directory_missing(self, directories: List[Sha1Git]) -> Iterable[Sha1Git]:
@@ -576,6 +597,8 @@
             self._objects[revision.id].append(("revision", revision.id))
             count += 1
 
+        self._cql_runner.increment_counter("revision", len(revisions))
+
         return {"revision:add": count}
 
     def revision_missing(self, revisions: List[Sha1Git]) -> Iterable[Sha1Git]:
@@ -635,6 +658,8 @@
             self._objects[rel.id].append(("release", rel.id))
             self._releases[rel.id] = rel
 
+        self._cql_runner.increment_counter("release", len(to_add))
+
         return {"release:add": len(to_add)}
 
     def release_missing(self, releases: List[Sha1Git]) -> Iterable[Sha1Git]:
@@ -661,6 +686,8 @@
             self._objects[snapshot.id].append(("snapshot", snapshot.id))
             count += 1
 
+        self._cql_runner.increment_counter("snapshot", len(snapshots))
+
         return {"snapshot:add": count}
 
     def snapshot_missing(self, snapshots: List[Sha1Git]) -> Iterable[Sha1Git]:
@@ -856,6 +883,8 @@
                 self.origin_add_one(origin)
                 added += 1
 
+        self._cql_runner.increment_counter("origin", len(origins))
+
         return {"origin:add": added}
 
     def origin_add_one(self, origin: Origin) -> str:
@@ -904,6 +933,8 @@
                 )
                 all_visits.append(visit)
 
+        self._cql_runner.increment_counter("origin_visit", len(all_visits))
+
         return all_visits
 
     def _origin_visit_status_add_one(self, visit_status: OriginVisitStatus) -> None:
@@ -1146,30 +1177,6 @@
         else:
             return None
 
-    def stat_counters(self):
-        keys = (
-            "content",
-            "directory",
-            "origin",
-            "origin_visit",
-            "person",
-            "release",
-            "revision",
-            "skipped_content",
-            "snapshot",
-        )
-        stats = {key: 0 for key in keys}
-        stats.update(
-            collections.Counter(
-                obj_type
-                for (obj_type, obj_id) in itertools.chain(*self._objects.values())
-            )
-        )
-        return stats
-
-    def refresh_stat_counters(self):
-        pass
-
     def raw_extrinsic_metadata_add(self, metadata: List[RawExtrinsicMetadata],) -> None:
         self.journal_writer.raw_extrinsic_metadata_add(metadata)
         for metadata_entry in metadata:
diff --git a/swh/storage/replay.py b/swh/storage/replay.py
--- a/swh/storage/replay.py
+++ b/swh/storage/replay.py
@@ -136,6 +136,6 @@
         method(model_objs)
     elif object_type in ("directory", "revision", "release", "snapshot", "origin",):
         method = getattr(storage, object_type + "_add")
-        method(object_converter_fn[object_type](o) for o in objects)
+        method([object_converter_fn[object_type](o) for o in objects])
     else:
         logger.warning("Received a series of %s, this should not happen", object_type)