diff --git a/swh/journal/client.py b/swh/journal/client.py
--- a/swh/journal/client.py
+++ b/swh/journal/client.py
@@ -7,6 +7,7 @@
 import logging
 import os
 import time
+from typing import Any, Dict, List, Optional, Set, Tuple, Union
 
 from confluent_kafka import Consumer, KafkaException, KafkaError
 
@@ -75,9 +76,17 @@
 
     """
     def __init__(
-            self, brokers, group_id, prefix=None, object_types=None,
-            max_messages=0, process_timeout=0, auto_offset_reset='earliest',
-            stop_on_eof=False, **kwargs):
+            self,
+            brokers: Union[str, List[str]],
+            group_id: str,
+            prefix: Optional[str] = None,
+            object_types: Optional[List[str]] = None,
+            max_messages: Optional[int] = None,
+            process_timeout: Optional[float] = None,
+            auto_offset_reset: str = 'earliest',
+            stop_on_eof: bool = False,
+            **kwargs
+    ):
         if prefix is None:
             prefix = DEFAULT_PREFIX
         if object_types is None:
@@ -161,7 +170,7 @@
 
         self.max_messages = max_messages
         self.process_timeout = process_timeout
-        self.eof_reached = set()
+        self.eof_reached: Set[Tuple[str, str]] = set()
 
         self._object_types = object_types
 
@@ -214,7 +223,7 @@
         return nb_messages
 
     def handle_messages(self, messages, worker_fn):
-        objects = defaultdict(list)
+        objects: Dict[str, List[Any]] = defaultdict(list)
         nb_processed = 0
 
         for message in messages:
diff --git a/swh/journal/tests/test_client.py b/swh/journal/tests/test_client.py
--- a/swh/journal/tests/test_client.py
+++ b/swh/journal/tests/test_client.py
@@ -37,14 +37,12 @@
     )
     producer.flush()
 
-    config = {
-        'brokers': 'localhost:%d' % kafka_server[1],
-        'group_id': kafka_consumer_group,
-        'prefix': kafka_prefix,
-        'max_messages': 1,
-    }
-    client = JournalClient(**config)
-
+    client = JournalClient(
+        brokers='localhost:%d' % kafka_server[1],
+        group_id=kafka_consumer_group,
+        prefix=kafka_prefix,
+        max_messages=1,
+    )
     worker_fn = MagicMock()
     client.process(worker_fn)
 
@@ -73,13 +71,13 @@
     )
     producer.flush()
 
-    config = {
-        'brokers': 'localhost:%d' % kafka_server[1],
-        'group_id': kafka_consumer_group,
-        'prefix': kafka_prefix,
-        'stop_on_eof': True,
-    }
-    client = JournalClient(**config)
+    client = JournalClient(
+        brokers='localhost:%d' % kafka_server[1],
+        group_id=kafka_consumer_group,
+        prefix=kafka_prefix,
+        max_messages=1,
+        stop_on_eof=True,
+    )
 
     worker_fn = MagicMock()
     client.process(worker_fn)
diff --git a/swh/journal/tests/test_replay.py b/swh/journal/tests/test_replay.py
--- a/swh/journal/tests/test_replay.py
+++ b/swh/journal/tests/test_replay.py
@@ -72,13 +72,12 @@
     producer.flush()
 
     # Fill the storage from Kafka
-    config = {
-        'brokers': 'localhost:%d' % kafka_server[1],
-        'group_id': kafka_consumer_group,
-        'prefix': kafka_prefix,
-        'max_messages': nb_sent,
-    }
-    replayer = JournalClient(**config)
+    replayer = JournalClient(
+        brokers='localhost:%d' % kafka_server[1],
+        group_id=kafka_consumer_group,
+        prefix=kafka_prefix,
+        max_messages=nb_sent,
+    )
     worker_fn = functools.partial(process_replay_objects, storage=storage)
     nb_inserted = 0
     while nb_inserted < nb_sent:
@@ -147,7 +146,7 @@
         writer.send('origin_visit', 'foo', visit)
 
     queue_size = len(queue)
-    assert replayer.max_messages == 0
+    assert replayer.max_messages is None
     replayer.max_messages = queue_size
 
     storage = get_storage(**storage_config)
diff --git a/swh/journal/tests/test_write_replay.py b/swh/journal/tests/test_write_replay.py
--- a/swh/journal/tests/test_write_replay.py
+++ b/swh/journal/tests/test_write_replay.py
@@ -79,7 +79,7 @@
                 pass
 
     queue_size = len(queue)
-    assert replayer.max_messages == 0
+    assert replayer.max_messages is None
     replayer.max_messages = queue_size
 
     storage2 = get_storage(**storage_config)
@@ -135,7 +135,7 @@
                 contents.append(obj)
 
     queue_size = len(queue)
-    assert replayer.max_messages == 0
+    assert replayer.max_messages is None
     replayer.max_messages = queue_size
 
     storage2 = get_storage(**storage_config)
diff --git a/swh/journal/tests/utils.py b/swh/journal/tests/utils.py
--- a/swh/journal/tests/utils.py
+++ b/swh/journal/tests/utils.py
@@ -69,7 +69,7 @@
     def __init__(self, queue, object_types=ACCEPTED_OBJECT_TYPES):
         self._object_types = object_types
         self.consumer = MockedKafkaConsumer(queue)
-        self.process_timeout = 0
-        self.max_messages = 0
+        self.process_timeout = None
+        self.max_messages = None
         self.value_deserializer = kafka_to_value
         self.stop_on_eof = False