Refactor BalancedAllocator.Balancer to LocalShardsBalancer (opensearc…

…h-project#4761)

* Refactor BalancedAllocator.Balancer to LocalShardsBalancer

Signed-off-by: Kunal Kotwani <kkotwani@amazon.com>

* Deprecate Balancer to maintain BWC

Signed-off-by: Kunal Kotwani <kkotwani@amazon.com>

Signed-off-by: Kunal Kotwani <kkotwani@amazon.com>
(cherry picked from commit 1d65485)

CHANGELOG.md

@@ -17,7 +17,6 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Support for labels on version bump PRs, skip label support for changelog verifier ([#4391](https://github.com/opensearch-project/OpenSearch/pull/4391))
 - Add a new node role 'search' which is dedicated to provide search capability ([#4689](https://github.com/opensearch-project/OpenSearch/pull/4689))
 - Introduce experimental searchable snapshot API ([#4680](https://github.com/opensearch-project/OpenSearch/pull/4680))
-
 ### Dependencies
 - Bumps `com.diffplug.spotless` from 6.9.1 to 6.10.0
 - Bumps `xmlbeans` from 5.1.0 to 5.1.1
@@ -57,9 +56,9 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - [Bug]: Alias filter lost after rollover ([#4499](https://github.com/opensearch-project/OpenSearch/pull/4499))
 - Fixing Gradle warnings associated with publishPluginZipPublicationToXxx tasks ([#4696](https://github.com/opensearch-project/OpenSearch/pull/4696))
 - Fixed randomly failing test ([4774](https://github.com/opensearch-project/OpenSearch/pull/4774))
-
 ### Security
 - CVE-2022-25857 org.yaml:snakeyaml DOS vulnerability ([#4341](https://github.com/opensearch-project/OpenSearch/pull/4341))
+
 ## [2.x]
 ### Added
 - Github workflow for changelog verification ([#4085](https://github.com/opensearch-project/OpenSearch/pull/4085))
@@ -77,6 +76,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Dependencies
 - Update Jackson Databind to 2.13.4.2 (addressing CVE-2022-42003) ([#4781](https://github.com/opensearch-project/OpenSearch/pull/4781))
 ### Changed
+- Refactored BalancedAllocator.Balancer to LocalShardsBalancer ([#4761](https://github.com/opensearch-project/OpenSearch/pull/4761))
 ### Deprecated
 ### Removed
 - Remove RepositoryData.MIN_VERSION support for next major release ([4729](https://github.com/opensearch-project/OpenSearch/pull/4729))

server/src/main/java/org/opensearch/cluster/routing/allocation/AllocationConstraints.java

@@ -6,6 +6,7 @@
 package org.opensearch.cluster.routing.allocation;
 
 import org.opensearch.cluster.routing.allocation.allocator.BalancedShardsAllocator;
+import org.opensearch.cluster.routing.allocation.allocator.ShardsBalancer;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -27,11 +28,11 @@ public AllocationConstraints() {
     }
 
     class ConstraintParams {
-        private BalancedShardsAllocator.Balancer balancer;
+        private ShardsBalancer balancer;
         private BalancedShardsAllocator.ModelNode node;
         private String index;
 
-        ConstraintParams(BalancedShardsAllocator.Balancer balancer, BalancedShardsAllocator.ModelNode node, String index) {
+        ConstraintParams(ShardsBalancer balancer, BalancedShardsAllocator.ModelNode node, String index) {
             this.balancer = balancer;
             this.node = node;
             this.index = index;
@@ -50,7 +51,7 @@ class ConstraintParams {
      * This weight function is used only in case of unassigned shards to avoid overloading a newly added node.
      * Weight calculation in other scenarios like shard movement and re-balancing remain unaffected by this function.
      */
-    public long weight(BalancedShardsAllocator.Balancer balancer, BalancedShardsAllocator.ModelNode node, String index) {
+    public long weight(ShardsBalancer balancer, BalancedShardsAllocator.ModelNode node, String index) {
         int constraintsBreached = 0;
         ConstraintParams params = new ConstraintParams(balancer, node, index);
         for (Predicate<ConstraintParams> predicate : constraintPredicates) {