[BUG] Allocation and Rebalance Constraints of WeightFunction are incorrectly reset to default if indexBalanceFactor, shardBalanceFactor or preferPrimaryShardRebalanceBuffer is updated

[KunjueYu]

Describe the bug

In BalancedShardsAllocator, the update consumers of PREFER_PRIMARY_SHARD_REBALANCE and PREFER_PRIMARY_SHARD_BALANCE directly modifies the constraint config of existing weightFunction. However, if indexBalanceFactor, shardBalanceFactor or preferPrimaryShardRebalanceBuffer is updated, a new weightFunction is constructed, which will discard the preferPrimary Constraint configs of previous weightFunction.

  /**
     * When primary shards balance is desired, enable primary shard balancing constraints
     * @param preferPrimaryShardBalance boolean to prefer balancing by primary shard
     */
    private void setPreferPrimaryShardBalance(boolean preferPrimaryShardBalance) {
        this.preferPrimaryShardBalance = preferPrimaryShardBalance;
        this.weightFunction.updateAllocationConstraint(INDEX_PRIMARY_SHARD_BALANCE_CONSTRAINT_ID, preferPrimaryShardBalance);
        this.weightFunction.updateAllocationConstraint(CLUSTER_PRIMARY_SHARD_BALANCE_CONSTRAINT_ID, preferPrimaryShardBalance);
        this.weightFunction.updateRebalanceConstraint(INDEX_PRIMARY_SHARD_BALANCE_CONSTRAINT_ID, preferPrimaryShardBalance);
    }

    private void setPreferPrimaryShardRebalance(boolean preferPrimaryShardRebalance) {
        this.preferPrimaryShardRebalance = preferPrimaryShardRebalance;
        this.weightFunction.updateRebalanceConstraint(CLUSTER_PRIMARY_SHARD_REBALANCE_CONSTRAINT_ID, preferPrimaryShardRebalance);
    }

 private void updateIndexBalanceFactor(float indexBalanceFactor) {
        this.indexBalanceFactor = indexBalanceFactor;
        updateWeightFunction();
    }

    private void updateShardBalanceFactor(float shardBalanceFactor) {
        this.shardBalanceFactor = shardBalanceFactor;
        updateWeightFunction();
    }

    private void updatePreferPrimaryShardBalanceBuffer(float preferPrimaryShardBalanceBuffer) {
        this.preferPrimaryShardRebalanceBuffer = preferPrimaryShardBalanceBuffer;
        updateWeightFunction();
    }

    private void updateWeightFunction() {
        weightFunction = new WeightFunction(this.indexBalanceFactor, this.shardBalanceFactor, this.preferPrimaryShardRebalanceBuffer);
    }

Related component

Cluster Manager

To Reproduce

1. set setting "PREFER_PRIMARY_SHARD_BALANCE" as true
2. set setting "SHARD_BALANCE_FACTOR_SETTING" as 0.5f (arbitrary value which differs from default value 0.45f)
3. the preferPrimary constraints of weight function will be incorrectly disabled.

Expected behavior

Updating indexBalanceFactor, shardBalanceFactor or preferPrimaryShardRebalanceBuffer will not reset preferPrimary constraints of weight function to default.

Additional Details

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Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]

Additional context
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[peternied]

[Triage - attendees 1 2 3 4 5 6 7 8]
@KunjueYu Thanks for creating this issue, we'd welcome a pull request to address this bug

[patelsmit32123]

@peternied I would like to take this up