First step towards incremental reduction of query responses (#23253)

Today all query results are buffered up until we received responses of
all shards. This can hold on to a significant amount of memory if the number of
shards is large. This commit adds a first step towards incrementally reducing
aggregations results if a, per search request, configurable amount of responses
are received. If enough query results have been received and buffered all so-far
received aggregation responses will be reduced and released to be GCed.

core/src/main/java/org/elasticsearch/action/search/AbstractSearchAsyncAction.java

@@ -42,7 +42,6 @@
 
 import java.util.List;
 import java.util.Map;
-import java.util.StringJoiner;
 import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Function;
@@ -61,7 +60,7 @@ abstract class AbstractSearchAsyncAction<Result extends SearchPhaseResult> exten
      **/
     private final Function<String, Transport.Connection> nodeIdToConnection;
     private final SearchTask task;
-    private final AtomicArray<Result> results;
+    private final SearchPhaseResults<Result> results;
     private final long clusterStateVersion;
     private final Map<String, AliasFilter> aliasFilter;
     private final Map<String, Float> concreteIndexBoosts;
@@ -76,7 +75,7 @@ protected AbstractSearchAsyncAction(String name, Logger logger, SearchTransportS
                                         Map<String, AliasFilter> aliasFilter, Map<String, Float> concreteIndexBoosts,
                                         Executor executor, SearchRequest request,
                                         ActionListener<SearchResponse> listener, GroupShardsIterator shardsIts, long startTime,
-                                        long clusterStateVersion, SearchTask task) {
+                                        long clusterStateVersion, SearchTask task, SearchPhaseResults<Result> resultConsumer) {
         super(name, request, shardsIts, logger);
         this.startTime = startTime;
         this.logger = logger;
@@ -87,9 +86,9 @@ protected AbstractSearchAsyncAction(String name, Logger logger, SearchTransportS
         this.listener = listener;
         this.nodeIdToConnection = nodeIdToConnection;
         this.clusterStateVersion = clusterStateVersion;
-        results = new AtomicArray<>(shardsIts.size());
         this.concreteIndexBoosts = concreteIndexBoosts;
         this.aliasFilter = aliasFilter;
+        this.results = resultConsumer;
     }
 
     /**
@@ -105,7 +104,7 @@ private long buildTookInMillis() {
      * This is the main entry point for a search. This method starts the search execution of the initial phase.
      */
     public final void start() {
-        if (results.length() == 0) {
+        if (getNumShards() == 0) {
             //no search shards to search on, bail with empty response
             //(it happens with search across _all with no indices around and consistent with broadcast operations)
             listener.onResponse(new SearchResponse(InternalSearchResponse.empty(), null, 0, 0, buildTookInMillis(),
@@ -130,8 +129,8 @@ public final void executeNextPhase(SearchPhase currentPhase, SearchPhase nextPha
             onPhaseFailure(currentPhase, "all shards failed", null);
         } else {
             if (logger.isTraceEnabled()) {
-                final String resultsFrom = results.asList().stream()
-                    .map(r -> r.value.shardTarget().toString()).collect(Collectors.joining(","));
+                final String resultsFrom = results.getSuccessfulResults()
+                    .map(r -> r.shardTarget().toString()).collect(Collectors.joining(","));
                 logger.trace("[{}] Moving to next phase: [{}], based on results from: {} (cluster state version: {})",
                     currentPhase.getName(), nextPhase.getName(), resultsFrom, clusterStateVersion);
             }
@@ -178,7 +177,7 @@ public final void onShardFailure(final int shardIndex, @Nullable SearchShardTarg
             synchronized (shardFailuresMutex) {
                 shardFailures = this.shardFailures.get(); // read again otherwise somebody else has created it?
                 if (shardFailures == null) { // still null so we are the first and create a new instance
-                    shardFailures = new AtomicArray<>(results.length());
+                    shardFailures = new AtomicArray<>(getNumShards());
                     this.shardFailures.set(shardFailures);
                 }
             }
@@ -194,7 +193,7 @@ public final void onShardFailure(final int shardIndex, @Nullable SearchShardTarg
             }
         }
 
