diff --git a/swh/journal/client.py b/swh/journal/client.py
--- a/swh/journal/client.py
+++ b/swh/journal/client.py
@@ -30,6 +30,17 @@
 ]
 
 
+def _error_cb(error):
+    if error.fatal():
+        raise KafkaException(error)
+    logger.info('Received non-fatal kafka error: %s', error)
+
+
+def _on_commit(error, partitions):
+    if error is not None:
+        _error_cb(error)
+
+
 class JournalClient:
     """A base client for the Software Heritage journal.
 
@@ -81,8 +92,8 @@
             'bootstrap.servers': ','.join(brokers),
             'auto.offset.reset': auto_offset_reset,
             'group.id': group_id,
-            'on_commit': self._on_commit,
-            'error_cb': self._error_cb,
+            'on_commit': _on_commit,
+            'error_cb': _error_cb,
             'enable.auto.commit': False,
             'logger': logger,
         }
@@ -105,15 +116,6 @@
 
         self._object_types = object_types
 
-    def _error_cb(self, error):
-        if error.fatal():
-            raise KafkaException(error)
-        logger.info('Received non-fatal kafka error: %s', error)
-
-    def _on_commit(self, error, partitions):
-        if error is not None:
-            self._error_cb(error)
-
     def process(self, worker_fn):
         """Polls Kafka for a batch of messages, and calls the worker_fn
         with these messages.