fix: consume only one record at a time after startup

If Karapace would expect to consume multiple records after startup,
there is extra latency of the consumption timeout as consumer does
not return control back to schema reader until it happens. This would
cause extra latency when storing schema.
On normal operating mode after startup there expectation is to
process single record at a time and return control immediately to
schema reader when record is available.

karapace/schema_reader.py

@@ -58,6 +58,16 @@
 KAFKA_CLIENT_CREATION_TIMEOUT_SECONDS: Final = 2.0
 SCHEMA_TOPIC_CREATION_TIMEOUT_SECONDS: Final = 5.0
 
+# For good startup performance the consumption of multiple
+# records for each consume round is essential.
+# Consumer default is 1 message for each consume call and after
+# startup the default is a good value. If consumer would expect
+# more messages it would return control back after timeout and
+# making schema storing latency to be `processing time + timeout`.
+MAX_MESSAGES_TO_CONSUME_ON_STARTUP: Final = 1000
+MAX_MESSAGES_TO_CONSUME_AFTER_STARTUP: Final = 1
+MESSAGE_CONSUME_TIMEOUT_SECONDS: Final = 0.2
+
 # Metric names
 METRIC_SCHEMA_TOPIC_RECORDS_PROCESSED_COUNT: Final = "karapace_schema_reader_records_processed"
 METRIC_SCHEMA_TOPIC_RECORDS_PER_KEYMODE_GAUGE: Final = "karapace_schema_reader_records_per_keymode"
@@ -120,11 +130,8 @@ def __init__(
     ) -> None:
         Thread.__init__(self, name="schema-reader")
         self.master_coordinator = master_coordinator
-        self.timeout_s = 0.2
-        # Consumer default is 1 message for each consume call
-        # For good startup performance the consumption of multiple
-        # records for each consume round is essential.
-        self.max_messages_to_process = 1000
+        self.timeout_s = MESSAGE_CONSUME_TIMEOUT_SECONDS
+        self.max_messages_to_process = MAX_MESSAGES_TO_CONSUME_ON_STARTUP
         self.config = config
 
         self.database = database
@@ -301,6 +308,7 @@ def _is_ready(self) -> bool:
         self.startup_previous_processed_offset = self.offset
         ready = self.offset >= self._highest_offset
         if ready:
+            self.max_messages_to_process = MAX_MESSAGES_TO_CONSUME_AFTER_STARTUP
             LOG.info("Ready in %s seconds", time.monotonic() - self.start_time)
         return ready
 

tests/unit/test_schema_reader.py

@@ -10,7 +10,13 @@
 from karapace.config import DEFAULTS
 from karapace.in_memory_database import InMemoryDatabase
 from karapace.offset_watcher import OffsetWatcher
-from karapace.schema_reader import KafkaSchemaReader, OFFSET_EMPTY, OFFSET_UNINITIALIZED
+from karapace.schema_reader import (
+    KafkaSchemaReader,
+    MAX_MESSAGES_TO_CONSUME_AFTER_STARTUP,
+    MAX_MESSAGES_TO_CONSUME_ON_STARTUP,
+    OFFSET_EMPTY,
+    OFFSET_UNINITIALIZED,
+)
 from tests.base_testcase import BaseTestCase
 from unittest.mock import Mock
 
@@ -154,3 +160,27 @@ def test_readiness_check(testcase: ReadinessTestCase) -> None:
 
     schema_reader.handle_messages()
     assert schema_reader.ready is testcase.expected
+
+
+def test_num_max_messages_to_consume_moved_to_one_after_ready() -> None:
+    key_formatter_mock = Mock()
+    consumer_mock = Mock()
+    consumer_mock.consume.return_value = []
+    # Return tuple (beginning, end), end offset is the next upcoming record offset
+    consumer_mock.get_watermark_offsets.return_value = (0, 1)
+
+    offset_watcher = OffsetWatcher()
+    schema_reader = KafkaSchemaReader(
+        config=DEFAULTS,
+        offset_watcher=offset_watcher,
+        key_formatter=key_formatter_mock,
+        master_coordinator=None,
+        database=InMemoryDatabase(),
+    )
+    schema_reader.consumer = consumer_mock
+    schema_reader.offset = 0
+    assert schema_reader.max_messages_to_process == MAX_MESSAGES_TO_CONSUME_ON_STARTUP
+
+    schema_reader.handle_messages()
+    assert schema_reader.ready is True
+    assert schema_reader.max_messages_to_process == MAX_MESSAGES_TO_CONSUME_AFTER_STARTUP