为什么 kafka 流线程在源主题分区更改时死亡？ 任何人都可以指出阅读材料吗？

Question

We increased the number of partitions to parallel process the messages as the throughput of the message was high. As soon as we increased the number of partitions all the streams thread which were subscribed to that topic died. We changed the consumer group id then we restarted the application it worked fine.

I know that the number of partitions changelog topic of application should be same as source topic. I would like to know the reason behind this.

I saw this link - https://issues.apache.org/jira/browse/KAFKA-6063?jql=project%20%3D%20KAFKA%20AND%20component%20%3D%20streams%20AND%20text%20~%20%22partition%22

Couldn't find the reason

https://github.com/apache/kafka/blob/fdc742b1ade420682911b3e336ae04827639cc04/streams/src/main/java/org/apache/kafka/streams/processor/internals/InternalTopicManager.java#L122

Basically, reason behind this if condition.

Answer 1

Input topic partitions define the level of parallelism, and if you have stateful operations like aggregation or join, the state of those operations in sharded. If you have X input topic partitions you get X tasks each with one state shard. Furthermore, state is backed by a changelog topic in Kafka with X partitions and each shard is using exactly one of those partitions.

If you change the number of input topic partitions to X+1, Kafka Streams tries to create X+1 tasks with X store shards, however the exiting changelog topic has only X partitions. Thus, the whole partitioning of your application breaks and Kafka Streams cannot guaranteed correct processing and thus shuts down with an error.

Also note, that Kafka Streams assume, that input data is partitioned by key. If you change the number of input topic partitions, the hash-based partitioning changes what may result in incorrect output, too.

In general, it's recommended to over-partition topics in the beginning to avoid this issue. If you really need to scale out, it is best to create a new topic with the new number of partitions, and start a copy of the application (with new application ID) in parallel. Afterwards, you update your upstream producer applications to write into the new topic, and finally shutdown the old application.

Answer 2

如果有状态客户端因此而失败，则删除更改日志主题和本地状态存储。