SERVER-18288 implement a common interface for OP_COMMAND and OP_QUERY

36 files changed, 2014 insertions, 155 deletions

diff --git a/src/mongo/base/error_codes.err b/src/mongo/base/error_codes.err
index 8bb00b55e39..d40b671d1df 100644
--- a/src/mongo/base/error_codes.err
+++ b/src/mongo/base/error_codes.err
@@ -124,6 +124,8 @@ error_code("DocumentValidationFailure", 121) # Only for the document validator o
 error_code("ReadAfterOptimeTimeout", 122)
 error_code("NotAReplicaSet", 123)
 error_code("IncompatibleElectionProtocol", 124)
+error_code("CommandFailed", 125)
+error_code("RPCProtocolNegotiationFailed", 126)
 
 # Non-sequential error codes (for compatibility only)
 error_code("NotMaster", 10107) #this comes from assert_util.h
diff --git a/src/mongo/rpc/SConscript b/src/mongo/rpc/SConscript
index 3bff42a61e6..20279380881 100644
--- a/src/mongo/rpc/SConscript
+++ b/src/mongo/rpc/SConscript
@@ -26,11 +26,38 @@ env.Library(
 
 env.Library(
     target=[
-        'request',
+        'protocol',
     ],
     source=[
-        'request.cpp',
-        'request_builder.cpp',
+        'protocol.cpp',
+    ],
+)
+
+env.Library(
+    target=[
+        'rpc',
+    ],
+    source=[
+        'factory.cpp',
+    ],
+    LIBDEPS=[
+        'command_reply',
+        'command_request',
+        'legacy_reply',
+        'legacy_request',
+        'metadata',
+        'protocol',
+        '$BUILD_DIR/mongo/db/server_parameters',
+    ],
+)
+
+env.Library(
+    target=[
+        'command_request',
+    ],
+    source=[
+        'command_request.cpp',
+        'command_request_builder.cpp',
     ],
     LIBDEPS=[
         'document_range',
@@ -40,11 +67,11 @@ env.Library(
 
 env.Library(
     target=[
-        'reply',
+        'legacy_request',
     ],
     source=[
-        'reply.cpp',
-        'reply_builder.cpp'
+        'legacy_request.cpp',
+        'legacy_request_builder.cpp',
     ],
     LIBDEPS=[
         'document_range',
@@ -52,18 +79,58 @@ env.Library(
     ],
 )
 
+env.Library(
+    target=[
+        'command_reply',
+    ],
+    source=[
+        'command_reply.cpp',
+        'command_reply_builder.cpp',
+        'reply_builder_interface.cpp',
+    ],
+    LIBDEPS=[
+        'document_range',
+        '$BUILD_DIR/mongo/util/net/network',
+    ],
+)
+
+env.Library(
+    target=[
+        'legacy_reply',
+    ],
+    source=[
+        'legacy_reply.cpp',
+        'legacy_reply_builder.cpp'
+    ],
+    LIBDEPS=[
+        'document_range',
+        '$BUILD_DIR/mongo/util/net/network',
+    ],
+)
+
+env.Library(
+    target=[
+        'metadata',
+    ],
+    source=[
+        'metadata.cpp'
+    ],
+)
+
 env.CppUnitTest(
     target=[
         'rpc_test',
     ],
     source=[
-        'reply_builder_test.cpp',
-        'reply_test.cpp',
-        'request_builder_test.cpp',
-        'request_test.cpp',
+        'command_reply_builder_test.cpp',
+        'command_reply_test.cpp',
+        'command_request_builder_test.cpp',
+        'command_request_test.cpp',
+        'protocol_test.cpp',
     ],
     LIBDEPS=[
-        'request',
-        'reply',
+        'command_request',
+        'command_reply',
+        'protocol',
     ],
 )
diff --git a/src/mongo/rpc/reply.cpp b/src/mongo/rpc/command_reply.cpp
index e45f72091b9..449308fbed3 100644
--- a/src/mongo/rpc/reply.cpp
+++ b/src/mongo/rpc/command_reply.cpp
@@ -28,7 +28,7 @@
 
 #include "mongo/platform/basic.h"
 
-#include "mongo/rpc/reply.h"
+#include "mongo/rpc/command_reply.h"
 
 #include <tuple>
 
@@ -38,15 +38,17 @@
 namespace mongo {
 namespace rpc {
 
-    Reply::Reply(const Message* message)
+    CommandReply::CommandReply(const Message* message)
         : _message(message) {
 
-        char* begin = _message->singleData().data();
+        const char* begin = _message->singleData().data();
         std::size_t length = _message->singleData().dataLen();
 
-        invariant(length <= MaxMessageSizeBytes); // checked by message_port.cpp
+        // This check failing would normally be operation fatal, but we expect it to have been
+        // done earlier in the network layer, so we make it an invariant.
+        invariant(length <= MaxMessageSizeBytes);
 
-        char* messageEnd = begin + length;
+        const char* messageEnd = begin + length;
         ConstDataRangeCursor cur(begin, messageEnd);
 
         // TODO(amidvidy): we don't currently handle BSON validation.
@@ -57,24 +59,24 @@ namespace rpc {
         _outputDocs = DocumentRange(cur.data(), messageEnd);
     }
 
-    const BSONObj& Reply::getMetadata() const {
+    const BSONObj& CommandReply::getMetadata() const {
         return _metadata;
     }
 
-    const BSONObj& Reply::getCommandReply() const {
+    const BSONObj& CommandReply::getCommandReply() const {
         return _commandReply;
     }
 
-    DocumentRange Reply::getOutputDocs() const {
+    DocumentRange CommandReply::getOutputDocs() const {
         return  _outputDocs;
     }
 
-    bool operator==(const Reply& lhs, const Reply& rhs) {
+    bool operator==(const CommandReply& lhs, const CommandReply& rhs) {
         return std::tie(lhs._metadata, lhs._commandReply, lhs._outputDocs) ==
                std::tie(rhs._metadata, rhs._commandReply, rhs._outputDocs);
     }
 
-    bool operator!=(const Reply& lhs, const Reply& rhs) {
+    bool operator!=(const CommandReply& lhs, const CommandReply& rhs) {
         return !(lhs == rhs);
     }
 
diff --git a/src/mongo/rpc/reply.h b/src/mongo/rpc/command_reply.h
index befd6d70719..8f888e6190d 100644
--- a/src/mongo/rpc/reply.h
+++ b/src/mongo/rpc/command_reply.h
@@ -28,8 +28,9 @@
 
 #pragma once
 
-#include "mongo/bson/bsonobj.h"
+#include "mongo/db/jsobj.h"
 #include "mongo/rpc/document_range.h"
+#include "mongo/rpc/reply_interface.h"
 
 namespace mongo {
     class Message;
@@ -45,7 +46,7 @@ namespace rpc {
      * explicit.
      * See SERVER-16730 for additional details.
      */
-    class Reply {
+    class CommandReply : public ReplyInterface {
     public:
         /**
          * Construct a Reply from a Message.
@@ -54,18 +55,18 @@ namespace rpc {
          *
          * The underlying Message also handles the wire-protocol header.
          */
-        explicit Reply(const Message* message);
+        explicit CommandReply(const Message* message);
 
         /**
          * Accessor for the metadata object. Metadata is generally used for information
          * that is independent of any specific command, e.g. auditing information.
          */
-        const BSONObj& getMetadata() const;
+        const BSONObj& getMetadata() const final;
 
         /**
          * The result of executing the command.
          */
-        const BSONObj& getCommandReply() const;
+        const BSONObj& getCommandReply() const final;
 
         /**
          * A variable number of BSON documents returned by the command. It is valid for the
@@ -77,10 +78,10 @@ namespace rpc {
          *    ... do stuff with doc
          * }
          */
-        DocumentRange getOutputDocs() const;
+        DocumentRange getOutputDocs() const final;
 
-        friend bool operator==(const Reply& lhs, const Reply& rhs);
-        friend bool operator!=(const Reply& lhs, const Reply& rhs);
+        friend bool operator==(const CommandReply& lhs, const CommandReply& rhs);
+        friend bool operator!=(const CommandReply& lhs, const CommandReply& rhs);
 
     private:
         const Message* _message;
diff --git a/src/mongo/rpc/reply_builder.cpp b/src/mongo/rpc/command_reply_builder.cpp
index cd214b33864..09d4156e09f 100644
--- a/src/mongo/rpc/reply_builder.cpp
+++ b/src/mongo/rpc/command_reply_builder.cpp
@@ -30,55 +30,59 @@
 
 #include <utility>
 
-#include "mongo/rpc/reply_builder.h"
+#include "mongo/rpc/command_reply_builder.h"
 #include "mongo/stdx/memory.h"
 #include "mongo/util/assert_util.h"
 
 namespace mongo {
 namespace rpc {
 
-    ReplyBuilder::ReplyBuilder()
+    CommandReplyBuilder::CommandReplyBuilder()
         : _message{stdx::make_unique<Message>()}
     {}
 
-    ReplyBuilder::ReplyBuilder(std::unique_ptr<Message> message)
+    CommandReplyBuilder::CommandReplyBuilder(std::unique_ptr<Message> message)
         : _message{std::move(message)}
     {}
 
-    ReplyBuilder& ReplyBuilder::setMetadata(const BSONObj& metadata) {
-        invariant(_buildState == BuildState::kMetadata);
+    CommandReplyBuilder& CommandReplyBuilder::setMetadata(BSONObj metadata) {
+        invariant(_state == State::kMetadata);
         metadata.appendSelfToBufBuilder(_builder);
-        _buildState = BuildState::kCommandReply;
+        _state = State::kCommandReply;
         return *this;
     }
 
