Simplified linear and RRF retrievers

docs/changelog/128633.yaml

@@ -0,0 +1,5 @@
+pr: 128633
+summary: Simplified linear and RRF retrievers
+area: Search
+type: enhancement
+issues: []

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

@@ -36,6 +36,7 @@
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
 import java.util.Objects;
@@ -65,7 +66,7 @@ protected CompoundRetrieverBuilder(List<RetrieverSource> innerRetrievers, int ra
 
     @SuppressWarnings("unchecked")
     public T addChild(RetrieverBuilder retrieverBuilder) {
-        innerRetrievers.add(new RetrieverSource(retrieverBuilder, null));
+        innerRetrievers.add(convertToRetrieverSource(retrieverBuilder));
         return (T) this;
     }
 
@@ -99,6 +100,11 @@ public final RetrieverBuilder rewrite(QueryRewriteContext ctx) throws IOExceptio
             throw new IllegalStateException("PIT is required");
         }
 
+        RetrieverBuilder rewritten = doRewrite(ctx);
+        if (rewritten != this) {
+            return rewritten;
+        }
+
         // Rewrite prefilters
         // We eagerly rewrite prefilters, because some of the innerRetrievers
         // could be compound too, so we want to propagate all the necessary filter information to them
@@ -290,6 +296,14 @@ public int rankWindowSize() {
         return rankWindowSize;
     }
 
+    public List<RetrieverSource> innerRetrievers() {
+        return Collections.unmodifiableList(innerRetrievers);
+    }
+
+    public static RetrieverSource convertToRetrieverSource(RetrieverBuilder retrieverBuilder) {
+        return new RetrieverSource(retrieverBuilder, null);
+    }
+
     protected final SearchSourceBuilder createSearchSourceBuilder(PointInTimeBuilder pit, RetrieverBuilder retrieverBuilder) {
         var sourceBuilder = new SearchSourceBuilder().pointInTimeBuilder(pit)
             .trackTotalHits(false)
@@ -316,6 +330,16 @@ protected SearchSourceBuilder finalizeSourceBuilder(SearchSourceBuilder sourceBu
         return sourceBuilder;
     }
 
+    /**
+     * Perform any custom rewrite logic necessary
+     *
+     * @param ctx The query rewrite context
+     * @return RetrieverBuilder the rewritten retriever
+     */
+    protected RetrieverBuilder doRewrite(QueryRewriteContext ctx) {
+        return this;
+    }
+
     private RankDoc[] getRankDocs(SearchResponse searchResponse) {
         int size = searchResponse.getHits().getHits().length;
         RankDoc[] docs = new RankDoc[size];

x-pack/plugin/inference/build.gradle

@@ -403,5 +403,5 @@ tasks.named("thirdPartyAudit").configure {
 }
 
 tasks.named('yamlRestTest') {
-  usesDefaultDistribution("to be triaged")
+  usesDefaultDistribution("Uses the inference API")
 }

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

@@ -30,3 +30,7 @@ dependencies {
 
   clusterPlugins project(':x-pack:plugin:inference:qa:test-service-plugin')
 }
+
+tasks.named('yamlRestTest') {
+  usesDefaultDistribution("Uses the inference API")
+}

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

@@ -18,6 +18,7 @@
 public class RankRRFFeatures implements FeatureSpecification {
 
     public static final NodeFeature LINEAR_RETRIEVER_SUPPORTED = new NodeFeature("linear_retriever_supported");
+    public static final NodeFeature SIMPLIFIED_RETRIEVER_FORMAT = new NodeFeature("simplified_retriever_format");
 
     @Override
     public Set<NodeFeature> getFeatures() {
@@ -26,6 +27,6 @@ public Set<NodeFeature> getFeatures() {
 
     @Override
     public Set<NodeFeature> getTestFeatures() {
-        return Set.of(INNER_RETRIEVERS_FILTER_SUPPORT, LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX);
+        return Set.of(INNER_RETRIEVERS_FILTER_SUPPORT, LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX, SIMPLIFIED_RETRIEVER_FORMAT);
     }
 }