Add a generic rescorer retriever based on the search request's rescore functionality (#118585)

This pull request introduces a new retriever called `rescorer`, which leverages the `rescore` functionality of the search request.  
The `rescorer` retriever re-scores only the top documents retrieved by its child retriever, offering fine-tuned scoring capabilities.  

All rescorers supported in the `rescore` section of a search request are available in this retriever, and the same format is used to define the rescore configuration.  

<details>
<summary>Example:</summary>

```yaml
  - do:
      search:
        index: test
        body:
          retriever:
            rescorer:
              rescore:
                window_size: 10
                query:
                  rescore_query:
                    rank_feature:
                      field: "features.second_stage"
                      linear: { }
                  query_weight: 0
              retriever:
                standard:
                  query:
                    rank_feature:
                      field: "features.first_stage"
                      linear: { }
          size: 2
```

</details>

Closes #118327

Co-authored-by: Liam Thompson <32779855+leemthompo@users.noreply.github.com>

docs/changelog/118585.yaml

@@ -0,0 +1,7 @@
+pr: 118585
+summary: Add a generic `rescorer` retriever based on the search request's rescore
+  functionality
+area: Ranking
+type: feature
+issues:
+ - 118327

docs/reference/search/retriever.asciidoc

@@ -22,6 +22,9 @@ A <<standard-retriever, retriever>> that replaces the functionality of a traditi
 `knn`::
 A <<knn-retriever, retriever>> that replaces the functionality of a <<search-api-knn, knn search>>.
 
+`rescorer`::
+A <<rescorer-retriever, retriever>> that replaces the functionality of the <<rescore, query rescorer>>.
+
 `rrf`::
 A <<rrf-retriever, retriever>> that produces top documents from <<rrf, reciprocal rank fusion (RRF)>>.
 
@@ -371,6 +374,122 @@ GET movies/_search
 ----
 // TEST[skip:uses ELSER]
 
+[[rescorer-retriever]]
+==== Rescorer Retriever
+
+The `rescorer` retriever re-scores only the results produced by its child retriever.
+For the `standard` and `knn` retrievers, the `window_size` parameter specifies the number of documents examined per shard.
+
+For compound retrievers like `rrf`, the `window_size` parameter defines the total number of documents examined globally.
+
+When using the `rescorer`, an error is returned if the following conditions are not met:
+
+* The minimum configured rescore's `window_size` is:
+** Greater than or equal to the `size` of the parent retriever for nested `rescorer` setups.
+** Greater than or equal to the `size` of the search request when used as the primary retriever in the tree.
+
+* And the maximum rescore's `window_size` is:
+** Smaller than or equal to the `size` or `rank_window_size` of the child retriever.
+
+[discrete]
+[[rescorer-retriever-parameters]]
+===== Parameters
+
+`rescore`::
+(Required. <<rescore, A rescorer definition or an array of rescorer definitions>>)
++
+Defines the <<rescore, rescorers>> applied sequentially to the top documents returned by the child retriever.
+
+`retriever`::
+(Required. <<retriever, retriever>>)
++
+Specifies the child retriever responsible for generating the initial set of top documents to be re-ranked.
+
+`filter`::
+(Optional. <<query-dsl, query object or list of query objects>>)
++
+Applies a <<query-dsl-bool-query, boolean query filter>> to the retriever, ensuring that all documents match the filter criteria without affecting their scores.
+
+[discrete]
+[[rescorer-retriever-example]]
+==== Example
+
+The `rescorer` retriever can be placed at any level within the retriever tree.
+The following example demonstrates a `rescorer` applied to the results produced by an `rrf` retriever:
+
+[source,console]
+----
+GET movies/_search
+{
+  "size": 10, <1>
+  "retriever": {
+    "rescorer": { <2>
+      "rescore": {
+        "query": { <3>
+          "window_size": 50, <4>
+          "rescore_query": {
+            "script_score": {
+              "script": {
+                "source": "cosineSimilarity(params.queryVector, 'product-vector_final_stage') + 1.0",
+                "params": {
+                  "queryVector": [-0.5, 90.0, -10, 14.8, -156.0]
+                }
+              }
+            }
+          }
+        }
+      },
+      "retriever": { <5>
+        "rrf": {
+          "rank_window_size": 100, <6>
+          "retrievers": [
+            {
+              "standard": {
+                "query": {
+                  "sparse_vector": {
+                    "field": "plot_embedding",
+                    "inference_id": "my-elser-model",
+                    "query": "films that explore psychological depths"
+                  }
+                }
+              }
+            },
+            {
+              "standard": {
+                "query": {
+                  "multi_match": {
+                    "query": "crime",
+                    "fields": [
+                      "plot",
+                      "title"
+                    ]
+                  }
+                }
+              }
+            },
+            {
+              "knn": {
+                "field": "vector",
+                "query_vector": [10, 22, 77],
+                "k": 10,
+                "num_candidates": 10
+              }
+            }
+          ]
+        }
+      }
+    }
+  }
+}
+----
+// TEST[skip:uses ELSER]
+<1> Specifies the number of top documents to return in the final response.
+<2> A `rescorer` retriever applied as the final step.
+<3> The definition of the `query` rescorer.
+<4> Defines the number of documents to rescore from the child retriever.
+<5> Specifies the child retriever definition.
+<6> Defines the number of documents returned by the `rrf` retriever, which limits the available documents to
+
 [[text-similarity-reranker-retriever]]
 ==== Text Similarity Re-ranker Retriever
 
@@ -777,4 +896,4 @@ When a retriever is specified as part of a search, the following elements are no
 * <<search-after, `search_after`>>
 * <<request-body-search-terminate-after, `terminate_after`>>
 * <<search-sort-param, `sort`>>
-* <<rescore, `rescore`>>
+* <<rescore, `rescore`>> use a <<rescorer-retriever, rescorer retriever>> instead

rest-api-spec/build.gradle

@@ -70,4 +70,5 @@ tasks.named("yamlRestCompatTestTransform").configure ({ task ->
   task.skipTest("search.vectors/41_knn_search_bbq_hnsw/Test knn search", "Scoring has changed in latest versions")
   task.skipTest("search.vectors/42_knn_search_bbq_flat/Test knn search", "Scoring has changed in latest versions")
   task.skipTest("synonyms/90_synonyms_reloading_for_synset/Reload analyzers for specific synonym set", "Can't work until auto-expand replicas is 0-1 for synonyms index")
+  task.skipTest("search/90_search_after/_shard_doc sort", "restriction has been lifted in latest versions")
 })

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.retrievers/30_rescorer_retriever.yml

