SERVER-43120 SERVER-43121 SERVER-43122 ReadWriteConcernDefaults component, setDefaultRWConcern and getDefaultRWConcern commands, check RWC default suitability

12 files changed, 658 insertions, 33 deletions

diff --git a/src/mongo/db/SConscript b/src/mongo/db/SConscript
index 08aa289e870..3546d8ca650 100644
--- a/src/mongo/db/SConscript
+++ b/src/mongo/db/SConscript
@@ -59,6 +59,7 @@ env.Library(
         '$BUILD_DIR/mongo/base',
         '$BUILD_DIR/mongo/db/index_names',
         '$BUILD_DIR/mongo/db/write_concern_options',
+        '$BUILD_DIR/mongo/db/read_write_concern_defaults',
     ]
 )
 
@@ -370,6 +371,18 @@ env.Library(
 )
 
 env.Library(
+    target="read_write_concern_defaults",
+    source=[
+        "read_write_concern_defaults.cpp",
+        env.Idlc('rw_concern_default.idl')[0],
+    ],
+    LIBDEPS=[
+        'write_concern_options',
+        'repl/read_concern_args',
+    ],
+)
+
+env.Library(
     target='service_context',
     source=[
         'baton.cpp',
diff --git a/src/mongo/db/commands/SConscript b/src/mongo/db/commands/SConscript
index c46add8de57..34ab19d0b46 100644
--- a/src/mongo/db/commands/SConscript
+++ b/src/mongo/db/commands/SConscript
@@ -142,9 +142,11 @@ 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],
+        env.Idlc('rwc_defaults_commands.idl')[0],
     ],
     LIBDEPS_PRIVATE=[
         '$BUILD_DIR/mongo/client/clientdriver_minimal',
diff --git a/src/mongo/db/commands/rwc_defaults_commands.cpp b/src/mongo/db/commands/rwc_defaults_commands.cpp
new file mode 100644
index 00000000000..d3b0a9d072d
--- /dev/null
+++ b/src/mongo/db/commands/rwc_defaults_commands.cpp
@@ -0,0 +1,126 @@
+/**
+ *    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.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/db/commands.h"
+#include "mongo/db/commands/rwc_defaults_commands_gen.h"
+#include "mongo/db/read_write_concern_defaults.h"
+
+namespace mongo {
+namespace {
+
+class SetDefaultRWConcernCommand : public TypedCommand<SetDefaultRWConcernCommand> {
+public:
+    AllowedOnSecondary secondaryAllowed(ServiceContext*) const override {
+        return AllowedOnSecondary::kNever;
+    }
+    bool adminOnly() const override {
+        return true;
+    }
+    std::string help() const override {
+        return "set the current read/write concern defaults (cluster-wide)";
+    }
+
+public:
+    using Request = SetDefaultRWConcern;
+
+    class Invocation final : public InvocationBase {
+    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);
+
+            auto& rwcDefaults = ReadWriteConcernDefaults::get(opCtx->getServiceContext());
+            return rwcDefaults.setConcerns(opCtx, rc, wc);
+        }
+
+    private:
+        bool supportsWriteConcern() const override {
+            return true;
+        }
+
+        void doCheckAuthorization(OperationContext*) const override {
+            // TODO: add and use privilege action
+        }
+
+        NamespaceString ns() const override {
+            return NamespaceString(request().getDbName(), "");
+        }
+    };
+} setDefaultRWConcernCommand;
+
+class GetDefaultRWConcernCommand : public TypedCommand<GetDefaultRWConcernCommand> {
+public:
+    AllowedOnSecondary secondaryAllowed(ServiceContext*) const override {
+        return AllowedOnSecondary::kAlways;
+    }
+    bool adminOnly() const override {
+        return true;
+    }
+    std::string help() const override {
+        return "get the current read/write concern defaults being applied by this node";
+    }
+
+public:
+    using Request = GetDefaultRWConcern;
+
+    class Invocation final : public InvocationBase {
+    public:
+        using InvocationBase::InvocationBase;
+
+        auto typedRun(OperationContext* opCtx) {
+            auto& rwcDefaults = ReadWriteConcernDefaults::get(opCtx->getServiceContext());
+            return rwcDefaults.getDefault();
+        }
+
+    private:
+        bool supportsWriteConcern() const override {
+            return false;
+        }
+
+        void doCheckAuthorization(OperationContext*) const override {
+            // TODO: add and use privilege action
+        }
+
+        NamespaceString ns() const override {
+            return NamespaceString(request().getDbName(), "");
+        }
+    };
+} getDefaultRWConcernCommand;
+}  // namespace
+}  // namespace mongo
diff --git a/src/mongo/db/commands/rwc_defaults_commands.idl b/src/mongo/db/commands/rwc_defaults_commands.idl
new file mode 100644
