Soft limit added for max number of search contexts

server/src/main/java/org/elasticsearch/common/settings/ClusterSettings.java

@@ -365,6 +365,7 @@ public void apply(Settings value, Settings current, Settings previous) {
                     SearchService.MAX_KEEPALIVE_SETTING,
                     MultiBucketConsumerService.MAX_BUCKET_SETTING,
                     SearchService.LOW_LEVEL_CANCELLATION_SETTING,
+                    SearchService.MAX_OPEN_CONTEXT,
                     Node.WRITE_PORTS_FILE_SETTING,
                     Node.NODE_NAME_SETTING,
                     Node.NODE_DATA_SETTING,

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

@@ -136,6 +136,14 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
             Setting.boolSetting("search.default_allow_partial_results", true, Property.Dynamic, Property.NodeScope);
 
 
+    /**
+     * A setting describing the maximum number of search contexts that can be created. The default
+     * maximum of 100 is defensive to prevent generating too many search contexts.
+     */
+    public static final Setting<Integer> MAX_OPEN_CONTEXT =
+        Setting.intSetting("search.max_open_context", 100, 0, Property.NodeScope);
+
+
     private final ThreadPool threadPool;
 
     private final ClusterService clusterService;
@@ -170,6 +178,8 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
 
     private final ConcurrentMapLong<SearchContext> activeContexts = ConcurrentCollections.newConcurrentMapLongWithAggressiveConcurrency();
 
+    private int numActiveContexts = 0;
+
     private final MultiBucketConsumerService multiBucketConsumerService;
 
     public SearchService(ClusterService clusterService, IndicesService indicesService,
@@ -547,6 +557,13 @@ private SearchContext findContext(long id, TransportRequest request) throws Sear
     }
 
     final SearchContext createAndPutContext(ShardSearchRequest request) throws IOException {
+        if (numActiveContexts >= MAX_OPEN_CONTEXT.get(settings)) {
+            throw new ElasticsearchException(
+                "Trying to create too many search contexts. Must be less than or equal to: [" +
+                    MAX_OPEN_CONTEXT.get(settings) + "]. This limit can be set by changing the ["
+                    + MAX_OPEN_CONTEXT.getKey() + "] setting.");
+        }
+
         SearchContext context = createContext(request);
         boolean success = false;
         try {
@@ -556,6 +573,7 @@ final SearchContext createAndPutContext(ShardSearchRequest request) throws IOExc
             }
             context.indexShard().getSearchOperationListener().onNewContext(context);
             success = true;
+            numActiveContexts++;
             return context;
         } finally {
             if (!success) {
@@ -566,6 +584,7 @@ final SearchContext createAndPutContext(ShardSearchRequest request) throws IOExc
 
     final SearchContext createContext(ShardSearchRequest request) throws IOException {
         final DefaultSearchContext context = createSearchContext(request, defaultSearchTimeout);
+
         try {
             if (request.scroll() != null) {
                 context.scrollContext(new ScrollContext());
@@ -657,6 +676,7 @@ public boolean freeContext(long id) {
             } finally {
                 context.close();
             }
+            numActiveContexts--;
             return true;
         }
         return false;

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

@@ -22,6 +22,7 @@
 
 import org.apache.lucene.search.Query;
 import org.apache.lucene.store.AlreadyClosedException;
+import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.index.IndexResponse;
 import org.elasticsearch.action.search.SearchPhaseExecutionException;
@@ -343,6 +344,42 @@ searchSourceBuilder, new String[0], false, new AliasFilter(null, Strings.EMPTY_A
         }
     }
 
+
+    /**
+     * test that creating more than the allowed number of search contexts throws an exception
+     */
+    public void testMaxOpenContexts() throws IOException {
+        createIndex("index");
+        final SearchService service = getInstanceFromNode(SearchService.class);
+        final IndicesService indicesService = getInstanceFromNode(IndicesService.class);
+        final IndexService indexService = indicesService.indexServiceSafe(resolveIndex("index"));
+        final IndexShard indexShard = indexService.getShard(0);
+
+        for (int i = 0; i < service.MAX_OPEN_CONTEXT.get(Settings.EMPTY); i++) {
+            SearchContext context = service.createAndPutContext(
+                new ShardSearchLocalRequest(
+                    indexShard.shardId(),
+                    1,
+                    SearchType.DEFAULT,
+                    new SearchSourceBuilder(),
+                    new String[0],
+                    false,
+                    new AliasFilter(null, Strings.EMPTY_ARRAY),
+                    1.0f, true)
+            );
+        }
+
+        ElasticsearchException ex = expectThrows(ElasticsearchException.class,
+            () -> service.createAndPutContext(new ShardSearchLocalRequest(indexShard.shardId(), 1, SearchType.DEFAULT,
+                new SearchSourceBuilder(), new String[0], false, new AliasFilter(null, Strings.EMPTY_ARRAY), 1.0f, true)));
+        assertEquals(
+            "Trying to create too many search contexts. Must be less than or equal to: [" +
+                service.MAX_OPEN_CONTEXT.get(Settings.EMPTY) + "]. " +
+                "This limit can be set by changing the [search.max_open_context] setting.",
+            ex.getMessage());
+    }
+
+
     public static class FailOnRewriteQueryPlugin extends Plugin implements SearchPlugin {
         @Override
         public List<QuerySpec<?>> getQueries() {