-    ReplyBuilder& ReplyBuilder::setCommandReply(const BSONObj& commandReply) {
-        invariant(_buildState == BuildState::kCommandReply);
+    CommandReplyBuilder& CommandReplyBuilder::setRawCommandReply(BSONObj commandReply) {
+        invariant(_state == State::kCommandReply);
         commandReply.appendSelfToBufBuilder(_builder);
-        _buildState = BuildState::kOutputDocs;
+        _state = State::kOutputDocs;
         return *this;
     }
 
-    ReplyBuilder& ReplyBuilder::addOutputDocs(DocumentRange outputDocs) {
-        invariant(_buildState == BuildState::kOutputDocs);
+    CommandReplyBuilder& CommandReplyBuilder::addOutputDocs(DocumentRange outputDocs) {
+        invariant(_state == State::kOutputDocs);
         auto rangeData = outputDocs.data();
         _builder.appendBuf(rangeData.data(), rangeData.length());
         // leave state as is as we can add as many outputDocs as we want.
         return *this;
     }
 
-    ReplyBuilder& ReplyBuilder::addOutputDoc(const BSONObj& outputDoc) {
-        invariant(_buildState == BuildState::kOutputDocs);
+    CommandReplyBuilder& CommandReplyBuilder::addOutputDoc(BSONObj outputDoc) {
+        invariant(_state == State::kOutputDocs);
         outputDoc.appendSelfToBufBuilder(_builder);
         return *this;
     }
 
-    std::unique_ptr<Message> ReplyBuilder::done() {
-        invariant(_buildState == BuildState::kOutputDocs);
+    ReplyBuilderInterface::State CommandReplyBuilder::getState() const {
+        return _state;
+    }
+
+    std::unique_ptr<Message> CommandReplyBuilder::done() {
+        invariant(_state == State::kOutputDocs);
         // TODO: we can elide a large copy here by transferring the internal buffer of
         // the BufBuilder to the Message.
         _message->setData(dbCommandReply, _builder.buf(), _builder.len());
-        _buildState = BuildState::kDone;
+        _state = State::kDone;
         return std::move(_message);
     }
 
diff --git a/src/mongo/rpc/reply_builder.h b/src/mongo/rpc/command_reply_builder.h
index 129edbc81f4..cf2cf01ee93 100644
--- a/src/mongo/rpc/reply_builder.h
+++ b/src/mongo/rpc/command_reply_builder.h
@@ -30,9 +30,11 @@
 
 #include <memory>
 
-#include "mongo/bson/bsonobj.h"
+#include "mongo/base/status_with.h"
+#include "mongo/db/jsobj.h"
 #include "mongo/rpc/document_range.h"
-#include "mongo/util/net/message.h" // need Message destructor for unique_ptr
+#include "mongo/rpc/reply_builder_interface.h"
+#include "mongo/util/net/message.h"
 
 namespace mongo {
 namespace rpc {
@@ -40,46 +42,41 @@ namespace rpc {
     /**
      * Constructs an OP_COMMANDREPLY message.
      */
-    class ReplyBuilder {
+    class CommandReplyBuilder : public ReplyBuilderInterface {
     public:
 
         /**
          * Constructs an OP_COMMANDREPLY in a new buffer.
          */
-        ReplyBuilder();
+        CommandReplyBuilder();
 
         /*
          * Constructs an OP_COMMANDREPLY in an existing buffer. Ownership of the buffer
-         * will be transfered to the ReplyBuilder.
+         * will be transfered to the CommandReplyBuilder.
          */
-        ReplyBuilder(std::unique_ptr<Message> message);
+        CommandReplyBuilder(std::unique_ptr<Message> message);
 
-        ReplyBuilder& setMetadata(const BSONObj& metadata);
-        ReplyBuilder& setCommandReply(const BSONObj& commandReply);
+        CommandReplyBuilder& setMetadata(BSONObj metadata) final;
+        CommandReplyBuilder& setRawCommandReply(BSONObj commandReply) final;
 
-        ReplyBuilder& addOutputDocs(DocumentRange outputDocs);
-        ReplyBuilder& addOutputDoc(const BSONObj& outputDoc);
+        CommandReplyBuilder& addOutputDocs(DocumentRange outputDocs) final;
+        CommandReplyBuilder& addOutputDoc(BSONObj outputDoc) final;
+
+        State getState() const final;
 
         /**
          * Writes data then transfers ownership of the message to the caller.
          * The behavior of calling any methods on the object is subsequently
          * undefined.
          */
-        std::unique_ptr<Message> done();
+        std::unique_ptr<Message> done() final;
 
     private:
         // Default values are all empty.
         BufBuilder _builder{};
         std::unique_ptr<Message> _message;
 
-        enum class BuildState {
-            kMetadata,
-            kCommandReply,
-            kOutputDocs,
-            kDone
-        };
-
-        BuildState _buildState{BuildState::kMetadata};
+        State _state{State::kMetadata};
     };
 
 }  // namespace rpc
diff --git a/src/mongo/rpc/reply_builder_test.cpp b/src/mongo/rpc/command_reply_builder_test.cpp
index 7302036fe4e..8771bf3691d 100644
--- a/src/mongo/rpc/reply_builder_test.cpp
+++ b/src/mongo/rpc/command_reply_builder_test.cpp
@@ -28,12 +28,11 @@
 
 #include "mongo/platform/basic.h"
 
-#include "mongo/bson/bsonobj.h"
-#include "mongo/bson/bsonobjbuilder.h"
 #include "mongo/bson/util/builder.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/rpc/command_reply.h"
+#include "mongo/rpc/command_reply_builder.h"
 #include "mongo/rpc/document_range.h"
-#include "mongo/rpc/reply.h"
-#include "mongo/rpc/reply_builder.h"
 #include "mongo/unittest/unittest.h"
 
 namespace {
@@ -47,7 +46,7 @@ namespace {
         auto metadata = metadataBob.done();
 
         BSONObjBuilder commandReplyBob{};
-        commandReplyBob.append("bar", "baz");
+        commandReplyBob.append("bar", "baz").append("ok", 1.0);
         auto commandReply = commandReplyBob.done();
 
         BSONObjBuilder outputDoc1Bob{};
@@ -69,14 +68,14 @@ namespace {
 
         rpc::DocumentRange outputDocRange{outputDocs.buf(), outputDocs.buf() + outputDocs.len()};
 
-        rpc::ReplyBuilder r;
+        rpc::CommandReplyBuilder r;
 
         auto msg = r.setMetadata(metadata)
                     .setCommandReply(commandReply)
                     .addOutputDocs(outputDocRange)
                     .done();
 
-        rpc::Reply parsed(msg.get());
+        rpc::CommandReply parsed(msg.get());
 
         ASSERT_EQUALS(parsed.getMetadata(), metadata);
         ASSERT_EQUALS(parsed.getCommandReply(), commandReply);
diff --git a/src/mongo/rpc/reply_test.cpp b/src/mongo/rpc/command_reply_test.cpp
index 44c0ad7896e..48930bbe1b9 100644
--- a/src/mongo/rpc/reply_test.cpp
+++ b/src/mongo/rpc/command_reply_test.cpp
@@ -34,9 +34,9 @@
 
 #include "mongo/base/data_type_endian.h"
 #include "mongo/base/data_view.h"
-#include "mongo/bson/bsonobjbuilder.h"
+#include "mongo/db/jsobj.h"
 #include "mongo/platform/cstdint.h"
-#include "mongo/rpc/reply.h"
+#include "mongo/rpc/command_reply.h"
 #include "mongo/stdx/memory.h"
 #include "mongo/unittest/unittest.h"
 #include "mongo/util/net/message.h"
@@ -105,7 +105,7 @@ namespace {
         auto outputDoc2 = outputDoc2Bob.done();
         writeObj(outputDoc2);
 
-        rpc::Reply opCmdReply{buildMessage()};
+        rpc::CommandReply opCmdReply{buildMessage()};
 
         ASSERT_EQUALS(opCmdReply.getMetadata(), metadata);
         ASSERT_EQUALS(opCmdReply.getCommandReply(), commandReply);
@@ -125,7 +125,7 @@ namespace {
     }
 
     TEST_F(ReplyTest, EmptyMessageThrows) {
-        ASSERT_THROWS(rpc::Reply{buildMessage()}, UserException);
+        ASSERT_THROWS(rpc::CommandReply{buildMessage()}, UserException);
     }
 
     TEST_F(ReplyTest, MetadataOnlyThrows) {
@@ -134,7 +134,7 @@ namespace {
         auto metadata = metadataBob.done();
         writeObj(metadata);
 
-        ASSERT_THROWS(rpc::Reply{buildMessage()}, UserException);
+        ASSERT_THROWS(rpc::CommandReply{buildMessage()}, UserException);
     }
 
     TEST_F(ReplyTest, MetadataInvalidLengthThrows) {
@@ -151,7 +151,7 @@ namespace {
         auto commandReply = commandReplyBob.done();
         writeObj(commandReply);
 
-        ASSERT_THROWS(rpc::Reply{buildMessage()}, UserException);
+        ASSERT_THROWS(rpc::CommandReply{buildMessage()}, UserException);
     }
 
     TEST_F(ReplyTest, InvalidLengthThrows) {
@@ -169,7 +169,7 @@ namespace {
         DataView(const_cast<char*>(commandReply.objdata())).write<LittleEndian<int32_t>>(100000);
         writeObj(commandReply, trueSize);
 
-        ASSERT_THROWS(rpc::Reply{buildMessage()}, UserException);
+        ASSERT_THROWS(rpc::CommandReply{buildMessage()}, UserException);
     }
 
 }
diff --git a/src/mongo/rpc/request.cpp b/src/mongo/rpc/command_request.cpp
index b5491928729..00a9fb7b477 100644
--- a/src/mongo/rpc/request.cpp
+++ b/src/mongo/rpc/command_request.cpp
@@ -28,7 +28,7 @@
 
 #include "mongo/platform/basic.h"
 
-#include "mongo/rpc/request.h"
+#include "mongo/rpc/command_request.h"
 
