SERVER-45437 Add readHedgingMode serverParameter

 create mode 100644 src/mongo/s/mongos_server_parameters.cpp
 create mode 100644 src/mongo/s/mongos_server_parameters.h

7 files changed, 137 insertions, 5 deletions

diff --git a/jstests/sharding/hedged_reads_server_parameters.js b/jstests/sharding/hedged_reads_server_parameters.js
index 3c5dd089171..dab38ab6013 100644
--- a/jstests/sharding/hedged_reads_server_parameters.js
+++ b/jstests/sharding/hedged_reads_server_parameters.js
@@ -34,5 +34,16 @@ st.restartMongos(0, {
 assert.commandWorked(st.s.getDB(dbName).runCommand(
     {query: {count: collName}, $readPreference: {mode: "secondaryPreferred", hedge: {}}}));
 
+// readHedgingMode "off".
+st.restartMongos(0, {
+    restart: true,
+    setParameter: {readHedgingMode: "off"},
+});
+
+assert.commandWorked(st.s.getDB(dbName).runCommand({
+    query: {distinct: collName, key: "x"},
+    $readPreference: {mode: "primaryPreferred", hedge: {}}
+}));
+
 st.stop();
 })();
diff --git a/src/mongo/s/SConscript b/src/mongo/s/SConscript
index ab5e0984fb6..55a58284272 100644
--- a/src/mongo/s/SConscript
+++ b/src/mongo/s/SConscript
@@ -413,6 +413,7 @@ env.Library(
 env.Library(
     target='mongos_server_parameters',
     source=[
+        'mongos_server_parameters.cpp',
         env.Idlc('mongos_server_parameters.idl')[0],
     ],
     LIBDEPS_PRIVATE=[
diff --git a/src/mongo/s/hedge_options_util.cpp b/src/mongo/s/hedge_options_util.cpp
index 2de268c9e79..9301a416bcf 100644
--- a/src/mongo/s/hedge_options_util.cpp
+++ b/src/mongo/s/hedge_options_util.cpp
@@ -39,7 +39,7 @@ boost::optional<executor::RemoteCommandRequestOnAny::HedgeOptions> extractHedgeO
     OperationContext* opCtx, const BSONObj& cmdObj) {
     const auto hedgingMode = ReadPreferenceSetting::get(opCtx).hedgingMode;
 
-    if (hedgingMode && hedgingMode->getEnabled()) {
+    if (gReadHedgingMode == kReadHedgingModeOn && hedgingMode && hedgingMode->getEnabled()) {
         boost::optional<int> maxTimeMS;
         if (auto cmdOptionMaxTimeMSField = cmdObj[QueryRequest::cmdOptionMaxTimeMS]) {
             maxTimeMS = uassertStatusOK(QueryRequest::parseMaxTimeMS(cmdOptionMaxTimeMSField));
diff --git a/src/mongo/s/hedge_options_util_test.cpp b/src/mongo/s/hedge_options_util_test.cpp
index 7aae8afadfe..1fd54e39082 100644
--- a/src/mongo/s/hedge_options_util_test.cpp
+++ b/src/mongo/s/hedge_options_util_test.cpp
@@ -104,16 +104,17 @@ protected:
 
     static inline const std::string kCollName = "testColl";
 
+    static inline const std::string kReadHedgingModeFieldName = "readHedgingMode";
     static inline const std::string kMaxTimeMSThresholdForHedgingFieldName =
         "maxTimeMSThresholdForHedging";
     static inline const std::string kHedgingDelayPercentageFieldName = "hedgingDelayPercentage";
     static inline const std::string kDefaultHedgingDelayMSFieldName = "defaultHedgingDelayMS";
 
     static inline const BSONObj kDefaultParameters =
-        BSON(kMaxTimeMSThresholdForHedgingFieldName
-             << kMaxTimeMSThresholdForHedgingDefault << kHedgingDelayPercentageFieldName
-             << kHedgingDelayPercentageDefault << kDefaultHedgingDelayMSFieldName
-             << kDefaultHedgingDelayMSDefault);
+        BSON(kReadHedgingModeFieldName
+             << "on" << kMaxTimeMSThresholdForHedgingFieldName << gMaxTimeMSThresholdForHedging
+             << kHedgingDelayPercentageFieldName << gHedgingDelayPercentage
+             << kDefaultHedgingDelayMSFieldName << gDefaultHedgingDelayMS);
 
 private:
     ServiceContext::UniqueServiceContext _serviceCtx = ServiceContext::make();
@@ -225,5 +226,18 @@ TEST_F(HedgeOptionsUtilTestFixture, SetAll) {
     checkHedgeOptions(parameters, BSON("find" << kCollName), rspObj, Milliseconds{800});
 }
 
