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From "Yuto Kawamura (JIRA)" <j...@apache.org>
Subject [jira] [Created] (KAFKA-4024) First metadata update always take retry.backoff.ms milliseconds to complete
Date Sat, 06 Aug 2016 09:15:20 GMT
Yuto Kawamura created KAFKA-4024:
------------------------------------

             Summary: First metadata update always take retry.backoff.ms milliseconds to complete
                 Key: KAFKA-4024
                 URL: https://issues.apache.org/jira/browse/KAFKA-4024
             Project: Kafka
          Issue Type: Bug
          Components: clients
    Affects Versions: 0.10.0.0, 0.9.0.1
            Reporter: Yuto Kawamura
            Assignee: Yuto Kawamura


Recently I updated our KafkaProducer configuration, specifically we adjusted {{retry.backoff.ms}}
from default(100ms) to 1000ms.
After that we observed that the first {{send()}} start taking longer than before, investigated
then found following facts.

Environment:
- Kafka broker 0.9.0.1
- Kafka producer 0.9.0.1

Our current version is 0.9.0.1 but it reproduced with latest build from trunk branch as well.

h2. TL;DR
The first {{KafkaProducer.send()}} always blocked {{retry.backoff.ms}} milliseconds, due to
unintentionally applied backoff on first metadata update.


h2. Proof
I wrote following test code and placed under the clients/main/java/

{code}
import java.util.Properties;
import java.util.concurrent.TimeUnit;

import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.Producer;
import org.apache.kafka.clients.producer.ProducerConfig;
import org.apache.kafka.common.serialization.ByteArraySerializer;

public final class KafkaProducerMetadataUpdateDurationTest {
    public static void main(String[] args) {
        Properties props = new Properties();
        props.setProperty(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
        props.setProperty(ProducerConfig.MAX_BLOCK_MS_CONFIG, "30000");
        String retryBackoffMs = System.getProperty("retry.backoff.ms");
        System.err.println("Experimenting with retry.backoff.ms = " + retryBackoffMs);
        props.setProperty(ProducerConfig.RETRY_BACKOFF_MS_CONFIG, retryBackoffMs);

        Producer<byte[], byte[]> producer =
                new KafkaProducer<>(props, new ByteArraySerializer(), new ByteArraySerializer());

        long t0 = System.nanoTime();
        try {
            producer.partitionsFor("test");
            long duration = System.nanoTime() - t0;
            System.err.println("Duration = " + TimeUnit.NANOSECONDS.toMillis(duration) + "
ms");
        } finally {
            producer.close();
        }
    }
}
{code}

Here's experiment log:
{code}
# Start zookeeper & kafka broker
./bin/zookeeper-server-start.sh config/zookeeper.properties
./bin/kafka-server-start.sh config/server.properties

# Create test topic
./bin/kafka-topics.sh --zookeeper localhost:2181 --create --topic test --replication-factor
1 --partitions 1

$ ./bin/kafka-run-class.sh -Dretry.backoff.ms=100 KafkaProducerMetadataUpdateDurationTest
Experimenting with retry.backoff.ms = 100
Duration = 175 ms

$ ./bin/kafka-run-class.sh -Dretry.backoff.ms=1000 KafkaProducerMetadataUpdateDurationTest
Experimenting with retry.backoff.ms = 1000
Duration = 1066 ms

$ ./bin/kafka-run-class.sh -Dretry.backoff.ms=10000 KafkaProducerMetadataUpdateDurationTest
Experimenting with retry.backoff.ms = 10000
Duration = 10070 ms
{code}

As you can see, duration of {{partitionsFor()}} increases linearly in proportion to the value
of {{retry.backoff.ms}}.

Here I describe the scenario that leads this behavior:
1. KafkaProducer initializes metadata with giving {{bootstrap.servers}} and the current timestamp:
https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/producer/KafkaProducer.java#L276
2. On the first {{send()}}, KafkaProducer requests metadata update due to missing partition
info: https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/producer/KafkaProducer.java#L527
3. But, DefaultMetadataUpdater doesn't actually send MetadataRequest, because {{metadata.timeToNextUpdate}}
returns a value lager than zero: https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/NetworkClient.java#L541-L548
4. {{Metadata.timeToNextUpdate}} returns lager one of time till metadata expiration or time
till backing off expiration but practially needUpdate is always true at the first time so
here the timeToAllowUpdate is always adopted, which never be zero until {{retry.backoff.ms}}
elapsed since the first {{metadata.update()}}: https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/Metadata.java#L116


This is because of kafka client tries to keep interval configured by {{retry.backoff.ms}}
between each metadata update so it's basically works fine from the second update but for the
first time, since it could never have the actual metadata(which is obtained by MetadaUpdate
request), this backing off isn't making sense and in fact it's harming our application by
blocking the first {{send()}} insanely long.



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