@@ -0,0 +1,225 @@
+setup:
+  - requires:
+      cluster_features: [ "search.retriever.rescorer.enabled" ]
+      reason: "Support for rescorer retriever"
+
+  - do:
+      indices.create:
+        index: test
+        body:
+          settings:
+            number_of_shards: 1
+            number_of_replicas: 0
+          mappings:
+            properties:
+              available:
+                type: boolean
+              features:
+                type: rank_features
+
+  - do:
+      bulk:
+        refresh: true
+        index: test
+        body:
+          - '{"index": {"_id": 1 }}'
+          - '{"features": { "first_stage": 1, "second_stage": 10}, "available": true, "group": 1}'
+          - '{"index": {"_id": 2 }}'
+          - '{"features": { "first_stage": 2, "second_stage": 9}, "available": false, "group": 1}'
+          - '{"index": {"_id": 3 }}'
+          - '{"features": { "first_stage": 3, "second_stage": 8}, "available": false, "group": 3}'
+          - '{"index": {"_id": 4 }}'
+          - '{"features": { "first_stage": 4, "second_stage": 7}, "available": true, "group": 1}'
+          - '{"index": {"_id": 5 }}'
+          - '{"features": { "first_stage": 5, "second_stage": 6}, "available": true, "group": 3}'
+          - '{"index": {"_id": 6 }}'
+          - '{"features": { "first_stage": 6, "second_stage": 5}, "available": false, "group": 2}'
+          - '{"index": {"_id": 7 }}'
+          - '{"features": { "first_stage": 7, "second_stage": 4}, "available": true, "group": 3}'
+          - '{"index": {"_id": 8 }}'
+          - '{"features": { "first_stage": 8, "second_stage": 3}, "available": true, "group": 1}'
+          - '{"index": {"_id": 9 }}'
+          - '{"features": { "first_stage": 9, "second_stage": 2}, "available": true, "group": 2}'
+          - '{"index": {"_id": 10 }}'
+          - '{"features": { "first_stage": 10, "second_stage": 1}, "available": false, "group": 1}'
+
+---
+"Rescorer retriever basic":
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 10
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.second_stage"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                standard:
+                  query:
+                    rank_feature:
+                      field: "features.first_stage"
+                      linear: { }
+          size: 2
+
+  - match: { hits.total.value: 10 }
+  - match: { hits.hits.0._id: "1" }
+  - match: { hits.hits.0._score: 10.0 }
+  - match: { hits.hits.1._id: "2" }
+  - match: { hits.hits.1._score: 9.0 }
+
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 3
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.second_stage"
+                      linear: {}
+                  query_weight: 0
+              retriever:
+                standard:
+                  query:
+                    rank_feature:
+                      field: "features.first_stage"
+                      linear: {}
+          size: 2
+
+  - match: {hits.total.value: 10}
+  - match: {hits.hits.0._id: "8"}
+  - match: { hits.hits.0._score: 3.0 }
+  - match: {hits.hits.1._id: "9"}
+  - match: { hits.hits.1._score: 2.0 }
+
+---
+"Rescorer retriever with pre-filters":
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              filter:
+                match:
+                  available: true
+              rescore:
+                window_size: 10
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.second_stage"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                standard:
+                  query:
+                    rank_feature:
+                      field: "features.first_stage"
+                      linear: { }
+          size: 2
+
+  - match: { hits.total.value: 6 }
+  - match: { hits.hits.0._id: "1" }
+  - match: { hits.hits.0._score: 10.0 }
+  - match: { hits.hits.1._id: "4" }
+  - match: { hits.hits.1._score: 7.0 }
+
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 4
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.second_stage"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                standard:
+                  filter:
+                    match:
+                      available: true
+                  query:
+                    rank_feature:
+                      field: "features.first_stage"
+                      linear: { }
+          size: 2
+
+  - match: { hits.total.value: 6 }
+  - match: { hits.hits.0._id: "5" }
+  - match: { hits.hits.0._score: 6.0 }
+  - match: { hits.hits.1._id: "7" }
+  - match: { hits.hits.1._score: 4.0 }
+
+---
+"Rescorer retriever and collapsing":
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 10
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.second_stage"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                standard:
+                  query:
+                    rank_feature:
+                      field: "features.first_stage"
+                      linear: { }
+          collapse:
+            field: group
+          size: 3
+
+  - match: { hits.total.value: 10 }
+  - match: { hits.hits.0._id: "1" }
+  - match: { hits.hits.0._score: 10.0 }
+  - match: { hits.hits.1._id: "3" }
+  - match: { hits.hits.1._score: 8.0 }
+  - match: { hits.hits.2._id: "6" }
+  - match: { hits.hits.2._score: 5.0 }
+
+---
+"Rescorer retriever and invalid window size":
+  - do:
+      catch: "/\\[rescorer\\] requires \\[window_size: 5\\] be greater than or equal to \\[size: 10\\]/"
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 5
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.second_stage"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                standard:
+                  query:
+                    rank_feature:
+                      field: "features.first_stage"
+                      linear: { }
+          size: 10

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search/90_search_after.yml

@@ -218,31 +218,6 @@
   - match: {hits.hits.0._source.timestamp: "2019-10-21 00:30:04.828740" }
   - match: {hits.hits.0.sort: [1571617804828740000] }
 
-
----
-"_shard_doc sort":
-  - requires:
-      cluster_features: ["gte_v7.12.0"]
-      reason: _shard_doc sort was added in 7.12
-
-  - do:
-      indices.create:
-        index:  test
-  - do:
-      index:
-        index:  test
-        id:     "1"
-        body:   { id: 1, foo: bar, age: 18 }
-
-  - do:
-      catch:      /\[_shard_doc\] sort field cannot be used without \[point in time\]/
-      search:
-        index: test
-        body:
-          size: 1
-          sort: ["_shard_doc"]
-          search_after: [ 0L ]
-
 ---
 "Format sort values":
   - requires:

server/src/internalClusterTest/java/org/elasticsearch/search/functionscore/QueryRescorerIT.java

@@ -38,6 +38,7 @@ import org.elasticsearch.search.SearchHits;
 import org.elasticsearch.search.collapse.CollapseBuilder;
 import org.elasticsearch.search.rescore.QueryRescoreMode;
 import org.elasticsearch.search.rescore.QueryRescorerBuilder;
+import org.elasticsearch.search.sort.FieldSortBuilder;
 import org.elasticsearch.search.sort.SortBuilders;
 import org.elasticsearch.test.ESIntegTestCase;
 import org.elasticsearch.xcontent.ParseField;
@@ -840,6 +841,20 @@ public class QueryRescorerIT extends ESIntegTestCase {
                 }
             }
         );
+
+        assertResponse(
+            prepareSearch().addSort(SortBuilders.scoreSort())
+                .addSort(new FieldSortBuilder(FieldSortBuilder.SHARD_DOC_FIELD_NAME))
+                .setTrackScores(true)
+                .addRescorer(new QueryRescorerBuilder(matchAllQuery()).setRescoreQueryWeight(100.0f), 50),
+            response -> {
+                assertThat(response.getHits().getTotalHits().value(), equalTo(5L));
+                assertThat(response.getHits().getHits().length, equalTo(5));
+                for (SearchHit hit : response.getHits().getHits()) {
+                    assertThat(hit.getScore(), equalTo(101f));
+                }
+            }
+        );
     }
 