+TEST_F(HedgeOptionsUtilTestFixture, ReadHedgingModeOff) {
+    const auto parameters =
+        BSON(kReadHedgingModeFieldName << "off" << kMaxTimeMSThresholdForHedgingFieldName << 1000
+                                       << kHedgingDelayPercentageFieldName << 50
+                                       << kDefaultHedgingDelayMSFieldName << 800);
+    const auto rspObj = BSON("mode"
+                             << "nearest"
+                             << "hedge" << BSONObj());
+
+    checkHedgeOptions(parameters, BSON("find" << kCollName << "maxTimeMS" << 500), rspObj);
+    checkHedgeOptions(parameters, BSON("find" << kCollName), rspObj);
+}
+
 }  // namespace
 }  // namespace mongo
diff --git a/src/mongo/s/mongos_server_parameters.cpp b/src/mongo/s/mongos_server_parameters.cpp
new file mode 100644
index 00000000000..de9d5507598
--- /dev/null
+++ b/src/mongo/s/mongos_server_parameters.cpp
@@ -0,0 +1,48 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    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
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    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 Server Side 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/s/mongos_server_parameters.h"
+
+#include "mongo/util/str.h"
+
+namespace mongo {
+
+/**
+ * Validation callback for setParameter 'readHedgingMode'.
+ */
+Status validateReadHedgingMode(const std::string& mode) {
+    if (mode != kReadHedgingModeOn && mode != kReadHedgingModeOff) {
+        return {ErrorCodes::BadValue,
+                str::stream() << "readHedgingMode must be either \"" << kReadHedgingModeOn
+                              << "\" or \"" << kReadHedgingModeOff << "\""};
+    }
+    return Status::OK();
+}
+
+}  // namespace mongo
diff --git a/src/mongo/s/mongos_server_parameters.h b/src/mongo/s/mongos_server_parameters.h
new file mode 100644
index 00000000000..9c2a580278b
--- /dev/null
+++ b/src/mongo/s/mongos_server_parameters.h
@@ -0,0 +1,46 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    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
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    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 Server Side 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 "mongo/platform/basic.h"
+
+#include "mongo/base/status.h"
+
+namespace mongo {
+
+constexpr auto kReadHedgingModeOn = "on";
+constexpr auto kReadHedgingModeOff = "off";
+
+/**
+ * Validation callback for setParameter 'readHedgingMode'.
+ */
+Status validateReadHedgingMode(const std::string& mode);
+
+}  // namespace mongo
diff --git a/src/mongo/s/mongos_server_parameters.idl b/src/mongo/s/mongos_server_parameters.idl
index 1724d0e13ba..aaac5f4d2dc 100644
--- a/src/mongo/s/mongos_server_parameters.idl
+++ b/src/mongo/s/mongos_server_parameters.idl
@@ -29,6 +29,8 @@
 
 global:
   cpp_namespace: "mongo"
+  cpp_includes:
+    - "mongo/s/mongos_server_parameters.h"
 
 imports:
     - "mongo/idl/basic_types.idl"
@@ -58,6 +60,16 @@ server_parameters:
     cpp_varname: "gWarmMinConnectionsInShardingTaskExecutorPoolOnStartupWaitMS"
     default: 2000 # 2secs
 
+  readHedgingMode:
+    description: >-
+        Enables hedged reads.
+    set_at: [ startup ]
+    cpp_vartype: std::string
+    cpp_varname: "gReadHedgingMode"
+    default: "on"
+    validator:
+      callback: "validateReadHedgingMode"
+
   maxTimeMSThresholdForHedging:
     description: >-
         The upper threshold for the expected running time of an operation (maxTimeMS)