index 00000000000..e4f2c2bb1b5
--- /dev/null
+++ b/src/mongo/db/commands/rwc_defaults_commands.idl
@@ -0,0 +1,54 @@
+# 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.
+#
+global:
+    cpp_namespace: "mongo"
+
+
+imports:
+    - "mongo/idl/basic_types.idl"
+    - "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)"
+        namespace: ignored
+        fields:
+            defaultReadConcern:
+                description: "The new default read concern"
+                type: ReadConcern
+                optional: true
+            defaultWriteConcern:
+                description: "The new default write concern"
+                type: WriteConcern
+                optional: true
+
+    getDefaultRWConcern:
+        description: "get the current read/write concern defaults being applied by this node"
+        namespace: ignored
diff --git a/src/mongo/db/read_write_concern_defaults.cpp b/src/mongo/db/read_write_concern_defaults.cpp
new file mode 100644
index 00000000000..828160e2e41
--- /dev/null
+++ b/src/mongo/db/read_write_concern_defaults.cpp
@@ -0,0 +1,158 @@
+/**
+ *    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.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/db/read_write_concern_defaults.h"
+
+#include "mongo/db/logical_clock.h"
+
+namespace mongo {
+
+void ReadWriteConcernDefaults::checkSuitabilityAsDefault(const ReadConcern& rc) {
+    uassert(ErrorCodes::BadValue,
+            str::stream() << "level: '" << ReadConcern::kSnapshotReadConcernStr
+                          << "' is not suitable for the default read concern",
+            rc.getLevel() != repl::ReadConcernLevel::kSnapshotReadConcern);
+    uassert(ErrorCodes::BadValue,
+            str::stream() << "level: '" << ReadConcern::kLinearizableReadConcernStr
+                          << "' is not suitable for the default read concern",
+            rc.getLevel() != repl::ReadConcernLevel::kLinearizableReadConcern);
+    uassert(ErrorCodes::BadValue,
+            str::stream() << "'" << ReadConcern::kAfterOpTimeFieldName
+                          << "' is not suitable for the default read concern",
+            !rc.getArgsOpTime());
+    uassert(ErrorCodes::BadValue,
+            str::stream() << "'" << ReadConcern::kAfterClusterTimeFieldName
+                          << "' is not suitable for the default read concern",
+            !rc.getArgsAfterClusterTime());
+    uassert(ErrorCodes::BadValue,
+            str::stream() << "'" << ReadConcern::kAtClusterTimeFieldName
+                          << "' is not suitable for the default read concern",
+            !rc.getArgsAtClusterTime());
+}
+
+void ReadWriteConcernDefaults::checkSuitabilityAsDefault(const WriteConcern& wc) {
+    uassert(ErrorCodes::BadValue,
+            "Unacknowledged write concern is not suitable for the default write concern",
+            !(wc.wMode.empty() && wc.wNumNodes < 1));
+}
+
+void ReadWriteConcernDefaults::_setDefault(WithLock, RWConcernDefault&& rwc) {
+    _defaults.erase(kReadWriteConcernEntry);
+    _defaults.emplace(kReadWriteConcernEntry, rwc);
+}
+
+RWConcernDefault ReadWriteConcernDefaults::setConcerns(OperationContext* opCtx,
+                                                       const boost::optional<ReadConcern>& rc,
+                                                       const boost::optional<WriteConcern>& wc) {
+    invariant(rc || wc);
+
+    if (rc) {
+        checkSuitabilityAsDefault(*rc);
+    }
+    if (wc) {
+        checkSuitabilityAsDefault(*wc);
+    }
+
+    auto now = opCtx->getServiceContext()->getFastClockSource()->now();
+    auto epoch = LogicalClock::get(opCtx->getServiceContext())->getClusterTime().asTimestamp();
+
+    RWConcernDefault rwc;
+    rwc.setDefaultReadConcern(rc);
+    rwc.setDefaultWriteConcern(wc);
+    rwc.setEpoch(epoch);
+    rwc.setSetTime(now);
+    rwc.setLocalSetTime(now);
+
+    stdx::lock_guard<Latch> lk(_mutex);
+
+    auto current = _getDefault(lk);
+    if (!rc && current) {
+        rwc.setDefaultReadConcern(current->getDefaultReadConcern());
+    }
+    if (!wc && current) {
+        rwc.setDefaultWriteConcern(current->getDefaultWriteConcern());
+    }
+    _setDefault(lk, std::move(rwc));
+    return *_getDefault(lk);
+}
+
+void ReadWriteConcernDefaults::invalidate() {
+    stdx::lock_guard<Latch> lk(_mutex);
+    _defaults.erase(kReadWriteConcernEntry);
+}
+
+boost::optional<RWConcernDefault> ReadWriteConcernDefaults::_getDefault(WithLock) const {