     record GroupDoc(String id, String group, float firstPassScore, float secondPassScore, boolean shouldFilter) {}
@@ -879,6 +894,10 @@ public class QueryRescorerIT extends ESIntegTestCase {
             .setQuery(fieldValueScoreQuery("firstPassScore"))
             .addRescorer(new QueryRescorerBuilder(fieldValueScoreQuery("secondPassScore")))
             .setCollapse(new CollapseBuilder("group"));
+        if (randomBoolean()) {
+            request.addSort(SortBuilders.scoreSort());
+            request.addSort(new FieldSortBuilder(FieldSortBuilder.SHARD_DOC_FIELD_NAME));
+        }
         assertResponse(request, resp -> {
             assertThat(resp.getHits().getTotalHits().value(), equalTo(5L));
             assertThat(resp.getHits().getHits().length, equalTo(3));
@@ -958,6 +977,10 @@ public class QueryRescorerIT extends ESIntegTestCase {
             .addRescorer(new QueryRescorerBuilder(fieldValueScoreQuery("secondPassScore")).setQueryWeight(0f).windowSize(numGroups))
             .setCollapse(new CollapseBuilder("group"))
             .setSize(Math.min(numGroups, 10));
+        if (randomBoolean()) {
+            request.addSort(SortBuilders.scoreSort());
+            request.addSort(new FieldSortBuilder(FieldSortBuilder.SHARD_DOC_FIELD_NAME));
+        }
         long expectedNumHits = numHits;
         assertResponse(request, resp -> {
             assertThat(resp.getHits().getTotalHits().value(), equalTo(expectedNumHits));

server/src/main/java/org/elasticsearch/search/DefaultSearchContext.java

@@ -73,6 +73,7 @@ import org.elasticsearch.search.query.QuerySearchResult;
 import org.elasticsearch.search.rank.context.QueryPhaseRankShardContext;
 import org.elasticsearch.search.rank.feature.RankFeatureResult;
 import org.elasticsearch.search.rescore.RescoreContext;
+import org.elasticsearch.search.rescore.RescorePhase;
 import org.elasticsearch.search.slice.SliceBuilder;
 import org.elasticsearch.search.sort.SortAndFormats;
 import org.elasticsearch.search.suggest.SuggestionSearchContext;
@@ -377,7 +378,7 @@ final class DefaultSearchContext extends SearchContext {
             );
         }
         if (rescore != null) {
-            if (sort != null) {
+            if (RescorePhase.validateSort(sort) == false) {
                 throw new IllegalArgumentException("Cannot use [sort] option in conjunction with [rescore].");
             }
             int maxWindow = indexService.getIndexSettings().getMaxRescoreWindow();

server/src/main/java/org/elasticsearch/search/SearchFeatures.java

@@ -23,4 +23,11 @@ public final class SearchFeatures implements FeatureSpecification {
     public Set<NodeFeature> getFeatures() {
         return Set.of(KnnVectorQueryBuilder.K_PARAM_SUPPORTED, LUCENE_10_0_0_UPGRADE);
     }
+
+    public static final NodeFeature RETRIEVER_RESCORER_ENABLED = new NodeFeature("search.retriever.rescorer.enabled");
+
+    @Override
+    public Set<NodeFeature> getTestFeatures() {
+        return Set.of(RETRIEVER_RESCORER_ENABLED);
+    }
 }

server/src/main/java/org/elasticsearch/search/SearchModule.java

@@ -231,6 +231,7 @@ import org.elasticsearch.search.rank.feature.RankFeatureShardResult;
 import org.elasticsearch.search.rescore.QueryRescorerBuilder;
 import org.elasticsearch.search.rescore.RescorerBuilder;
 import org.elasticsearch.search.retriever.KnnRetrieverBuilder;
+import org.elasticsearch.search.retriever.RescorerRetrieverBuilder;
 import org.elasticsearch.search.retriever.RetrieverBuilder;
 import org.elasticsearch.search.retriever.RetrieverParserContext;
 import org.elasticsearch.search.retriever.StandardRetrieverBuilder;
@@ -1080,6 +1081,7 @@ public class SearchModule {
     private void registerRetrieverParsers(List<SearchPlugin> plugins) {
         registerRetriever(new RetrieverSpec<>(StandardRetrieverBuilder.NAME, StandardRetrieverBuilder::fromXContent));
         registerRetriever(new RetrieverSpec<>(KnnRetrieverBuilder.NAME, KnnRetrieverBuilder::fromXContent));
+        registerRetriever(new RetrieverSpec<>(RescorerRetrieverBuilder.NAME, RescorerRetrieverBuilder::fromXContent));
 
         registerFromPlugin(plugins, SearchPlugin::getRetrievers, this::registerRetriever);
     }

server/src/main/java/org/elasticsearch/search/builder/SearchSourceBuilder.java

@@ -48,9 +48,7 @@ import org.elasticsearch.search.retriever.RetrieverBuilder;
 import org.elasticsearch.search.retriever.RetrieverParserContext;
 import org.elasticsearch.search.searchafter.SearchAfterBuilder;
 import org.elasticsearch.search.slice.SliceBuilder;
-import org.elasticsearch.search.sort.FieldSortBuilder;
 import org.elasticsearch.search.sort.ScoreSortBuilder;
-import org.elasticsearch.search.sort.ShardDocSortField;
 import org.elasticsearch.search.sort.SortBuilder;
 import org.elasticsearch.search.sort.SortBuilders;
 import org.elasticsearch.search.sort.SortOrder;
@@ -2341,18 +2339,6 @@ public final class SearchSourceBuilder implements Writeable, ToXContentObject, R
                 validationException = rescorer.validate(this, validationException);
             }
         }
-
-        if (pointInTimeBuilder() == null && sorts() != null) {
-            for (var sortBuilder : sorts()) {
-                if (sortBuilder instanceof FieldSortBuilder fieldSortBuilder
-                    && ShardDocSortField.NAME.equals(fieldSortBuilder.getFieldName())) {
-                    validationException = addValidationError(
-                        "[" + FieldSortBuilder.SHARD_DOC_FIELD_NAME + "] sort field cannot be used without [point in time]",
-                        validationException
-                    );
-                }
-            }
-        }
         return validationException;
     }
 }

server/src/main/java/org/elasticsearch/search/query/QueryPhaseCollectorManager.java

