From 4fa908b7df3a36fae872e34a770de88e85328dd7 Mon Sep 17 00:00:00 2001
From: Prabhakar Sithanandam <prabhakar.s87@gmail.com>
Date: Sat, 6 Apr 2024 13:55:40 +0530
Subject: [PATCH] Pre computing hash code for WeightedRouting to avoid
 recomputing it (#13099)

WeightedRoutingKey is used as key in the HashMap to retreive the shard
routings, for every usage the hash key is computed. WeightedRoutingKey
internally uses WeightedRouting. As the number of the shards getting
queried increases, the computation cost goes high.

Computing the hash key during object creation as the object is immutable
and using it through out the object thus saving on the additional computation.

Signed-off-by: Prabhakar Sithanandam <prabhakar.s87@gmail.com>
Signed-off-by: Shivansh Arora <hishiv@amazon.com>
---
 .../cluster/routing/WeightedRouting.java      | 20 +++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/server/src/main/java/org/opensearch/cluster/routing/WeightedRouting.java b/server/src/main/java/org/opensearch/cluster/routing/WeightedRouting.java
index 468fac08d2946..6f0e4fe90cfff 100644
--- a/server/src/main/java/org/opensearch/cluster/routing/WeightedRouting.java
+++ b/server/src/main/java/org/opensearch/cluster/routing/WeightedRouting.java
@@ -14,6 +14,7 @@
 import org.opensearch.core.common.io.stream.Writeable;
 
 import java.io.IOException;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
@@ -25,27 +26,26 @@
  */
 @PublicApi(since = "2.4.0")
 public class WeightedRouting implements Writeable {
-    private String attributeName;
-    private Map<String, Double> weights;
+    private final String attributeName;
+    private final Map<String, Double> weights;
+    private final int hashCode;
 
     public WeightedRouting() {
-        this.attributeName = "";
-        this.weights = new HashMap<>(3);
+        this("", new HashMap<>(3));
     }
 
     public WeightedRouting(String attributeName, Map<String, Double> weights) {
         this.attributeName = attributeName;
-        this.weights = weights;
+        this.weights = Collections.unmodifiableMap(weights);
+        this.hashCode = Objects.hash(this.attributeName, this.weights);
     }
 
     public WeightedRouting(WeightedRouting weightedRouting) {
-        this.attributeName = weightedRouting.attributeName();
-        this.weights = weightedRouting.weights;
+        this(weightedRouting.attributeName(), weightedRouting.weights);
     }
 
     public WeightedRouting(StreamInput in) throws IOException {
-        attributeName = in.readString();
-        weights = (Map<String, Double>) in.readGenericValue();
+        this(in.readString(), (Map<String, Double>) in.readGenericValue());
     }
 
     public boolean isSet() {
@@ -70,7 +70,7 @@ public boolean equals(Object o) {
 
     @Override
     public int hashCode() {
-        return Objects.hash(attributeName, weights);
+        return hashCode;
     }
 
     @Override