+    if (_defaults.find(kReadWriteConcernEntry) == _defaults.end()) {
+        return boost::none;
+    }
+    return _defaults.at(kReadWriteConcernEntry);
+}
+
+RWConcernDefault ReadWriteConcernDefaults::getDefault() const {
+    auto current = ([&]() {
+        stdx::lock_guard<Latch> lk(_mutex);
+        return _getDefault(lk);
+    })();
+    if (!current) {
+        return RWConcernDefault{};
+    }
+    return *current;
+}
+
+boost::optional<ReadWriteConcernDefaults::ReadConcern>
+ReadWriteConcernDefaults::getDefaultReadConcern() const {
+    auto current = getDefault();
+    return current.getDefaultReadConcern();
+}
+
+boost::optional<ReadWriteConcernDefaults::WriteConcern>
+ReadWriteConcernDefaults::getDefaultWriteConcern() const {
+    auto current = getDefault();
+    return current.getDefaultWriteConcern();
+}
+
+
+namespace {
+
+const auto getReadWriteConcernDefaults =
+    ServiceContext::declareDecoration<ReadWriteConcernDefaults>();
+
+}  // namespace
+
+ReadWriteConcernDefaults& ReadWriteConcernDefaults::get(ServiceContext* service) {
+    return getReadWriteConcernDefaults(service);
+}
+
+ReadWriteConcernDefaults& ReadWriteConcernDefaults::get(ServiceContext& service) {
+    return getReadWriteConcernDefaults(service);
+}
+
+}  // namespace mongo
diff --git a/src/mongo/db/read_write_concern_defaults.h b/src/mongo/db/read_write_concern_defaults.h
new file mode 100644
index 00000000000..f7ce2f9bfee
--- /dev/null
+++ b/src/mongo/db/read_write_concern_defaults.h
@@ -0,0 +1,102 @@
+/**
+ *    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.
+ */
+
+#pragma once
+
+#include <map>
+
+#include "mongo/db/operation_context.h"
+#include "mongo/db/repl/read_concern_args.h"
+#include "mongo/db/rw_concern_default_gen.h"
+#include "mongo/db/service_context.h"
+#include "mongo/db/write_concern_options.h"
+#include "mongo/platform/mutex.h"
+#include "mongo/util/concurrency/with_lock.h"
+
+namespace mongo {
+
+/**
+ * Class to manage Read Concern and Write Concern (RWC) defaults.
+ */
+class ReadWriteConcernDefaults {
+public:
+    /**
+     * Consistent names for the classes that define a ReadConcern and a WriteConcern.
+     */
+    using ReadConcern = repl::ReadConcernArgs;
+    using WriteConcern = WriteConcernOptions;
+
+    static constexpr StringData readConcernFieldName = ReadConcern::kReadConcernFieldName;
+    static constexpr StringData writeConcernFieldName = WriteConcern::kWriteConcernField;
+
+    static ReadWriteConcernDefaults& get(ServiceContext* service);
+    static ReadWriteConcernDefaults& get(ServiceContext& service);
+
+    ReadWriteConcernDefaults() = default;
+    ~ReadWriteConcernDefaults() = default;
+
+    static void checkSuitabilityAsDefault(const ReadConcern& rc);
+    static void checkSuitabilityAsDefault(const WriteConcern& wc);
+
+    /**
+     * 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.
+     */
+    RWConcernDefault setConcerns(OperationContext* opCtx,
+                                 const boost::optional<ReadConcern>& rc,
+                                 const boost::optional<WriteConcern>& wc);
+
+    /**
+     * Invalidates the cached RWC defaults, causing them to be refreshed.
+     *
+     * After this call returns, the read methods below (getDefault, getDefaultReadConcern,
+     * getDefaultWriteConcern) may continue returning the invalidated defaults, until they have been
+     * replaced by the refreshed values.  This is to avoid stalling CRUD ops (and other ops that
+     * need RC/WC) during refresh.
+     */
+    void invalidate();
+
+    RWConcernDefault getDefault() const;
+    boost::optional<ReadConcern> getDefaultReadConcern() const;
+    boost::optional<WriteConcern> getDefaultWriteConcern() const;
+
+private:
+    enum Type { kReadWriteConcernEntry };
+
+    void _setDefault(WithLock, RWConcernDefault&& rwc);
+    boost::optional<RWConcernDefault> _getDefault(WithLock) const;
+
+    // Protects access to the private members below.
+    mutable Mutex _mutex = MONGO_MAKE_LATCH("ReadWriteConcernDefaults::_mutex");
+
+    std::map<Type, RWConcernDefault> _defaults;
+};
+
+}  // namespace mongo
diff --git a/src/mongo/db/repl/read_concern_args.cpp b/src/mongo/db/repl/read_concern_args.cpp
index bb060b5a4dc..772ceeb76da 100644
--- a/src/mongo/db/repl/read_concern_args.cpp
+++ b/src/mongo/db/repl/read_concern_args.cpp
@@ -45,23 +45,6 @@ using std::string;
 namespace mongo {
 namespace repl {
 
