diff --git a/swh/storage/cassandra/storage.py b/swh/storage/cassandra/storage.py
--- a/swh/storage/cassandra/storage.py
+++ b/swh/storage/cassandra/storage.py
@@ -153,7 +153,9 @@
         contents = [attr.evolve(c, ctime=now()) for c in content]
         return self._content_add(list(contents), with_data=True)
 
-    def content_update(self, content, keys=[]):
+    def content_update(
+        self, contents: List[Dict[str, Any]], keys: List[str] = []
+    ) -> None:
         raise NotImplementedError(
             "content_update is not supported by the Cassandra backend"
         )
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
@@ -226,10 +226,12 @@
         content = [attr.evolve(c, ctime=now()) for c in content]
         return self._content_add(content, with_data=True)
 
-    def content_update(self, content, keys=[]):
-        self.journal_writer.content_update(content)
+    def content_update(
+        self, contents: List[Dict[str, Any]], keys: List[str] = []
+    ) -> None:
+        self.journal_writer.content_update(contents)
 
-        for cont_update in content:
+        for cont_update in contents:
             cont_update = cont_update.copy()
             sha1 = cont_update.pop("sha1")
             for old_key in self._content_indexes["sha1"][sha1]:
diff --git a/swh/storage/interface.py b/swh/storage/interface.py
--- a/swh/storage/interface.py
+++ b/swh/storage/interface.py
@@ -93,12 +93,14 @@
         ...
 
     @remote_api_endpoint("content/update")
-    def content_update(self, content, keys=[]):
+    def content_update(
+        self, contents: List[Dict[str, Any]], keys: List[str] = []
+    ) -> None:
         """Update content blobs to the storage. Does nothing for unknown
         contents or skipped ones.
 
         Args:
-            content (iterable): iterable of dictionaries representing
+            content: iterable of dictionaries representing
                 individual pieces of content to update. Each dictionary has the
                 following keys:
 
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -236,15 +236,17 @@
 
     @timed
     @db_transaction()
-    def content_update(self, content, keys=[], db=None, cur=None):
+    def content_update(
+        self, contents: List[Dict[str, Any]], keys: List[str] = [], db=None, cur=None
+    ) -> None:
         # TODO: Add a check on input keys. How to properly implement
         # this? We don't know yet the new columns.
-        self.journal_writer.content_update(content)
+        self.journal_writer.content_update(contents)
 
         db.mktemp("content", cur)
         select_keys = list(set(db.content_get_metadata_keys).union(set(keys)))
         with convert_validation_exceptions():
-            db.copy_to(content, "tmp_content", select_keys, cur)
+            db.copy_to(contents, "tmp_content", select_keys, cur)
             db.content_update_from_temp(keys_to_update=keys, cur=cur)
 
     @timed