 #include <string>
 #include <utility>
@@ -36,7 +36,7 @@
 #include "mongo/base/data_range_cursor.h"
 #include "mongo/base/data_type_string_data.h"
 #include "mongo/base/data_type_terminated.h"
-#include "mongo/bson/bsonobj.h"
+#include "mongo/db/jsobj.h"
 #include "mongo/util/assert_util.h"
 #include "mongo/util/mongoutils/str.h"
 #include "mongo/util/net/message.h"
@@ -54,7 +54,7 @@ namespace rpc {
 
     }  // namespace
 
-    Request::Request(const Message* message)
+    CommandRequest::CommandRequest(const Message* message)
         : _message(message) {
         char* begin = _message->singleData().data();
         std::size_t length = _message->singleData().dataLen();
@@ -87,27 +87,27 @@ namespace rpc {
         _inputDocs = DocumentRange{cur.data(), messageEnd};
     }
 
-    StringData Request::getDatabase() const {
+    StringData CommandRequest::getDatabase() const {
         return _database;
     }
 
-    StringData Request::getCommandName() const {
+    StringData CommandRequest::getCommandName() const {
         return _commandName;
     }
 
-    const BSONObj& Request::getMetadata() const {
+    const BSONObj& CommandRequest::getMetadata() const {
         return _metadata;
     }
 
-    const BSONObj& Request::getCommandArgs() const {
+    const BSONObj& CommandRequest::getCommandArgs() const {
         return _commandArgs;
     }
 
-    DocumentRange Request::getInputDocs() const {
+    DocumentRange CommandRequest::getInputDocs() const {
         return _inputDocs;
     }
 
-    bool operator==(const Request& lhs, const Request& rhs) {
+    bool operator==(const CommandRequest& lhs, const CommandRequest& rhs) {
         return std::tie(lhs._database,
                         lhs._commandName,
                         lhs._metadata,
@@ -120,7 +120,7 @@ namespace rpc {
                         rhs._inputDocs);
     }
 
-    bool operator!=(const Request& lhs, const Request& rhs) {
+    bool operator!=(const CommandRequest& lhs, const CommandRequest& rhs) {
         return !(lhs == rhs);
     }
 
diff --git a/src/mongo/rpc/request.h b/src/mongo/rpc/command_request.h
index 12891354958..b96323477c2 100644
--- a/src/mongo/rpc/request.h
+++ b/src/mongo/rpc/command_request.h
@@ -29,8 +29,9 @@
 #pragma once
 
 #include "mongo/base/string_data.h"
-#include "mongo/bson/bsonobj.h"
+#include "mongo/db/jsobj.h"
 #include "mongo/rpc/document_range.h"
+#include "mongo/rpc/request_interface.h"
 
 namespace mongo {
     class Message;
@@ -43,34 +44,36 @@ namespace rpc {
      *
      * TODO: BSON validation. See SERVER-18167 for details.
      */
-    class Request {
+    class CommandRequest : public RequestInterface {
     public:
         /**
          * Construct a Request from a Message. Underlying message MUST outlive the Request.
          * Required fields are parsed eagerly, inputDocs are parsed lazily.
          */
-        explicit Request(const Message* message);
+        explicit CommandRequest(const Message* message);
+
+        ~CommandRequest() = default;
 
         /**
          * The database that the command is to be executed on.
          */
-        StringData getDatabase() const;
+        StringData getDatabase() const final;
 
         /**
          * The name of the command to execute.
          */
-        StringData getCommandName() const;
+        StringData getCommandName() const final;
 
         /**
          * The metadata associated with the command request. This is information that is
          * independent of any specific command, i.e. auditing information.
          */
-        const BSONObj& getMetadata() const;
+        const BSONObj& getMetadata() const final;
 
         /**
          * The arguments to the command - this is passed to the command's run() method.
          */
-        const BSONObj& getCommandArgs() const;
+        const BSONObj& getCommandArgs() const final;
 
         /**
          * A variable number of BSON documents to pass to the command. It is valid for
@@ -82,10 +85,10 @@ namespace rpc {
          *    ... do stuff with doc
          * }
          */
-        DocumentRange getInputDocs() const;
+        DocumentRange getInputDocs() const final;
 
-        friend bool operator==(const Request& lhs, const Request& rhs);
-        friend bool operator!=(const Request& lhs, const Request& rhs);
+        friend bool operator==(const CommandRequest& lhs, const CommandRequest& rhs);
+        friend bool operator!=(const CommandRequest& lhs, const CommandRequest& rhs);
 
     private:
         const Message* _message;
diff --git a/src/mongo/rpc/request_builder.cpp b/src/mongo/rpc/command_request_builder.cpp
index bbbc842ab96..b65ddc3f0b3 100644
--- a/src/mongo/rpc/request_builder.cpp
+++ b/src/mongo/rpc/command_request_builder.cpp
@@ -28,7 +28,7 @@
 
 #include "mongo/platform/basic.h"
 
-#include "mongo/rpc/request_builder.h"
+#include "mongo/rpc/command_request_builder.h"
 
 #include <utility>
 
@@ -38,61 +38,68 @@
 namespace mongo {
 namespace rpc {
 
-    RequestBuilder::RequestBuilder()
+    CommandRequestBuilder::CommandRequestBuilder()
         : _message{stdx::make_unique<Message>()}
     {}
 
-    RequestBuilder::RequestBuilder(std::unique_ptr<Message> message)
+    CommandRequestBuilder::~CommandRequestBuilder()
+    {}
+
+    CommandRequestBuilder::CommandRequestBuilder(std::unique_ptr<Message> message)
         : _message{std::move(message)}
     {}
 
-    RequestBuilder& RequestBuilder::setDatabase(StringData database) {
-        invariant(_buildState == BuildState::kDatabase);
+    CommandRequestBuilder& CommandRequestBuilder::setDatabase(StringData database) {
+        invariant(_state == State::kDatabase);
         _builder.appendStr(database);
-        _buildState = BuildState::kCommandName;
+        _state = State::kCommandName;
         return *this;
     }
 
-    RequestBuilder& RequestBuilder::setCommandName(StringData commandName) {
-        invariant(_buildState == BuildState::kCommandName);
+    CommandRequestBuilder& CommandRequestBuilder::setCommandName(StringData commandName) {
+        invariant(_state == State::kCommandName);
         _builder.appendStr(commandName);
-        _buildState = BuildState::kMetadata;
+        _state = State::kMetadata;
         return *this;
     }
 
-    RequestBuilder& RequestBuilder::setMetadata(const BSONObj& metadata) {
-        invariant(_buildState == BuildState::kMetadata);
+    CommandRequestBuilder& CommandRequestBuilder::setMetadata(BSONObj metadata) {
+        invariant(_state == State::kMetadata);
         metadata.appendSelfToBufBuilder(_builder);
-        _buildState = BuildState::kCommandArgs;
+        _state = State::kCommandArgs;
         return *this;
     }
 
-    RequestBuilder& RequestBuilder::setCommandArgs(const BSONObj& commandArgs) {
-        invariant(_buildState == BuildState::kCommandArgs);
+    CommandRequestBuilder& CommandRequestBuilder::setCommandArgs(BSONObj commandArgs) {
+        invariant(_state == State::kCommandArgs);
         commandArgs.appendSelfToBufBuilder(_builder);
-        _buildState = BuildState::kInputDocs;
+        _state = State::kInputDocs;
         return *this;
     }
 
-    RequestBuilder& RequestBuilder::addInputDocs(DocumentRange inputDocs) {
-        invariant(_buildState == BuildState::kInputDocs);
+    CommandRequestBuilder& CommandRequestBuilder::addInputDocs(DocumentRange inputDocs) {
+        invariant(_state == State::kInputDocs);
         auto rangeData = inputDocs.data();
         _builder.appendBuf(rangeData.data(), rangeData.length());
         return *this;
     }
 
-    RequestBuilder& RequestBuilder::addInputDoc(const BSONObj& inputDoc) {
-        invariant(_buildState == BuildState::kInputDocs);
+    CommandRequestBuilder& CommandRequestBuilder::addInputDoc(BSONObj inputDoc) {
+        invariant(_state == State::kInputDocs);
         inputDoc.appendSelfToBufBuilder(_builder);
         return *this;
     }
 
-    std::unique_ptr<Message> RequestBuilder::done() {
-        invariant(_buildState == BuildState::kInputDocs);
+    RequestBuilderInterface::State CommandRequestBuilder::getState() const {
+        return _state;
+    }
+
+    std::unique_ptr<Message> CommandRequestBuilder::done() {
+        invariant(_state == State::kInputDocs);
         // TODO: we can elide a large copy here by transferring the internal buffer of
         // the BufBuilder to the Message.
         _message->setData(dbCommand, _builder.buf(), _builder.len());
-        _buildState = BuildState::kDone;
+        _state = State::kDone;
         return std::move(_message);
     }
 
diff --git a/src/mongo/rpc/request_builder.h b/src/mongo/rpc/command_request_builder.h
index 5715667ab35..12298a48f3c 100644
--- a/src/mongo/rpc/request_builder.h
+++ b/src/mongo/rpc/command_request_builder.h
@@ -31,8 +31,9 @@
 #include <memory>
 
 #include "mongo/base/string_data.h"
-#include "mongo/bson/bsonobj.h"
+#include "mongo/db/jsobj.h"
 #include "mongo/rpc/document_range.h"
+#include "mongo/rpc/request_builder_interface.h"
 #include "mongo/util/net/message.h"
 
 namespace mongo {
@@ -41,49 +42,48 @@ namespace rpc {
     /**
      * Constructs an OP_COMMAND message.
      */
-    class RequestBuilder {
+    class CommandRequestBuilder : public RequestBuilderInterface {
     public:
 
         /**
          * Constructs an OP_COMMAND in a new buffer.
          */
-        RequestBuilder();
+        CommandRequestBuilder();
+
+        ~CommandRequestBuilder() final;
 
