Status

Current state: ["Under Discussion"]

Discussion thread: here

JIRA: KAFKA-15777

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Describe the problems you are trying to solve.

When remote storage is enabled for a topic, and the consumer is doing backfill, then the data might be read from the remote storage. Currently, Kafka remote storage supports only reading the first remote partition in the given FETCH request. The default value of max.partition.fetch.bytes  is configured to 1 MB. Reading one MB of data per FETCH request, increases the round-trip time for the consumer and the cloud storages are often tuned to read data in chunks of 4 MB. 

The proposal is to introduce remote.max.partition.fetch.bytes  in the Consumer config which allows the user to tune the value depending on their storage plugin. The user might want to optimize the number of calls to remote storage vs amount of bytes returned back to the client in the FETCH response.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

A public interface is any change to the following:

• Introduce new remote.max.partition.fetch.bytes consumer config, with default value to 1 MB.
• Bump FetchRequest to v18 to add the RemotePartitionMaxBytes field to propagate the value configured on the consumer to the broker. 

• Binary log format

• The network protocol and api behavior

• Any class in the public packages under clientsConfiguration, especially client configuration

  • org/apache/kafka/common/serialization

  • org/apache/kafka/common

  • org/apache/kafka/common/errors

  • org/apache/kafka/clients/producer

  • org/apache/kafka/clients/consumer (eventually, once stable)

• Monitoring

• Command line tools and arguments

• Anything else that will likely break existing users in some way when they upgrade

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

The proposal is to introduce a new Consumer config: remote.max.partition.fetch.bytes to configure the number of bytes returned from remote storage.  


 The reason for not reusing the existing the max.partition.fetch.bytes  config.

1. The default value of fetch.max.bytes  is configured to 50 MB and max.partition.fetch.bytes is 1 MB.
2. If the user tunes the max.partition.fetch.bytes value to 4 MB, then it applies for all the FETCH requests. (ie) Reading the data from local storage too.
3. Assume that the consumer is reading data for a topic with 64 partitions and 16 partition leaders are colocated in a single broker. Then:
   1. Broker allocates 16 instances of 4 MB buffers which will impact the Young Gen and Old Gen GC.
   2. Broker does not use BufferPool to allocate the buffers. 
4. Since

1. The existing  cannot be reused to read from remote storage. 

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.