Status

Current state: "Under Discussion"

Discussion thread: here

JIRA: here

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

Motivation

Kafka provides high throughput in each component. In the producer, there are 2 key configs to support the high throughput:

1. batch.size: it specifies a maximum batch size in bytes (default 16384)
2. linger.ms: it specifies a maximum duration to fill the batch in milliseconds (default 0 or no delay)

Records are sent until either of above 2 thresholds are reached.

However, when we set the batch size, we'll run into one trade-off situation:

1. either we have higher throughput + more memory waste,
2. or we have slower throughput + less memory waste.

Why is that?

We can check the description for "batch.size" in documentation here. Here's the last paragraph I extracted from the document:

A small batch size will make batching less common and may reduce throughput (a batch size of zero will disable batching entirely). A very large batch size may use memory a bit more wastefully as we will always allocate a buffer of the specified batch size in anticipation of additional records.

That explains why we have the trade-off when setting this config.

In this KIP, I'm going to introduce 2 new configuration for users to config:

1. batch.initial.size: It specifies the initial batch size when new batch created (default is the same as "batch.size", which means no buffer expansion will happen)
2. batch.reallocation.factor: it specifies the factor when we try to reallocate the new buffer (default is 2)

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:

• 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.

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?

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.