         /**
          * Construct an OP_COMMAND in an existing buffer. Ownership of the buffer will be
-         * transfered to the RequestBuilder.
+         * transfered to the CommandRequestBuilder.
          */
-        RequestBuilder(std::unique_ptr<Message> message);
+        CommandRequestBuilder(std::unique_ptr<Message> message);
+
+        CommandRequestBuilder& setDatabase(StringData database) final;
+
+        CommandRequestBuilder& setCommandName(StringData commandName) final;
+
+        CommandRequestBuilder& setMetadata(BSONObj metadata) final;
 
-        RequestBuilder& setDatabase(StringData database);
-        RequestBuilder& setCommandName(StringData commandName);
-        RequestBuilder& setMetadata(const BSONObj& metadata);
-        RequestBuilder& setCommandArgs(const BSONObj& commandArgs);
+        CommandRequestBuilder& setCommandArgs(BSONObj commandArgs) final;
 
-        RequestBuilder& addInputDocs(DocumentRange inputDocs);
-        RequestBuilder& addInputDoc(const BSONObj& inputDoc);
+        CommandRequestBuilder& addInputDocs(DocumentRange inputDocs) final;
+
+        CommandRequestBuilder& addInputDoc(BSONObj inputDoc) final;
+
+        State getState() const final;
 
         /**
          * Writes data then transfers ownership of the message to the caller.
          * The behavior of calling any methods on the object is subsequently
          * undefined.
          */
-        std::unique_ptr<Message> done();
+        std::unique_ptr<Message> done() final;
 
     private:
         BufBuilder _builder{};
         std::unique_ptr<Message> _message;
 
-        enum class BuildState {
-            kDatabase,
-            kCommandName,
-            kMetadata,
-            kCommandArgs,
-            kInputDocs,
-            kDone
-        };
-
-        BuildState _buildState{BuildState::kDatabase};
+        State _state{State::kDatabase};
     };
 
 }  // rpc
diff --git a/src/mongo/rpc/request_builder_test.cpp b/src/mongo/rpc/command_request_builder_test.cpp
index 1d47ad79c9b..10a4f2d8795 100644
--- a/src/mongo/rpc/request_builder_test.cpp
+++ b/src/mongo/rpc/command_request_builder_test.cpp
@@ -28,12 +28,10 @@
 
 #include "mongo/platform/basic.h"
 
-#include "mongo/bson/bsonobj.h"
-#include "mongo/bson/bsonobjbuilder.h"
-#include "mongo/bson/util/builder.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/rpc/command_request.h"
+#include "mongo/rpc/command_request_builder.h"
 #include "mongo/rpc/document_range.h"
-#include "mongo/rpc/request.h"
-#include "mongo/rpc/request_builder.h"
 #include "mongo/unittest/unittest.h"
 
 namespace {
@@ -72,7 +70,7 @@ namespace {
 
         rpc::DocumentRange inputDocRange{inputDocs.buf(), inputDocs.buf() + inputDocs.len()};
 
-        rpc::RequestBuilder r;
+        rpc::CommandRequestBuilder r;
 
         auto msg = r.setDatabase(databaseName)
                     .setCommandName(commandName)
@@ -81,7 +79,7 @@ namespace {
                     .addInputDocs(inputDocRange)
                     .done();
 
-        rpc::Request parsed(msg.get());
+        rpc::CommandRequest parsed(msg.get());
 
         ASSERT_EQUALS(parsed.getDatabase(), databaseName);
         ASSERT_EQUALS(parsed.getCommandName(), commandName);
diff --git a/src/mongo/rpc/request_test.cpp b/src/mongo/rpc/command_request_test.cpp
index da7c2d316db..7f5c5b52214 100644
--- a/src/mongo/rpc/request_test.cpp
+++ b/src/mongo/rpc/command_request_test.cpp
@@ -32,8 +32,8 @@
 #include <string>
 #include <vector>
 
-#include "mongo/bson/bsonobjbuilder.h"
-#include "mongo/rpc/request.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/rpc/command_request.h"
 #include "mongo/unittest/unittest.h"
 #include "mongo/util/net/message.h"
 
@@ -86,7 +86,7 @@ namespace {
         Message toSend;
         toSend.setData(dbCommand, opCommandData.data(), opCommandData.size());
 
-        rpc::Request opCmd{&toSend};
+        rpc::CommandRequest opCmd{&toSend};
 
         ASSERT_EQUALS(opCmd.getCommandName(), commandName);
         ASSERT_EQUALS(opCmd.getDatabase(), database);
@@ -107,4 +107,3 @@ namespace {
         ASSERT_TRUE(inputDocRangeIter == inputDocRange.end());
     }
 }  // namespace
-
diff --git a/src/mongo/rpc/document_range.h b/src/mongo/rpc/document_range.h
index 04f00179fdd..aa8d3131b13 100644
--- a/src/mongo/rpc/document_range.h
+++ b/src/mongo/rpc/document_range.h
@@ -33,7 +33,7 @@
 
 #include "mongo/base/data_range.h"
 #include "mongo/base/data_range_cursor.h"
-#include "mongo/bson/bsonobj.h"
+#include "mongo/db/jsobj.h"
 
