diff --git a/swh/storage/buffer.py b/swh/storage/buffer.py
--- a/swh/storage/buffer.py
+++ b/swh/storage/buffer.py
@@ -6,12 +6,15 @@
 from functools import partial
 from typing import Dict, Iterable, Mapping, Sequence, Tuple
 
+from typing_extensions import Literal
+
 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: Tuple[str, ...] = (
+LObjectType = Literal["content", "skipped_content", "directory", "revision", "release"]
+OBJECT_TYPES: Tuple[LObjectType, ...] = (
     "content",
     "skipped_content",
     "directory",
@@ -60,7 +63,7 @@
 
         self._buffer_thresholds = {**DEFAULT_BUFFER_THRESHOLDS, **min_batch_size}
 
-        self._objects: Dict[str, Dict[Tuple[str, ...], BaseModel]] = {
+        self._objects: Dict[LObjectType, Dict[Tuple[str, ...], BaseModel]] = {
             k: {} for k in OBJECT_TYPES
         }
         self._contents_size: int = 0
@@ -103,7 +106,11 @@
         )
 
     def object_add(
-        self, objects: Sequence[BaseModel], *, object_type: str, keys: Iterable[str],
+        self,
+        objects: Sequence[BaseModel],
+        *,
+        object_type: LObjectType,
+        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.
@@ -118,7 +125,9 @@
 
         return {}
 
-    def flush(self, object_types: Sequence[str] = OBJECT_TYPES) -> Dict[str, int]:
+    def flush(
+        self, object_types: Sequence[LObjectType] = OBJECT_TYPES
+    ) -> Dict[str, int]:
         summary: Dict[str, int] = self.storage.flush(object_types)
         for object_type in object_types:
             buffer_ = self._objects[object_type]
@@ -132,7 +141,7 @@
 
         return summary
 
-    def clear_buffers(self, object_types: Sequence[str] = OBJECT_TYPES) -> None:
+    def clear_buffers(self, object_types: Sequence[LObjectType] = OBJECT_TYPES) -> None:
         """Clear objects from current buffer.
 
         WARNING: