diff --git a/swh/objstorage/api/client.py b/swh/objstorage/api/client.py
--- a/swh/objstorage/api/client.py
+++ b/swh/objstorage/api/client.py
@@ -3,12 +3,14 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+from typing import Iterator, Optional
+
 from swh.core.api import RPCClient
 from swh.core.utils import iter_chunks
 from swh.model import hashutil
+from swh.objstorage.constants import DEFAULT_LIMIT, ID_DIGEST_LENGTH
 from swh.objstorage.exc import Error, ObjNotFoundError, ObjStorageAPIError
-from swh.objstorage.interface import ObjStorageInterface
-from swh.objstorage.objstorage import DEFAULT_LIMIT, ID_DIGEST_LENGTH
+from swh.objstorage.interface import ObjId, ObjStorageInterface
 
 
 class RemoteObjStorage(RPCClient):
@@ -28,13 +30,17 @@
     reraise_exceptions = [ObjNotFoundError, Error]
     backend_class = ObjStorageInterface
 
-    def restore(self, content, obj_id):
+    def restore(self: ObjStorageInterface, content: bytes, obj_id: ObjId):
         return self.add(content, obj_id, check_presence=False)
 
     def __iter__(self):
         yield from self.list_content()
 
-    def list_content(self, last_obj_id=None, limit=DEFAULT_LIMIT):
+    def list_content(
+        self,
+        last_obj_id: Optional[ObjId] = None,
+        limit: int = DEFAULT_LIMIT,
+    ) -> Iterator[ObjId]:
         params = {"limit": limit}
         if last_obj_id:
             params["last_obj_id"] = hashutil.hash_to_hex(last_obj_id)
diff --git a/swh/objstorage/backends/azure.py b/swh/objstorage/backends/azure.py
--- a/swh/objstorage/backends/azure.py
+++ b/swh/objstorage/backends/azure.py
@@ -8,7 +8,7 @@
 import datetime
 from itertools import product
 import string
-from typing import Dict, Optional, Union
+from typing import Dict, Iterator, List, Optional, Union
 import warnings
 
 from azure.core.exceptions import ResourceExistsError, ResourceNotFoundError
@@ -21,6 +21,7 @@
 
 from swh.model import hashutil
 from swh.objstorage.exc import Error, ObjNotFoundError
+from swh.objstorage.interface import ObjId
 from swh.objstorage.objstorage import (
     ObjStorage,
     compressors,
@@ -205,7 +206,7 @@
         """
         return sum(1 for i in self)
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         """Add an obj in storage if it's not there already."""
         if check_presence and obj_id in self:
             return obj_id
@@ -229,14 +230,14 @@
 
         return obj_id
 
-    def restore(self, content, obj_id):
+    def restore(self, content: bytes, obj_id: ObjId):
         """Restore a content."""
         if obj_id in self:
             self.delete(obj_id)
 
         return self.add(content, obj_id, check_presence=False)
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         """retrieve blob's content if found."""
         return call_async(self._get_async, obj_id)
 
@@ -286,18 +287,18 @@
                 ]
             )
 
-    def get_batch(self, obj_ids):
+    def get_batch(self, obj_ids: List[ObjId]) -> Iterator[Optional[bytes]]:
         """Retrieve objects' raw content in bulk from storage, concurrently."""
         return call_async(self._get_batch_async, obj_ids)
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         """Check the content integrity."""
         obj_content = self.get(obj_id)
         content_obj_id = compute_hash(obj_content)
         if content_obj_id != obj_id:
             raise Error(obj_id)
 
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         """Delete an object."""
         super().delete(obj_id)  # Check delete permission
         hex_obj_id = self._internal_id(obj_id)
diff --git a/swh/objstorage/backends/generator.py b/swh/objstorage/backends/generator.py
--- a/swh/objstorage/backends/generator.py
+++ b/swh/objstorage/backends/generator.py
@@ -1,7 +1,9 @@
 from itertools import count, islice, repeat
 import logging
 import random
+from typing import Iterator, Optional
 