 namespace mongo {
 namespace rpc {
diff --git a/src/mongo/rpc/factory.cpp b/src/mongo/rpc/factory.cpp
new file mode 100644
index 00000000000..1edca3950b3
--- /dev/null
+++ b/src/mongo/rpc/factory.cpp
@@ -0,0 +1,70 @@
+/**
+ *    Copyright (C) 2015 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/rpc/factory.h"
+
+#include "mongo/rpc/command_reply.h"
+#include "mongo/rpc/command_request_builder.h"
+#include "mongo/rpc/legacy_reply.h"
+#include "mongo/rpc/legacy_request_builder.h"
+#include "mongo/rpc/protocol.h"
+#include "mongo/stdx/memory.h"
+#include "mongo/util/assert_util.h"
+
+namespace mongo {
+namespace rpc {
+
+    std::unique_ptr<RequestBuilderInterface> makeRequestBuilder(ProtocolSet clientProtos,
+                                                                ProtocolSet serverProtos) {
+        switch (uassertStatusOK(negotiate(clientProtos, serverProtos))) {
+        case Protocol::kOpQuery:
+            return stdx::make_unique<LegacyRequestBuilder>();
+        case Protocol::kOpCommandV1:
+            return stdx::make_unique<CommandRequestBuilder>();
+        default:
+            MONGO_UNREACHABLE;
+        }
+    }
+
+    std::unique_ptr<ReplyInterface> makeReply(const Message* unownedMessage,
+                                              ProtocolSet clientProtos,
+                                              ProtocolSet serverProtos) {
+        switch (uassertStatusOK(negotiate(clientProtos, serverProtos))) {
+        case Protocol::kOpQuery:
+            return stdx::make_unique<LegacyReply>(unownedMessage);
+        case Protocol::kOpCommandV1:
+            return stdx::make_unique<CommandReply>(unownedMessage);
+        default:
+            MONGO_UNREACHABLE;
+        }
+    }
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/factory.h b/src/mongo/rpc/factory.h
new file mode 100644
index 00000000000..0e178a54ecb
--- /dev/null
+++ b/src/mongo/rpc/factory.h
@@ -0,0 +1,60 @@
+/**
+ *    Copyright (C) 2015 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 "mongo/rpc/protocol.h"
+
+#include <memory>
+
+/**
+ * Utilities to construct the correct concrete rpc class based on what the remote server
+ * supports, and what the client has been configured to do.
+ */
+
+namespace mongo {
+    class Message;
+
+namespace rpc {
+    class ReplyInterface;
+    class RequestBuilderInterface;
+
+    /**
+     * Returns the appropriate concrete RequestBuilder. Throws if one cannot be chosen.
+     */
+    std::unique_ptr<RequestBuilderInterface> makeRequestBuilder(ProtocolSet clientProtos,
+                                                                ProtocolSet serverProtos);
+    /**
+     * Returns the appropriate concrete Reply. Throws if one cannot be chosen.
+     */
+    std::unique_ptr<ReplyInterface> makeReply(const Message* unownedMessage,
+                                              ProtocolSet clientProtos,
+                                              ProtocolSet serverProtos);
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/legacy_reply.cpp b/src/mongo/rpc/legacy_reply.cpp
new file mode 100644
index 00000000000..1dc40c83cd6
--- /dev/null
+++ b/src/mongo/rpc/legacy_reply.cpp
@@ -0,0 +1,82 @@
+/**
+ *    Copyright (C) 2015 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/rpc/legacy_reply.h"
+
+#include <utility>
+
+#include "mongo/util/assert_util.h"
+#include "mongo/util/mongoutils/str.h"
+
+namespace mongo {
+namespace rpc {
+
+    LegacyReply::LegacyReply(const Message* message)
+        : _message(std::move(message)) {
+        invariant(message->operation() == opReply);
+
+        QueryResult::View qr = _message->singleData().view2ptr();
+
+        // should be checked by caller.
+        invariant(qr.msgdata().getOperation() == opReply);
+
+        uassert(ErrorCodes::BadValue,
+                str::stream() << "Got legacy command reply with a bad cursorId field,"
+                              << " expected a value of 0 but got " << qr.getCursorId(),
+                qr.getCursorId() == 0);
+
+        uassert(ErrorCodes::BadValue,
+                str::stream() << "Got legacy command reply with a bad nReturned field,"
+                              << " expected a value of 1 but got " << qr.getNReturned(),
+                qr.getNReturned() == 1);
+
+        uassert(ErrorCodes::BadValue,
+                str::stream() << "Got legacy command reply with a bad startingFrom field,"
+                              << " expected a value of 0 but got " << qr.getStartingFrom(),
+                qr.getStartingFrom() == 0);
+        // TODO bson validation
+        _commandReply = BSONObj(qr.data());
+    }
+
+    const BSONObj& LegacyReply::getMetadata() const {
+        return _metadataPlaceholder;
+    }
+
+    const BSONObj& LegacyReply::getCommandReply() const {
+        return _commandReply;
+    }
+
+    DocumentRange LegacyReply::getOutputDocs() const {
+        // return empty range
+        return DocumentRange{};
+    }
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/legacy_reply.h b/src/mongo/rpc/legacy_reply.h
new file mode 100644
index 00000000000..fddc050a71b
--- /dev/null
+++ b/src/mongo/rpc/legacy_reply.h
@@ -0,0 +1,87 @@
+/**
+ *    Copyright (C) 2015 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 "mongo/db/dbmessage.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/rpc/document_range.h"
+#include "mongo/rpc/reply_interface.h"
+
+namespace mongo {
+    class Message;
+
+namespace rpc {
+
+    /**
+     * Immutable view of an OP_REPLY legacy-style command reply.
+     *
+     * TODO: BSON validation (SERVER-18167)
+     */
+    class LegacyReply : public ReplyInterface {
+    public:
+
+        /**
+         * Construct a Reply from a Message.
+         * The underlying message MUST outlive the Reply.
+         */
+        explicit LegacyReply(const Message* message);
+
+        /**
+         * Accessor for the metadata object. Metadata is generally used for information
+         * that is independent of any specific command, e.g. auditing information.
+         */
+        const BSONObj& getMetadata() const final;
+
+        /**
+         * The result of executing the command.
+         */
+        const BSONObj& getCommandReply() const final;
+
+        /**
+         * A variable number of BSON documents returned by the command. It is valid for the
+         * returned range to be empty.
+         *
+         * Example usage:
+         *
+         * for (auto&& doc : reply.getOutputDocs()) {
+         *    ... do stuff with doc
+         * }
+         */
+        DocumentRange getOutputDocs() const final;
+
+    private:
+        const Message* _message;
+
+        // TODO: SERVER-18236
+        BSONObj _metadataPlaceholder{};
+        BSONObj _commandReply{}; // will hold unowned
+    };
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/legacy_reply_builder.cpp b/src/mongo/rpc/legacy_reply_builder.cpp
new file mode 100644
index 00000000000..4ed4ff88886
--- /dev/null
+++ b/src/mongo/rpc/legacy_reply_builder.cpp
@@ -0,0 +1,101 @@
+/**
+ *    Copyright (C) 2015 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/db/dbmessage.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/rpc/legacy_reply_builder.h"
+#include "mongo/stdx/memory.h"
+#include "mongo/util/assert_util.h"
+
+namespace mongo {
+namespace rpc {
+
+    LegacyReplyBuilder::LegacyReplyBuilder()
+        : LegacyReplyBuilder(stdx::make_unique<Message>())
+    {}
+
+    LegacyReplyBuilder::LegacyReplyBuilder(std::unique_ptr<Message> message)
+        : _message{std::move(message)} {
+        _builder.skip(sizeof(QueryResult::Value));
+    }
+
+    LegacyReplyBuilder::~LegacyReplyBuilder() {}
+
+    LegacyReplyBuilder& LegacyReplyBuilder::setMetadata(BSONObj metadata) {
+        invariant(_state == State::kMetadata);
+        // no op for now: SERVER-18236
+        _state = State::kCommandReply;
+        return *this;
+    }
+
+    LegacyReplyBuilder& LegacyReplyBuilder::setRawCommandReply(BSONObj commandReply) {
+        invariant(_state == State::kCommandReply);
+        commandReply.appendSelfToBufBuilder(_builder);
+        _state = State::kOutputDocs;
+        return *this;
+    }
+
+    LegacyReplyBuilder& LegacyReplyBuilder::addOutputDocs(DocumentRange outputDocs) {
+        invariant(_state == State::kOutputDocs);
+        // no op
+        return *this;
+    }
+
+    LegacyReplyBuilder& LegacyReplyBuilder::addOutputDoc(BSONObj outputDoc) {
+        invariant(_state == State::kOutputDocs);
+        // no op
+        return *this;
+    }
+
+    ReplyBuilderInterface::State LegacyReplyBuilder::getState() const {
+        return _state;
+    }
+
+    std::unique_ptr<Message> LegacyReplyBuilder::done() {
+        invariant(_state == State::kOutputDocs);
+        std::unique_ptr<Message> message = stdx::make_unique<Message>();
+
+        QueryResult::View qr = _builder.buf();
+        qr.setResultFlagsToOk();
+        qr.msgdata().setLen(_builder.len());
+        qr.msgdata().setOperation(opReply);
+        qr.setCursorId(0);
+        qr.setStartingFrom(0);
+        qr.setNReturned(1);
+        _builder.decouple();
+
+        message->setData(qr.view2ptr(), true);
+
+        _state = State::kDone;
+        return std::move(message);
+    }
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/legacy_reply_builder.h b/src/mongo/rpc/legacy_reply_builder.h
new file mode 100644
index 00000000000..35dc49a911e
--- /dev/null
+++ b/src/mongo/rpc/legacy_reply_builder.h
@@ -0,0 +1,65 @@
+/**
+ *    Copyright (C) 2015 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 <memory>
+
+#include "mongo/base/status_with.h"
+#include "mongo/bson/util/builder.h"
+#include "mongo/rpc/document_range.h"
+#include "mongo/rpc/reply_builder_interface.h"
+
+namespace mongo {
+namespace rpc {
+
+    class LegacyReplyBuilder : public ReplyBuilderInterface {
+    public:
+
+        LegacyReplyBuilder();
+        LegacyReplyBuilder(std::unique_ptr<Message>);
+        ~LegacyReplyBuilder() final;
+
+        LegacyReplyBuilder& setMetadata(BSONObj metadata) final;
+        LegacyReplyBuilder& setRawCommandReply(BSONObj commandReply) final;
+
+        LegacyReplyBuilder& addOutputDocs(DocumentRange outputDocs) final;
+        LegacyReplyBuilder& addOutputDoc(BSONObj outputDoc) final;
+
+        State getState() const final;
+
+        std::unique_ptr<Message> done() final;
+
+    private:
+        BufBuilder _builder{};
+        std::unique_ptr<Message> _message;
+        State _state{State::kMetadata};
+    };
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/legacy_request.cpp b/src/mongo/rpc/legacy_request.cpp
new file mode 100644
index 00000000000..bb79928abb9
--- /dev/null
+++ b/src/mongo/rpc/legacy_request.cpp
@@ -0,0 +1,78 @@
+/**
+ *    Copyright (C) 2015 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 <utility>
+
+#include "mongo/db/namespace_string.h"
+#include "mongo/rpc/legacy_request.h"
+#include "mongo/rpc/metadata.h"
+#include "mongo/util/assert_util.h"
+
+namespace mongo {
+namespace rpc {
