[CICP-51496] Retire ResourceConfig rebalance strategy

[sjainit]

Issues

• Addresses the ResourceConfig configuration-surface cleanup under CICP-51496.

Dedicated ticket creation is blocked by Jira's expired Datavault identity token, including after an authentication retry. This PR links to the existing epic; it does not close the epic.

Description

Retire only the unused ResourceConfig copy of REBALANCE_STRATEGY, exposed through org.apache.helix.api.config.RebalanceConfig.

• Remove its enum constant, backing field, getter/setter, record parsing and typed serialization.
• Keep IdealState strategy selection unchanged. The multi-ZK fixture already uses IdealState's enum through merged [CICP-52135] Retire three ResourceConfig rebalance settings #322; it is no longer part of this PR's diff.
• Preserve existing raw ResourceConfig fields as opaque metadata. The typed wrapper and constructors/builders consuming its serialized map no longer emit the retired setting; generic raw-record APIs remain unchanged.
• Leave STATE_MODEL_FACTORY_NAME, task handling and other retained settings untouched.
  Resource-level timer support was removed upstream in Remove resource-level REBALANCE_TIMER_PERIOD #300 and remains absent;
  cluster-level periodic rebalance remains supported.
• Preserve merged [CICP-52135] Retire three ResourceConfig rebalance settings #322's delay/mode/rebalancer-class retirement. Retain the RebalanceConfig wrapper and legacy enum types for compatibility, but no supported settings remain in the wrapper. getConfigsMap() returns a fresh mutable empty map; null constructor records remain rejected.

Includes dev at 37fe529ddb57fb4a860a3728202ef07be92f9dfe, including merged #322 and #317. Conflicts are resolved with a normal merge commit, without rewriting the PR's existing history. The final diff contains four files and retires only ResourceConfig strategy support; it is not stacked on the still-open #323 or #325.

Code audit: current Helix strategy selection reads IdealState, including AutoRebalancer and DelayedAutoRebalancer. No current runtime strategy consumer was found for ResourceConfig's composed RebalanceConfig. The paired strategy/factory downstream audit inspected 13 pinned source files across nine repositories; no affected MP strategy API caller was identified among those candidates. Inspected Espresso/Pinot and Python clients use IdealState or application-owned configuration types. This is bounded source evidence, not proof about every deployed binary, reflection path or unindexed caller; downstream builds were not run.

Saved September 23 census: across 86,624 successful ResourceConfig reads (52,248 prod, 12,080 corp, 22,296 EI), strategy was absent in prod/EI and present in two corp endpoint observations:

Namespace / cluster / resource	Endpoints	ResourceConfig strategy	Paired IdealState
ambry / view-aggregator-cluster / Ambry-corp-VIEW	corp-ltx1, corp-lva1	CrushEdRebalanceStrategy	WagedRebalancer, no strategy field

These are not equal duplicates: current Helix does not consume the ResourceConfig value. Do not copy it into IdealState automatically. The original writer was not identified; the endpoints may alias one underlying record. This is saved census evidence, not a fresh live scan, and discovery/read/churn gaps limit fleet-wide absence claims. No live records are changed by this PR.

Tests

• Local code review completed
• Added TestRebalanceConfig.testLegacyStrategyIsNotSerialized with absent, empty, known-class and invalid-class values.
• Added TestRebalanceConfig.testEmptyWrapperReturnsIndependentConfigMaps and testNullRecordIsRejected.
• Adapted merged retirement coverage for an empty wrapper, preserving legacy timer/delay/mode handling, mode-name enum compatibility and strategy round trips.
• Added TestResourceConfig.testLegacyStrategyPreservedInRecordButNotRebuilt, testConstructorDoesNotWriteLegacyStrategy and testMergeDoesNotMigrateOrOverrideStrategy.

JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk11.0.21-2-msft.jdk/Contents/Home \
mvn -B -pl helix-core -am test \
  -Dtest=TestRebalanceConfig,TestResourceConfig,TestIdealState,TestDelayedRebalanceUtil \
  -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false

Result: 61 tests passed, zero failures/errors/skips; BUILD SUCCESS. Coverage includes typed serialization, original-record preservation, constructors/builders, conflicting IdealState/ResourceConfig values, retained factory support, empty-map behavior and null-record rejection. The expected invalid-IdealState log comes from the existing negative test.

The reactor compiled 616 core production sources for the existing JDK 8/JDK 11 targets and 572 test sources, including the multi-ZK fixture already updated upstream. That integration fixture was compiled, not executed. javap confirmed removal of the wrapper strategy methods/enum constant and retention of IdealState strategy methods, ResourceConfig factory/wrapper APIs and cluster timer support. The resource-level timer APIs remain absent. No full-suite, downstream-build or mutation-test claim.

Changes that Break Backward Compatibility (Optional)

Removed public APIs:

• RebalanceConfig.RebalanceConfigProperty.REBALANCE_STRATEGY
• RebalanceConfig.getRebalanceStrategy()
• RebalanceConfig.setRebalanceStrategy(String)

Callers must update and recompile; already compiled clients referencing removed members are binary-incompatible. The RebalanceConfigProperty enum type remains available with no constants, and the deprecated RebalanceMode names remain available. Use IdealState's corresponding enum/getter/setter for supported strategy selection, but do not blindly migrate previously ignored ResourceConfig values.

RebalanceConfig.getConfigsMap() and ResourceConfig constructors/builders using that map no longer reproduce the legacy strategy field. Wrapping an existing ZNRecord or merging ResourceConfig with IdealState preserves the raw field; it neither affects strategy selection nor overwrites IdealState. No automatic data deletion or migration.

Documentation (Optional)

Added a README compatibility/migration section and clarified the RebalanceConfig class documentation.

Commits

• Commit references CICP-51496 and explains the compatibility boundary.
• Commit includes the Copilot co-author trailer.

Code Quality

• Changes follow the surrounding Helix Java/TestNG formatting and naming conventions.
• git diff --check passes; no dependency changes, mocks or production fallback logic.

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Tests generated with unit-tests plugin

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