Kafka-consumer. commitSync vs commitAsync

问题

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

The drawback is that while commitSync() will retry the commit until it either succeeds or encounters a non-retriable failure, commitAsync() will not retry.

This phrase is not clear to me. I suppose that consumer sends commit request to broker and in case if the broker doesn't respond within some timeout it means that the commit failed. Am I wrong?

Can you clarify the difference of commitSync and commitAsync in details?
Also, please provide use cases when which commit type should I prefer.

回答1:

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.

回答2:

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)

来源：https://stackoverflow.com/questions/46546174/kafka-consumer-commitsync-vs-commitasync

标签

java

apache-kafka

offset

kafka-consumer-api