Retriever builder is also modified according to the new changes:

Author: mridula-s109

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/LinearRetrieverBuilder.java

@@ -83,7 +83,8 @@ public final class LinearRetrieverBuilder extends CompoundRetrieverBuilder<Linea
             List<LinearRetrieverComponent> retrieverComponents = args[0] == null ? List.of() : (List<LinearRetrieverComponent>) args[0];
             List<String> fields = (List<String>) args[1];
             String query = (String) args[2];
-            ScoreNormalizer normalizer = args[3] == null ? DEFAULT_NORMALIZER : ScoreNormalizer.valueOf((String) args[3]);
+            String normalizerName = (String) args[3];
+            ScoreNormalizer normalizer = normalizerName == null ? null : ScoreNormalizer.valueOf(normalizerName);
             int rankWindowSize = args[4] == null ? RankBuilder.DEFAULT_RANK_WINDOW_SIZE : (int) args[4];
 
             int index = 0;
@@ -94,6 +95,7 @@ public final class LinearRetrieverBuilder extends CompoundRetrieverBuilder<Linea
             for (LinearRetrieverComponent component : retrieverComponents) {
                 innerRetrievers.add(RetrieverSource.from(component.retriever));
                 weights[index] = component.weight;
+                normalizers[index] = component.normalizer;
                 index++;
             }
             return new LinearRetrieverBuilder(innerRetrievers, fields, query, normalizer, rankWindowSize, weights, normalizers);
@@ -224,6 +226,27 @@ public ActionRequestValidationException validate(
             );
         }
 
+        if (normalizer != null) {
+            for (ScoreNormalizer perRetrieverNormalizer : normalizers) {
+                boolean isExplicitSubNormalizer = perRetrieverNormalizer != null && !perRetrieverNormalizer.equals(DEFAULT_NORMALIZER);
+                boolean isMismatch = isExplicitSubNormalizer && !perRetrieverNormalizer.equals(normalizer);
+                if (isMismatch) {
+                    validationException = addValidationError(
+                        String.format(
+                            Locale.ROOT,
+                            "[%s] top-level [%s] is [%s] but a sub-retriever specifies [%s]",
+                            getName(),
+                            NORMALIZER_FIELD.getPreferredName(),
+                            normalizer.getName(),
+                            perRetrieverNormalizer.getName()
+                        ),
+                        validationException
+                    );
+                    break;
+                }
+            }
+        }
+
         return validationException;
     }
 
@@ -358,6 +381,14 @@ protected RetrieverBuilder doRewrite(QueryRewriteContext ctx) {
                 rewritten = new StandardRetrieverBuilder(new MatchNoneQueryBuilder());
             }
         }
+        if (normalizer != null) {
+            ScoreNormalizer[] newNormalizers = new ScoreNormalizer[normalizers.length];
+            for (int i = 0; i < normalizers.length; i++) {
+                newNormalizers[i] = (normalizers[i] == null || normalizers[i].equals(DEFAULT_NORMALIZER))
+                    ? normalizer : normalizers[i];
+            }
+            return new LinearRetrieverBuilder(innerRetrievers, fields, query, null, rankWindowSize, weights, newNormalizers);
+        }
 
         return rewritten;
     }
@@ -383,6 +414,9 @@ public void doToXContent(XContentBuilder builder, Params params) throws IOExcept
                 builder.startObject();
                 builder.field(LinearRetrieverComponent.RETRIEVER_FIELD.getPreferredName(), entry.retriever());
                 builder.field(LinearRetrieverComponent.WEIGHT_FIELD.getPreferredName(), weights[index]);
+                if (normalizers[index] != null && !normalizers[index].equals(DEFAULT_NORMALIZER)) {
+                    builder.field(LinearRetrieverComponent.NORMALIZER_FIELD.getPreferredName(), normalizers[index].getName());
+                }
                 builder.endObject();
                 index++;
             }
@@ -399,7 +433,7 @@ public void doToXContent(XContentBuilder builder, Params params) throws IOExcept
         if (query != null) {
             builder.field(QUERY_FIELD.getPreferredName(), query);
         }
-        if (normalizer != null) {
+        if (normalizer != null && !normalizer.equals(DEFAULT_NORMALIZER)) {
             builder.field(NORMALIZER_FIELD.getPreferredName(), normalizer.getName());
         }