Merge branch '2.x' into backport/backport-4597-to-2.x

Author: heemin32

CHANGELOG.md

@@ -58,9 +58,9 @@ 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))
@@ -79,6 +79,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Update Jackson Databind to 2.13.4.2 (addressing CVE-2022-42003) ([#4781](https://github.com/opensearch-project/OpenSearch/pull/4781))
 - Install and configure Log4j JUL Adapter for Lucene 9.4 ([#4754](https://github.com/opensearch-project/OpenSearch/pull/4754))
 ### Changed
+- Refactored BalancedAllocator.Balancer to LocalShardsBalancer ([#4818](https://github.com/opensearch-project/OpenSearch/pull/4818))
 ### 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) {

server/src/main/java/org/opensearch/cluster/routing/allocation/allocator/BalancedShardsAllocator.java

@@ -0,0 +1,75 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.cluster.routing.allocation.allocator;
+
+import org.opensearch.cluster.routing.ShardRouting;
+import org.opensearch.cluster.routing.allocation.AllocateUnassignedDecision;
+import org.opensearch.cluster.routing.allocation.MoveDecision;
+
+/**
+ * <p>
+ * A {@link ShardsBalancer} helps the {@link BalancedShardsAllocator} to perform allocation and balancing
+ * operations on the cluster.
+ * </p>
+ *
+ * @opensearch.internal
+ */
+public abstract class ShardsBalancer {
+
+    /**
+     * Performs allocation of unassigned shards on nodes within the cluster.
+     */
+    abstract void allocateUnassigned();
+
+    /**
+     * Moves shards that cannot be allocated to a node anymore.
+     */
+    abstract void moveShards();
+
+    /**
+     *  Balances the nodes on the cluster model.
+     */
+    abstract void balance();
+
+    /**
+     * Make a decision for allocating an unassigned shard.
+     * @param shardRouting the shard for which the decision has to be made
+     * @return the allocation decision
+     */
+    abstract AllocateUnassignedDecision decideAllocateUnassigned(ShardRouting shardRouting);
+
+    /**
+     * Makes a decision on whether to move a started shard to another node.
+     * @param shardRouting the shard for which the decision has to be made
+     * @return a move decision for the shard
+     */
+    abstract MoveDecision decideMove(ShardRouting shardRouting);
+
+    /**
+     * Makes a decision about moving a single shard to a different node to form a more
+     * optimally balanced cluster.
+     * @param shardRouting the shard for which the move decision has to be made
+     * @return a move decision for the shard
+     */
+    abstract MoveDecision decideRebalance(ShardRouting shardRouting);
+
+    /**
+     * Returns the average of shards per node for the given index
+     */
+    public float avgShardsPerNode() {
+        return Float.MAX_VALUE;
+    }
+
+    /**
+     * Returns the global average of shards per node
+     */
+    public float avgShardsPerNode(String index) {
+        return Float.MAX_VALUE;
+    }
+}

server/src/main/java/org/opensearch/cluster/routing/allocation/allocator/LocalShardsBalancer.java

@@ -10,6 +10,8 @@
 
 import org.opensearch.cluster.OpenSearchAllocationTestCase;
 import org.opensearch.cluster.routing.allocation.allocator.BalancedShardsAllocator;
+import org.opensearch.cluster.routing.allocation.allocator.LocalShardsBalancer;
+import org.opensearch.cluster.routing.allocation.allocator.ShardsBalancer;
 import org.opensearch.common.settings.ClusterSettings;
 import org.opensearch.common.settings.Settings;
 
@@ -45,7 +47,7 @@ public void testSettings() {
      * for IndexShardPerNode constraint satisfied and breached.
      */
     public void testIndexShardsPerNodeConstraint() {
-        BalancedShardsAllocator.Balancer balancer = mock(BalancedShardsAllocator.Balancer.class);
+        ShardsBalancer balancer = mock(LocalShardsBalancer.class);
         BalancedShardsAllocator.ModelNode node = mock(BalancedShardsAllocator.ModelNode.class);
         AllocationConstraints constraints = new AllocationConstraints();
 

server/src/main/java/org/opensearch/cluster/routing/allocation/allocator/ShardsBalancer.java

@@ -43,7 +43,7 @@
 import org.opensearch.cluster.routing.ShardRouting;
 import org.opensearch.cluster.routing.ShardRoutingState;
 import org.opensearch.cluster.routing.allocation.allocator.BalancedShardsAllocator;
-import org.opensearch.cluster.routing.allocation.allocator.BalancedShardsAllocator.Balancer;
+import org.opensearch.cluster.routing.allocation.allocator.ShardsBalancer;
 import org.opensearch.cluster.routing.allocation.decider.AllocationDecider;
 import org.opensearch.cluster.routing.allocation.decider.AllocationDeciders;
 import org.opensearch.cluster.routing.allocation.decider.Decision;
@@ -65,7 +65,7 @@
 import static org.hamcrest.Matchers.startsWith;
 
 /**
- * Tests for balancing a single shard, see {@link Balancer#decideRebalance(ShardRouting)}.
+ * Tests for balancing a single shard, see {@link ShardsBalancer#decideRebalance(ShardRouting)}.
  */
 public class BalancedSingleShardTests extends OpenSearchAllocationTestCase {