SERVER-43720 Add default read write concern commands to mongos and config server

14 files changed, 574 insertions, 26 deletions

diff --git a/jstests/auth/lib/commands_lib.js b/jstests/auth/lib/commands_lib.js
index 91eeaefe1da..9c5485aeafa 100644
--- a/jstests/auth/lib/commands_lib.js
+++ b/jstests/auth/lib/commands_lib.js
@@ -3756,6 +3756,16 @@ var authCommandsLib = {
           ]
         },
         {
+          testname: "getDefaultRWConcern",
+          command: {getDefaultRWConcern: 1},
+          skipUnlessReplicaSet: true,
+          testcases: [
+              {runOnDb: adminDbName, roles: roles_all, privileges: []},
+              {runOnDb: firstDbName, roles: {}},
+              {runOnDb: secondDbName, roles: {}}
+          ]
+        },
+        {
           testname: "getDiagnosticData",
           command: {getDiagnosticData: 1},
           testcases: [
@@ -5094,6 +5104,16 @@ var authCommandsLib = {
           ]
         },
         {
+          testname: "setDefaultRWConcern",
+          command: {setDefaultRWConcern: 1, defaultReadConcern: {level: "local"}},
+          skipUnlessReplicaSet: true,
+          testcases: [
+              {runOnDb: adminDbName, roles: roles_all, privileges: []},
+              {runOnDb: firstDbName, roles: {}},
+              {runOnDb: secondDbName, roles: {}}
+          ]
+        },
+        {
           testname: "setFeatureCompatibilityVersion",
           command: {setFeatureCompatibilityVersion: "x"},
           testcases: [
diff --git a/jstests/sharding/read_write_concern_defaults_commands_api.js b/jstests/sharding/read_write_concern_defaults_commands_api.js
new file mode 100644
index 00000000000..984443b5842
--- /dev/null
+++ b/jstests/sharding/read_write_concern_defaults_commands_api.js
@@ -0,0 +1,326 @@
+// Tests the basic API of the getDefaultRWConcern and setDefaultRWConcern commands against different
+// topologies.
+//
+// @tags: [requires_fcv_44]
+(function() {
+"use strict";
+
+// Asserts a set/get default RWC command response contains the expected fields. Assumes a default
+// read or write concern has been set previously.
+function verifyResponseFields(res, {expectRC, expectWC}) {
+    // These fields are always set once a read or write concern has been set at least once.
+    const expectedFields = ["epoch", "setTime", "localSetTime"];
+    const unexpectedFields = [];
+
+    if (expectRC) {
+        expectedFields.push("defaultReadConcern");
+    } else {
+        unexpectedFields.push("defaultReadConcern");
+    }
+
+    if (expectWC) {
+        expectedFields.push("defaultWriteConcern");
+    } else {
+        unexpectedFields.push("defaultWriteConcern");
+    }
+
+    assert.hasFields(res, expectedFields);
+    unexpectedFields.forEach(field => {
+        assert(!res.hasOwnProperty(field),
+               `response unexpectedly had field '${field}', res: ${tojson(res)}`);
+    });
+}
+
+function verifyDefaultRWCommandsInvalidInput(conn) {
+    //
+    // Test invalid parameters for getDefaultRWConcern.
+    //
+
+    // Invalid inMemory.
+    assert.commandFailedWithCode(conn.adminCommand({getDefaultRWConcern: 1, inMemory: "true"}),
+                                 ErrorCodes.TypeMismatch);
+
+    //
+    // Test invalid parameters for setDefaultRWConcern.
+    //
+
+    // Must include either wc or rc.
+    assert.commandFailedWithCode(conn.adminCommand({setDefaultRWConcern: 1}), ErrorCodes.BadValue);
+
+    // Invalid write concern.
+    assert.commandFailedWithCode(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultWriteConcern: 1}),
+        ErrorCodes.TypeMismatch);
+
+    // w less than 1.
+    assert.commandFailedWithCode(conn.adminCommand({
+        setDefaultRWConcern: 1,
+        defaultWriteConcern: {w: 0},
+    }),
+                                 ErrorCodes.BadValue);
+
+    // Invalid read concern.
+    assert.commandFailedWithCode(conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: 1}),