@@ -58,6 +58,7 @@ import org.elasticsearch.search.internal.SearchContext;
 import org.elasticsearch.search.profile.query.CollectorResult;
 import org.elasticsearch.search.profile.query.InternalProfileCollector;
 import org.elasticsearch.search.rescore.RescoreContext;
+import org.elasticsearch.search.rescore.RescorePhase;
 import org.elasticsearch.search.sort.SortAndFormats;
 
 import java.io.IOException;
@@ -238,7 +239,7 @@ abstract class QueryPhaseCollectorManager implements CollectorManager<Collector,
             int numDocs = Math.min(searchContext.from() + searchContext.size(), totalNumDocs);
             final boolean rescore = searchContext.rescore().isEmpty() == false;
             if (rescore) {
-                assert searchContext.sort() == null;
+                assert RescorePhase.validateSort(searchContext.sort());
                 for (RescoreContext rescoreContext : searchContext.rescore()) {
                     numDocs = Math.max(numDocs, rescoreContext.getWindowSize());
                 }

server/src/main/java/org/elasticsearch/search/rescore/RescorePhase.java

@@ -13,6 +13,7 @@ import org.apache.lucene.search.FieldDoc;
 import org.apache.lucene.search.ScoreDoc;
 import org.apache.lucene.search.SortField;
 import org.apache.lucene.search.TopDocs;
+import org.apache.lucene.search.TopFieldDocs;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.search.SearchShardTask;
 import org.elasticsearch.common.lucene.search.TopDocsAndMaxScore;
@@ -22,9 +23,12 @@ import org.elasticsearch.search.internal.ContextIndexSearcher;
 import org.elasticsearch.search.internal.SearchContext;
 import org.elasticsearch.search.query.QueryPhase;
 import org.elasticsearch.search.query.SearchTimeoutException;
+import org.elasticsearch.search.sort.ShardDocSortField;
+import org.elasticsearch.search.sort.SortAndFormats;
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
 
@@ -39,15 +43,27 @@ public class RescorePhase {
         if (context.size() == 0 || context.rescore() == null || context.rescore().isEmpty()) {
             return;
         }
-
+        if (validateSort(context.sort()) == false) {
+            throw new IllegalStateException("Cannot use [sort] option in conjunction with [rescore], missing a validate?");
+        }
         TopDocs topDocs = context.queryResult().topDocs().topDocs;
         if (topDocs.scoreDocs.length == 0) {
             return;
         }
+        // Populate FieldDoc#score using the primary sort field (_score) to ensure compatibility with top docs rescoring
+        Arrays.stream(topDocs.scoreDocs).forEach(t -> {
+            if (t instanceof FieldDoc fieldDoc) {
+                fieldDoc.score = (float) fieldDoc.fields[0];
+            }
+        });
         TopFieldGroups topGroups = null;
+        TopFieldDocs topFields = null;
         if (topDocs instanceof TopFieldGroups topFieldGroups) {
-            assert context.collapse() != null;
+            assert context.collapse() != null && validateSortFields(topFieldGroups.fields);
             topGroups = topFieldGroups;
+        } else if (topDocs instanceof TopFieldDocs topFieldDocs) {
+            assert validateSortFields(topFieldDocs.fields);
+            topFields = topFieldDocs;
         }
         try {
             Runnable cancellationCheck = getCancellationChecks(context);
@@ -56,17 +72,18 @@ public class RescorePhase {
                 topDocs = ctx.rescorer().rescore(topDocs, context.searcher(), ctx);
                 // It is the responsibility of the rescorer to sort the resulted top docs,
                 // here we only assert that this condition is met.
-                assert context.sort() == null && topDocsSortedByScore(topDocs) : "topdocs should be sorted after rescore";
+                assert topDocsSortedByScore(topDocs) : "topdocs should be sorted after rescore";
                 ctx.setCancellationChecker(null);
             }
+            /**
+             * Since rescorers are building top docs with score only, we must reconstruct the {@link TopFieldGroups}
+             * or {@link TopFieldDocs} using their original version before rescoring.
+             */
             if (topGroups != null) {
                 assert context.collapse() != null;
-                /**
+                topDocs = rewriteTopFieldGroups(topGroups, topDocs);
-                 * Since rescorers don't preserve collapsing, we must reconstruct the group and field
+            } else if (topFields != null) {
-                 * values from the originalTopGroups to create a new {@link TopFieldGroups} from the
+                topDocs = rewriteTopFieldDocs(topFields, topDocs);
-                 * rescored top documents.
-                 */
-                topDocs = rewriteTopGroups(topGroups, topDocs);
             }
             context.queryResult()
                 .topDocs(new TopDocsAndMaxScore(topDocs, topDocs.scoreDocs[0].score), context.queryResult().sortValueFormats());
@@ -81,29 +98,84 @@ public class RescorePhase {
         }
     }
 
-    private static TopFieldGroups rewriteTopGroups(TopFieldGroups originalTopGroups, TopDocs rescoredTopDocs) {
+    /**
-        assert originalTopGroups.fields.length == 1 && SortField.FIELD_SCORE.equals(originalTopGroups.fields[0])
+     * Returns whether the provided {@link SortAndFormats} can be used to rescore
-            : "rescore must always sort by score descending";
+     * top documents.
+     */
+    public static boolean validateSort(SortAndFormats sortAndFormats) {
+        if (sortAndFormats == null) {
+            return true;
+        }
+        return validateSortFields(sortAndFormats.sort.getSort());
+    }
+
+    private static boolean validateSortFields(SortField[] fields) {
+        if (fields[0].equals(SortField.FIELD_SCORE) == false) {
+            return false;
+        }
+        if (fields.length == 1) {
+            return true;
+        }
+
+        // The ShardDocSortField can be used as a tiebreaker because it maintains
+        // the natural document ID order within the shard.
+        if (fields[1] instanceof ShardDocSortField == false || fields[1].getReverse()) {
+            return false;
+        }
+        return true;
+    }
+
+    private static TopFieldDocs rewriteTopFieldDocs(TopFieldDocs originalTopFieldDocs, TopDocs rescoredTopDocs) {
+        Map<Integer, FieldDoc> docIdToFieldDoc = Maps.newMapWithExpectedSize(originalTopFieldDocs.scoreDocs.length);
+        for (int i = 0; i < originalTopFieldDocs.scoreDocs.length; i++) {
+            docIdToFieldDoc.put(originalTopFieldDocs.scoreDocs[i].doc, (FieldDoc) originalTopFieldDocs.scoreDocs[i]);
+        }
+        var newScoreDocs = new FieldDoc[rescoredTopDocs.scoreDocs.length];
+        int pos = 0;
+        for (var doc : rescoredTopDocs.scoreDocs) {
+            newScoreDocs[pos] = docIdToFieldDoc.get(doc.doc);
+            newScoreDocs[pos].score = doc.score;
+            newScoreDocs[pos].fields[0] = newScoreDocs[pos].score;
+            pos++;
+        }
+        return new TopFieldDocs(originalTopFieldDocs.totalHits, newScoreDocs, originalTopFieldDocs.fields);
+    }
+
+    private static TopFieldGroups rewriteTopFieldGroups(TopFieldGroups originalTopGroups, TopDocs rescoredTopDocs) {
+        var newFieldDocs = rewriteFieldDocs((FieldDoc[]) originalTopGroups.scoreDocs, rescoredTopDocs.scoreDocs);
+
         Map<Integer, Object> docIdToGroupValue = Maps.newMapWithExpectedSize(originalTopGroups.scoreDocs.length);
         for (int i = 0; i < originalTopGroups.scoreDocs.length; i++) {
             docIdToGroupValue.put(originalTopGroups.scoreDocs[i].doc, originalTopGroups.groupValues[i]);
         }
-        var newScoreDocs = new FieldDoc[rescoredTopDocs.scoreDocs.length];
         var newGroupValues = new Object[originalTopGroups.groupValues.length];
         int pos = 0;
         for (var doc : rescoredTopDocs.scoreDocs) {
-            newScoreDocs[pos] = new FieldDoc(doc.doc, doc.score, new Object[] { doc.score });
             newGroupValues[pos++] = docIdToGroupValue.get(doc.doc);
         }
         return new TopFieldGroups(
             originalTopGroups.field,
             originalTopGroups.totalHits,
-            newScoreDocs,
+            newFieldDocs,
             originalTopGroups.fields,
             newGroupValues
         );
     }
 
