cpoerschke commented on code in PR #3418:
URL: https://github.com/apache/solr/pull/3418#discussion_r2231263967


##########
solr/core/src/java/org/apache/solr/handler/component/CombinedQueryComponent.java:
##########
@@ -0,0 +1,572 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.solr.handler.component;
+
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.io.StringWriter;
+import java.lang.invoke.MethodHandles;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
+import org.apache.lucene.search.Explanation;
+import org.apache.lucene.search.Query;
+import org.apache.lucene.search.Sort;
+import org.apache.lucene.search.SortField;
+import org.apache.solr.client.solrj.SolrServerException;
+import org.apache.solr.common.SolrDocument;
+import org.apache.solr.common.SolrDocumentList;
+import org.apache.solr.common.SolrException;
+import org.apache.solr.common.params.CombinerParams;
+import org.apache.solr.common.params.CursorMarkParams;
+import org.apache.solr.common.params.ShardParams;
+import org.apache.solr.common.params.SolrParams;
+import org.apache.solr.common.util.NamedList;
+import org.apache.solr.common.util.SimpleOrderedMap;
+import org.apache.solr.common.util.StrUtils;
+import org.apache.solr.core.SolrCore;
+import org.apache.solr.response.BasicResultContext;
+import org.apache.solr.response.ResultContext;
+import org.apache.solr.response.SolrQueryResponse;
+import org.apache.solr.schema.IndexSchema;
+import org.apache.solr.schema.SchemaField;
+import org.apache.solr.search.DocListAndSet;
+import org.apache.solr.search.QueryResult;
+import org.apache.solr.search.SolrReturnFields;
+import org.apache.solr.search.SortSpec;
+import org.apache.solr.search.combine.QueryAndResponseCombiner;
+import org.apache.solr.search.combine.ReciprocalRankFusion;
+import org.apache.solr.util.SolrResponseUtil;
+import org.apache.solr.util.plugin.SolrCoreAware;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The CombinedQueryComponent class extends QueryComponent and provides 
support for executing
+ * multiple queries and combining their results.
+ */
+public class CombinedQueryComponent extends QueryComponent implements 
SolrCoreAware {
+
+  public static final String COMPONENT_NAME = "combined_query";
+  protected NamedList<?> initParams;
+  private Map<String, QueryAndResponseCombiner> combiners = new 
ConcurrentHashMap<>();
+  private int maxCombinerQueries;
+  private static final Logger log = 
LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
+
+  @Override
+  public void init(NamedList<?> args) {
+    super.init(args);
+    this.initParams = args;
+    this.maxCombinerQueries = CombinerParams.DEFAULT_MAX_COMBINER_QUERIES;
+  }
+
+  @Override
+  public void inform(SolrCore core) {
+    if (initParams != null && initParams.size() > 0) {
+      log.info("Initializing CombinedQueryComponent");
+      NamedList<?> all = (NamedList<?>) initParams.get("combiners");
+      for (int i = 0; i < all.size(); i++) {
+        String name = all.getName(i);
+        NamedList<?> combinerConfig = (NamedList<?>) all.getVal(i);
+        String className = (String) combinerConfig.get("class");
+        QueryAndResponseCombiner combiner =
+            core.getResourceLoader().newInstance(className, 
QueryAndResponseCombiner.class);
+        combiner.init(combinerConfig);
+        combiners.computeIfAbsent(name, combinerName -> combiner);
+      }
+      Object maxQueries = initParams.get("maxCombinerQueries");
+      if (maxQueries != null) {
+        this.maxCombinerQueries = Integer.parseInt(maxQueries.toString());
+      }
+    }
+    combiners.computeIfAbsent(
+        CombinerParams.RECIPROCAL_RANK_FUSION,
+        key -> {
+          ReciprocalRankFusion reciprocalRankFusion = new 
ReciprocalRankFusion();
