[KAFKA-12776] Producer sends messages out-of-order inspite of enabling idempotence

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Information Provided
• Affects Version/s: 2.6.0, 2.7.0
• Component/s: producer
• Environment: Linux RHEL 7.9 and Ubuntu 20.04
• Assignee: Unassigned
• Reporter: NEERAJ VAIDYA
• Created: 12/May/21 20:51
• Updated: 20/May/21 11:08
• Resolved: 20/May/21 11:08

Description

I have an Apache Kafka 2.6 Producer which writes to topic-A (TA). 

My application is basically a Spring boot web-application which accepts JSON payloads via HTTP and then pushes each to a Kafka topic. I also use Spring Cloud Stream Kafka in the application to create and use a Producer.

For one of my failure handling test cases, I shutdown the Kafka cluster while my applications are running. (Note : No messages have been published to the Kafka cluster before I stop the cluster)

When the producer application tries to write messages to TA, it cannot because the cluster is down and hence (I assume) buffers the messages. Let's say it receives 4 messages m1,m2,m3,m4 in increasing time order. (i.e. m1 is first and m4 is last).

When I bring the Kafka cluster back online, the producer sends the buffered messages to the topic, but they are not in order. I receive for example, m2 then m3 then m1 and then m4.

Why is that ? Is it because the buffering in the producer is multi-threaded with each producing to the topic at the same time ?

My project code is attached herewith.

I can confirm that I have enabled idempotence. I have also tried with ```max.in.flight.requests=1```