+                                 ErrorCodes.TypeMismatch);
+
+    // Non-existent level.
+    assert.commandFailedWithCode(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: {level: "dummy"}}),
+        ErrorCodes.FailedToParse);
+
+    // Unsupported level.
+    assert.commandFailedWithCode(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: {level: "linearizable"}}),
+        ErrorCodes.BadValue);
+    assert.commandFailedWithCode(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: {level: "snapshot"}}),
+        ErrorCodes.BadValue);
+
+    // Fields other than level.
+    assert.commandFailedWithCode(conn.adminCommand({
+        setDefaultRWConcern: 1,
+        defaultReadConcern: {level: "local", afterClusterTime: Timestamp(50, 1)}
+    }),
+                                 ErrorCodes.BadValue);
+    assert.commandFailedWithCode(conn.adminCommand({
+        setDefaultRWConcern: 1,
+        defaultReadConcern: {level: "snapshot", atClusterTime: Timestamp(50, 1)}
+    }),
+                                 ErrorCodes.BadValue);
+    assert.commandFailedWithCode(conn.adminCommand({
+        setDefaultRWConcern: 1,
+        defaultReadConcern: {level: "local", afterOpTime: {ts: Timestamp(50, 1), t: 1}}
+    }),
+                                 ErrorCodes.BadValue);
+}
+
+// Sets a default read and write concern.
+function setDefaultRWConcern(conn) {
+    assert.commandWorked(conn.adminCommand({
+        setDefaultRWConcern: 1,
+        defaultReadConcern: {level: "local"},
+        defaultWriteConcern: {w: 1}
+    }));
+}
+
+// Unsets the default read and write concerns.
+function unsetDefaultRWConcern(conn) {
+    assert.commandWorked(conn.adminCommand(
+        {setDefaultRWConcern: 1, defaultReadConcern: {}, defaultWriteConcern: {}}));
+}
+
+// Verifies no fields are returned if neither a default read nor write concern has been set.
+function verifyDefaultResponses(conn) {
+    const res = assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1}));
+    const inMemoryRes =
+        assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1, inMemory: true}));
+
+    const unexpectedFields =
+        ["defaultReadConcern", "defaultWriteConcern", "epoch", "setTime", "localSetTime"];
+    unexpectedFields.forEach(field => {
+        assert(!res.hasOwnProperty(field),
+               `response unexpectedly had field '${field}', res: ${tojson(res)}`);
+        assert(!inMemoryRes.hasOwnProperty(field),
+               `inMemory=true response unexpectedly had field '${field}', res: ${tojson(res)}`);
+    });
+}
+
+function verifyDefaultRWCommandsValidInput(conn) {
+    //
+    // Test parameters for getDefaultRWConcern.
+    //
+
+    // No parameters is allowed.
+    assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1}));
+
+    // inMemory parameter is allowed.
+    assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1, inMemory: true}));
+    assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1, inMemory: false}));
+
+    //
+    // Test parameters for setDefaultRWConcern.
+    //
+
+    // Setting only rc is allowed.
+    assert.commandWorked(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: {level: "local"}}));
+    assert.commandWorked(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: {level: "majority"}}));
+
+    // Setting only wc is allowed.
+    assert.commandWorked(conn.adminCommand({setDefaultRWConcern: 1, defaultWriteConcern: {w: 1}}));
+    assert.commandWorked(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultWriteConcern: {w: 1, j: false}}));
+    assert.commandWorked(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultWriteConcern: {w: "majority"}}));
+
+    // Setting both wc and rc is allowed.
+    assert.commandWorked(conn.adminCommand({
+        setDefaultRWConcern: 1,
+        defaultWriteConcern: {w: 1},
+        defaultReadConcern: {level: "local"}
+    }));
+
+    // Empty write concern is allowed.
+    assert.commandWorked(conn.adminCommand({setDefaultRWConcern: 1, defaultWriteConcern: {}}));