+    private static FieldDoc[] rewriteFieldDocs(FieldDoc[] originalTopDocs, ScoreDoc[] rescoredTopDocs) {
+        Map<Integer, FieldDoc> docIdToFieldDoc = Maps.newMapWithExpectedSize(rescoredTopDocs.length);
+        Arrays.stream(originalTopDocs).forEach(d -> docIdToFieldDoc.put(d.doc, d));
+        var newDocs = new FieldDoc[rescoredTopDocs.length];
+        int pos = 0;
+        for (var doc : rescoredTopDocs) {
+            newDocs[pos] = docIdToFieldDoc.get(doc.doc);
+            newDocs[pos].score = doc.score;
+            newDocs[pos].fields[0] = doc.score;
+            pos++;
+        }
+        return newDocs;
+    }
+
     /**
      * Returns true if the provided docs are sorted by score.
      */

server/src/main/java/org/elasticsearch/search/rescore/RescorerBuilder.java

@@ -39,7 +39,7 @@ public abstract class RescorerBuilder<RB extends RescorerBuilder<RB>>
 
     protected Integer windowSize;
 
-    private static final ParseField WINDOW_SIZE_FIELD = new ParseField("window_size");
+    public static final ParseField WINDOW_SIZE_FIELD = new ParseField("window_size");
 
     /**
      * Construct an empty RescoreBuilder.

server/src/main/java/org/elasticsearch/search/retriever/CompoundRetrieverBuilder.java

@@ -32,10 +32,12 @@ import org.elasticsearch.search.sort.FieldSortBuilder;
 import org.elasticsearch.search.sort.ScoreSortBuilder;
 import org.elasticsearch.search.sort.ShardDocSortField;
 import org.elasticsearch.search.sort.SortBuilder;
+import org.elasticsearch.xcontent.ParseField;
 
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Locale;
 import java.util.Objects;
 
 import static org.elasticsearch.action.ValidateActions.addValidationError;
@@ -49,6 +51,8 @@ public abstract class CompoundRetrieverBuilder<T extends CompoundRetrieverBuilde
 
     public static final NodeFeature INNER_RETRIEVERS_FILTER_SUPPORT = new NodeFeature("inner_retrievers_filter_support");
 
+    public static final ParseField RANK_WINDOW_SIZE_FIELD = new ParseField("rank_window_size");
+
     public record RetrieverSource(RetrieverBuilder retriever, SearchSourceBuilder source) {}
 
     protected final int rankWindowSize;
@@ -81,6 +85,14 @@ public abstract class CompoundRetrieverBuilder<T extends CompoundRetrieverBuilde
         return true;
     }
 
+    /**
+     * Retrieves the {@link ParseField} used to configure the {@link CompoundRetrieverBuilder#rankWindowSize}
+     * at the REST layer.
+     */
+    public ParseField getRankWindowSizeField() {
+        return RANK_WINDOW_SIZE_FIELD;
+    }
+
     @Override
     public final RetrieverBuilder rewrite(QueryRewriteContext ctx) throws IOException {
         if (ctx.getPointInTimeBuilder() == null) {
@@ -209,14 +221,14 @@ public abstract class CompoundRetrieverBuilder<T extends CompoundRetrieverBuilde
         validationException = super.validate(source, validationException, isScroll, allowPartialSearchResults);
         if (source.size() > rankWindowSize) {
             validationException = addValidationError(
-                "["
+                String.format(
-                    + this.getName()
+                    Locale.ROOT,
-                    + "] requires [rank_window_size: "
+                    "[%s] requires [%s: %d] be greater than or equal to [size: %d]",
-                    + rankWindowSize
+                    getName(),
-                    + "]"
+                    getRankWindowSizeField().getPreferredName(),
-                    + " be greater than or equal to [size: "
+                    rankWindowSize,
-                    + source.size()
+                    source.size()
-                    + "]",
+                ),
                 validationException
             );
         }
@@ -231,6 +243,21 @@ public abstract class CompoundRetrieverBuilder<T extends CompoundRetrieverBuilde
         }
         for (RetrieverSource innerRetriever : innerRetrievers) {
             validationException = innerRetriever.retriever().validate(source, validationException, isScroll, allowPartialSearchResults);
+            if (innerRetriever.retriever() instanceof CompoundRetrieverBuilder<?> compoundChild) {
+                if (rankWindowSize > compoundChild.rankWindowSize) {
+                    String errorMessage = String.format(
+                        Locale.ROOT,
+                        "[%s] requires [%s: %d] to be smaller than or equal to its sub retriever's %s [%s: %d]",
+                        this.getName(),
+                        getRankWindowSizeField().getPreferredName(),
+                        rankWindowSize,
+                        compoundChild.getName(),
+                        compoundChild.getRankWindowSizeField(),
+                        compoundChild.rankWindowSize
+                    );
+                    validationException = addValidationError(errorMessage, validationException);
+                }
+            }
         }
         return validationException;
     }

server/src/main/java/org/elasticsearch/search/retriever/RescorerRetrieverBuilder.java

