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Motivation

We've received quite a lot of feedback on the consumer side features over the past few months. Some of them are improvements to the current consumer design and some are simply new feature/API requests. I have attempted to write up the requirements that I've heard on this wiki - https://cwiki.apache.org/confluence/display/KAFKA/Consumer+Client+Re- Kafka 0.9 Consumer Rewrite Design
This would involve some significant changes to the consumer APIs, so we would like to collect feedback on the proposal from our community. Since the list of changes is not small, we would like to understand if some features are preferred over others, and more importantly, if some features are not required at all.

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1. The current version of the high level consumer suffers from herd and split brain problems, where multiple consumers in a group run a distributed algorithm to agree on the same partition ownership decision. Due to different view of the zookeeper data, they run into conflicts that makes the rebalancing attempt fail. But there is no way for a consumer to verify if a rebalancing operation completed successfully on the entire group. This also leads to some potential bugs in the rebalancing logic, for example, https://issues.apache.org/jira/browse/KAFKA-242
2. This can be mediated by moving the failure detection and rebalancing logic to a centralized highly-available co-ordinator - https://cwiki.apache.org/confluence/display/KAFKA/Central+Consumer+Coordination Kafka 0.9 Consumer Rewrite Design

We think the first two requirements are the prerequisite of the rest. So we have proceeded by trying to design a centralized coordinator for consumer rebalancing without Zookeeper, for details please read here.

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Allow manual partition assignment

1. There are a number of stateful data systems would like to manually assign partitions to consumers. The main motive is to enable them to keep some local per-partition state since the mapping from their consumer to partition never changes; also there are some use cases where it makes sense to co-locate brokers and consumer processes, hence would be nice to optimize the automatic partition assignment algorithm to consider co-location. Examples of such systems are databases, search indexers etc
2. A side effect of this requirement is wanting to turn off automatic rebalancing in the high level consumer.
3. This feature depends on the central co-ordination feature since it is cannot be correctly and easily implemented with the current distributed co-ordination model.

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Allow manual offset management

1. Some systems require offset management in a custom database, at specific intervals. Overall, the requirement is to have access to the message metadata like topic, partition, offset of the message, and to be able to provide per-partition offsets on consumer startup.
2. This would require designing new consumer APIs that allow providing offsets on startup and return message metadata with the consumer iterator.
3. One thing that needs to be thought through is if the consumer client can be allowed to pick manual offset management for some, but not all topics. One option is to allow the consumer to pick one offset management only. This could potentially make the API a bit simpler
4. This feature depends on the central co-ordination feature since it is cannot be correctly and easily implemented with the current distributed co-ordination model.

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