-        if (results.get(shardIndex) != null) {
+        if (results.hasResult(shardIndex)) {
             assert failure == null : "shard failed before but shouldn't: " + failure;
             successfulOps.decrementAndGet(); // if this shard was successful before (initial phase) we have to adjust the counter
         }
@@ -207,22 +206,22 @@ public final void onShardFailure(final int shardIndex, @Nullable SearchShardTarg
      * @param exception the exception explaining or causing the phase failure
      */
     private void raisePhaseFailure(SearchPhaseExecutionException exception) {
-        for (AtomicArray.Entry<Result> entry : results.asList()) {
+        results.getSuccessfulResults().forEach((entry) -> {
             try {
-                Transport.Connection connection = nodeIdToConnection.apply(entry.value.shardTarget().getNodeId());
-                sendReleaseSearchContext(entry.value.id(), connection);
+                Transport.Connection connection = nodeIdToConnection.apply(entry.shardTarget().getNodeId());
+                sendReleaseSearchContext(entry.id(), connection);
             } catch (Exception inner) {
                 inner.addSuppressed(exception);
                 logger.trace("failed to release context", inner);
             }
-        }
+        });
         listener.onFailure(exception);
     }
 
     @Override
     public final void onShardSuccess(int shardIndex, Result result) {
         successfulOps.incrementAndGet();
-        results.set(shardIndex, result);
+        results.consumeResult(shardIndex, result);
         if (logger.isTraceEnabled()) {
             logger.trace("got first-phase result from {}", result != null ? result.shardTarget() : null);
         }
@@ -242,7 +241,7 @@ public final void onPhaseDone() {
 
     @Override
     public final int getNumShards() {
-        return results.length();
+        return results.getNumShards();
     }
 
     @Override
@@ -262,7 +261,7 @@ public final SearchRequest getRequest() {
 
     @Override
     public final SearchResponse buildSearchResponse(InternalSearchResponse internalSearchResponse, String scrollId) {
-        return new SearchResponse(internalSearchResponse, scrollId, results.length(), successfulOps.get(),
+        return new SearchResponse(internalSearchResponse, scrollId, getNumShards(), successfulOps.get(),
             buildTookInMillis(), buildShardFailures());
     }
 
@@ -310,6 +309,5 @@ public final ShardSearchTransportRequest buildShardSearchRequest(ShardIterator s
      *                executed shard request
      * @param context the search context for the next phase
      */
-    protected abstract SearchPhase getNextPhase(AtomicArray<Result> results, SearchPhaseContext context);
-
+    protected abstract SearchPhase getNextPhase(SearchPhaseResults<Result> results, SearchPhaseContext context);
 }

core/src/main/java/org/elasticsearch/action/search/CountedCollector.java

@@ -19,7 +19,6 @@
 package org.elasticsearch.action.search;
 
 import org.elasticsearch.common.Nullable;
-import org.elasticsearch.common.util.concurrent.AtomicArray;
 import org.elasticsearch.common.util.concurrent.CountDown;
 import org.elasticsearch.search.SearchPhaseResult;
 import org.elasticsearch.search.SearchShardTarget;
@@ -30,17 +29,13 @@
  * where the given index is used to set the result on the array.
  */
 final class CountedCollector<R extends SearchPhaseResult> {
-    private final AtomicArray<R> resultArray;
+    private final ResultConsumer<R> resultConsumer;
     private final CountDown counter;
     private final Runnable onFinish;
     private final SearchPhaseContext context;
 