+
+    // Empty read concern is allowed.
+    assert.commandWorked(conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: {}}));
+}
+
+function verifyDefaultRWCommandsSuccessfulResponses(conn) {
+    //
+    // Test responses for getDefaultRWConcern.
+    //
+
+    // When neither read nor write concern is set.
+    unsetDefaultRWConcern(conn);
+    verifyResponseFields(assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1})),
+                         {expectRC: false, expectWC: false});
+    verifyResponseFields(
+        assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1, inMemory: true})),
+        {expectRC: false, expectWC: false});
+
+    // When only read concern is set.
+    assert.commandWorked(conn.adminCommand(
+        {setDefaultRWConcern: 1, defaultReadConcern: {level: "local"}, defaultWriteConcern: {}}));
+    verifyResponseFields(assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1})),
+                         {expectRC: true, expectWC: false});
+    verifyResponseFields(
+        assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1, inMemory: true})),
+        {expectRC: true, expectWC: false});
+
+    // When only write concern is set.
+    assert.commandWorked(conn.adminCommand(
+        {setDefaultRWConcern: 1, defaultReadConcern: {}, defaultWriteConcern: {w: 1}}));
+    verifyResponseFields(assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1})),
+                         {expectRC: false, expectWC: true});
+    verifyResponseFields(
+        assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1, inMemory: true})),
+        {expectRC: false, expectWC: true});
+
+    // When both read and write concern are set.
+    assert.commandWorked(conn.adminCommand({
+        setDefaultRWConcern: 1,
+        defaultReadConcern: {level: "local"},
+        defaultWriteConcern: {w: 1}
+    }));
+    verifyResponseFields(assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1})),
+                         {expectRC: true, expectWC: true});
+    verifyResponseFields(
+        assert.commandWorked(conn.adminCommand({getDefaultRWConcern: 1, inMemory: true})),
+        {expectRC: true, expectWC: true});
+
+    //
+    // Test responses for setDefaultRWConcern.
+    //
+
+    // When unsetting both read and write concern.
+    setDefaultRWConcern(conn);
+    verifyResponseFields(
+        assert.commandWorked(conn.adminCommand(
+            {setDefaultRWConcern: 1, defaultReadConcern: {}, defaultWriteConcern: {}})),
+        {expectRC: false, expectWC: false});
+
+    // When unsetting only read concern.
+    setDefaultRWConcern(conn);
+    verifyResponseFields(
+        assert.commandWorked(conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: {}})),
+        {expectRC: false, expectWC: true});
+
+    // When unsetting only write concern.
+    setDefaultRWConcern(conn);
+    verifyResponseFields(
+        assert.commandWorked(conn.adminCommand({setDefaultRWConcern: 1, defaultWriteConcern: {}})),
+        {expectRC: true, expectWC: false});
+
+    // When setting only write concern.
+    unsetDefaultRWConcern(conn);
+    verifyResponseFields(assert.commandWorked(conn.adminCommand(
+                             {setDefaultRWConcern: 1, defaultWriteConcern: {w: 1}})),
+                         {expectRC: false, expectWC: true});
+
+    // When setting only read concern.
+    unsetDefaultRWConcern(conn);
+    verifyResponseFields(assert.commandWorked(conn.adminCommand(
+                             {setDefaultRWConcern: 1, defaultReadConcern: {level: "local"}})),
+                         {expectRC: true, expectWC: false});
+
+    // When setting both read and write concern.
+    unsetDefaultRWConcern(conn);
+    verifyResponseFields(assert.commandWorked(conn.adminCommand({
+        setDefaultRWConcern: 1,
+        defaultReadConcern: {level: "local"},
+        defaultWriteConcern: {w: 1}
+    })),
+                         {expectRC: true, expectWC: true});