+
+    LegacyRequest::LegacyRequest(const Message *message)
+        : _message(std::move(message))
+        , _dbMessage(*message)
+        , _queryMessage(_dbMessage)
+        , _database(NamespaceString(_queryMessage.ns).db().toString()) {
+
+        std::tie(_upconvertedCommandArgs, _upconvertedMetadata) = uassertStatusOK(
+            metadata::upconvertRequest(std::move(_queryMessage.query),
+                                       std::move(_queryMessage.queryOptions))
+        );
+    }
+
+    LegacyRequest::~LegacyRequest() = default;
+
+    StringData LegacyRequest::getDatabase() const {
+        return _database;
+    }
+
+    StringData LegacyRequest::getCommandName() const {
+        return _upconvertedCommandArgs.firstElement().fieldNameStringData();
+    }
+
+    const BSONObj& LegacyRequest::getMetadata() const {
+        // TODO SERVER-18236
+        return _upconvertedMetadata;
+    }
+
+    const BSONObj& LegacyRequest::getCommandArgs() const {
+        return _upconvertedCommandArgs;
+    }
+
+    DocumentRange LegacyRequest::getInputDocs() const {
+        // return an empty document range.
+        return DocumentRange{};
+    }
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/legacy_request.h b/src/mongo/rpc/legacy_request.h
new file mode 100644
index 00000000000..93567a6fe6c
--- /dev/null
+++ b/src/mongo/rpc/legacy_request.h
@@ -0,0 +1,103 @@
+/**
+ *    Copyright (C) 2015 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 "mongo/base/string_data.h"
+#include "mongo/db/dbmessage.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/rpc/document_range.h"
+#include "mongo/rpc/request_interface.h"
+
+namespace mongo {
+    class Message;
+
+namespace rpc {
+
+    /**
+     * An immutable view of an OP_QUERY command request. The underlying bytes are owned
+     * by a mongo::Message, which must outlive any LegacyRequest instances created from it.
+     *
+     */
+    class LegacyRequest : public RequestInterface {
+    public:
+        /**
+         * Construct a Request from a Message. Underlying message MUST outlive the Request.
+         * Required fields are parsed eagerly, inputDocs are parsed lazily.
+         */
+        explicit LegacyRequest(const Message* message);
+
+        ~LegacyRequest() final;
+
+        /**
+         * The database that the command is to be executed on.
+         */
+        StringData getDatabase() const final;
+
+        /**
+         * The name of the command to execute.
+         */
+        StringData getCommandName() const final;
+
+        /**
+         * The metadata associated with the command request. This is information that is
+         * independent of any specific command, i.e. auditing information.
+         */
+        const BSONObj& getMetadata() const final;
+
+        /**
+         * The arguments to the command - this is passed to the command's run() method.
+         */
+        const BSONObj& getCommandArgs() const final;
+
+        /**
+         * A variable number of BSON documents to pass to the command. It is valid for
+         * the returned range to be empty.
+         *
+         * Example usage:
+         *
+         * for (auto&& doc : req.getInputDocs()) {
+         *    ... do stuff with doc
+         * }
+         */
+        DocumentRange getInputDocs() const final;
+
+    private:
+        const Message* _message;
+        // TODO: metadata will be handled in SERVER-18236
+        // for now getMetadata() is a no op
+        DbMessage _dbMessage;
+        QueryMessage _queryMessage;
+        std::string _database;
+
+        BSONObj _upconvertedMetadata;
+        BSONObj _upconvertedCommandArgs;
+    };
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/legacy_request_builder.cpp b/src/mongo/rpc/legacy_request_builder.cpp
new file mode 100644
index 00000000000..f1978583df8
--- /dev/null
+++ b/src/mongo/rpc/legacy_request_builder.cpp
@@ -0,0 +1,121 @@
+/**
+ *    Copyright (C) 2015 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/rpc/legacy_request_builder.h"
+
+#include <utility>
+#include <tuple>
+
+#include "mongo/db/namespace_string.h"
+#include "mongo/rpc/metadata.h"
+#include "mongo/stdx/memory.h"
+#include "mongo/util/assert_util.h"
+
+namespace mongo {
+namespace rpc {
+
+    LegacyRequestBuilder::LegacyRequestBuilder()
+        : _message{stdx::make_unique<Message>()}
+    {}
+
+    LegacyRequestBuilder::~LegacyRequestBuilder()
+    {}
+
+    LegacyRequestBuilder::LegacyRequestBuilder(std::unique_ptr<Message> message)
+        : _message{std::move(message)}
+    {}
+
+    LegacyRequestBuilder& LegacyRequestBuilder::setDatabase(StringData database) {
+        invariant(_state == State::kDatabase);
+        _ns = NamespaceString(database, "$cmd").toString();
+        _state = State::kCommandName;
+        return *this;
+    }
+
+    LegacyRequestBuilder& LegacyRequestBuilder::setCommandName(StringData commandName) {
+        invariant(_state == State::kCommandName);
+        // no op, as commandName is the first element of commandArgs
+        _state = State::kMetadata;
+        return *this;
+    }
+
+    LegacyRequestBuilder& LegacyRequestBuilder::setMetadata(BSONObj metadata) {
+        invariant(_state == State::kMetadata);
+        _metadata = std::move(metadata);
+        _state = State::kCommandArgs;
+        return *this;
+    }
+
+    LegacyRequestBuilder& LegacyRequestBuilder::setCommandArgs(BSONObj commandArgs) {
+        invariant(_state == State::kCommandArgs);
+
+        BSONObj legacyCommandArgs;
+        int queryOptions;
+
+        std::tie(legacyCommandArgs, queryOptions) = uassertStatusOK(
+            metadata::downconvertRequest(std::move(commandArgs),
+                                         std::move(_metadata))
+        );
+
+        _builder.appendNum(queryOptions); // queryOptions
+        _builder.appendStr(_ns);
+        _builder.appendNum(0); // nToSkip
+        _builder.appendNum(1); // nToReturn
+
+        legacyCommandArgs.appendSelfToBufBuilder(_builder);
+        _state = State::kInputDocs;
+        return *this;
+    }
+
+    LegacyRequestBuilder& LegacyRequestBuilder::addInputDocs(DocumentRange inputDocs) {
+        invariant(_state == State::kInputDocs);
+        // no op
+        return *this;
+    }
+
+    LegacyRequestBuilder& LegacyRequestBuilder::addInputDoc(BSONObj inputDoc) {
+        invariant(_state == State::kInputDocs);
+        // no op
+        return *this;
+    }
+
+    RequestBuilderInterface::State LegacyRequestBuilder::getState() const {
+        return _state;
+    }
+
+    std::unique_ptr<Message> LegacyRequestBuilder::done() {
+        invariant(_state == State::kInputDocs);
+        _message->setData(dbQuery, _builder.buf(), _builder.len());
+        _state = State::kDone;
+        return std::move(_message);
+    }
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/legacy_request_builder.h b/src/mongo/rpc/legacy_request_builder.h
new file mode 100644
index 00000000000..0beed3b9a71
--- /dev/null
+++ b/src/mongo/rpc/legacy_request_builder.h
@@ -0,0 +1,75 @@
+/**
+ *    Copyright (C) 2015 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 <memory>
+
+#include "mongo/base/string_data.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/rpc/document_range.h"
+#include "mongo/rpc/request_builder_interface.h"
+#include "mongo/util/net/message.h"
+
+namespace mongo {
+namespace rpc {
+
+    class LegacyRequestBuilder : public RequestBuilderInterface {
+    public:
+        LegacyRequestBuilder();
+        ~LegacyRequestBuilder() final;
+
+        LegacyRequestBuilder(std::unique_ptr<Message>);
+
+        LegacyRequestBuilder& setDatabase(StringData database) final;
+        LegacyRequestBuilder& setCommandName(StringData commandName) final;
+        LegacyRequestBuilder& setMetadata(BSONObj metadata) final;
+        LegacyRequestBuilder& setCommandArgs(BSONObj commandArgs) final;
+
+        LegacyRequestBuilder& addInputDocs(DocumentRange inputDocs) final;
+        LegacyRequestBuilder& addInputDoc(BSONObj inputDoc) final;
+
+        State getState() const final;
+
+        std::unique_ptr<Message> done() final;
+
+    private:
+        std::unique_ptr<Message> _message;
+        BufBuilder _builder{};
+
+        // we need to stash this as we need commandArgs to
+        // upconvert.
+        BSONObj _metadata;
+
+        std::string _ns{}; // copied to in setDatabase
+
+        State _state{State::kDatabase};
+    };
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/metadata.cpp b/src/mongo/rpc/metadata.cpp
new file mode 100644
index 00000000000..055dfd07984
--- /dev/null
+++ b/src/mongo/rpc/metadata.cpp
@@ -0,0 +1,69 @@
+/**
+ *    Copyright (C) 2015 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/rpc/metadata.h"
+
+#include "mongo/client/dbclientinterface.h"
+#include "mongo/db/jsobj.h"
+
+namespace mongo {
+namespace rpc {
+namespace metadata {
+
+    BSONObj empty() {
+        return BSONObj();
+    }
+
+    const char kSecondaryOk[] = "$secondaryOk";
+
+    StatusWith<CommandAndMetadata> upconvertRequest(BSONObj legacyCmdObj, int queryFlags) {
+        BSONObjBuilder metadataBob;
+
+        // note second check may be erroneous: see SERVER-18194
+        if ((queryFlags & QueryOption_SlaveOk)) {
+            metadataBob.append(kSecondaryOk, 1);
+        }
+
+        return std::make_tuple(std::move(legacyCmdObj), std::move(metadataBob.obj()));
+    }
+
+    StatusWith<LegacyCommandAndFlags> downconvertRequest(BSONObj cmdObj, BSONObj metadata) {
+        int flags = 0;
+
+        if (metadata.hasField(kSecondaryOk)) {
+            flags |= QueryOption_SlaveOk;
+        }
+
+        return std::make_tuple(std::move(cmdObj), flags);
+    }
+
+}  // namespace metadata
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/metadata.h b/src/mongo/rpc/metadata.h
new file mode 100644
index 00000000000..183dedffc5d
--- /dev/null
+++ b/src/mongo/rpc/metadata.h
@@ -0,0 +1,93 @@
+/**
+ *    Copyright (C) 2015 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 <tuple>
+
+#include "mongo/base/status_with.h"
+
+namespace mongo {
+    class BSONObj;
+
+/**
+ * Utilities for converting metadata between the legacy OP_QUERY format and the new
+ * OP_COMMAND format.
+ *
+ * Metadata consists of information independent of any particular command such as:
+ *
+ * Request/Reply/Both | (legacy) OP_QUERY format         | OP_COMMAND format
+ *__________________________________________________________________________________________________
+ * Request            | the slaveOk bit                  | $secondaryOk on metadata obj
+ * Request            | $readPreference field of command | $readPreference on metadata obj
+ * Request            | $impersonatedUsers on command obj| $impersonatedUsers on metadata obj
+ * Request            | $impersonatedRoles on command obj| $impersonatedRoles on metadata obj
+ * Request            | maxTimeMS on command obj         | $maxTimeMS on metadata obj
+ * Reply              | $gleStats field on command reply | $gleStats on metadata obj