-namespace {
-
-constexpr StringData kLocalReadConcernStr = "local"_sd;
-constexpr StringData kMajorityReadConcernStr = "majority"_sd;
-constexpr StringData kLinearizableReadConcernStr = "linearizable"_sd;
-constexpr StringData kAvailableReadConcernStr = "available"_sd;
-constexpr StringData kSnapshotReadConcernStr = "snapshot"_sd;
-
-}  // unnamed namespace
-
-const string ReadConcernArgs::kReadConcernFieldName("readConcern");
-const string ReadConcernArgs::kAfterOpTimeFieldName("afterOpTime");
-const string ReadConcernArgs::kAfterClusterTimeFieldName("afterClusterTime");
-const string ReadConcernArgs::kAtClusterTimeFieldName("atClusterTime");
-
-const string ReadConcernArgs::kLevelFieldName("level");
-
 const OperationContext::Decoration<ReadConcernArgs> handle =
     OperationContext::declareDecoration<ReadConcernArgs>();
 
@@ -122,8 +105,6 @@ boost::optional<LogicalTime> ReadConcernArgs::getArgsAtClusterTime() const {
 }
 
 Status ReadConcernArgs::initialize(const BSONElement& readConcernElem) {
-    invariant(isEmpty());  // only legal to call on uninitialized object.
-
     if (readConcernElem.eoo()) {
         return Status::OK();
     }
@@ -135,7 +116,11 @@ Status ReadConcernArgs::initialize(const BSONElement& readConcernElem) {
                       str::stream() << kReadConcernFieldName << " field should be an object");
     }
 
-    BSONObj readConcernObj = readConcernElem.Obj();
+    return parse(readConcernElem.Obj());
+}
+
+Status ReadConcernArgs::parse(const BSONObj& readConcernObj) {
+    invariant(isEmpty());  // only legal to call on uninitialized object.
     for (auto&& field : readConcernObj) {
         auto fieldName = field.fieldNameStringData();
         if (fieldName == kAfterOpTimeFieldName) {
@@ -248,6 +233,12 @@ Status ReadConcernArgs::initialize(const BSONElement& readConcernElem) {
     return Status::OK();
 }
 