+}
+
+// Verifies the error code returned by connections to nodes that do not support the get/set default
+// rw concern commands.
+function verifyDefaultRWCommandsFailWithCode(conn, {failureCode}) {
+    assert.commandFailedWithCode(conn.adminCommand({getDefaultRWConcern: 1}), failureCode);
+    assert.commandFailedWithCode(
+        conn.adminCommand({setDefaultRWConcern: 1, defaultReadConcern: {level: "local"}}),
+        failureCode);
+}
+
+jsTestLog("Testing standalone mongod...");
+{
+    const standalone = MongoRunner.runMongod();
+
+    // Standalone node fails.
+    verifyDefaultRWCommandsFailWithCode(standalone, {failureCode: 51300});
+
+    MongoRunner.stopMongod(standalone);
+}
+
+jsTestLog("Testing standalone replica set...");
+{
+    const rst = new ReplSetTest({nodes: 2});
+    rst.startSet();
+    rst.initiate();
+
+    // Primary succeeds.
+    verifyDefaultResponses(rst.getPrimary());
+    verifyDefaultRWCommandsValidInput(rst.getPrimary());
+    verifyDefaultRWCommandsInvalidInput(rst.getPrimary());
+    verifyDefaultRWCommandsSuccessfulResponses(rst.getPrimary());
+
+    // Secondary succeeds.
+    assert.commandWorked(rst.getSecondary().adminCommand({getDefaultRWConcern: 1}));
+    // TODO SERVER-44890 Assert setDefaultRWConcern fails with NotMaster instead.
+    assert.commandWorked(rst.getSecondary().adminCommand(
+        {setDefaultRWConcern: 1, defaultReadConcern: {level: "local"}}));
+
+    rst.stopSet();
+}
+
+jsTestLog("Testing sharded cluster...");
+{
+    const st = new ShardingTest({shards: 1, rs: {nodes: 2}});
+
+    // Mongos succeeds.
+    verifyDefaultResponses(st.s);
+    verifyDefaultRWCommandsValidInput(st.s);
+    verifyDefaultRWCommandsInvalidInput(st.s);
+    verifyDefaultRWCommandsSuccessfulResponses(st.s);
+
+    // Shard node fails.
+    verifyDefaultRWCommandsFailWithCode(st.rs0.getPrimary(), {failureCode: 51301});
+    verifyDefaultRWCommandsFailWithCode(st.rs0.getSecondary(), {failureCode: 51301});
+
+    // Config server primary succeeds.
+    verifyDefaultRWCommandsValidInput(st.configRS.getPrimary());
+    verifyDefaultRWCommandsInvalidInput(st.configRS.getPrimary());
+    verifyDefaultRWCommandsSuccessfulResponses(st.configRS.getPrimary());
+
+    // Config server secondary succeeds.
+    assert.commandWorked(st.configRS.getSecondary().adminCommand({getDefaultRWConcern: 1}));
+    // TODO SERVER-44890 Assert setDefaultRWConcern fails instead.
+    assert.commandWorked(st.configRS.getSecondary().adminCommand(
+        {setDefaultRWConcern: 1, defaultReadConcern: {level: "local"}}));
+
+    st.stop();
+}
+})();
diff --git a/src/mongo/db/commands/SConscript b/src/mongo/db/commands/SConscript
index 1e1b927a553..f309978cb67 100644
--- a/src/mongo/db/commands/SConscript
+++ b/src/mongo/db/commands/SConscript
@@ -142,7 +142,6 @@ env.Library(
         'logical_session_server_status_section.cpp',
         'mr_common.cpp',
         'reap_logical_session_cache_now.cpp',
-        'rwc_defaults_commands.cpp',
         'traffic_recording_cmds.cpp',
         'user_management_commands_common.cpp',
         env.Idlc('drop_connections.idl')[0],
@@ -369,6 +368,7 @@ env.Library(
         "oplog_note.cpp",
         "resize_oplog.cpp",
         "restart_catalog_command.cpp",
+        'rwc_defaults_commands.cpp',
         "set_feature_compatibility_version_command.cpp",
         "set_index_commit_quorum_command.cpp",
         "shutdown_d.cpp",
diff --git a/src/mongo/db/commands/rwc_defaults_commands.cpp b/src/mongo/db/commands/rwc_defaults_commands.cpp
index e251d4dfdc5..b6fb559c9a8 100644
--- a/src/mongo/db/commands/rwc_defaults_commands.cpp
+++ b/src/mongo/db/commands/rwc_defaults_commands.cpp
@@ -34,11 +34,24 @@
 #include "mongo/db/commands.h"
 #include "mongo/db/commands/rwc_defaults_commands_gen.h"
 #include "mongo/db/read_write_concern_defaults.h"
+#include "mongo/db/repl/read_concern_args.h"
+#include "mongo/db/repl/replication_coordinator.h"
 #include "mongo/util/log.h"
 