+          reciprocalRankFusion.init(initParams);
+          return reciprocalRankFusion;
+        });
+  }
+
+  /**
+   * Overrides the prepare method to handle combined queries.
+   *
+   * @param rb the ResponseBuilder to prepare
+   * @throws IOException if an I/O error occurs during preparation
+   */
+  @Override
+  public void prepare(ResponseBuilder rb) throws IOException {
+    if (rb instanceof CombinedQueryResponseBuilder crb) {
+      SolrParams params = crb.req.getParams();
+      String[] queriesToCombineKeys = 
params.getParams(CombinerParams.COMBINER_QUERY);
+      if (queriesToCombineKeys.length > maxCombinerQueries) {
+        throw new SolrException(
+            SolrException.ErrorCode.BAD_REQUEST,
+            "Too many queries to combine: limit is " + maxCombinerQueries);
+      }
+      for (String queryKey : queriesToCombineKeys) {
+        final var unparsedQuery = params.get(queryKey);
+        ResponseBuilder rbNew = new ResponseBuilder(rb.req, new 
SolrQueryResponse(), rb.components);
+        rbNew.setQueryString(unparsedQuery);
+        super.prepare(rbNew);
+        crb.responseBuilders.add(rbNew);
+      }
+    }
+    super.prepare(rb);
+  }
+
+  /**
+   * Overrides the process method to handle CombinedQueryResponseBuilder 
instances. This method
+   * processes the responses from multiple shards, combines them using the 
specified
+   * QueryAndResponseCombiner strategy, and sets the appropriate results and 
metadata in the
+   * CombinedQueryResponseBuilder.
+   *
+   * @param rb the ResponseBuilder object to process
+   * @throws IOException if an I/O error occurs during processing
+   */
+  @Override
+  public void process(ResponseBuilder rb) throws IOException {
+    if (rb instanceof CombinedQueryResponseBuilder crb) {
+      boolean partialResults = false;
+      boolean segmentTerminatedEarly = false;
+      boolean setMaxHitsTerminatedEarly = false;
+      List<QueryResult> queryResults = new ArrayList<>();
+      for (ResponseBuilder rbNow : crb.responseBuilders) {
+        // propagating from global ResponseBuilder, so that if in case cursor 
is needed for
+        // retrieving the next batch of documents
+        // which might have duplicate results from the previous cursorMark as 
we are dealing with
+        // multiple queries
+        rbNow.setCursorMark(crb.getCursorMark());
+        super.process(rbNow);
+        DocListAndSet docListAndSet = rbNow.getResults();
+        QueryResult queryResult = new QueryResult();
+        queryResult.setDocListAndSet(docListAndSet);
+        queryResults.add(queryResult);
+        partialResults |= queryResult.isPartialResults();
+        if (queryResult.getSegmentTerminatedEarly() != null) {
+          segmentTerminatedEarly |= queryResult.getSegmentTerminatedEarly();
+        }
+        if (queryResult.getMaxHitsTerminatedEarly() != null) {
+          setMaxHitsTerminatedEarly |= queryResult.getMaxHitsTerminatedEarly();
+        }
+      }
+      QueryAndResponseCombiner combinerStrategy =
+          QueryAndResponseCombiner.getImplementation(rb.req.getParams(), 
combiners);
+      QueryResult combinedQueryResult = combinerStrategy.combine(queryResults, 
rb.req.getParams());

Review Comment:
   subjective/clarity: the params being passed to both `getImplementation` and 
to `combine` jumps out.
   
   Would it be sufficient to pass the algorithm parameter?
   ```suggestion
         QueryAndResponseCombiner combinerStrategy =
             
QueryAndResponseCombiner.getImplementation(rb.req.getParams().get(CombinerParams.COMBINER_ALGORITHM,
 CombinerParams.DEFAULT_COMBINER), combiners);
         QueryResult combinedQueryResult = 
combinerStrategy.combine(queryResults, rb.req.getParams());
   ```



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