@@ -0,0 +1,173 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.search.retriever;
+
+import org.apache.lucene.search.ScoreDoc;
+import org.elasticsearch.common.ParsingException;
+import org.elasticsearch.index.query.QueryBuilder;
+import org.elasticsearch.search.builder.SearchSourceBuilder;
+import org.elasticsearch.search.rank.RankDoc;
+import org.elasticsearch.search.rescore.RescorerBuilder;
+import org.elasticsearch.xcontent.ConstructingObjectParser;
+import org.elasticsearch.xcontent.ObjectParser;
+import org.elasticsearch.xcontent.ParseField;
+import org.elasticsearch.xcontent.XContentBuilder;
+import org.elasticsearch.xcontent.XContentParser;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+import static org.elasticsearch.search.builder.SearchSourceBuilder.RESCORE_FIELD;
+import static org.elasticsearch.xcontent.ConstructingObjectParser.constructorArg;
+
+/**
+ * A {@link CompoundRetrieverBuilder} that re-scores only the results produced by its child retriever.
+ */
+public final class RescorerRetrieverBuilder extends CompoundRetrieverBuilder<RescorerRetrieverBuilder> {
+
+    public static final String NAME = "rescorer";
+    public static final ParseField RETRIEVER_FIELD = new ParseField("retriever");
+
+    @SuppressWarnings("unchecked")
+    public static final ConstructingObjectParser<RescorerRetrieverBuilder, RetrieverParserContext> PARSER = new ConstructingObjectParser<>(
+        NAME,
+        args -> new RescorerRetrieverBuilder((RetrieverBuilder) args[0], (List<RescorerBuilder<?>>) args[1])
+    );
+
+    static {
+        PARSER.declareNamedObject(constructorArg(), (parser, context, n) -> {
+            RetrieverBuilder innerRetriever = parser.namedObject(RetrieverBuilder.class, n, context);
+            context.trackRetrieverUsage(innerRetriever.getName());
+            return innerRetriever;
+        }, RETRIEVER_FIELD);
+        PARSER.declareField(constructorArg(), (parser, context) -> {
+            if (parser.currentToken() == XContentParser.Token.START_ARRAY) {
+                List<RescorerBuilder<?>> rescorers = new ArrayList<>();
+                while ((parser.nextToken()) != XContentParser.Token.END_ARRAY) {
+                    rescorers.add(RescorerBuilder.parseFromXContent(parser, name -> context.trackRescorerUsage(name)));
+                }
+                return rescorers;
+            } else if (parser.currentToken() == XContentParser.Token.START_OBJECT) {
+                return List.of(RescorerBuilder.parseFromXContent(parser, name -> context.trackRescorerUsage(name)));
+            } else {
+                throw new IllegalArgumentException(
+                    "Unknown format for [rescorer.rescore], expects an object or an array of objects, got: " + parser.currentToken()
+                );
+            }
+        }, RESCORE_FIELD, ObjectParser.ValueType.OBJECT_ARRAY);
+        RetrieverBuilder.declareBaseParserFields(NAME, PARSER);
+    }
+
+    public static RescorerRetrieverBuilder fromXContent(XContentParser parser, RetrieverParserContext context) throws IOException {
+        try {
+            return PARSER.apply(parser, context);
+        } catch (Exception e) {
+            throw new ParsingException(parser.getTokenLocation(), e.getMessage(), e);
+        }
+    }
+
+    private final List<RescorerBuilder<?>> rescorers;
+
+    public RescorerRetrieverBuilder(RetrieverBuilder retriever, List<RescorerBuilder<?>> rescorers) {
+        super(List.of(new RetrieverSource(retriever, null)), extractMinWindowSize(rescorers));
+        if (rescorers.isEmpty()) {
+            throw new IllegalArgumentException("Missing rescore definition");
+        }
+        this.rescorers = rescorers;
+    }
+
+    private RescorerRetrieverBuilder(RetrieverSource retriever, List<RescorerBuilder<?>> rescorers) {
+        super(List.of(retriever), extractMinWindowSize(rescorers));
+        this.rescorers = rescorers;
+    }
+
+    /**
+     * The minimum window size is used as the {@link CompoundRetrieverBuilder#rankWindowSize},
+     * the final number of top documents to return in this retriever.
+     */
+    private static int extractMinWindowSize(List<RescorerBuilder<?>> rescorers) {
+        int windowSize = Integer.MAX_VALUE;
+        for (var rescore : rescorers) {
+            windowSize = Math.min(rescore.windowSize() == null ? RescorerBuilder.DEFAULT_WINDOW_SIZE : rescore.windowSize(), windowSize);
+        }
+        return windowSize;
+    }
+
+    @Override
+    public String getName() {
+        return NAME;
+    }
+
+    @Override
+    public ParseField getRankWindowSizeField() {
+        return RescorerBuilder.WINDOW_SIZE_FIELD;
+    }
+
+    @Override
+    protected SearchSourceBuilder finalizeSourceBuilder(SearchSourceBuilder source) {
+        /**
+         * The re-scorer is passed downstream because this query operates only on
+         * the top documents retrieved by the child retriever.
+         *
+         * - If the sub-retriever is a {@link CompoundRetrieverBuilder}, only the top
+         *   documents are re-scored since they are already determined at this stage.
+         * - For other retrievers that do not require a rewrite, the re-scorer's window
+         *   size is applied per shard. As a result, more documents are re-scored
+         *   compared to the final top documents produced by these retrievers in isolation.
+         */
+        for (var rescorer : rescorers) {
+            source.addRescorer(rescorer);
+        }
+        return source;
+    }
+
+    @Override
+    public void doToXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.field(RETRIEVER_FIELD.getPreferredName(), innerRetrievers.getFirst().retriever());
+        builder.startArray(RESCORE_FIELD.getPreferredName());
+        for (RescorerBuilder<?> rescorer : rescorers) {
+            rescorer.toXContent(builder, params);
+        }
+        builder.endArray();
+    }
+
+    @Override
+    protected RescorerRetrieverBuilder clone(List<RetrieverSource> newChildRetrievers, List<QueryBuilder> newPreFilterQueryBuilders) {
+        var newInstance = new RescorerRetrieverBuilder(newChildRetrievers.get(0), rescorers);
+        newInstance.preFilterQueryBuilders = newPreFilterQueryBuilders;
+        return newInstance;
+    }
+
+    @Override
+    protected RankDoc[] combineInnerRetrieverResults(List<ScoreDoc[]> rankResults) {
+        assert rankResults.size() == 1;
+        ScoreDoc[] scoreDocs = rankResults.getFirst();
+        RankDoc[] rankDocs = new RankDoc[scoreDocs.length];
+        for (int i = 0; i < scoreDocs.length; i++) {
+            ScoreDoc scoreDoc = scoreDocs[i];
+            rankDocs[i] = new RankDoc(scoreDoc.doc, scoreDoc.score, scoreDoc.shardIndex);
+            rankDocs[i].rank = i + 1;
+        }
+        return rankDocs;
+    }
+
+    @Override
+    public boolean doEquals(Object o) {
+        RescorerRetrieverBuilder that = (RescorerRetrieverBuilder) o;
+        return super.doEquals(o) && Objects.equals(rescorers, that.rescorers);
+    }
+
+    @Override
+    public int doHashCode() {
+        return Objects.hash(super.doHashCode(), rescorers);
+    }
+}

server/src/main/java/org/elasticsearch/search/retriever/RetrieverBuilder.java

@@ -63,7 +63,7 @@ public abstract class RetrieverBuilder implements Rewriteable<RetrieverBuilder>,
         AbstractObjectParser<? extends RetrieverBuilder, RetrieverParserContext> parser
     ) {
         parser.declareObjectArray(
-            (r, v) -> r.preFilterQueryBuilders = v,
+            (r, v) -> r.preFilterQueryBuilders = new ArrayList<>(v),
             (p, c) -> AbstractQueryBuilder.parseTopLevelQuery(p, c::trackQueryUsage),
             PRE_FILTER_FIELD
         );

server/src/test/java/org/elasticsearch/search/DefaultSearchContextTests.java

@@ -23,6 +23,7 @@ import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.MatchNoDocsQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.Sort;
+import org.apache.lucene.search.SortField;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.tests.index.RandomIndexWriter;
 import org.apache.lucene.tests.store.BaseDirectoryWrapper;
@@ -245,7 +246,10 @@ public class DefaultSearchContextTests extends MapperServiceTestCase {
                 // resultWindow not greater than maxResultWindow and both rescore and sort are not null
                 context1.from(0);
                 DocValueFormat docValueFormat = mock(DocValueFormat.class);
-                SortAndFormats sortAndFormats = new SortAndFormats(new Sort(), new DocValueFormat[] { docValueFormat });
+                SortAndFormats sortAndFormats = new SortAndFormats(
+                    new Sort(new SortField[] { SortField.FIELD_DOC }),
+                    new DocValueFormat[] { docValueFormat }
+                );
                 context1.sort(sortAndFormats);
 