 namespace mongo {
 namespace {
 
+void assertNotStandaloneOrShardServer(OperationContext* opCtx, StringData cmdName) {
+    const auto replCoord = repl::ReplicationCoordinator::get(opCtx);
+    uassert(51300,
+            str::stream() << "'" << cmdName << "' is not supported on standalone nodes.",
+            replCoord->isReplEnabled());
+
+    uassert(51301,
+            str::stream() << "'" << cmdName << "' is not supported on shard nodes.",
+            serverGlobalParams.clusterRole != ClusterRole::ShardServer);
+}
+
 class SetDefaultRWConcernCommand : public TypedCommand<SetDefaultRWConcernCommand> {
 public:
     AllowedOnSecondary secondaryAllowed(ServiceContext*) const override {
@@ -62,17 +75,11 @@ public:
         using InvocationBase::InvocationBase;
 
         auto typedRun(OperationContext* opCtx) {
-            auto rc = request().getDefaultReadConcern();
-            auto wc = request().getDefaultWriteConcern();
-            uassert(ErrorCodes::BadValue,
-                    str::stream() << "At least one of the \""
-                                  << SetDefaultRWConcern::kDefaultReadConcernFieldName << "\" or \""
-                                  << SetDefaultRWConcern::kDefaultWriteConcernFieldName
-                                  << "\" fields must be present",
-                    rc || wc);
+            assertNotStandaloneOrShardServer(opCtx, SetDefaultRWConcern::kCommandName);
 
             auto& rwcDefaults = ReadWriteConcernDefaults::get(opCtx->getServiceContext());
-            auto newDefaults = rwcDefaults.setConcerns(opCtx, rc, wc);
+            auto newDefaults = rwcDefaults.setConcerns(
+                opCtx, request().getDefaultReadConcern(), request().getDefaultWriteConcern());
             log() << "successfully set RWC defaults to " << newDefaults.toBSON();
             return newDefaults;
         }
@@ -83,7 +90,7 @@ public:
         }
 
         void doCheckAuthorization(OperationContext*) const override {
-            // TODO: add and use privilege action
+            // TODO SERVER-45038: add and use privilege action
         }
 
         NamespaceString ns() const override {
@@ -112,6 +119,10 @@ public:
         using InvocationBase::InvocationBase;
 
         auto typedRun(OperationContext* opCtx) {
+            assertNotStandaloneOrShardServer(opCtx, GetDefaultRWConcern::kCommandName);
+
+            // TODO SERVER-43720 Implement inMemory option.
+
             auto& rwcDefaults = ReadWriteConcernDefaults::get(opCtx->getServiceContext());
             return rwcDefaults.getDefault(opCtx);
         }
@@ -122,7 +133,7 @@ public:
         }
 
         void doCheckAuthorization(OperationContext*) const override {
-            // TODO: add and use privilege action
+            // TODO SERVER-45038: add and use privilege action
         }
 
         NamespaceString ns() const override {
diff --git a/src/mongo/db/commands/rwc_defaults_commands.idl b/src/mongo/db/commands/rwc_defaults_commands.idl
index e4f2c2bb1b5..ad8efef676f 100644
--- a/src/mongo/db/commands/rwc_defaults_commands.idl
+++ b/src/mongo/db/commands/rwc_defaults_commands.idl
@@ -34,7 +34,6 @@ imports:
     - "mongo/db/repl/read_concern_args.idl"
     - "mongo/db/write_concern_options.idl"
 
-
 commands:
     setDefaultRWConcern:
         description: "set the current read/write concern defaults (cluster-wide)"
@@ -52,3 +51,8 @@ commands:
     getDefaultRWConcern:
         description: "get the current read/write concern defaults being applied by this node"
         namespace: ignored
+        fields:
+            inMemory:
+                type: bool
+                description: "If true, return the locally cached read/write concern defaults"
+                optional: true
diff --git a/src/mongo/db/read_write_concern_defaults.cpp b/src/mongo/db/read_write_concern_defaults.cpp
index 1e2c2197c83..aaf861e0a03 100644
--- a/src/mongo/db/read_write_concern_defaults.cpp
+++ b/src/mongo/db/read_write_concern_defaults.cpp
@@ -83,7 +83,12 @@ void ReadWriteConcernDefaults::_setDefault(RWConcernDefault&& rwc) {
 RWConcernDefault ReadWriteConcernDefaults::setConcerns(OperationContext* opCtx,
                                                        const boost::optional<ReadConcern>& rc,
                                                        const boost::optional<WriteConcern>& wc) {
-    invariant(rc || wc);
+    uassert(ErrorCodes::BadValue,
+            str::stream() << "At least one of the \""
+                          << RWConcernDefault::kDefaultReadConcernFieldName << "\" or \""
+                          << RWConcernDefault::kDefaultWriteConcernFieldName
+                          << "\" fields must be present",
+            rc || wc);
 
     if (rc) {
         checkSuitabilityAsDefault(*rc);
@@ -96,8 +101,12 @@ RWConcernDefault ReadWriteConcernDefaults::setConcerns(OperationContext* opCtx,
     auto epoch = LogicalClock::get(opCtx->getServiceContext())->getClusterTime().asTimestamp();
 