+ *
+ * TODO: currently only $secondaryOk (request only) is handled. SERVER-18236 will cover the rest.
+ */
+namespace rpc {
+namespace metadata {
+
+    /**
+     * Returns an empty metadata object.
+     */
+    BSONObj empty();
+
+    /**
+     * The field name for the secondaryOk metadata field.
+     */
+    extern const char kSecondaryOk[];
+
+    /**
+     * A command object and a corresponding metadata object.
+     */
+    using CommandAndMetadata = std::tuple<BSONObj, BSONObj>;
+
+    /**
+     * A legacy command object and a corresponding query flags bitfield. The legacy command object
+     * may contain metadata fields, so it cannot safely be passed to a command's run method.
+     */
+    using LegacyCommandAndFlags = std::tuple<BSONObj, int>;
+
+    /**
+     * Given a legacy command object and a query flags bitfield, attempts to parse and remove
+     * the metadata from the command object and construct a corresponding metadata object.
+     */
+    StatusWith<CommandAndMetadata> upconvertRequest(BSONObj legacyCmdObj, int queryFlags);
+
+    /**
+     * Given a command object and a metadata object, attempts to construct a legacy command
+     * object and query flags bitfield augmented with the given metadata.
+     */
+    StatusWith<LegacyCommandAndFlags> downconvertRequest(BSONObj cmdObj, BSONObj metadata);
+
+}  // namespace metadata
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/protocol.cpp b/src/mongo/rpc/protocol.cpp
new file mode 100644
index 00000000000..dae77abce3b
--- /dev/null
+++ b/src/mongo/rpc/protocol.cpp
@@ -0,0 +1,117 @@
+/**
+ *    Copyright (C) 2015 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/rpc/protocol.h"
+
+#include <algorithm>
+#include <iterator>
+
+#include "mongo/base/string_data.h"
+#include "mongo/util/mongoutils/str.h"
+
+namespace mongo {
+namespace rpc {
+
+namespace {
+
+    /**
+     * Protocols supported by order of preference.
+     */
+    const Protocol kPreferredProtos[] = {
+        Protocol::kOpCommandV1,
+        Protocol::kOpQuery
+    };
+
+    const char kNone[] = "none";
+    const char kOpQueryOnly[] = "opQueryOnly";
+    const char kOpCommandOnly[] = "opCommandOnly";
+    const char kAll[] = "all";
+
+}  // namespace
+
+    StatusWith<Protocol> negotiate(ProtocolSet fst, ProtocolSet snd) {
+        using std::begin;
+        using std::end;
+
+        ProtocolSet common = fst & snd;
+
+        auto it =
+            std::find_if(begin(kPreferredProtos), end(kPreferredProtos), [common](Protocol p) {
+                return common & static_cast<ProtocolSet>(p);
+        });
+
+        if (it == end(kPreferredProtos)) {
+            return Status(ErrorCodes::RPCProtocolNegotiationFailed,
+                          "No common protocol found.");
+        }
+        return *it;
+    }
+
+    StatusWith<StringData> toString(ProtocolSet protocols) {
+        switch (protocols) {
+        case supports::kNone:
+            return StringData(kNone);
+        case supports::kOpQueryOnly:
+            return StringData(kOpQueryOnly);
+        case supports::kOpCommandOnly:
+            return StringData(kOpCommandOnly);
+        case supports::kAll:
+            return StringData(kAll);
+        default:
+            return Status(ErrorCodes::BadValue,
+                          str::stream() << "Can not convert ProtocolSet " << protocols
+                                        << " to a string, only the predefined ProtocolSet "
+                                        << "constants 'none' (0x0), 'opQueryOnly' (0x1), "
+                                        << "'opCommandOnly' (0x2), and 'all' (0x3) are supported.");
+        }
+    }
+
+    StatusWith<ProtocolSet> parseProtocolSet(StringData repr) {
+        if (repr == kNone) {
+            return supports::kNone;
+        }
+        else if (repr == kOpQueryOnly) {
+            return supports::kOpQueryOnly;
+        }
+        else if (repr == kOpCommandOnly) {
+            return supports::kOpCommandOnly;
+        }
+        else if (repr == kAll) {
+            return supports::kAll;
+        }
+        return Status(ErrorCodes::BadValue,
+                      str::stream() << "Can not parse a ProtocolSet from " << repr
+                                    << " only the predefined ProtocolSet constants "
+                                    << "'none' (0x0), 'opQueryOnly' (0x1), 'opCommandOnly' (0x2), "
+                                    << "and 'all' (0x3) are supported.");
+    }
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/protocol.h b/src/mongo/rpc/protocol.h
new file mode 100644
index 00000000000..ba370a90e69
--- /dev/null
+++ b/src/mongo/rpc/protocol.h
@@ -0,0 +1,99 @@
+/**
+ *    Copyright (C) 2015 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 <string>
+#include <type_traits>
+
+#include "mongo/base/status_with.h"
+#include "mongo/platform/cstdint.h"
+
+namespace mongo {
+namespace rpc {
+
+    /**
+     * Bit flags representing support for a particular RPC protocol.
+     * This is just an internal representation, and is never transmitted over the wire. It should
+     * never be used for any other feature detection in favor of max/min wire version.
+     *
+     * A new protocol must be added as the highest order bit flag so that it is prioritized in
+     * negotiation.
+     */
+    enum class Protocol : std::uint64_t {
+
+        /**
+         * The pre-3.2 OP_QUERY on db.$cmd protocol
+         */
+        kOpQuery = 1 << 0,
+
+        /**
+         * The post-3.2 OP_COMMAND protocol.
+         */
+        kOpCommandV1 = 1 << 1,
+    };
+
+    /**
+     * Bitfield representing a set of supported RPC protocols.
+     */
+    using ProtocolSet = std::underlying_type<Protocol>::type;
+
+/**
+ * This namespace contains predefined bitfields for common levels of protocol support.
+ */
+namespace supports {
+
+    const ProtocolSet kNone = ProtocolSet{0};
+    const ProtocolSet kOpQueryOnly = static_cast<ProtocolSet>(Protocol::kOpQuery);
+    const ProtocolSet kOpCommandOnly = static_cast<ProtocolSet>(Protocol::kOpCommandV1);
+    const ProtocolSet kAll = kOpQueryOnly | kOpCommandOnly;
+
+}  // namespace supports
+
+    /**
+     * Returns the newest protocol supported by two parties.
+     */
+    StatusWith<Protocol> negotiate(ProtocolSet fst, ProtocolSet snd);
+
+    /**
+     * Converts a ProtocolSet to a string. Currently only the predefined ProtocolSets in the
+     * 'supports' namespace are supported.
+     * 
+     * This intentionally does not conform to the STL 'to_string' convention so that it will
+     * not conflict with the to_string overload for uint64_t.
+     */
+    StatusWith<StringData> toString(ProtocolSet protocols);
+
+    /**
+     * Parses a ProtocolSet from a string. Currently only the predefined ProtocolSets in the
+     * 'supports' namespace are supported
+     */
+    StatusWith<ProtocolSet> parseProtocolSet(StringData repr);
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/protocol_test.cpp b/src/mongo/rpc/protocol_test.cpp
new file mode 100644
index 00000000000..86211717eb7
--- /dev/null
+++ b/src/mongo/rpc/protocol_test.cpp
@@ -0,0 +1,66 @@
+/**
+ *    Copyright (C) 2015 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/base/status.h"
+#include "mongo/rpc/protocol.h"
+#include "mongo/unittest/unittest.h"
+
+namespace {
+
+    using namespace mongo::rpc;
+
+    // Checks if negotiation of the first to protocol sets results in the 'proto'
+    const auto assert_negotiated = [](ProtocolSet fst, ProtocolSet snd, Protocol proto) {
+        auto negotiated = negotiate(fst, snd);
+        ASSERT_TRUE(negotiated.isOK());
+        ASSERT_TRUE(negotiated.getValue() == proto);
+    };
+
+    TEST(Protocol, SuccessfulNegotiation) {
+        assert_negotiated(supports::kAll, supports::kAll, Protocol::kOpCommandV1);
+        assert_negotiated(supports::kAll, supports::kOpCommandOnly, Protocol::kOpCommandV1);
+        assert_negotiated(supports::kAll, supports::kOpQueryOnly, Protocol::kOpQuery);
+    }
+
+    // Checks that negotiation fails
+    const auto assert_not_negotiated = [](ProtocolSet fst, ProtocolSet snd) {
+        auto proto = negotiate(fst, snd);
+        ASSERT_TRUE(!proto.isOK());
+        ASSERT_TRUE(proto.getStatus().code() == mongo::ErrorCodes::RPCProtocolNegotiationFailed);
+    };
+
+    TEST(Protocol, FailedNegotiation) {
+        assert_not_negotiated(supports::kOpQueryOnly, supports::kOpCommandOnly);
+        assert_not_negotiated(supports::kAll, supports::kNone);
+        assert_not_negotiated(supports::kOpQueryOnly, supports::kNone);
+        assert_not_negotiated(supports::kOpCommandOnly, supports::kNone);
+    }
+
+}  // namespace
diff --git a/src/mongo/rpc/reply_builder_interface.cpp b/src/mongo/rpc/reply_builder_interface.cpp
new file mode 100644
index 00000000000..1b4b5aadbda
--- /dev/null
+++ b/src/mongo/rpc/reply_builder_interface.cpp
@@ -0,0 +1,85 @@
+/**
+ *    Copyright (C) 2015 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/rpc/reply_builder_interface.h"
+
+#include <utility>
+
+#include "mongo/base/status_with.h"
+#include "mongo/db/jsobj.h"
+
+namespace mongo {
+namespace rpc {
+
+    namespace {
+        const char kOKField[] = "ok";
+        const char kCodeField[] = "code";
+        const char kErrorField[] = "errmsg";
+
+        // similar to appendCommandStatus (duplicating logic here to avoid cyclic library
+        // dependency)
+        BSONObj augmentReplyWithStatus(const Status& status, const BSONObj& reply) {
+            BSONObjBuilder bob;
+            bob.appendElements(reply);
+
+            if (!reply.hasField(kOKField)) {
+                bob.append(kOKField, status.isOK() ? 1.0 : 0.0);
+            }
+
+            if (status.isOK()) {
+                return bob.obj();
+            }
+
+            if (!reply.hasField(kErrorField)) {
+                bob.append(kErrorField, status.reason());
+            }
+
+            if (!reply.hasField(kCodeField)) {
+                bob.append(kCodeField, status.code());
+            }
+
+            return bob.obj();
+        }
+    }
+
+    ReplyBuilderInterface&
+    ReplyBuilderInterface::setCommandReply(StatusWith<BSONObj> commandReply) {
+        auto reply = commandReply.isOK() ? std::move(commandReply.getValue()) : BSONObj();
+        return setRawCommandReply(augmentReplyWithStatus(commandReply.getStatus(), reply));
+    }
+
+    ReplyBuilderInterface&
+    ReplyBuilderInterface::setCommandReply(Status nonOKStatus, BSONObj extraErrorInfo) {
+        invariant(!nonOKStatus.isOK());
+        return setRawCommandReply(augmentReplyWithStatus(nonOKStatus, extraErrorInfo));
+    }
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/reply_builder_interface.h b/src/mongo/rpc/reply_builder_interface.h
new file mode 100644
index 00000000000..00942fe1dac
--- /dev/null
+++ b/src/mongo/rpc/reply_builder_interface.h
@@ -0,0 +1,125 @@
+/**
+ *    Copyright (C) 2015 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 <memory>
+