                 RescoreContext rescoreContext = mock(RescoreContext.class);

server/src/test/java/org/elasticsearch/search/retriever/RescorerRetrieverBuilderParsingTests.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.search.retriever;
+
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.search.SearchModule;
+import org.elasticsearch.search.rescore.QueryRescorerBuilderTests;
+import org.elasticsearch.search.rescore.RescorerBuilder;
+import org.elasticsearch.test.AbstractXContentTestCase;
+import org.elasticsearch.usage.SearchUsage;
+import org.elasticsearch.xcontent.NamedXContentRegistry;
+import org.elasticsearch.xcontent.XContentParser;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+
+import static java.util.Collections.emptyList;
+
+public class RescorerRetrieverBuilderParsingTests extends AbstractXContentTestCase<RescorerRetrieverBuilder> {
+    private static List<NamedXContentRegistry.Entry> xContentRegistryEntries;
+
+    @BeforeClass
+    public static void init() {
+        xContentRegistryEntries = new SearchModule(Settings.EMPTY, emptyList()).getNamedXContents();
+    }
+
+    @AfterClass
+    public static void afterClass() throws Exception {
+        xContentRegistryEntries = null;
+    }
+
+    @Override
+    protected RescorerRetrieverBuilder createTestInstance() {
+        int num = randomIntBetween(1, 3);
+        List<RescorerBuilder<?>> rescorers = new ArrayList<>();
+        for (int i = 0; i < num; i++) {
+            rescorers.add(QueryRescorerBuilderTests.randomRescoreBuilder());
+        }
+        return new RescorerRetrieverBuilder(TestRetrieverBuilder.createRandomTestRetrieverBuilder(), rescorers);
+    }
+
+    @Override
+    protected RescorerRetrieverBuilder doParseInstance(XContentParser parser) throws IOException {
+        return (RescorerRetrieverBuilder) RetrieverBuilder.parseTopLevelRetrieverBuilder(
+            parser,
+            new RetrieverParserContext(new SearchUsage(), n -> true)
+        );
+    }
+
+    @Override
+    protected boolean supportsUnknownFields() {
+        return false;
+    }
+
+    @Override
+    protected NamedXContentRegistry xContentRegistry() {
+        List<NamedXContentRegistry.Entry> entries = new ArrayList<>(xContentRegistryEntries);
+        entries.add(
+            new NamedXContentRegistry.Entry(
+                RetrieverBuilder.class,
+                TestRetrieverBuilder.TEST_SPEC.getName(),
+                (p, c) -> TestRetrieverBuilder.TEST_SPEC.getParser().fromXContent(p, (RetrieverParserContext) c),
+                TestRetrieverBuilder.TEST_SPEC.getName().getForRestApiVersion()
+            )
+        );
+        return new NamedXContentRegistry(entries);
+    }
+}

x-pack/plugin/ent-search/src/main/java/org/elasticsearch/xpack/application/rules/retriever/QueryRuleRetrieverBuilder.java

@@ -50,7 +50,6 @@ public final class QueryRuleRetrieverBuilder extends CompoundRetrieverBuilder<Qu
     public static final ParseField RULESET_IDS_FIELD = new ParseField("ruleset_ids");
     public static final ParseField MATCH_CRITERIA_FIELD = new ParseField("match_criteria");
     public static final ParseField RETRIEVER_FIELD = new ParseField("retriever");
-    public static final ParseField RANK_WINDOW_SIZE_FIELD = new ParseField("rank_window_size");
 
     @SuppressWarnings("unchecked")
     public static final ConstructingObjectParser<QueryRuleRetrieverBuilder, RetrieverParserContext> PARSER = new ConstructingObjectParser<>(

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/rank/textsimilarity/TextSimilarityRankRetrieverBuilder.java

@@ -47,7 +47,6 @@ public class TextSimilarityRankRetrieverBuilder extends CompoundRetrieverBuilder
     public static final ParseField INFERENCE_ID_FIELD = new ParseField("inference_id");
     public static final ParseField INFERENCE_TEXT_FIELD = new ParseField("inference_text");
     public static final ParseField FIELD_FIELD = new ParseField("field");
-    public static final ParseField RANK_WINDOW_SIZE_FIELD = new ParseField("rank_window_size");
 
     public static final ConstructingObjectParser<TextSimilarityRankRetrieverBuilder, RetrieverParserContext> PARSER =
         new ConstructingObjectParser<>(TextSimilarityRankBuilder.NAME, args -> {

x-pack/plugin/rank-rrf/build.gradle

@@ -22,6 +22,7 @@ dependencies {
   testImplementation(testArtifact(project(xpackModule('core'))))
   testImplementation(testArtifact(project(':server')))
 
+  clusterModules project(':modules:mapper-extras')
   clusterModules project(xpackModule('rank-rrf'))
   clusterModules project(xpackModule('inference'))
   clusterModules project(':modules:lang-painless')

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

@@ -48,7 +48,6 @@ public final class RRFRetrieverBuilder extends CompoundRetrieverBuilder<RRFRetri
     public static final NodeFeature RRF_RETRIEVER_COMPOSITION_SUPPORTED = new NodeFeature("rrf_retriever_composition_supported");
 
     public static final ParseField RETRIEVERS_FIELD = new ParseField("retrievers");
-    public static final ParseField RANK_WINDOW_SIZE_FIELD = new ParseField("rank_window_size");
     public static final ParseField RANK_CONSTANT_FIELD = new ParseField("rank_constant");
 
     public static final int DEFAULT_RANK_CONSTANT = 60;

x-pack/plugin/rank-rrf/src/yamlRestTest/java/org/elasticsearch/xpack/rank/rrf/RRFRankClientYamlTestSuiteIT.java

@@ -21,6 +21,7 @@ public class RRFRankClientYamlTestSuiteIT extends ESClientYamlSuiteTestCase {
     @ClassRule
     public static ElasticsearchCluster cluster = ElasticsearchCluster.local()
         .nodes(2)
+        .module("mapper-extras")
         .module("rank-rrf")
         .module("lang-painless")
         .module("x-pack-inference")

x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/rrf/900_rrf_with_rescorer.yml