     RWConcernDefault rwc;
-    rwc.setDefaultReadConcern(rc);
-    rwc.setDefaultWriteConcern(wc);
+    if (rc && !rc->isEmpty()) {
+        rwc.setDefaultReadConcern(rc);
+    }
+    if (wc && !wc->usedDefaultW) {
+        rwc.setDefaultWriteConcern(wc);
+    }
     rwc.setEpoch(epoch);
     rwc.setSetTime(now);
     rwc.setLocalSetTime(now);
diff --git a/src/mongo/db/read_write_concern_defaults.h b/src/mongo/db/read_write_concern_defaults.h
index de21b27836d..761d8a08fbc 100644
--- a/src/mongo/db/read_write_concern_defaults.h
+++ b/src/mongo/db/read_write_concern_defaults.h
@@ -82,6 +82,7 @@ public:
      * Interface when an admin has run the command to change the defaults.
      * At least one of the `rc` or `wc` params must be set.
      * Will generate and use a new epoch and setTime for the updated defaults, which are returned.
+     * Validates the supplied read and write concerns can serve as defaults.
      */
     RWConcernDefault setConcerns(OperationContext* opCtx,
                                  const boost::optional<ReadConcern>& rc,
diff --git a/src/mongo/db/repl/read_concern_args.cpp b/src/mongo/db/repl/read_concern_args.cpp
index d7ef1ce6ff1..45e826482ee 100644
--- a/src/mongo/db/repl/read_concern_args.cpp
+++ b/src/mongo/db/repl/read_concern_args.cpp
@@ -82,6 +82,12 @@ BSONObj ReadConcernArgs::toBSON() const {
     return bob.obj();
 }
 
+BSONObj ReadConcernArgs::toBSONInner() const {
+    BSONObjBuilder bob;
+    _appendInfoInner(&bob);
+    return bob.obj();
+}
+
 bool ReadConcernArgs::isEmpty() const {
     return !_afterClusterTime && !_opTime && !_atClusterTime && !_level;
 }
@@ -261,25 +267,27 @@ bool ReadConcernArgs::isSpeculativeMajority() const {
         _majorityReadMechanism == MajorityReadMechanism::kSpeculative;
 }
 
-void ReadConcernArgs::appendInfo(BSONObjBuilder* builder) const {
-    BSONObjBuilder rcBuilder(builder->subobjStart(kReadConcernFieldName));
-
+void ReadConcernArgs::_appendInfoInner(BSONObjBuilder* builder) const {
     if (_level) {
-        rcBuilder.append(kLevelFieldName, readConcernLevels::toString(_level.get()));
+        builder->append(kLevelFieldName, readConcernLevels::toString(_level.get()));
     }
 
     if (_opTime) {
-        _opTime->append(&rcBuilder, kAfterOpTimeFieldName.toString());
+        _opTime->append(builder, kAfterOpTimeFieldName.toString());
     }
 
     if (_afterClusterTime) {
-        rcBuilder.append(kAfterClusterTimeFieldName, _afterClusterTime->asTimestamp());
+        builder->append(kAfterClusterTimeFieldName, _afterClusterTime->asTimestamp());
     }
 
     if (_atClusterTime) {
-        rcBuilder.append(kAtClusterTimeFieldName, _atClusterTime->asTimestamp());
+        builder->append(kAtClusterTimeFieldName, _atClusterTime->asTimestamp());
     }
+}
 
+void ReadConcernArgs::appendInfo(BSONObjBuilder* builder) const {
+    BSONObjBuilder rcBuilder(builder->subobjStart(kReadConcernFieldName));
+    _appendInfoInner(&rcBuilder);
     rcBuilder.done();
 }
 
diff --git a/src/mongo/db/repl/read_concern_args.h b/src/mongo/db/repl/read_concern_args.h
index 110624649bd..8a34b4248b9 100644
--- a/src/mongo/db/repl/read_concern_args.h
+++ b/src/mongo/db/repl/read_concern_args.h
@@ -130,7 +130,7 @@ public:
     bool isSpeculativeMajority() const;
 
     /**
-     * Appends level and afterOpTime.
+     * Appends level, afterOpTime, and any other sub-fields in a 'readConcern' sub-object.
      */
     void appendInfo(BSONObjBuilder* builder) const;
 
