4 files changed, 83 insertions, 1 deletions
diff --git a/src/mongo/s/commands/cluster_pipeline_cmd.cpp b/src/mongo/s/commands/cluster_pipeline_cmd.cpp
index 48544a41c3d..16ef0b05a55 100644
--- a/src/mongo/s/commands/cluster_pipeline_cmd.cpp
+++ b/src/mongo/s/commands/cluster_pipeline_cmd.cpp
@@ -51,6 +51,7 @@
 #include "mongo/s/commands/cluster_commands_common.h"
 #include "mongo/s/config.h"
 #include "mongo/s/grid.h"
+#include "mongo/s/query/cluster_query_knobs.h"
 #include "mongo/s/query/store_possible_cursor.h"
 #include "mongo/s/stale_exception.h"
 #include "mongo/util/log.h"
@@ -237,7 +238,8 @@ public:
         // Run merging command on random shard, unless a stage needs the primary shard. Need to use
         // ShardConnection so that the merging mongod is sent the config servers on connection init.
         auto& prng = txn->getClient()->getPrng();
-        const auto& mergingShardId = needPrimaryShardMerger
+        const auto& mergingShardId =
+            (needPrimaryShardMerger || internalQueryAlwaysMergeOnPrimaryShard)
             ? conf->getPrimaryId()
             : shardResults[prng.nextInt32(shardResults.size())].shardTargetId;
         const auto mergingShard = grid.shardRegistry()->getShard(txn, mergingShardId);
diff --git a/src/mongo/s/query/SConscript b/src/mongo/s/query/SConscript
index 18022507aa2..fd27f991f01 100644
--- a/src/mongo/s/query/SConscript
+++ b/src/mongo/s/query/SConscript
@@ -6,6 +6,7 @@ env.Library(
     target="cluster_query",
     source=[
         "cluster_find.cpp",
+        "cluster_query_knobs.cpp",
     ],
     LIBDEPS=[
         '$BUILD_DIR/mongo/db/commands',
diff --git a/src/mongo/s/query/cluster_query_knobs.cpp b/src/mongo/s/query/cluster_query_knobs.cpp
new file mode 100644
index 00000000000..76e82b4f914
--- /dev/null
+++ b/src/mongo/s/query/cluster_query_knobs.cpp
@@ -0,0 +1,39 @@
+/**
+ * Copyright (C) 2016 MongoDB Inc.
+ *
+ * This program is free software: you can redistribute it and/or  modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * As a special exception, the copyright holders give permission to link the
+ * code of portions of this program with the OpenSSL library under certain
+ * conditions as described in each individual source file and distribute
+ * linked combinations including the program with the OpenSSL library. You
+ * must comply with the GNU Affero General Public License in all respects
+ * for all of the code used other than as permitted herein. If you modify
+ * file(s) with this exception, you may extend this exception to your
+ * version of the file(s), but you are not obligated to do so. If you do not
+ * wish to do so, delete this exception statement from your version. If you
+ * delete this exception statement from all source files in the program,
+ * then also delete it in the license file.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/s/query/cluster_query_knobs.h"
+
+#include "mongo/db/server_parameters.h"
+
+namespace mongo {
+
+MONGO_EXPORT_SERVER_PARAMETER(internalQueryAlwaysMergeOnPrimaryShard, bool, false);
+
+}  // namespace mongo
diff --git a/src/mongo/s/query/cluster_query_knobs.h b/src/mongo/s/query/cluster_query_knobs.h
new file mode 100644
index 00000000000..76df18fde4d
--- /dev/null
+++ b/src/mongo/s/query/cluster_query_knobs.h
@@ -0,0 +1,40 @@
+/**
+ * Copyright (C) 2016 MongoDB Inc.
+ *
+ * This program is free software: you can redistribute it and/or  modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * As a special exception, the copyright holders give permission to link the
+ * code of portions of this program with the OpenSSL library under certain
+ * conditions as described in each individual source file and distribute
+ * linked combinations including the program with the OpenSSL library. You
+ * must comply with the GNU Affero General Public License in all respects
+ * for all of the code used other than as permitted herein. If you modify
+ * file(s) with this exception, you may extend this exception to your
+ * version of the file(s), but you are not obligated to do so. If you do not
+ * wish to do so, delete this exception statement from your version. If you
+ * delete this exception statement from all source files in the program,
+ * then also delete it in the license file.
+ */
+
+#pragma once
+
+#include <atomic>
+
+namespace mongo {
+
+// If set to true on mongos, all aggregations delivered to the mongos which require a merging shard
+// will select the primary shard as the merger. False by default, which means that the merging shard
+// will be selected randomly amongst the shards participating in the query.
+extern std::atomic<bool> internalQueryAlwaysMergeOnPrimaryShard;  // NOLINT
+
+}  // namespace mongo