-    CountedCollector(AtomicArray<R> resultArray, int expectedOps, Runnable onFinish, SearchPhaseContext context) {
-        if (expectedOps > resultArray.length()) {
-            throw new IllegalStateException("unexpected number of operations. got: " + expectedOps + " but array size is: "
-                + resultArray.length());
-        }
-        this.resultArray = resultArray;
+    CountedCollector(ResultConsumer<R> resultConsumer, int expectedOps, Runnable onFinish, SearchPhaseContext context) {
+        this.resultConsumer = resultConsumer;
         this.counter = new CountDown(expectedOps);
         this.onFinish = onFinish;
         this.context = context;
@@ -63,7 +58,7 @@ void countDown() {
     void onResult(int index, R result, SearchShardTarget target) {
         try {
             result.shardTarget(target);
-            resultArray.set(index, result);
+            resultConsumer.consume(index, result);
         } finally {
             countDown();
         }
@@ -80,4 +75,12 @@ void onFailure(final int shardIndex, @Nullable SearchShardTarget shardTarget, Ex
             countDown();
         }
     }
+
+    /**
+     * A functional interface to plug in shard result consumers to this collector
+     */
+    @FunctionalInterface
+    public interface ResultConsumer<R extends SearchPhaseResult> {
+        void consume(int shardIndex, R result);
+    }
 }

core/src/main/java/org/elasticsearch/action/search/DfsQueryPhase.java

@@ -40,18 +40,19 @@
  * @see CountedCollector#onFailure(int, SearchShardTarget, Exception)
  */
 final class DfsQueryPhase extends SearchPhase {
-    private final AtomicArray<QuerySearchResultProvider> queryResult;
+    private final InitialSearchPhase.SearchPhaseResults<QuerySearchResultProvider> queryResult;
     private final SearchPhaseController searchPhaseController;
     private final AtomicArray<DfsSearchResult> dfsSearchResults;
-    private final Function<AtomicArray<QuerySearchResultProvider>, SearchPhase> nextPhaseFactory;
+    private final Function<InitialSearchPhase.SearchPhaseResults<QuerySearchResultProvider>, SearchPhase> nextPhaseFactory;
     private final SearchPhaseContext context;
     private final SearchTransportService searchTransportService;
 
     DfsQueryPhase(AtomicArray<DfsSearchResult> dfsSearchResults,
                   SearchPhaseController searchPhaseController,
-                  Function<AtomicArray<QuerySearchResultProvider>, SearchPhase> nextPhaseFactory, SearchPhaseContext context) {
+                  Function<InitialSearchPhase.SearchPhaseResults<QuerySearchResultProvider>, SearchPhase> nextPhaseFactory,
+                  SearchPhaseContext context) {
         super("dfs_query");
-        this.queryResult = new AtomicArray<>(dfsSearchResults.length());
+        this.queryResult = searchPhaseController.newSearchPhaseResults(context.getRequest(), context.getNumShards());
         this.searchPhaseController = searchPhaseController;
         this.dfsSearchResults = dfsSearchResults;
         this.nextPhaseFactory = nextPhaseFactory;
@@ -64,7 +65,8 @@ public void run() throws IOException {
         // TODO we can potentially also consume the actual per shard results from the initial phase here in the aggregateDfs
         // to free up memory early
         final AggregatedDfs dfs = searchPhaseController.aggregateDfs(dfsSearchResults);
-        final CountedCollector<QuerySearchResultProvider> counter = new CountedCollector<>(queryResult, dfsSearchResults.asList().size(),
+        final CountedCollector<QuerySearchResultProvider> counter = new CountedCollector<>(queryResult::consumeResult,
+            dfsSearchResults.asList().size(),
             () -> {
                 context.executeNextPhase(this, nextPhaseFactory.apply(queryResult));
             }, context);

core/src/main/java/org/elasticsearch/action/search/FetchSearchPhase.java

@@ -49,29 +49,31 @@ final class FetchSearchPhase extends SearchPhase {
     private final Function<SearchResponse, SearchPhase> nextPhaseFactory;
     private final SearchPhaseContext context;
     private final Logger logger;
+    private final InitialSearchPhase.SearchPhaseResults<QuerySearchResultProvider> resultConsumer;
 