+ReadConcernArgs ReadConcernArgs::fromBSONThrows(const BSONObj& readConcernObj) {
+    ReadConcernArgs rc;
+    uassertStatusOK(rc.parse(readConcernObj));
+    return rc;
+}
+
 void ReadConcernArgs::setMajorityReadMechanism(MajorityReadMechanism mechanism) {
     invariant(*_level == ReadConcernLevel::kMajorityReadConcern);
     _majorityReadMechanism = mechanism;
@@ -297,7 +288,7 @@ void ReadConcernArgs::appendInfo(BSONObjBuilder* builder) const {
     }
 
     if (_opTime) {
-        _opTime->append(&rcBuilder, kAfterOpTimeFieldName);
+        _opTime->append(&rcBuilder, kAfterOpTimeFieldName.toString());
     }
 
     if (_afterClusterTime) {
diff --git a/src/mongo/db/repl/read_concern_args.h b/src/mongo/db/repl/read_concern_args.h
index 775877f9b34..310cfc59432 100644
--- a/src/mongo/db/repl/read_concern_args.h
+++ b/src/mongo/db/repl/read_concern_args.h
@@ -48,11 +48,17 @@ namespace repl {
 
 class ReadConcernArgs {
 public:
-    static const std::string kReadConcernFieldName;
-    static const std::string kAfterOpTimeFieldName;
-    static const std::string kAfterClusterTimeFieldName;
-    static const std::string kAtClusterTimeFieldName;
-    static const std::string kLevelFieldName;
+    static constexpr StringData kReadConcernFieldName = "readConcern"_sd;
+    static constexpr StringData kAfterOpTimeFieldName = "afterOpTime"_sd;
+    static constexpr StringData kAfterClusterTimeFieldName = "afterClusterTime"_sd;
+    static constexpr StringData kAtClusterTimeFieldName = "atClusterTime"_sd;
+    static constexpr StringData kLevelFieldName = "level"_sd;
+
+    static constexpr StringData kLocalReadConcernStr = "local"_sd;
+    static constexpr StringData kMajorityReadConcernStr = "majority"_sd;
+    static constexpr StringData kLinearizableReadConcernStr = "linearizable"_sd;
+    static constexpr StringData kAvailableReadConcernStr = "available"_sd;
+    static constexpr StringData kSnapshotReadConcernStr = "snapshot"_sd;
 
     /**
      * Represents the internal mechanism an operation uses to satisfy 'majority' read concern.
@@ -104,6 +110,13 @@ public:
     Status initialize(const BSONElement& readConcernElem);
 