+#include "mongo/base/disallow_copying.h"
+#include "mongo/base/status_with.h"
+
+namespace mongo {
+    class BSONObj;
+    class Message;
+
+namespace rpc {
+    class DocumentRange;
+
+    /**
+     * Constructs an RPC Reply.
+     */
+    class ReplyBuilderInterface {
+        MONGO_DISALLOW_COPYING(ReplyBuilderInterface);
+    public:
+
+        /**
+         * Reply builders must have their fields set in order as they are immediately written into
+         * the underlying message buffer. This enum represents the next field that can be written
+         * into the builder. Note that when the builder is in state 'kInputDocs', multiple input
+         * docs can be added. After the builder's done() method is called it is in state 'kDone',
+         * and no further methods can be called.
+         */
+        enum class State {
+            kMetadata,
+            kCommandReply,
+            kOutputDocs,
+            kDone
+        };
+
+        virtual ~ReplyBuilderInterface() = default;
+
+        virtual ReplyBuilderInterface& setMetadata(BSONObj metadata) = 0;
+
+        /**
+         * Sets the raw command reply. This should probably not be used in favor of the
+         * variants that accept a Status or StatusWith.
+         */
+        virtual ReplyBuilderInterface& setRawCommandReply(BSONObj reply) = 0;
+
+        /**
+         * Sets the reply for this command. If an engaged StatusWith<BSONObj> is passed, the command
+         * reply will be set to the contained BSONObj, augmented with the element {ok, 1.0} if it
+         * does not already have an "ok" field. If a disengaged StatusWith<BSONObj> is passed, the
+         * command reply will be set to {ok: 0.0, code: <code of status>,
+         *                               errmsg: <reason of status>}
+         */
+        ReplyBuilderInterface& setCommandReply(StatusWith<BSONObj> commandReply);
+
+        /**
+         * Sets the reply for this command. The status parameter must be non-OK. The reply for
+         * this command will be set to an object containing all the fields in extraErrorInfo,
+         * augmented with {ok: 0.0} , {code: <code of status>}, and {errmsg: <reason of status>}.
+         * If any of the fields "ok", "code", or "errmsg" already exist in extraErrorInfo, they
+         * will be left as-is in the command reply. This use of this form is intended for
+         * interfacing with legacy code that adds additional data to a failed command reply and
+         * its use is discouraged in new code.
+         */
+        ReplyBuilderInterface& setCommandReply(Status nonOKStatus, BSONObj extraErrorInfo);
+
+        /**
+         * Add a range of output documents to the reply. This method can be called multiple times
+         * before calling done().
+         */
+        virtual ReplyBuilderInterface& addOutputDocs(DocumentRange outputDocs) = 0;
+
+        /**
+         * Add a single output document to the reply. This method can be called multiple times
+         * before calling done().
+         */
+        virtual ReplyBuilderInterface& addOutputDoc(BSONObj outputDoc) = 0;
+
+        /**
+         * Gets the state of the builder. As the builder will simply crash the process if it is ever
+         * put in an invalid state, it isn't neccessary to call this method for correctness. Rather
+         * it may be helpful to explicitly assert that the builder is in a certain state to make
+         * code that manipulates the builder more readable.
+         */
+        virtual State getState() const = 0;
+
+        /**
+         * Writes data then transfers ownership of the message to the caller. The behavior of
+         * calling any methods on the builder is subsequently undefined.
+         */
+        virtual std::unique_ptr<Message> done() = 0;
+
+    protected:
+        ReplyBuilderInterface() = default;
+    };
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/reply_interface.h b/src/mongo/rpc/reply_interface.h
new file mode 100644
index 00000000000..3fd5644f785
--- /dev/null
+++ b/src/mongo/rpc/reply_interface.h
@@ -0,0 +1,76 @@
+/**
+ *    Copyright (C) 2015 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 "mongo/base/disallow_copying.h"
+
+namespace mongo {
+    class BSONObj;
+    class Message;
+
+namespace rpc {
+    class DocumentRange;
+
+    /**
+     * An immutable view of an RPC Reply.
+     */
+    class ReplyInterface {
+        MONGO_DISALLOW_COPYING(ReplyInterface);
+    public:
+        virtual ~ReplyInterface() = default;
+
+        /**
+         * Accessor for the metadata object. Metadata is generally used for information
+         * that is independent of any specific command, e.g. auditing information.
+         */
+        virtual const BSONObj& getMetadata() const = 0;
+
+        /**
+         * The result of executing the command.
+         */
+        virtual const BSONObj& getCommandReply() const = 0;
+
+        /**
+         * A variable number of BSON documents returned by the command. It is valid for the
+         * returned range to be empty.
+         *
+         * Example usage:
+         *
+         * for (auto&& doc : reply.getOutputDocs()) {
+         *    ... do stuff with doc
+         * }
+         */
+        virtual DocumentRange getOutputDocs() const = 0;
+
+    protected:
+        ReplyInterface() = default;
+    };
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/request_builder_interface.h b/src/mongo/rpc/request_builder_interface.h
new file mode 100644
index 00000000000..39623e12a10
--- /dev/null
+++ b/src/mongo/rpc/request_builder_interface.h
@@ -0,0 +1,118 @@
+/**
+ *    Copyright (C) 2015 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 <memory>
+
+#include "mongo/base/disallow_copying.h"
+
+namespace mongo {
+    class Message;
+    class BSONObj;
+    class StringData;
+
+namespace rpc {
+    class DocumentRange;
+
+    /**
+     * Constructs an RPC request.
+     */
+    class RequestBuilderInterface {
+        MONGO_DISALLOW_COPYING(RequestBuilderInterface);
+    public:
+
+        /**
+         * Request builders must have their fields set in order as they are immediately written into
+         * the underlying message buffer. This enum represents the next field that can be written
+         * into the builder. Note that when the builder is in state 'kInputDocs', multiple input
+         * docs can be added. After the builder's done() method is called it is in state 'kDone',
+         * and no further methods can be called.
+         */
+        enum class State {
+            kDatabase,
+            kCommandName,
+            kMetadata,
+            kCommandArgs,
+            kInputDocs,
+            kDone
+        };
+
+        virtual ~RequestBuilderInterface() = default;
+
+        /**
+         * Sets the database that the command will be executed against.
+         */
+        virtual RequestBuilderInterface& setDatabase(StringData database) = 0;
+
+        /**
+         * Sets the name of the command to execute.
+         */
+        virtual RequestBuilderInterface& setCommandName(StringData commandName) = 0;
+
+        /**
+         * Sets the metadata associated with this command request - see metadata.h for details.
+         */
+        virtual RequestBuilderInterface& setMetadata(BSONObj metadata) = 0;
+
+        /**
+         * Sets the arguments to pass to the command.
+         */
+        virtual RequestBuilderInterface& setCommandArgs(BSONObj commandArgs) = 0;
+
+        /**
+         * Add a range of input documents to the request. This method can be called multiple times
+         * before calling done().
+         */
+        virtual RequestBuilderInterface& addInputDocs(DocumentRange inputDocs) = 0;
+
+        /**
+         * Add a single output document to the request. This method can be called multiple times
+         * before calling done().
+         */
+        virtual RequestBuilderInterface& addInputDoc(BSONObj inputDoc) = 0;
+
+        /**
+         * Get the state of the builder. This method is intended to enable debug or invariant
+         * checks that the builder is in the correct state.
+         */
+        virtual State getState() const = 0;
+
+        /**
+         * Writes data then transfers ownership of the message to the caller.
+         * The behavior of calling any methods on the object is subsequently
+         * undefined.
+         */
+        virtual std::unique_ptr<Message> done() = 0;
+
+    protected:
+        RequestBuilderInterface() = default;
+    };
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/rpc/request_interface.h b/src/mongo/rpc/request_interface.h
new file mode 100644
index 00000000000..27f407f3f5c
--- /dev/null
+++ b/src/mongo/rpc/request_interface.h
@@ -0,0 +1,88 @@
+/**
+ *    Copyright (C) 2015 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 "mongo/base/disallow_copying.h"
+
+namespace mongo {
+    class BSONObj;
+    class Message;
+    class StringData;
+
+namespace rpc {
+    class DocumentRange;
+
+    /**
+     * An immutable view of an RPC message.
+     */
+    class RequestInterface {
+        MONGO_DISALLOW_COPYING(RequestInterface);
+    public:
+        virtual ~RequestInterface() = default;
+
+        /**
+         * Gets the database that the command is to be executed on.
+         */
+        virtual StringData getDatabase() const = 0;
+
+        /**
+         * Gets the name of the command to execute.
+         */
+        virtual StringData getCommandName() const = 0;
+
+        /**
+         * Gets the metadata associated with the command request. This is information that is
+         * independent of any specific command, i.e. auditing information. See metadata.h for
+         * further details.
+         */
+        virtual const BSONObj& getMetadata() const = 0;
+
+        /**
+         * Gets the arguments to the command - this is passed to the command's run() method.
+         */
+        virtual const BSONObj& getCommandArgs() const = 0;
+
+        /**
+         * A variable number of BSON documents to pass to the command. It is valid for
+         * the returned range to be empty.
+         *
+         * Example usage:
+         *
+         * for (auto&& doc : req.getInputDocs()) {
+         *    ... do stuff with doc
+         * }
+         */
+        virtual DocumentRange getInputDocs() const = 0;
+
+    protected:
+        RequestInterface() = default;
+    };
+
+}  // namespace rpc
+}  // namespace mongo
diff --git a/src/mongo/util/net/message.h b/src/mongo/util/net/message.h
index c1627550724..b3573026f84 100644
--- a/src/mongo/util/net/message.h
+++ b/src/mongo/util/net/message.h
@@ -82,6 +82,8 @@ namespace mongo {
         case dbGetMore: return "getmore";
         case dbDelete: return "remove";
         case dbKillCursors: return "killcursors";
+        case dbCommand: return "command";
+        case dbCommandReply: return "commandReply";
         default:
             massert( 16141, str::stream() << "cannot translate opcode " << op, !op );
             return "";