-    FetchSearchPhase(AtomicArray<QuerySearchResultProvider> queryResults,
+    FetchSearchPhase(InitialSearchPhase.SearchPhaseResults<QuerySearchResultProvider> resultConsumer,
                      SearchPhaseController searchPhaseController,
                      SearchPhaseContext context) {
-        this(queryResults, searchPhaseController, context,
+        this(resultConsumer, searchPhaseController, context,
             (response) -> new ExpandSearchPhase(context, response, // collapse only happens if the request has inner hits
                 (finalResponse) -> sendResponsePhase(finalResponse, context)));
     }
 
-    FetchSearchPhase(AtomicArray<QuerySearchResultProvider> queryResults,
+    FetchSearchPhase(InitialSearchPhase.SearchPhaseResults<QuerySearchResultProvider> resultConsumer,
                      SearchPhaseController searchPhaseController,
                      SearchPhaseContext context, Function<SearchResponse, SearchPhase> nextPhaseFactory) {
         super("fetch");
-        if (context.getNumShards() != queryResults.length()) {
+        if (context.getNumShards() != resultConsumer.getNumShards()) {
             throw new IllegalStateException("number of shards must match the length of the query results but doesn't:"
-                + context.getNumShards() + "!=" + queryResults.length());
+                + context.getNumShards() + "!=" + resultConsumer.getNumShards());
         }
-        this.fetchResults = new AtomicArray<>(queryResults.length());
+        this.fetchResults = new AtomicArray<>(resultConsumer.getNumShards());
         this.searchPhaseController = searchPhaseController;
-        this.queryResults = queryResults;
+        this.queryResults = resultConsumer.results;
         this.nextPhaseFactory =  nextPhaseFactory;
         this.context = context;
         this.logger = context.getLogger();
+        this.resultConsumer = resultConsumer;
 
     }
 
@@ -99,7 +101,7 @@ private void innerRun() throws IOException {
         ScoreDoc[] sortedShardDocs = searchPhaseController.sortDocs(isScrollSearch, queryResults);
         String scrollId = isScrollSearch ? TransportSearchHelper.buildScrollId(queryResults) : null;
         List<AtomicArray.Entry<QuerySearchResultProvider>> queryResultsAsList = queryResults.asList();
-        final SearchPhaseController.ReducedQueryPhase reducedQueryPhase = searchPhaseController.reducedQueryPhase(queryResultsAsList);
+        final SearchPhaseController.ReducedQueryPhase reducedQueryPhase = resultConsumer.reduce();
         final boolean queryAndFetchOptimization = queryResults.length() == 1;
         final Runnable finishPhase = ()
             -> moveToNextPhase(searchPhaseController, sortedShardDocs, scrollId, reducedQueryPhase, queryAndFetchOptimization ?
@@ -119,7 +121,7 @@ private void innerRun() throws IOException {
                 final ScoreDoc[] lastEmittedDocPerShard = isScrollSearch ?
                     searchPhaseController.getLastEmittedDocPerShard(reducedQueryPhase, sortedShardDocs, numShards)
                     : null;
-                final CountedCollector<FetchSearchResult> counter = new CountedCollector<>(fetchResults,
+                final CountedCollector<FetchSearchResult> counter = new CountedCollector<>(fetchResults::set,
                     docIdsToLoad.length, // we count down every shard in the result no matter if we got any results or not
                     finishPhase, context);
                 for (int i = 0; i < docIdsToLoad.length; i++) {

core/src/main/java/org/elasticsearch/action/search/InitialSearchPhase.java

@@ -28,12 +28,14 @@
 import org.elasticsearch.cluster.routing.ShardIterator;
 import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.common.Nullable;
+import org.elasticsearch.common.util.concurrent.AtomicArray;
 import org.elasticsearch.search.SearchPhaseResult;
 import org.elasticsearch.search.SearchShardTarget;
 import org.elasticsearch.transport.ConnectTransportException;
 
 import java.io.IOException;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.Stream;
 