@@ -165,10 +165,16 @@ public:
 
     boost::optional<LogicalTime> getArgsAtClusterTime() const;
     BSONObj toBSON() const;
+    BSONObj toBSONInner() const;
     std::string toString() const;
 
 private:
     /**
+     * Appends level, afterOpTime, and the other "inner" fields of the read concern args.
+     */
+    void _appendInfoInner(BSONObjBuilder* builder) const;
+
+    /**
      *  Read data after the OpTime of an operation on this replica set. Deprecated.
      *  The only user is for read-after-optime calls using the config server optime.
      */
diff --git a/src/mongo/db/repl/read_concern_args.idl b/src/mongo/db/repl/read_concern_args.idl
index 1954f6dddf2..e791e370bdd 100644
--- a/src/mongo/db/repl/read_concern_args.idl
+++ b/src/mongo/db/repl/read_concern_args.idl
@@ -40,5 +40,5 @@ types:
         description: "An object representing a read concern."
         bson_serialization_type: object
         cpp_type: "mongo::repl::ReadConcernArgs"
-        serializer: "mongo::repl::ReadConcernArgs::toBSON"
+        serializer: "mongo::repl::ReadConcernArgs::toBSONInner"
         deserializer: "mongo::repl::ReadConcernArgs::fromBSONThrows"
diff --git a/src/mongo/db/rw_concern_default.idl b/src/mongo/db/rw_concern_default.idl
index 396c12264c1..94aca23c7b9 100644
--- a/src/mongo/db/rw_concern_default.idl
+++ b/src/mongo/db/rw_concern_default.idl
@@ -38,6 +38,7 @@ imports:
 structs:
     RWConcernDefault:
         description: "Represents a set of read/write concern defaults, and associated metadata"
+        strict: false
         fields:
             defaultReadConcern:
                 description: "The default read concern"
diff --git a/src/mongo/s/commands/SConscript b/src/mongo/s/commands/SConscript
index d9c0f9b6f12..a279e7535ee 100644
--- a/src/mongo/s/commands/SConscript
+++ b/src/mongo/s/commands/SConscript
@@ -79,6 +79,7 @@ env.Library(
         'cluster_repl_set_get_status_cmd.cpp',
         'cluster_reset_error_cmd.cpp',
         'cluster_restart_catalog_command.cpp',
+        'cluster_rwc_defaults_commands.cpp',
         'cluster_set_index_commit_quorum_cmd.cpp',
         'cluster_set_feature_compatibility_version_cmd.cpp',
         'cluster_set_free_monitoring.cpp' if get_option("enable-free-mon") == 'on' else [],
@@ -116,6 +117,7 @@ env.Library(
         '$BUILD_DIR/mongo/db/ftdc/ftdc_server',
         '$BUILD_DIR/mongo/db/shared_request_handling',
         '$BUILD_DIR/mongo/db/logical_session_cache_impl',
+        '$BUILD_DIR/mongo/db/read_write_concern_defaults',
         '$BUILD_DIR/mongo/db/pipeline/aggregation',
         '$BUILD_DIR/mongo/db/query/command_request_response',
         '$BUILD_DIR/mongo/db/query/map_reduce_output_format',
diff --git a/src/mongo/s/commands/cluster_rwc_defaults_commands.cpp b/src/mongo/s/commands/cluster_rwc_defaults_commands.cpp
new file mode 100644
index 00000000000..8286a8d42c4
--- /dev/null
+++ b/src/mongo/s/commands/cluster_rwc_defaults_commands.cpp
@@ -0,0 +1,160 @@
+/**
+ *    Copyright (C) 2019-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.
+ */
+
+#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kCommand
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/db/auth/authorization_session.h"
+#include "mongo/db/commands.h"
+#include "mongo/db/commands/rwc_defaults_commands_gen.h"
+#include "mongo/db/namespace_string.h"
+#include "mongo/db/read_write_concern_defaults.h"
+#include "mongo/db/rw_concern_default_gen.h"
+#include "mongo/s/cluster_commands_helpers.h"
+#include "mongo/s/grid.h"
+#include "mongo/util/log.h"
+
+namespace mongo {
+namespace {
+
+/**
+ * Implements the setDefaultRWConcern command on mongos. Inherits from BasicCommand because this
+ * command forwards the user's request to the config server and does not need to parse it.
+ */
+class ClusterSetDefaultRWConcernCommand : public BasicCommand {
+public:
+    ClusterSetDefaultRWConcernCommand() : BasicCommand("setDefaultRWConcern") {}
+
+    bool run(OperationContext* opCtx,
+             const std::string& dbName,
+             const BSONObj& cmdObj,
+             BSONObjBuilder& result) override {
+        auto configShard = Grid::get(opCtx)->shardRegistry()->getConfigShard();
+        auto cmdResponse = uassertStatusOK(configShard->runCommandWithFixedRetryAttempts(
+            opCtx,
+            ReadPreferenceSetting(ReadPreference::PrimaryOnly),
+            NamespaceString::kAdminDb.toString(),
+            CommandHelpers::appendMajorityWriteConcern(
+                CommandHelpers::filterCommandRequestForPassthrough(cmdObj)),
+            Shard::RetryPolicy::kNotIdempotent));
+
+        uassertStatusOK(cmdResponse.commandStatus);
+        uassertStatusOK(cmdResponse.writeConcernStatus);
+
+        CommandHelpers::filterCommandReplyForPassthrough(cmdResponse.response, &result);
+        return true;
+    }
+
+    bool supportsWriteConcern(const BSONObj& cmd) const override {
+        return true;
+    }
+
+    Status checkAuthForOperation(OperationContext* opCtx,
+                                 const std::string& dbname,
+                                 const BSONObj& cmdObj) const override {
+        // TODO SERVER-45038: add and use privilege action
+        return Status::OK();
+    }
+
+    std::string help() const override {
+        return "Sets the default read or write concern for a cluster";
+    }
+
+    bool adminOnly() const override {
+        return true;
+    }
+
+    AllowedOnSecondary secondaryAllowed(ServiceContext*) const override {
+        return AllowedOnSecondary::kNever;
+    }
+} clusterSetDefaultRWConcernCommand;
+
+/**
+ * Implements the getDefaultRWConcern command on mongos.
+ */
+class ClusterGetDefaultRWConcernCommand final
+    : public TypedCommand<ClusterGetDefaultRWConcernCommand> {
+public:
+    using Request = GetDefaultRWConcern;
+    using Response = RWConcernDefault;
+
+    class Invocation final : public InvocationBase {
+    public:
+        using InvocationBase::InvocationBase;
+
+        Response typedRun(OperationContext* opCtx) {
+            // TODO SERVER-43720 Implement inMemory option.
+
+            GetDefaultRWConcern configsvrRequest;
+            configsvrRequest.setDbName(request().getDbName());
+
+            auto configShard = Grid::get(opCtx)->shardRegistry()->getConfigShard();
+            auto cmdResponse = uassertStatusOK(configShard->runCommandWithFixedRetryAttempts(
+                opCtx,
+                ReadPreferenceSetting(ReadPreference::PrimaryOnly),
+                NamespaceString::kAdminDb.toString(),
+                applyReadWriteConcern(opCtx, this, configsvrRequest.toBSON({})),
+                Shard::RetryPolicy::kIdempotent));
+
+            uassertStatusOK(cmdResponse.commandStatus);
+
+            return Response::parse(IDLParserErrorContext("ClusterGetDefaultRWConcernResponse"),
+                                   cmdResponse.response);
+        }
+
+    private:
+        bool supportsWriteConcern() const override {
+            return false;
+        }
+
+        void doCheckAuthorization(OperationContext*) const override {
+            // TODO SERVER-45038: add and use privilege action
+        }
+
+        NamespaceString ns() const override {
+            return NamespaceString(request().getDbName(), "");
+        }
+    };
+
+    std::string help() const override {
+        return "Gets the default read or write concern for a cluster";
+    }
+
+    bool adminOnly() const override {
+        return true;
+    }
+
+    AllowedOnSecondary secondaryAllowed(ServiceContext*) const override {
+        return AllowedOnSecondary::kNever;
+    }
+} clusterGetDefaultRWConcernCommand;
+
+}  // namespace
+}  // namespace mongo
diff --git a/src/mongo/shell/assert.js b/src/mongo/shell/assert.js
index 53e283b540a..07bf0c31bfd 100644
--- a/src/mongo/shell/assert.js
+++ b/src/mongo/shell/assert.js
@@ -284,7 +284,7 @@ assert = (function() {
 
         if (count != arr.length) {
             doassert(_buildAssertionMessage(
-                msg, "None of values from " + tojson(arr) + " was in " + tojson(result)));
+                msg, "Not all of the values from " + tojson(arr) + " were in " + tojson(result)));
         }
     };