backup: add command line flag to override replication factor in resto…

…re command

src/karapace/backup/api.py

@@ -373,13 +373,19 @@ def _handle_restore_topic(
     instruction: RestoreTopic,
     config: Config,
     skip_topic_creation: bool = False,
+    override_replication_factor: int | None = None,
 ) -> None:
     if skip_topic_creation:
         return
+    repl_factor = instruction.replication_factor
+    if override_replication_factor is not None:
+        LOG.info("Overriding replication factor with: %d (was: %d)",
+            override_replication_factor, instruction.replication_factor)
+        repl_factor = override_replication_factor
     if not _maybe_create_topic(
         config=config,
         name=instruction.topic_name,
-        replication_factor=instruction.replication_factor,
+        replication_factor=repl_factor,
         topic_configs=instruction.topic_configs,
     ):
         raise BackupTopicAlreadyExists(f"Topic to restore '{instruction.topic_name}' already exists")
@@ -426,6 +432,7 @@ def restore_backup(
     backup_location: ExistingFile,
     topic_name: TopicName,
     skip_topic_creation: bool = False,
+    override_replication_factor: int | None = None,
 ) -> None:
     """Restores a backup from the specified location into the configured topic.
 
@@ -475,7 +482,7 @@ def _check_producer_exception() -> None:
                 _handle_restore_topic_legacy(instruction, config, skip_topic_creation)
                 producer = stack.enter_context(_producer(config, instruction.topic_name))
             elif isinstance(instruction, RestoreTopic):
-                _handle_restore_topic(instruction, config, skip_topic_creation)
+                _handle_restore_topic(instruction, config, skip_topic_creation, override_replication_factor)
                 producer = stack.enter_context(_producer(config, instruction.topic_name))
             elif isinstance(instruction, ProducerSend):
                 if producer is None:

src/karapace/backup/cli.py

@@ -76,6 +76,15 @@ def parse_args() -> argparse.Namespace:
         ),
     )
 
+    parser_restore.add_argument(
+        "--override-replication-factor",
+        help=(
+            "Override the replication factor that is save in the backup. This is needed when restoring a backup from a"
+            "downsized cluster (like scaling down from 6 to 3 nodes). This has effect only for V3 backups."
+        ),
+        type=int,
+    )
+
     return parser.parse_args()
 
 
@@ -115,6 +124,7 @@ def dispatch(args: argparse.Namespace) -> None:
                 backup_location=api.locate_backup_file(location),
                 topic_name=api.normalize_topic_name(args.topic, config),
                 skip_topic_creation=args.skip_topic_creation,
+                override_replication_factor=args.override_replication_factor,
             )
         except BackupDataRestorationError:
             traceback.print_exc()

tests/integration/backup/test_v3_backup.py

@@ -4,7 +4,7 @@
 """
 from __future__ import annotations
 
-from aiokafka.errors import UnknownTopicOrPartitionError
+from aiokafka.errors import InvalidReplicationFactorError, UnknownTopicOrPartitionError
 from collections.abc import Iterator
 from confluent_kafka import Message, TopicPartition
 from confluent_kafka.admin import NewTopic
@@ -698,6 +698,58 @@ def __exit__(self, exc_type, exc_value, exc_traceback):
             )
 
 
+def test_backup_restoration_override_replication_factor(
+    admin_client: KafkaAdminClient,
+    kafka_servers: KafkaServers,
+    producer: KafkaProducer,
+    new_topic: NewTopic,
+) -> None:
+    topic_name = new_topic.topic
+    backup_directory = Path(__file__).parent.parent.resolve() / "test_data" / "backup_v3_single_partition" / topic_name
+    metadata_path = backup_directory / f"{topic_name}.metadata"
+    config = set_config_defaults(
+        {
+            "bootstrap_uri": kafka_servers.bootstrap_servers,
+            "topic_name": new_topic.topic,
+        }
+    )
+
+    # pupulate the topic and create a backup
+    for i in range(10):
+        producer.send(
+            new_topic.topic,
+            key=f"message-key-{i}",
+            value=f"message-value-{i}-" + 1000 * "X",
+        )
+    producer.flush()
+    api.create_backup(
+        config=config,
+        backup_location=backup_directory,
+        topic_name=api.normalize_topic_name(None, config),
+        version=BackupVersion.V3,
+        replication_factor=6,
+    )
+
+    # make sure topic doesn't exist beforehand.
+    _delete_topic(admin_client, topic_name)
+
+    # assert that the restore would fail without the replication factor override
+    with pytest.raises(InvalidReplicationFactorError):
+        api.restore_backup(
+            config=config,
+            backup_location=metadata_path,
+            topic_name=TopicName(topic_name),
+        )
+
+    # finally restore the backup with override
+    api.restore_backup(
+        config=config,
+        backup_location=metadata_path,
+        topic_name=TopicName(topic_name),
+        override_replication_factor=1,
+    )
+
+
 def no_color_env() -> dict[str, str]:
     env = os.environ.copy()
     try: