[GitHub] [incubator-pinot] npawar commented on a change in pull request #4397: Adding Support for Kafka 2.0 Consumer

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npawar commented on a change in pull request #4397: Adding Support for Kafka 2.0 Consumer
URL: https://github.com/apache/incubator-pinot/pull/4397#discussion_r302680527
 
 

 ##########
 File path: pinot-connectors/pinot-connector-kafka-2.0/src/main/java/org/apache/pinot/core/realtime/impl/kafka2/Kafka2ConsumerManager.java
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+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.pinot.core.realtime.impl.kafka2;
+
+import com.google.common.util.concurrent.Uninterruptibles;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.IdentityHashMap;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Properties;
+import java.util.concurrent.TimeUnit;
+import org.apache.commons.lang3.tuple.ImmutableTriple;
+import org.apache.kafka.clients.consumer.ConsumerConfig;
+import org.apache.kafka.clients.consumer.KafkaConsumer;
+import org.apache.kafka.common.serialization.BytesDeserializer;
+import org.apache.kafka.common.serialization.StringDeserializer;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+/**
+ * Manager for Kafka consumers that reuses consumers and delays their shutdown.
+ *
+ * This is a workaround for the current realtime design flaw where any issue while flushing/committing offsets causes
+ * duplicate or dropped events. Kafka consumption is driven by the controller, which assigns a realtime segment to the
+ * servers; when a server is assigned a new realtime segment, it creates a Kafka consumer, consumes until it reaches a
+ * threshold then flushes to disk, writes metadata to helix indicating the segment is completed, commits Kafka offsets
+ * to ZK and then shuts down the consumer. The controller notices the metadata write and reassigns a segment to the
+ * server, so that it can keep on consuming.
+ *
+ * This logic is flawed if committing Kafka offsets fails, at which time the committed state is unknown. The proper fix
+ * would be to just keep on using that consumer and try committing our offsets later, but we recreate a new Kafka
+ * consumer whenever we get a new segment and also keep the old consumer around, leading to half the events being
+ * assigned, due to Kafka rebalancing the partitions between the two consumers (one of which is not actually reading
+ * anything anymore). Because that logic is stateless and driven by Helix, there's no real clean way to keep the
+ * consumer alive and pass it to the next segment.
+ *
+ * This class and long comment is to work around this issue by keeping the consumer alive for a little bit instead of
+ * shutting it down immediately, so that the next segment assignment can pick up the same consumer. This way, even if
+ * committing the offsets fails, we can still pick up the same consumer the next time we get a segment assigned to us
+ * by the controller and hopefully commit our offsets the next time we flush to disk.
+ *
+ * This temporary code should be completely removed by the time we redesign the consumption to use the lower level
+ * Kafka APIs.
+ */
+public class Kafka2ConsumerManager {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Kafka2ConsumerManager.class);
+  private static final Long IN_USE = -1L;
+  private static final long CONSUMER_SHUTDOWN_DELAY_MILLIS = TimeUnit.SECONDS.toMillis(60); // One minute
+  private static final Map<ImmutableTriple<String, String, String>, KafkaConsumer> CONSUMER_FOR_CONFIG_KEY =
+      new HashMap<>();
+  private static final IdentityHashMap<KafkaConsumer, Long> CONSUMER_RELEASE_TIME = new IdentityHashMap<>();
+
+  public static KafkaConsumer acquireKafkaConsumerForConfig(Kafka2HighLevelStreamConfig kafka2HighLevelStreamConfig) {
+    final ImmutableTriple<String, String, String> configKey =
+        new ImmutableTriple<>(kafka2HighLevelStreamConfig.getKafkaTopicName(), kafka2HighLevelStreamConfig.getGroupId(),
+            kafka2HighLevelStreamConfig.getBootstrapServers());
+
+    synchronized (Kafka2ConsumerManager.class) {
+      // If we have the consumer and it's not already acquired, return it, otherwise error out if it's already acquired
+      if (CONSUMER_FOR_CONFIG_KEY.containsKey(configKey)) {
+        KafkaConsumer kafkaConsumer = CONSUMER_FOR_CONFIG_KEY.get(configKey);
+        if (CONSUMER_RELEASE_TIME.get(kafkaConsumer).equals(IN_USE)) {
+          throw new RuntimeException("Consumer/iterator " + kafkaConsumer + " already in use!");
+        } else {
+          LOGGER.info("Reusing kafka consumer/iterator with id {}", kafkaConsumer);
+          CONSUMER_RELEASE_TIME.put(kafkaConsumer, IN_USE);
+          return kafkaConsumer;
+        }
+      }
+
+      LOGGER.info("Creating new kafka consumer and iterator for topic {}",
+          kafka2HighLevelStreamConfig.getKafkaTopicName());
+
+      // Create the consumer
+
+      Properties consumerProp = new Properties();
+      consumerProp.putAll(kafka2HighLevelStreamConfig.getKafkaConsumerProperties());
+      consumerProp.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, kafka2HighLevelStreamConfig.getBootstrapServers());
+      consumerProp.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
+      consumerProp.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, BytesDeserializer.class.getName());
+      KafkaConsumer consumer = new KafkaConsumer<>(consumerProp);
+      consumer.subscribe(Collections.singletonList(kafka2HighLevelStreamConfig.getKafkaTopicName()));
+
+      // Mark both the consumer and iterator as acquired
+      CONSUMER_FOR_CONFIG_KEY.put(configKey, consumer);
+      CONSUMER_RELEASE_TIME.put(consumer, IN_USE);
+
+      LOGGER.info("Created consumer/iterator with id {} for topic {}", consumer,
+          kafka2HighLevelStreamConfig.getKafkaTopicName());
+
+      return consumer;
+    }
+  }
+
+  public static void releaseKafkaConsumer(final KafkaConsumer kafkaConsumer) {
+    synchronized (Kafka2ConsumerManager.class) {
+      // Release the consumer, mark it for shutdown in the future
+      final long releaseTime = System.currentTimeMillis() + CONSUMER_SHUTDOWN_DELAY_MILLIS;
+      CONSUMER_RELEASE_TIME.put(kafkaConsumer, releaseTime);
+
+      LOGGER.info("Marking consumer/iterator with id {} for release at {}", kafkaConsumer, releaseTime);
 
 Review comment:
   This is just "consumer" right? No need to say "consumer/iterator"
   Same comment throughout the file

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