diff --git a/swh/storage/buffer.py b/swh/storage/buffer.py
--- a/swh/storage/buffer.py
+++ b/swh/storage/buffer.py
@@ -4,20 +4,20 @@
 # See top-level LICENSE file for more information
 
 from functools import partial
-from typing import Collection, Dict, Iterable, List, Mapping, Optional, Tuple
+from typing import Dict, Iterable, Mapping, Sequence, Tuple
 
 from swh.core.utils import grouper
 from swh.model.model import BaseModel, Content, SkippedContent
 from swh.storage import get_storage
 from swh.storage.interface import StorageInterface
 
-OBJECT_TYPES: List[str] = [
+OBJECT_TYPES: Tuple[str, ...] = (
     "content",
     "skipped_content",
     "directory",
     "revision",
     "release",
-]
+)
 
 DEFAULT_BUFFER_THRESHOLDS: Dict[str, int] = {
     "content": 10000,
@@ -74,7 +74,7 @@
             raise AttributeError(key)
         return getattr(self.storage, key)
 
-    def content_add(self, contents: Collection[Content]) -> Dict:
+    def content_add(self, contents: Sequence[Content]) -> Dict:
         """Push contents to write to the storage in the buffer.
 
         Following policies apply:
@@ -95,7 +95,7 @@
 
         return stats
 
-    def skipped_content_add(self, contents: Collection[SkippedContent]) -> Dict:
+    def skipped_content_add(self, contents: Sequence[SkippedContent]) -> Dict:
         return self.object_add(
             contents,
             object_type="skipped_content",
@@ -103,7 +103,7 @@
         )
 
     def object_add(
-        self, objects: Collection[BaseModel], *, object_type: str, keys: Iterable[str],
+        self, objects: Sequence[BaseModel], *, object_type: str, keys: Iterable[str],
     ) -> Dict[str, int]:
         """Push objects to write to the storage in the buffer. Flushes the
         buffer to the storage if the threshold is hit.
@@ -118,11 +118,8 @@
 
         return {}
 
-    def flush(self, object_types: Optional[List[str]] = None) -> Dict[str, int]:
+    def flush(self, object_types: Sequence[str] = OBJECT_TYPES) -> Dict[str, int]:
         summary: Dict[str, int] = self.storage.flush(object_types)
-        if object_types is None:
-            object_types = OBJECT_TYPES
-
         for object_type in object_types:
             buffer_ = self._objects[object_type]
             batches = grouper(buffer_.values(), n=self._buffer_thresholds[object_type])
@@ -135,7 +132,7 @@
 
         return summary
 
-    def clear_buffers(self, object_types: Optional[List[str]] = None) -> None:
+    def clear_buffers(self, object_types: Sequence[str] = OBJECT_TYPES) -> None:
         """Clear objects from current buffer.
 
         WARNING:
@@ -145,13 +142,10 @@
             you want to continue your processing.
 
         """
-        if object_types is None:
-            object_types = OBJECT_TYPES
-
         for object_type in object_types:
             buffer_ = self._objects[object_type]
             buffer_.clear()
             if object_type == "content":
                 self._contents_size = 0
 
-        return self.storage.clear_buffers(object_types)
+        self.storage.clear_buffers(object_types)
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
@@ -9,7 +9,18 @@
 import json
 import random
 import re
-from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Tuple, Union
+from typing import (
+    Any,
+    Callable,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Sequence,
+    Set,
+    Tuple,
+    Union,
+)
 
 import attr
 
@@ -1303,11 +1314,11 @@
         else:
             return None
 
-    def clear_buffers(self, object_types: Optional[List[str]] = None) -> None:
+    def clear_buffers(self, object_types: Sequence[str]) -> None:
         """Do nothing
 