+from swh.objstorage.interface import ObjId
 from swh.objstorage.objstorage import DEFAULT_LIMIT, ObjStorage
 
 logger = logging.getLogger(__name__)
@@ -211,7 +213,11 @@
     def delete(self, obj_id, *args, **kwargs):
         return True
 
-    def list_content(self, last_obj_id=None, limit=DEFAULT_LIMIT):
+    def list_content(
+        self,
+        last_obj_id: Optional[ObjId] = None,
+        limit: int = DEFAULT_LIMIT,
+    ) -> Iterator[ObjId]:
         it = iter(self)
         if last_obj_id:
             next(it)
diff --git a/swh/objstorage/backends/http.py b/swh/objstorage/backends/http.py
--- a/swh/objstorage/backends/http.py
+++ b/swh/objstorage/backends/http.py
@@ -4,12 +4,14 @@
 # See top-level LICENSE file for more information
 
 import logging
+from typing import Iterator, Optional
 from urllib.parse import urljoin
 
 import requests
 
 from swh.model import hashutil
 from swh.objstorage import exc
+from swh.objstorage.interface import ObjId
 from swh.objstorage.objstorage import (
     DEFAULT_LIMIT,
     ObjStorage,
@@ -53,19 +55,23 @@
     def __len__(self):
         raise exc.NonIterableObjStorage("__len__")
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         raise exc.ReadOnlyObjStorage("add")
 
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         raise exc.ReadOnlyObjStorage("delete")
 
-    def restore(self, content, obj_id):
+    def restore(self, content: bytes, obj_id: ObjId):
         raise exc.ReadOnlyObjStorage("restore")
 
-    def list_content(self, last_obj_id=None, limit=DEFAULT_LIMIT):
+    def list_content(
+        self,
+        last_obj_id: Optional[ObjId] = None,
+        limit: int = DEFAULT_LIMIT,
+    ) -> Iterator[ObjId]:
         raise exc.NonIterableObjStorage("__len__")
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         try:
             resp = self.session.get(self._path(obj_id))
             resp.raise_for_status()
@@ -81,7 +87,7 @@
                 raise exc.Error("Corrupt object %s: trailing data found" % hex_obj_id)
         return ret
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         # Check the content integrity
         obj_content = self.get(obj_id)
         content_obj_id = compute_hash(obj_content)
diff --git a/swh/objstorage/backends/in_memory.py b/swh/objstorage/backends/in_memory.py
--- a/swh/objstorage/backends/in_memory.py
+++ b/swh/objstorage/backends/in_memory.py
@@ -4,6 +4,7 @@
 # See top-level LICENSE file for more information
 
 from swh.objstorage.exc import Error, ObjNotFoundError
+from swh.objstorage.interface import ObjId
 from swh.objstorage.objstorage import ObjStorage, compute_hash
 
 
@@ -27,7 +28,7 @@
     def __iter__(self):
         return iter(sorted(self.state))
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         if check_presence and obj_id in self:
             return obj_id
 
@@ -35,20 +36,19 @@
 
         return obj_id
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         if obj_id not in self:
             raise ObjNotFoundError(obj_id)
 
         return self.state[obj_id]
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         if obj_id not in self:
             raise ObjNotFoundError(obj_id)
         if compute_hash(self.state[obj_id]) != obj_id:
-            raise Error("Corrupt object %s" % obj_id)
-        return True
+            raise Error("Corrupt object %s" % obj_id.hex())
 
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         super().delete(obj_id)  # Check delete permission
         if obj_id not in self:
             raise ObjNotFoundError(obj_id)
diff --git a/swh/objstorage/backends/libcloud.py b/swh/objstorage/backends/libcloud.py
--- a/swh/objstorage/backends/libcloud.py
+++ b/swh/objstorage/backends/libcloud.py
@@ -15,6 +15,7 @@
 
 from swh.model import hashutil
 from swh.objstorage.exc import Error, ObjNotFoundError
+from swh.objstorage.interface import ObjId
 from swh.objstorage.objstorage import (
     ObjStorage,
     compressors,
@@ -61,7 +62,7 @@
     def __init__(
         self,
         container_name: str,
-        compression: Optional[str] = None,
+        compression: str = "gzip",
         path_prefix: Optional[str] = None,
         **kwargs,
     ):
@@ -156,17 +157,17 @@
         """
         return sum(1 for i in self)
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         if check_presence and obj_id in self:
             return obj_id
 
         self._put_object(content, obj_id)
         return obj_id
 
-    def restore(self, content, obj_id):
+    def restore(self, content: bytes, obj_id: ObjId):
         return self.add(content, obj_id, check_presence=False)
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         obj = b"".join(self._get_object(obj_id).as_stream())
         d = decompressors[self.compression]()
         ret = d.decompress(obj)
@@ -175,7 +176,7 @@
             raise Error("Corrupt object %s: trailing data found" % hex_obj_id)
         return ret
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         # Check that the file exists, as _get_object raises ObjNotFoundError
         self._get_object(obj_id)
         # Check the content integrity
@@ -184,7 +185,7 @@
         if content_obj_id != obj_id:
             raise Error(obj_id)
 
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         super().delete(obj_id)  # Check delete permission
         obj = self._get_object(obj_id)
         return self.driver.delete_object(obj)
diff --git a/swh/objstorage/backends/pathslicing.py b/swh/objstorage/backends/pathslicing.py
--- a/swh/objstorage/backends/pathslicing.py
+++ b/swh/objstorage/backends/pathslicing.py
@@ -1,26 +1,20 @@
-# Copyright (C) 2015-2019  The Software Heritage developers
+# Copyright (C) 2015-2022  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from collections.abc import Iterator
 from contextlib import contextmanager
 from itertools import islice
 import os
 import random
 import tempfile
-from typing import List
+from typing import Iterable, Iterator, List, Optional
 
 from swh.model import hashutil
+from swh.objstorage.constants import DEFAULT_LIMIT, ID_HASH_ALGO, ID_HEXDIGEST_LENGTH
 from swh.objstorage.exc import Error, ObjNotFoundError
-from swh.objstorage.objstorage import (
-    DEFAULT_LIMIT,
-    ID_HASH_ALGO,
-    ID_HEXDIGEST_LENGTH,
-    ObjStorage,
-    compressors,
-    decompressors,
-)
+from swh.objstorage.interface import ObjId
+from swh.objstorage.objstorage import ObjStorage, compressors, decompressors
 
 BUFSIZ = 1048576
 
@@ -188,11 +182,11 @@
 
         return True
 
-    def __contains__(self, obj_id):
+    def __contains__(self, obj_id: ObjId) -> bool:
         hex_obj_id = hashutil.hash_to_hex(obj_id)
         return os.path.isfile(self.slicer.get_path(hex_obj_id))
 
-    def __iter__(self):
+    def __iter__(self) -> Iterator[bytes]:
         """Iterate over the object identifiers currently available in the
         storage.
 
@@ -217,7 +211,7 @@
 
         return obj_iterator()
 
-    def __len__(self):
+    def __len__(self) -> int:
         """Compute the number of objects available in the storage.
 
         Warning: this currently uses `__iter__`, its warning about bad
@@ -228,23 +222,25 @@
         """
         return sum(1 for i in self)
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(
+        self,
+        content: bytes,
+        obj_id: ObjId,
+        check_presence: bool = True,
+    ) -> ObjId:
         if check_presence and obj_id in self:
             # If the object is already present, return immediately.
             return obj_id
 
         hex_obj_id = hashutil.hash_to_hex(obj_id)
-        if not isinstance(content, Iterator):
-            content = [content]
         compressor = compressors[self.compression]()
         with self._write_obj_file(hex_obj_id) as f:
-            for chunk in content:
-                f.write(compressor.compress(chunk))
+            f.write(compressor.compress(content))
             f.write(compressor.flush())
 
         return obj_id
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         if obj_id not in self:
             raise ObjNotFoundError(obj_id)
 
@@ -260,7 +256,7 @@
 
         return out
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         try:
             data = self.get(obj_id)
         except OSError:
@@ -282,7 +278,7 @@
                 % (hashutil.hash_to_hex(obj_id), hashutil.hash_to_hex(actual_obj_id))
             )
 
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         super().delete(obj_id)  # Check delete permission
         if obj_id not in self:
             raise ObjNotFoundError(obj_id)
@@ -296,7 +292,7 @@
 
     # Management methods
 
-    def get_random(self, batch_size):
+    def get_random(self, batch_size: int) -> Iterable[ObjId]:
         def get_random_content(self, batch_size):
             """Get a batch of content inside a single directory.
 
@@ -334,7 +330,9 @@
             yield lambda c: f.write(compressor.compress(c))
             f.write(compressor.flush())
 
-    def list_content(self, last_obj_id=None, limit=DEFAULT_LIMIT):
+    def list_content(
+        self, last_obj_id: Optional[ObjId] = None, limit: int = DEFAULT_LIMIT
+    ) -> Iterator[ObjId]:
         if last_obj_id:
             it = self.iter_from(last_obj_id)
         else:
diff --git a/swh/objstorage/backends/seaweedfs/objstorage.py b/swh/objstorage/backends/seaweedfs/objstorage.py
--- a/swh/objstorage/backends/seaweedfs/objstorage.py
+++ b/swh/objstorage/backends/seaweedfs/objstorage.py
@@ -7,9 +7,11 @@
 from itertools import islice
 import logging
 import os
+from typing import Iterator, Optional
 
 from swh.model import hashutil
 from swh.objstorage.exc import Error, ObjNotFoundError
+from swh.objstorage.interface import ObjId
 from swh.objstorage.objstorage import (
     DEFAULT_LIMIT,
     ObjStorage,
@@ -72,27 +74,26 @@
         """
         return sum(1 for i in self)
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         if check_presence and obj_id in self:
             return obj_id
 
         def compressor(data):
             comp = compressors[self.compression]()
-            for chunk in data:
-                yield comp.compress(chunk)
+            yield comp.compress(data)
             yield comp.flush()
 
-        if isinstance(content, bytes):
-            content = [content]
+        assert isinstance(
+            content, bytes
+        ), "list of content chunks is not supported anymore"
 
-        # XXX should handle streaming correctly...
         self.wf.put(io.BytesIO(b"".join(compressor(content))), self._path(obj_id))
         return obj_id
 
-    def restore(self, content, obj_id):
+    def restore(self, content: bytes, obj_id: ObjId):
         return self.add(content, obj_id, check_presence=False)
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         try:
             obj = self.wf.get(self._path(obj_id))
         except Exception:
@@ -105,21 +106,25 @@
             raise Error("Corrupt object %s: trailing data found" % hex_obj_id)
         return ret
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         # Check the content integrity
         obj_content = self.get(obj_id)
         content_obj_id = compute_hash(obj_content)
         if content_obj_id != obj_id:
             raise Error(obj_id)
 
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         super().delete(obj_id)  # Check delete permission
         if obj_id not in self:
             raise ObjNotFoundError(obj_id)
         self.wf.delete(self._path(obj_id))
         return True
 
-    def list_content(self, last_obj_id=None, limit=DEFAULT_LIMIT):
+    def list_content(
+        self,
+        last_obj_id: Optional[ObjId] = None,
+        limit: int = DEFAULT_LIMIT,
+    ) -> Iterator[ObjId]:
         if last_obj_id:
             objid = hashutil.hash_to_hex(last_obj_id)
             lastfilename = objid
diff --git a/swh/objstorage/backends/winery/objstorage.py b/swh/objstorage/backends/winery/objstorage.py
--- a/swh/objstorage/backends/winery/objstorage.py
+++ b/swh/objstorage/backends/winery/objstorage.py
@@ -7,6 +7,7 @@
 from multiprocessing import Process
 
 from swh.objstorage import exc
+from swh.objstorage.interface import ObjId
 from swh.objstorage.objstorage import ObjStorage
 
 from .roshard import ROShard
@@ -28,7 +29,7 @@
     def uninit(self):
         self.winery.uninit()
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         return self.winery.get(obj_id)
 
     def check_config(self, *, check_write):
@@ -37,13 +38,13 @@
     def __contains__(self, obj_id):
         return obj_id in self.winery
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         return self.winery.add(content, obj_id, check_presence)
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         return self.winery.check(obj_id)
 
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         raise PermissionError("Delete is not allowed.")
 
 
@@ -74,7 +75,7 @@
             self.shards[name] = shard
         return self.shards[name]
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         shard_info = self.base.get(obj_id)
         if shard_info is None:
             raise exc.ObjNotFoundError(obj_id)
@@ -140,7 +141,7 @@
         self.shard.uninit()
         super().uninit()
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         if check_presence and obj_id in self:
             return obj_id
 
@@ -157,7 +158,7 @@
 
         return obj_id
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         # load all shards packing == True and not locked (i.e. packer
         # was interrupted for whatever reason) run pack for each of them
         pass
diff --git a/swh/objstorage/constants.py b/swh/objstorage/constants.py
new file mode 100644
--- /dev/null
+++ b/swh/objstorage/constants.py
@@ -0,0 +1,17 @@
+# Copyright (C) 2015-2022  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+from typing import Literal
+
+ID_HASH_ALGO: Literal["sha1"] = "sha1"
+
+ID_HEXDIGEST_LENGTH = 40
+"""Size in bytes of the hash hexadecimal representation."""
+
+ID_DIGEST_LENGTH = 20
+"""Size in bytes of the hash"""
+
+DEFAULT_LIMIT = 10000
+"""Default number of results of ``list_content``."""
diff --git a/swh/objstorage/factory.py b/swh/objstorage/factory.py
--- a/swh/objstorage/factory.py
+++ b/swh/objstorage/factory.py
@@ -15,7 +15,7 @@
 from swh.objstorage.backends.seaweedfs import SeaweedFilerObjStorage
 from swh.objstorage.multiplexer import MultiplexerObjStorage, StripingObjStorage
 from swh.objstorage.multiplexer.filter import add_filters
-from swh.objstorage.objstorage import ID_HEXDIGEST_LENGTH, ObjStorage  # noqa
+from swh.objstorage.objstorage import ObjStorage
 
 __all__ = ["get_objstorage", "ObjStorage"]
 
diff --git a/swh/objstorage/interface.py b/swh/objstorage/interface.py
--- a/swh/objstorage/interface.py
+++ b/swh/objstorage/interface.py
@@ -3,12 +3,15 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from typing import Dict
+from typing import Dict, Iterable, Iterator, List, Optional
 
 from typing_extensions import Protocol, runtime_checkable
 
 from swh.core.api import remote_api_endpoint
-from swh.objstorage.objstorage import DEFAULT_LIMIT
+from swh.objstorage.constants import DEFAULT_LIMIT
+
+ObjId = bytes
+"""Type of object ids, which should be a sha1 hash."""
 
 
 @runtime_checkable
@@ -48,11 +51,11 @@
         ...
 
     @remote_api_endpoint("content/contains")
-    def __contains__(self, obj_id):
+    def __contains__(self, obj_id: ObjId) -> bool:
         """Indicate if the given object is present in the storage.
 
         Args:
-            obj_id (bytes): object identifier.
+            obj_id: object identifier.
 
         Returns:
             True if and only if the object is present in the current object
@@ -62,12 +65,12 @@
         ...
 
     @remote_api_endpoint("content/add")
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         """Add a new object to the object storage.
 
         Args:
-            content (bytes): object's raw content to add in storage.
-            obj_id (bytes): checksum of [bytes] using [ID_HASH_ALGO]
+            content: object's raw content to add in storage.
+            obj_id: checksum of [bytes] using [ID_HASH_ALGO]
                 algorithm. It is trusted to match the bytes.
             check_presence (bool): indicate if the presence of the
                 content should be verified before adding the file.
@@ -92,7 +95,7 @@
         """
         ...
 
-    def restore(self, content, obj_id):
+    def restore(self, content: bytes, obj_id: ObjId):
         """Restore a content that have been corrupted.
 
         This function is identical to add but does not check if
@@ -101,21 +104,17 @@
         suitable for most cases.
 
         Args:
-            content (bytes): object's raw content to add in storage
-            obj_id (bytes): checksum of `bytes` as computed by
-                ID_HASH_ALGO. When given, obj_id will be trusted to
-                match bytes. If missing, obj_id will be computed on
-                the fly.
-
+            content: object's raw content to add in storage
+            obj_id: dict of hashes of the content (or only the sha1, for legacy clients)
         """
         ...
 
     @remote_api_endpoint("content/get")
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         """Retrieve the content of a given object.
 
         Args:
-            obj_id (bytes): object id.
+            obj_id: object id.
 
         Returns:
             the content of the requested object as bytes.
@@ -127,7 +126,7 @@
         ...
 
     @remote_api_endpoint("content/get/batch")
-    def get_batch(self, obj_ids):
+    def get_batch(self, obj_ids: List[ObjId]) -> Iterator[Optional[bytes]]:
         """Retrieve objects' raw content in bulk from storage.
 
         Note: This function does have a default implementation in
@@ -138,7 +137,7 @@
         can be overridden to perform a more efficient operation.
 
         Args:
-            obj_ids ([bytes]: list of object ids.
+            obj_ids: list of object ids.
 
         Returns:
             list of resulting contents, or None if the content could
@@ -149,14 +148,14 @@
         ...
 
     @remote_api_endpoint("content/check")
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         """Perform an integrity check for a given object.
 
         Verify that the file object is in place and that the content matches
         the object id.
 
         Args:
-            obj_id (bytes): object identifier.
+            obj_id: object identifier.
 
         Raises:
             ObjNotFoundError: if the requested object is missing.
@@ -166,11 +165,11 @@
         ...
 
     @remote_api_endpoint("content/delete")
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         """Delete an object.
 
         Args:
-            obj_id (bytes): object identifier.
+            obj_id: object identifier.
 
         Raises:
             ObjNotFoundError: if the requested object is missing.
@@ -181,34 +180,35 @@
     # Management methods
 
     @remote_api_endpoint("content/get/random")
-    def get_random(self, batch_size):
+    def get_random(self, batch_size: int) -> Iterable[ObjId]:
         """Get random ids of existing contents.
 
         This method is used in order to get random ids to perform
         content integrity verifications on random contents.
 
         Args:
-            batch_size (int): Number of ids that will be given
+            batch_size: Number of ids that will be given
 
         Yields:
-            An iterable of ids (bytes) of contents that are in the
-            current object storage.
+            ids of contents that are in the current object storage.
 
         """
         ...
 
-    def __iter__(self):
+    def __iter__(self) -> Iterator[ObjId]:
         ...
 
-    def list_content(self, last_obj_id=None, limit=DEFAULT_LIMIT):
+    def list_content(
+        self, last_obj_id: Optional[ObjId] = None, limit: int = DEFAULT_LIMIT
+    ) -> Iterator[ObjId]:
         """Generates known object ids.
 
         Args:
-            last_obj_id (bytes): object id from which to iterate from
+            last_obj_id: object id from which to iterate from
                  (excluded).
             limit (int): max number of object ids to generate.
 
         Generates:
-            obj_id (bytes): object ids.
+            obj_id: object ids.
         """
         ...
diff --git a/swh/objstorage/multiplexer/multiplexer_objstorage.py b/swh/objstorage/multiplexer/multiplexer_objstorage.py
--- a/swh/objstorage/multiplexer/multiplexer_objstorage.py
+++ b/swh/objstorage/multiplexer/multiplexer_objstorage.py
@@ -6,9 +6,10 @@
 import queue
 import random
 import threading
-from typing import Dict
+from typing import Dict, Iterable
 
 from swh.objstorage.exc import ObjNotFoundError
+from swh.objstorage.interface import ObjId
 from swh.objstorage.objstorage import ObjStorage
 
 
@@ -222,7 +223,7 @@
 
         return obj_iterator()
 
-    def add(self, content, obj_id, check_presence=True):
+    def add(self, content: bytes, obj_id: ObjId, check_presence: bool = True) -> ObjId:
         """Add a new object to the object storage.
 
         If the adding step works in all the storages that accept this content,
@@ -255,6 +256,8 @@
                 continue
             return result
 
+        assert False, "No backend objstorage configured"
+
     def add_batch(self, contents, check_presence=True) -> Dict:
         """Add a batch of new objects to the object storage."""
         write_threads = list(self.get_write_threads())
@@ -275,7 +278,7 @@
             "object:add:bytes": summed["object:add:bytes"] // len(results),
         }
 
-    def restore(self, content, obj_id):
+    def restore(self, content: bytes, obj_id: ObjId):
         return self.wrap_call(
             self.get_write_threads(obj_id),
             "restore",
@@ -283,7 +286,7 @@
             obj_id=obj_id,
         ).pop()
 
-    def get(self, obj_id):
+    def get(self, obj_id: ObjId) -> bytes:
         for storage in self.get_read_threads(obj_id):
             try:
                 return storage.get(obj_id)
@@ -292,7 +295,7 @@
         # If no storage contains this content, raise the error
         raise ObjNotFoundError(obj_id)
 
-    def check(self, obj_id):
+    def check(self, obj_id: ObjId) -> None:
         nb_present = 0
         for storage in self.get_read_threads(obj_id):
             try:
@@ -308,11 +311,11 @@
         if nb_present == 0:
             raise ObjNotFoundError(obj_id)
 
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         super().delete(obj_id)  # Check delete permission
         return all(self.wrap_call(self.get_write_threads(obj_id), "delete", obj_id))
 
-    def get_random(self, batch_size):
+    def get_random(self, batch_size: int) -> Iterable[ObjId]:
         storages_set = [storage for storage in self.storages if len(storage) > 0]
         if len(storages_set) <= 0:
             return []
diff --git a/swh/objstorage/objstorage.py b/swh/objstorage/objstorage.py
--- a/swh/objstorage/objstorage.py
+++ b/swh/objstorage/objstorage.py
@@ -7,23 +7,14 @@
 import bz2
 from itertools import dropwhile, islice
 import lzma
-from typing import Dict
+from typing import Callable, Dict, Iterable, Iterator, List, Optional
 import zlib
 
 from swh.model import hashutil
 
+from .constants import DEFAULT_LIMIT, ID_HASH_ALGO
 from .exc import ObjNotFoundError
-
-ID_HASH_ALGO = "sha1"
-
-ID_HEXDIGEST_LENGTH = 40
-"""Size in bytes of the hash hexadecimal representation."""
-
-ID_DIGEST_LENGTH = 20
-"""Size in bytes of the hash"""
-
-DEFAULT_LIMIT = 10000
-"""Default number of results of ``list_content``."""
+from .interface import ObjId, ObjStorageInterface
 
 
 def compute_hash(content, algo=ID_HASH_ALGO):
@@ -56,28 +47,43 @@
 
 
 class NullDecompressor:
-    def decompress(self, data):
+    def decompress(self, data: bytes) -> bytes:
         return data
 
     @property
-    def unused_data(self):
+    def unused_data(self) -> bytes:
         return b""
 
 
-decompressors = {
-    "bz2": bz2.BZ2Decompressor,
-    "lzma": lzma.LZMADecompressor,
-    "gzip": lambda: zlib.decompressobj(wbits=31),
-    "zlib": zlib.decompressobj,
-    "none": NullDecompressor,
+class _CompressorProtocol:
+    def compress(self, data: bytes) -> bytes:
+        ...
+
+    def flush(self) -> bytes:
+        ...
+
+
+class _DecompressorProtocol:
+    def decompress(self, data: bytes) -> bytes:
+        ...
+
+    unused_data: bytes
+
+
+decompressors: Dict[str, Callable[[], _DecompressorProtocol]] = {
+    "bz2": bz2.BZ2Decompressor,  # type: ignore
+    "lzma": lzma.LZMADecompressor,  # type: ignore
+    "gzip": lambda: zlib.decompressobj(wbits=31),  # type: ignore
+    "zlib": zlib.decompressobj,  # type: ignore
+    "none": NullDecompressor,  # type: ignore
 }
 
-compressors = {
-    "bz2": bz2.BZ2Compressor,
-    "lzma": lzma.LZMACompressor,
-    "gzip": lambda: zlib.compressobj(wbits=31),
-    "zlib": zlib.compressobj,
-    "none": NullCompressor,
+compressors: Dict[str, Callable[[], _CompressorProtocol]] = {
+    "bz2": bz2.BZ2Compressor,  # type: ignore
+    "lzma": lzma.LZMACompressor,  # type: ignore
+    "gzip": lambda: zlib.compressobj(wbits=31),  # type: ignore
+    "zlib": zlib.compressobj,  # type: ignore
+    "none": NullCompressor,  # type: ignore
 }
 
 
@@ -87,19 +93,7 @@
         # it becomes needed
         self.allow_delete = allow_delete
 
-    @abc.abstractmethod
-    def check_config(self, *, check_write):
-        pass
-
-    @abc.abstractmethod
-    def __contains__(self, obj_id):
-        pass
-
-    @abc.abstractmethod
-    def add(self, content, obj_id, check_presence=True):
-        pass
-
-    def add_batch(self, contents, check_presence=True) -> Dict:
+    def add_batch(self: ObjStorageInterface, contents, check_presence=True) -> Dict:
         summary = {"object:add": 0, "object:add:bytes": 0}
         for obj_id, content in contents.items():
             if check_presence and obj_id in self:
@@ -109,15 +103,13 @@
             summary["object:add:bytes"] += len(content)
         return summary
 
-    def restore(self, content, obj_id):
+    def restore(self: ObjStorageInterface, content: bytes, obj_id: ObjId):
         # check_presence to false will erase the potential previous content.
         return self.add(content, obj_id, check_presence=False)
 
-    @abc.abstractmethod
-    def get(self, obj_id):
-        pass
-
-    def get_batch(self, obj_ids):
+    def get_batch(
+        self: ObjStorageInterface, obj_ids: List[ObjId]
+    ) -> Iterator[Optional[bytes]]:
         for obj_id in obj_ids:
             try:
                 yield self.get(obj_id)
@@ -125,23 +117,19 @@
                 yield None
 
     @abc.abstractmethod
-    def check(self, obj_id):
-        pass
-
-    @abc.abstractmethod
-    def delete(self, obj_id):
+    def delete(self, obj_id: ObjId):
         if not self.allow_delete:
             raise PermissionError("Delete is not allowed.")
 
-    # Management methods
-
-    def get_random(self, batch_size):
+    def get_random(self, batch_size: int) -> Iterable[ObjId]:
         pass
 
-    # Streaming methods
-
-    def list_content(self, last_obj_id=None, limit=DEFAULT_LIMIT):
+    def list_content(
+        self: ObjStorageInterface,
+        last_obj_id: Optional[ObjId] = None,
+        limit: int = DEFAULT_LIMIT,
+    ) -> Iterator[ObjId]:
         it = iter(self)
-        if last_obj_id:
-            it = dropwhile(lambda x: x <= last_obj_id, it)
+        if last_obj_id is not None:
+            it = dropwhile(last_obj_id.__ge__, it)
         return islice(it, limit)
diff --git a/swh/objstorage/tests/test_objstorage_pathslicing.py b/swh/objstorage/tests/test_objstorage_pathslicing.py
--- a/swh/objstorage/tests/test_objstorage_pathslicing.py
+++ b/swh/objstorage/tests/test_objstorage_pathslicing.py
@@ -10,8 +10,8 @@
 
 from swh.model import hashutil
 from swh.objstorage import exc
+from swh.objstorage.constants import ID_DIGEST_LENGTH
 from swh.objstorage.factory import get_objstorage
-from swh.objstorage.objstorage import ID_DIGEST_LENGTH
 
 from .objstorage_testing import ObjStorageTestFixture