 /**
  * This is an abstract base class that encapsulates the logic to fan out to all shards in provided {@link GroupShardsIterator}
@@ -213,4 +215,53 @@ private void onShardResult(int shardIndex, String nodeId, FirstResult result, Sh
      * @param listener the listener to notify on response
      */
     protected abstract void executePhaseOnShard(ShardIterator shardIt, ShardRouting shard, ActionListener<FirstResult> listener);
+
+    /**
+     * This class acts as a basic result collection that can be extended to do on-the-fly reduction or result processing
+     */
+    static class SearchPhaseResults<Result extends SearchPhaseResult> {
+        final AtomicArray<Result> results;
+
+        SearchPhaseResults(int size) {
+            results = new AtomicArray<>(size);
+        }
+
+        /**
+         * Returns the number of expected results this class should collect
+         */
+        final int getNumShards() {
+            return results.length();
+        }
+
+        /**
+         * A stream of all non-null (successful) shard results
+         */
+        final Stream<Result> getSuccessfulResults() {
+            return results.asList().stream().map(e -> e.value);
+        }
+
+        /**
+         * Consumes a single shard result
+         * @param shardIndex the shards index, this is a 0-based id that is used to establish a 1 to 1 mapping to the searched shards
+         * @param result the shards result
+         */
+        void consumeResult(int shardIndex, Result result) {
+            assert results.get(shardIndex) == null : "shardIndex: " + shardIndex + " is already set";
+            results.set(shardIndex, result);
+        }
+
+        /**
+         * Returns <code>true</code> iff a result if present for the given shard ID.
+         */
+        final boolean hasResult(int shardIndex) {
+            return results.get(shardIndex) != null;
+        }
+
+        /**
+         * Reduces the collected results
+         */
+        SearchPhaseController.ReducedQueryPhase reduce() {
+            throw new UnsupportedOperationException("reduce is not supported");
+        }
+    }
 }

core/src/main/java/org/elasticsearch/action/search/SearchDfsQueryThenFetchAsyncAction.java

@@ -24,7 +24,6 @@
 import org.elasticsearch.cluster.routing.GroupShardsIterator;
 import org.elasticsearch.cluster.routing.ShardIterator;
 import org.elasticsearch.cluster.routing.ShardRouting;
-import org.elasticsearch.common.util.concurrent.AtomicArray;
 import org.elasticsearch.search.dfs.DfsSearchResult;
 import org.elasticsearch.search.internal.AliasFilter;
 import org.elasticsearch.transport.Transport;
@@ -43,7 +42,7 @@ final class SearchDfsQueryThenFetchAsyncAction extends AbstractSearchAsyncAction
                                        ActionListener<SearchResponse> listener, GroupShardsIterator shardsIts, long startTime,
                                        long clusterStateVersion, SearchTask task) {
         super("dfs", logger, searchTransportService, nodeIdToConnection, aliasFilter, concreteIndexBoosts, executor,
-                request, listener, shardsIts, startTime, clusterStateVersion, task);
+                request, listener, shardsIts, startTime, clusterStateVersion, task, new SearchPhaseResults<>(shardsIts.size()));
         this.searchPhaseController = searchPhaseController;
     }
 
@@ -54,8 +53,8 @@ protected void executePhaseOnShard(ShardIterator shardIt, ShardRouting shard, Ac
     }
 
     @Override
-    protected SearchPhase getNextPhase(AtomicArray<DfsSearchResult> results, SearchPhaseContext context) {
-        return new DfsQueryPhase(results, searchPhaseController,
+    protected SearchPhase getNextPhase(SearchPhaseResults<DfsSearchResult> results, SearchPhaseContext context) {
+        return new DfsQueryPhase(results.results, searchPhaseController,
             (queryResults) -> new FetchSearchPhase(queryResults, searchPhaseController, context), context);
     }
 }

core/src/main/java/org/elasticsearch/action/search/SearchPhaseContext.java

@@ -114,4 +114,5 @@ default void sendReleaseSearchContext(long contextId, Transport.Connection conne
      * a response is returned to the user indicating that all shards have failed.
      */
     void executeNextPhase(SearchPhase currentPhase, SearchPhase nextPhase);
+
 }