diff --git a/swh/storage/backfill.py b/swh/storage/backfill.py
--- a/swh/storage/backfill.py
+++ b/swh/storage/backfill.py
@@ -453,7 +453,7 @@
         return str(bound)
 
 
-MANDATORY_KEYS = ["brokers", "storage_dbconn", "prefix", "client_id"]
+MANDATORY_KEYS = ["storage_dbconn", "journal_writer"]
 
 
 class JournalBackfiller:
@@ -520,11 +520,7 @@
         )
 
         db = BaseDb.connect(self.config["storage_dbconn"])
-        writer = KafkaJournalWriter(
-            brokers=self.config["brokers"],
-            prefix=self.config["prefix"],
-            client_id=self.config["client_id"],
-        )
+        writer = KafkaJournalWriter(**self.config["journal_writer"])
         for range_start, range_end in RANGE_GENERATORS[object_type](
             start_object, end_object
         ):
diff --git a/swh/storage/tests/test_backfill.py b/swh/storage/tests/test_backfill.py
--- a/swh/storage/tests/test_backfill.py
+++ b/swh/storage/tests/test_backfill.py
@@ -16,9 +16,11 @@
 from swh.storage.tests.test_replay import check_replayed
 
 TEST_CONFIG = {
-    "brokers": ["localhost"],
-    "prefix": "swh.tmp_journal.new",
-    "client_id": "swh.journal.client.test",
+    "journal_writer": {
+        "brokers": ["localhost"],
+        "prefix": "swh.tmp_journal.new",
+        "client_id": "swh.journal.client.test",
+    },
     "storage_dbconn": "service=swh-dev",
 }
 
@@ -205,9 +207,11 @@
 
     # now apply the backfiller on the storage to fill the journal under prefix2
     backfiller_config = {
-        "brokers": [kafka_server],
-        "client_id": "kafka_writer-2",
-        "prefix": prefix2,
+        "journal_writer": {
+            "brokers": [kafka_server],
+            "client_id": "kafka_writer-2",
+            "prefix": prefix2,
+        },
         "storage_dbconn": swh_storage_backend_config["db"],
     }