[Draft] KAFKA-19393: Consolidate consumer assignment state views.

[chickenchickenlove]

Description

• Related Issue - https://issues.apache.org/jira/browse/KAFKA-19393
  Currently, Both SubscriptionState and AbstractMembershipManager have states related with Assignment as their member fields.
  So, it can be confusing point because which one is source of truth.
  Therefore, It would be better to consolidate consumer assignment state views.

Discussion Points.

In this PR, AbstractMembershipManager has still currentAssignments and currentTargetAssignments.
The LocalAssignment includes information of assignment and information of protocol between consumer - broker.
So, I'm not sure that AssignState or SubscriptionState knows information of protocol between consumer - broker.

So, apart from that, I’ve extracted all the remaining assignment-related data into AssignState.
In addition to, IMHO, Also, since AssignState is a subdomain of Subscription in terms of meaning, it has been extracted as an internal state within Subscription.

I made a commit based on my thoughts!
It would like to hear feedback from others.

Please take a look, @lianetm and other reviewer 🙇‍♂️

[chickenchickenlove]

@lianetm Hi!
Sorry to bother you.
I made a Draft PR and I want to discuss with you about extracting AssingState from MembershipManager.
When you have time, could you give me your opinion about Discussion Points in PR descriptions? 🙇‍♂️

[lianetm]

Hey @chickenchickenlove , thanks for starting this! Sure, I will take a look. Very busy now with the 4.2 deadlines starting this week, but will come back to this one asap.

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