refactor: move kafka initialization off calling thread (#95)

Author: aaron-steinfeld

kafka-event-listener/src/main/java/org/hypertrace/core/kafka/event/listener/KafkaLiveEventListenerCallable.java

@@ -16,18 +16,18 @@
 import lombok.extern.slf4j.Slf4j;
 import org.apache.kafka.clients.consumer.Consumer;
 import org.apache.kafka.clients.consumer.ConsumerRecords;
-import org.apache.kafka.common.PartitionInfo;
 import org.apache.kafka.common.TopicPartition;
 import org.apache.kafka.common.errors.InterruptException;
 import org.hypertrace.core.serviceframework.metrics.PlatformMetricsRegistry;
 
 @Slf4j
 class KafkaLiveEventListenerCallable<K, V> implements Callable<Void> {
+
   private static final String EVENT_CONSUMER_ERROR_COUNT = "event.consumer.error.count";
-  private final List<TopicPartition> topicPartitions;
   private final Consumer<K, V> kafkaConsumer;
   private final Duration pollTimeout;
   private final Counter errorCounter;
+  private final String topic;
   private final ConcurrentLinkedQueue<BiConsumer<? super K, ? super V>> callbacks;
 
   KafkaLiveEventListenerCallable(
@@ -41,16 +41,9 @@ class KafkaLiveEventListenerCallable<K, V> implements Callable<Void> {
         kafkaConfig.hasPath(POLL_TIMEOUT)
             ? kafkaConfig.getDuration(POLL_TIMEOUT)
             : Duration.ofSeconds(30);
-    String topic = kafkaConfig.getString(TOPIC_NAME);
+    this.topic = kafkaConfig.getString(TOPIC_NAME);
     this.kafkaConsumer = kafkaConsumer;
-    // fetch partitions and seek to end of partitions to consume live events
-    List<PartitionInfo> partitions = kafkaConsumer.partitionsFor(topic);
-    topicPartitions =
-        partitions.stream()
-            .map(p -> new TopicPartition(p.topic(), p.partition()))
-            .collect(Collectors.toList());
-    kafkaConsumer.assign(topicPartitions);
-    kafkaConsumer.seekToEnd(topicPartitions);
+
     this.errorCounter =
         PlatformMetricsRegistry.registerCounter(
             consumerName + "." + EVENT_CONSUMER_ERROR_COUNT, Collections.emptyMap());
@@ -60,8 +53,20 @@ void addCallback(BiConsumer<? super K, ? super V> callbackFunction) {
     callbacks.add(callbackFunction);
   }
 
+  private List<TopicPartition> initializePartitions() {
+    // fetch partitions and seek to end of partitions to consume live events
+    List<TopicPartition> topicPartitions =
+        kafkaConsumer.partitionsFor(this.topic).stream()
+            .map(p -> new TopicPartition(p.topic(), p.partition()))
+            .collect(Collectors.toUnmodifiableList());
+    kafkaConsumer.assign(topicPartitions);
+    kafkaConsumer.seekToEnd(topicPartitions);
+    return topicPartitions;
+  }
+
   @Override
   public Void call() {
+    List<TopicPartition> topicPartitions = this.initializePartitions();
     do {
       try {
         ConsumerRecords<K, V> records = kafkaConsumer.poll(pollTimeout);