         """
         return None
 
-    def flush(self, object_types: Optional[List[str]] = None) -> Dict:
+    def flush(self, object_types: Sequence[str]) -> Dict[str, int]:
         return {}
diff --git a/swh/storage/cli.py b/swh/storage/cli.py
--- a/swh/storage/cli.py
+++ b/swh/storage/cli.py
@@ -11,7 +11,8 @@
 
 import click
 
-from swh.core.cli import CONTEXT_SETTINGS, swh as swh_cli_group
+from swh.core.cli import CONTEXT_SETTINGS
+from swh.core.cli import swh as swh_cli_group
 
 try:
     from systemd.daemon import notify
diff --git a/swh/storage/interface.py b/swh/storage/interface.py
--- a/swh/storage/interface.py
+++ b/swh/storage/interface.py
@@ -5,7 +5,7 @@
 
 import datetime
 from enum import Enum
-from typing import Any, Dict, Iterable, List, Optional, Tuple, TypeVar, Union
+from typing import Any, Dict, Iterable, List, Optional, Sequence, Tuple, TypeVar, Union
 
 from typing_extensions import Protocol, TypedDict, runtime_checkable
 
@@ -1185,7 +1185,7 @@
         ...
 
     @remote_api_endpoint("clear/buffer")
-    def clear_buffers(self, object_types: Optional[List[str]] = None) -> None:
+    def clear_buffers(self, object_types: Sequence[str]) -> None:
         """For backend storages (pg, storage, in-memory), this is a noop operation. For proxy
         storages (especially filter, buffer), this is an operation which cleans internal
         state.
@@ -1193,7 +1193,7 @@
         """
 
     @remote_api_endpoint("flush")
-    def flush(self, object_types: Optional[List[str]] = None) -> Dict:
+    def flush(self, object_types: Sequence[str]) -> Dict[str, int]:
         """For backend storages (pg, storage, in-memory), this is expected to be a noop
         operation. For proxy storages (especially buffer), this is expected to trigger
         actual writes to the backend.
diff --git a/swh/storage/postgresql/storage.py b/swh/storage/postgresql/storage.py
--- a/swh/storage/postgresql/storage.py
+++ b/swh/storage/postgresql/storage.py
@@ -9,7 +9,7 @@
 from contextlib import contextmanager
 import datetime
 import itertools
-from typing import Any, Counter, Dict, Iterable, List, Optional, Tuple, Union
+from typing import Any, Counter, Dict, Iterable, List, Optional, Sequence, Tuple, Union
 
 import attr
 import psycopg2
@@ -1396,13 +1396,13 @@
             return None
         return MetadataAuthority.from_dict(dict(zip(db.metadata_authority_cols, row)))
 
-    def clear_buffers(self, object_types: Optional[List[str]] = None) -> None:
+    def clear_buffers(self, object_types: Sequence[str]) -> None:
         """Do nothing
 
         """
         return None
 
-    def flush(self, object_types: Optional[List[str]] = None) -> Dict:
+    def flush(self, object_types: Sequence[str]) -> Dict[str, int]:
         return {}
 
     def _get_authority_id(self, authority: MetadataAuthority, db, cur):
diff --git a/swh/storage/retry.py b/swh/storage/retry.py
--- a/swh/storage/retry.py
+++ b/swh/storage/retry.py
@@ -5,7 +5,7 @@
 
 import logging
 import traceback
-from typing import Dict, Iterable, List, Optional
+from typing import Dict, Iterable, List
 
 from tenacity import retry, stop_after_attempt, wait_random_exponential
 
@@ -129,12 +129,3 @@
     @swh_retry
     def snapshot_add(self, snapshots: List[Snapshot]) -> Dict:
         return self.storage.snapshot_add(snapshots)
-
-    def clear_buffers(self, object_types: Optional[List[str]] = None) -> None:
-        return self.storage.clear_buffers(object_types)
-
-    def flush(self, object_types: Optional[List[str]] = None) -> Dict:
-        """Specific case for buffer proxy storage failing to flush data
-
-        """
-        return self.storage.flush(object_types)
diff --git a/swh/storage/tests/test_postgresql.py b/swh/storage/tests/test_postgresql.py
--- a/swh/storage/tests/test_postgresql.py
+++ b/swh/storage/tests/test_postgresql.py
@@ -248,13 +248,13 @@
         """Calling clear buffers on real storage does nothing
 
         """
-        assert swh_storage.clear_buffers() is None
+        assert swh_storage.clear_buffers([]) is None
 
     def test_flush(self, swh_storage):
         """Calling clear buffers on real storage does nothing
 
         """
-        assert swh_storage.flush() == {}
+        assert swh_storage.flush([]) == {}
 
     def test_dbversion(self, swh_storage):
         with swh_storage.db() as db: