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1. Data consume and produce scales are no longer coupled. This means we could save money by configuring a reasonable input topic with decent amount of partitions.
2. Better avoid partition level hotkeys. When a specific key is processing really slow, the decoupled key based consumption could bypass it and make progress on other keys.

3. No operation overhead for scaling out. Users just need to add more consumer/stream capacity to unblock even there are fewer consumers.

Proposed Changes

We want to clarify beforehand that this KIP would be a starting point of a transformational change on the Kafka client consumption semantics. It's not possible to have all the design details rolling out in one shot, instead the hope is to breakdown 

Use case scenario

Public Interfaces

The 

Compatibility, Deprecation, and Migration Plan

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