@@ -0,0 +1,409 @@
+setup:
+  - requires:
+      cluster_features: [ "search.retriever.rescorer.enabled" ]
+      reason: "Support for rescorer retriever"
+  - do:
+      indices.create:
+        index: test
+        body:
+          settings:
+            number_of_shards: 3
+          mappings:
+            properties:
+              available:
+                type: boolean
+              features:
+                type: rank_features
+
+  - do:
+      bulk:
+        refresh: true
+        index: test
+        body:
+          - '{"index": {"_id": 1 }}'
+          - '{"features": { "first_query": 1, "second_query": 3, "final_score": 7}, "available": true}'
+          - '{"index": {"_id": 2 }}'
+          - '{"features": { "first_query": 5, "second_query": 7, "final_score": 4}, "available": false}'
+          - '{"index": {"_id": 3 }}'
+          - '{"features": { "first_query": 6, "second_query": 5, "final_score": 3}, "available": false}'
+          - '{"index": {"_id": 4 }}'
+          - '{"features": { "first_query": 3, "second_query": 2, "final_score": 2}, "available": true}'
+          - '{"index": {"_id": 5 }}'
+          - '{"features": { "first_query": 2, "second_query": 1, "final_score": 1}, "available": true}'
+          - '{"index": {"_id": 6 }}'
+          - '{"features": { "first_query": 4, "second_query": 4, "final_score": 8}, "available": false}'
+          - '{"index": {"_id": 7 }}'
+          - '{"features": { "first_query": 7, "second_query": 10, "final_score": 9}, "available": true}'
+          - '{"index": {"_id": 8 }}'
+          - '{"features": { "first_query": 8, "second_query": 8, "final_score": 10}, "available": true}'
+          - '{"index": {"_id": 9 }}'
+          - '{"features": { "first_query": 9, "second_query": 9, "final_score": 5}, "available": true}'
+          - '{"index": {"_id": 10 }}'
+          - '{"features": { "first_query": 10, "second_query": 6, "final_score": 6}, "available": false}'
+
+---
+"RRF with rescorer retriever basic":
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 10
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.final_score"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                rrf:
+                  rank_window_size: 10
+                  retrievers: [
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.first_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    },
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.second_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    }
+                  ]
+          size: 3
+
+  - match:  { hits.total.value: 10 }
+  - length: { hits.hits: 3}
+  - match:  { hits.hits.0._id: "8" }
+  - match:  { hits.hits.0._score: 10.0 }
+  - match:  { hits.hits.1._id: "7" }
+  - match:  { hits.hits.1._score: 9.0 }
+  - match:  { hits.hits.2._id: "6" }
+  - match:  { hits.hits.2._score: 8.0 }
+
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 5
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.final_score"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                rrf:
+                  rank_window_size: 5
+                  retrievers: [
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.first_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    },
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.second_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    }
+                  ]
+          size: 3
+
+  - match:  { hits.total.value: 10 }
+  - length: { hits.hits: 3}
+  - match:  { hits.hits.0._id: "8" }
+  - match:  { hits.hits.0._score: 10.0 }
+  - match:  { hits.hits.1._id: "7" }
+  - match:  { hits.hits.1._score: 9.0 }
+  - match:  { hits.hits.2._id: "10" }
+  - match:  { hits.hits.2._score: 6.0 }
+
+---
+"RRF with rescorer retriever and prefilters":
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              filter:
+                match:
+                  available: true
+              rescore:
+                window_size: 5
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.final_score"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                rrf:
+                  rank_window_size: 5
+                  retrievers: [
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.first_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    },
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.second_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    }
+                  ]
+          size: 3
+
+  - match:  { hits.total.value: 6 }
+  - length: { hits.hits: 3}
+  - match:  { hits.hits.0._id: "8" }
+  - match:  { hits.hits.0._score: 10.0 }
+  - match:  { hits.hits.1._id: "7" }
+  - match:  { hits.hits.1._score: 9.0 }
+  - match:  { hits.hits.2._id: "1" }
+  - match:  { hits.hits.2._score: 7.0 }
+
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              filter:
+                match:
+                  available: true
+              rescore:
+                window_size: 5
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.final_score"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                rrf:
+                  rank_window_size: 5
+                  retrievers: [
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.first_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    },
+                    {
+                      standard: {
+                        filter: {
+                          match: {
+                            available: true
+                          }
+                        },
+                        query: {
+                          rank_feature: {
+                            field: "features.second_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    }
+                  ]
+          size: 3
+
+  - match:  { hits.total.value: 6 }
+  - length: { hits.hits: 3}
+  - match:  { hits.hits.0._id: "8" }
+  - match:  { hits.hits.0._score: 10.0 }
+  - match:  { hits.hits.1._id: "7" }
+  - match:  { hits.hits.1._score: 9.0 }
+  - match:  { hits.hits.2._id: "1" }
+  - match:  { hits.hits.2._score: 7.0 }
+
+---
+"RRF with rescorer retriever and aggs":
+  - do:
+      search:
+        index: test
+        body:
+          aggs:
+            1:
+              terms:
+                field: available
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 5
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.final_score"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                rrf:
+                  rank_window_size: 5
+                  retrievers: [
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.first_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    },
+                    {
+                      standard: {
+                        filter: {
+                          match: {
+                            available: true
+                          }
+                        },
+                        query: {
+                          rank_feature: {
+                            field: "features.second_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    }
+                  ]
+          size: 3
+
+  - match:  { hits.total.value: 10 }
+  - length: { hits.hits: 3}
+  - match:  { hits.hits.0._id: "8" }
+  - match:  { hits.hits.0._score: 10.0 }
+  - match:  { hits.hits.1._id: "7" }
+  - match:  { hits.hits.1._score: 9.0 }
+  - match:  { hits.hits.2._id: "1" }
+  - match:  { hits.hits.2._score: 7.0 }
+  - length: { aggregations.1.buckets: 2}
+  - match:  { aggregations.1.buckets.0.key: 1}
+  - match:  { aggregations.1.buckets.0.doc_count: 6}
+  - match:  { aggregations.1.buckets.1.key: 0 }
+  - match:  { aggregations.1.buckets.1.doc_count: 4 }
+
+---
+"RRF with rescorer retriever and invalid window size":
+  - do:
+      catch: "/\\[rescorer\\] requires \\[window_size: 5\\] be greater than or equal to \\[size: 10\\]/"
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 5
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.final_score"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                rrf:
+                  rank_window_size: 5
+                  retrievers: [
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.first_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    },
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.second_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    }
+                  ]
+          size: 10
+
+  - do:
+      catch: "/\\[rescorer\\] requires \\[window_size: 10\\] to be smaller than or equal to its sub retriever's rrf \\[rank_window_size: 5\\]/"
+      search:
+        index: test
+        body:
+          retriever:
+            rescorer:
+              rescore:
+                window_size: 10
+                query:
+                  rescore_query:
+                    rank_feature:
+                      field: "features.final_score"
+                      linear: { }
+                  query_weight: 0
+              retriever:
+                rrf:
+                  rank_window_size: 5
+                  retrievers: [
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.first_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    },
+                    {
+                      standard: {
+                        query: {
+                          rank_feature: {
+                            field: "features.second_query",
+                            linear: { }
+                          }
+                        }
+                      }
+                    }
+                  ]
+          size: 5