Kafka-consumer. commitSync vs commitAsync

Question

The quote from https://www.safaribooksonline.com/library/view/kafka-the-definitive/9781491936153/ch04.html#callout_kafka_consumers__reading_data_from_kafka_CO2-1

Answer 1

commitAync will not retry because if it retries it will make a mess.

Imagine that you are trying to commit offset 20 (async), and it did not commit (failed), and then the next poll block tries to commit the offset 40 (async), and it succeeded.

Now, commit offset 20 is still waiting to commit, if it reties and succeed it will make a mess.

The mess is that the committed offset should be 40 not 20.

Answer 2

As it is said in the API documentation:

---

• commitSync

This is a synchronous commits and will block until either the commit succeeds or an unrecoverable error is encountered (in which case it is thrown to the caller).

That means, the commitSync is a blocking method. Calling it will block your thread until it either succeeds or fails.

For example,

while (true) {
    ConsumerRecords<String, String> records = consumer.poll(100);
    for (ConsumerRecord<String, String> record : records) {
        System.out.printf("offset = %d, key = %s, value = %s", record.offset(), record.key(), record.value());
        consumer.commitSync();
    }
}

For each iteration in the for-loop, only after consumer.commitSync() successfully returns or interrupted with exception thrown, your code will move to the next iteration.

---

• commitAsync

This is an asynchronous call and will not block. Any errors encountered are either passed to the callback (if provided) or discarded.

That means, the commitAsync is a non-blocking method. Calling it will not block your thread. Instead, it will continue processing the following instructions, no matter whether it will succeed or fail eventually.

For example, similar to previous example, but here we use commitAsync:

while (true) {
    ConsumerRecords<String, String> records = consumer.poll(100);
    for (ConsumerRecord<String, String> record : records) {
        System.out.printf("offset = %d, key = %s, value = %s", record.offset(), record.key(), record.value());
        consumer.commitAsync(callback);
    }
}

For each iteration in the for-loop, no matter what will happen to consumer.commitAsync() eventually, your code will move to the next iteration. And, the result of the commit is going to be handled by the callback function you defined.

---

Trade-offs: latency vs. data consistency

• If you have to ensure the data consistency, choose commitSync() because it will make sure that, before doing any further actions, you will know whether the offset commit is successful or failed. But because it is sync and blocking, you will spend more time on waiting for the commit to be finished, which leads to high latency.
• If you are ok of certain data inconsistency and want to have low latency, choose commitAsync() because it will not wait to be finished. Instead, it will just send out the commit request and handle the response from Kafka (success or failure) later, and meanwhile, your code will continue executing.

This is all generally speaking, the actually behaviour will depend on your actual code and where you are calling the method.

Answer 3

Robust Retry handling with commitAsync()

In the book "Kafka - The Definitive Guide", there is a hint on how to mitigate the potential problem of commiting lower offsets due to an asynchronous commit:

Retrying Async Commits: A simple pattern to get commit order right for asynchronous retries is to use a monotonically increasing sequence number. Increase the sequence number every time you commit and add the sequence number at the time of the commit to the commitAsync callback. When you’re getting ready to send a retry, check if the commit sequence number the callback got is equal to the instance variable; if it is, there was no newer commit and it is safe to retry. If the instance sequence number is higher, don’t retry because a newer commit was already sent.

The following code depicts a possible solution:

import java.util._
import java.time.Duration
import org.apache.kafka.clients.consumer.{ConsumerConfig, ConsumerRecord, KafkaConsumer, OffsetAndMetadata, OffsetCommitCallback}
import org.apache.kafka.common.{KafkaException, TopicPartition}
import collection.JavaConverters._

object AsyncCommitWithCallback extends App {

  // define topic
  val topic = "myOutputTopic"

  // set properties
  val props = new Properties()
  props.put(ConsumerConfig.GROUP_ID_CONFIG, "AsyncCommitter")
  props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092")
  // [set more properties...]
  

  // create KafkaConsumer and subscribe
  val consumer = new KafkaConsumer[String, String](props)
  consumer.subscribe(List(topic).asJavaCollection)

  // initialize global counter
  val atomicLong = new AtomicLong(0)

  // consume message
  try {
    while(true) {
      val records = consumer.poll(Duration.ofMillis(1)).asScala

      if(records.nonEmpty) {
        for (data <- records) {
          // do something with the records
        }
        consumer.commitAsync(new KeepOrderAsyncCommit)
      }

    }
  } catch {
    case ex: KafkaException => ex.printStackTrace()
  } finally {
    consumer.commitSync()
    consumer.close()
  }


  class KeepOrderAsyncCommit extends OffsetCommitCallback {
    // keeping position of this callback instance
    val position = atomicLong.incrementAndGet()

    override def onComplete(offsets: util.Map[TopicPartition, OffsetAndMetadata], exception: Exception): Unit = {
      // retrying only if no other commit incremented the global counter
      if(exception != null){
        if(position == atomicLong.get) {
          consumer.commitAsync(this)
        }
      }
    }
  }

}

Answer 4

Both commitSync and commitAsync uses kafka offset management feature and both has demerits. If the message processing succeeds and commit offset failed(not atomic) and at same time partition re balancing happens, your processed message gets processed again(duplicate processing) by some other consumer. If you are okay with duplicate message processing, then you can go for commitAsync(because it doesn't block and provide low latency, and it provides a higher order commit. so you should be okay). Otherwise go for a custom offset management that takes care of atomicity while processing and updating the offset(use an external offset storage)