     /**
+     * Initializes the object by parsing the actual readConcern sub-object.
+     */
+    Status parse(const BSONObj& readConcernObj);
+
+    static ReadConcernArgs fromBSONThrows(const BSONObj& readConcernObj);
+
+    /**
      * Sets the mechanism we should use to satisfy 'majority' reads.
      *
      * Invalid to call unless the read concern level is 'kMajorityReadConcern'.
diff --git a/src/mongo/db/repl/read_concern_args.idl b/src/mongo/db/repl/read_concern_args.idl
new file mode 100644
index 00000000000..1954f6dddf2
--- /dev/null
+++ b/src/mongo/db/repl/read_concern_args.idl
@@ -0,0 +1,44 @@
+# 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.
+#
+global:
+    cpp_namespace: "mongo"
+    cpp_includes:
+        - "mongo/db/repl/read_concern_args.h"
+
+
+imports:
+    - "mongo/idl/basic_types.idl"
+
+
+types:
+    ReadConcern:
+        description: "An object representing a read concern."
+        bson_serialization_type: object
+        cpp_type: "mongo::repl::ReadConcernArgs"
+        serializer: "mongo::repl::ReadConcernArgs::toBSON"
+        deserializer: "mongo::repl::ReadConcernArgs::fromBSONThrows"
diff --git a/src/mongo/db/rw_concern_default.idl b/src/mongo/db/rw_concern_default.idl
new file mode 100644
index 00000000000..396c12264c1
--- /dev/null
+++ b/src/mongo/db/rw_concern_default.idl
@@ -0,0 +1,61 @@
+# 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.
+#
+global:
+    cpp_namespace: "mongo"
+
+
+imports:
+    - "mongo/idl/basic_types.idl"
+    - "mongo/db/repl/read_concern_args.idl"
+    - "mongo/db/write_concern_options.idl"
+
+
+structs:
+    RWConcernDefault:
+        description: "Represents a set of read/write concern defaults, and associated metadata"
+        fields:
+            defaultReadConcern:
+                description: "The default read concern"
+                type: ReadConcern
+                optional: true
+            defaultWriteConcern:
+                description: "The default write concern"
+                type: WriteConcern
+                optional: true
+            epoch:
+                description: "The epoch (unique generation id) of when the default read or write concern was last set"
+                type: timestamp
+                optional: true
+            setTime:
+                description: "The wallclock time when the default read or write concern was last set by an administrator"
+                type: date
+                optional: true
+            localSetTime:
+                description: "The wallclock time when this node updated its understanding of the default read or write concern"
+                type: date
+                optional: true
diff --git a/src/mongo/db/write_concern_options.cpp b/src/mongo/db/write_concern_options.cpp
index 055fa23a5ee..d92dbfed25c 100644
--- a/src/mongo/db/write_concern_options.cpp
+++ b/src/mongo/db/write_concern_options.cpp
@@ -159,20 +159,37 @@ Status WriteConcernOptions::parse(const BSONObj& obj) {
     return Status::OK();
 }
 
-WriteConcernOptions WriteConcernOptions::deserializerForIDL(const BSONObj& obj) {
-    WriteConcernOptions writeConcernOptions;
-    uassertStatusOK(writeConcernOptions.parse(obj));
-    return writeConcernOptions;
-}
+namespace {
 
-StatusWith<WriteConcernOptions> WriteConcernOptions::extractWCFromCommand(
-    const BSONObj& cmdObj, const WriteConcernOptions& defaultWC) {
+/**
+ * Construct a WriteConcernOptions based on an optional default WC object, in preparation for
+ * parsing out of a command object or IDL.
+ */
+WriteConcernOptions constructWCFromDefault(
+    const WriteConcernOptions& defaultWC = WriteConcernOptions()) {
     WriteConcernOptions writeConcern = defaultWC;
     writeConcern.usedDefault = true;
     writeConcern.usedDefaultW = true;
     if (writeConcern.wNumNodes == 0 && writeConcern.wMode.empty()) {
         writeConcern.wNumNodes = 1;
     }
+    return writeConcern;
+}
+
+}  // namespace
+
+
+WriteConcernOptions WriteConcernOptions::deserializerForIDL(const BSONObj& obj) {
+    WriteConcernOptions writeConcern = constructWCFromDefault();
+    if (!obj.isEmpty()) {
+        uassertStatusOK(writeConcern.parse(obj));
+    }
+    return writeConcern;
+}
+
+StatusWith<WriteConcernOptions> WriteConcernOptions::extractWCFromCommand(
+    const BSONObj& cmdObj, const WriteConcernOptions& defaultWC) {
+    WriteConcernOptions writeConcern = constructWCFromDefault(defaultWC);
 
     // Return the default write concern if no write concern is provided. We check for the existence
     // of the write concern field up front in order to avoid the expense of constructing an error
diff --git a/src/mongo/db/write_concern_options.idl b/src/mongo/db/write_concern_options.idl
new file mode 100644
index 00000000000..07c3cdbffc4
--- /dev/null
+++ b/src/mongo/db/write_concern_options.idl
@@ -0,0 +1,44 @@
+# 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.
+#
+global:
+    cpp_namespace: "mongo"
+    cpp_includes:
+        - "mongo/db/write_concern_options.h"
+
+
+imports:
+    - "mongo/idl/basic_types.idl"
+
+
+types:
+    WriteConcern:
+        description: "An object representing a write concern."
+        bson_serialization_type: object
+        cpp_type: "mongo::WriteConcernOptions"
+        serializer: "mongo::WriteConcernOptions::toBSON"
+        deserializer: "mongo::WriteConcernOptions::deserializerForIDL"