SERVER-28814 Replace ServerSelectionMetadata with just ReadPreferenceSetting

29 files changed, 381 insertions, 881 deletions

diff --git a/src/mongo/client/SConscript b/src/mongo/client/SConscript
index 89fb9b75516..fa9f3dcff45 100644
--- a/src/mongo/client/SConscript
+++ b/src/mongo/client/SConscript
@@ -37,6 +37,7 @@ env.Library(
     ],
     LIBDEPS=[
         '$BUILD_DIR/mongo/bson/util/bson_extract',
+        '$BUILD_DIR/mongo/db/service_context'
     ],
 )
 
diff --git a/src/mongo/client/dbclient_rs.cpp b/src/mongo/client/dbclient_rs.cpp
index 219069bb9dc..cd80a61f79d 100644
--- a/src/mongo/client/dbclient_rs.cpp
+++ b/src/mongo/client/dbclient_rs.cpp
@@ -43,7 +43,7 @@
 #include "mongo/client/sasl_client_authenticate.h"
 #include "mongo/db/dbmessage.h"
 #include "mongo/db/jsobj.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
+#include "mongo/stdx/memory.h"
 #include "mongo/util/log.h"
 
 namespace mongo {
@@ -104,30 +104,19 @@ const size_t MAX_RETRY = 3;
  *
  * @throws AssertionException if the read preference object is malformed
  */
-ReadPreferenceSetting* _extractReadPref(const BSONObj& query, int queryOptions) {
-    if (Query::hasReadPreference(query)) {
-        BSONElement readPrefElement;
-
-        if (query.hasField(Query::ReadPrefField.name())) {
-            readPrefElement = query[Query::ReadPrefField.name()];
-        } else {
-            readPrefElement = query["$queryOptions"][Query::ReadPrefField.name()];
-        }
-
-        uassert(16381, "$readPreference should be an object", readPrefElement.isABSONObj());
-
-        const BSONObj& prefDoc = readPrefElement.Obj();
-
-        auto readPrefSetting = uassertStatusOK(ReadPreferenceSetting::fromBSON(prefDoc));
-
-        return new ReadPreferenceSetting(std::move(readPrefSetting));
-    }
-
+std::unique_ptr<ReadPreferenceSetting> _extractReadPref(const BSONObj& query, int queryOptions) {
     // Default read pref is primary only or secondary preferred with slaveOK
-    ReadPreference pref = queryOptions & QueryOption_SlaveOk
-        ? mongo::ReadPreference::SecondaryPreferred
-        : mongo::ReadPreference::PrimaryOnly;
-    return new ReadPreferenceSetting(pref, TagSet());
+    const auto defaultReadPref = queryOptions & QueryOption_SlaveOk
+        ? ReadPreference::SecondaryPreferred
+        : ReadPreference::PrimaryOnly;
+
+    auto readPrefContainingObj = query;
+    if (auto elem = query["$queryOptions"]) {
+        // The readPreference is embedded in the $queryOptions field.
+        readPrefContainingObj = elem.Obj();
+    }
+    return stdx::make_unique<ReadPreferenceSetting>(uassertStatusOK(
+        ReadPreferenceSetting::fromContainingBSON(readPrefContainingObj, defaultReadPref)));
 }
 
 }  // namespace
@@ -273,7 +262,9 @@ bool _isSecondaryQuery(const string& ns,
     // Only certain commands are supported for secondary operation.
 
     BSONObj actualQueryObj;
-    if (strcmp(queryObj.firstElement().fieldName(), "query") == 0) {
+    if (strcmp(queryObj.firstElement().fieldName(), "$query") == 0) {
+        actualQueryObj = queryObj["$query"].embeddedObject();
+    } else if (strcmp(queryObj.firstElement().fieldName(), "query") == 0) {
         actualQueryObj = queryObj["query"].embeddedObject();
     } else {
         actualQueryObj = queryObj;
@@ -532,7 +523,7 @@ unique_ptr<DBClientCursor> DBClientReplicaSet::query(const string& ns,
     shared_ptr<ReadPreferenceSetting> readPref(_extractReadPref(query.obj, queryOptions));
     if (_isSecondaryQuery(ns, query.obj, *readPref)) {
         LOG(3) << "dbclient_rs query using secondary or tagged node selection in "
-               << _getMonitor()->getName() << ", read pref is " << readPref->toBSON()
+               << _getMonitor()->getName() << ", read pref is " << readPref->toString()
                << " (primary : "
                << (_master.get() != NULL ? _master->getServerAddress() : "[not cached]")
                << ", lastTagged : " << (_lastSlaveOkConn.get() != NULL
@@ -584,7 +575,7 @@ BSONObj DBClientReplicaSet::findOne(const string& ns,
     shared_ptr<ReadPreferenceSetting> readPref(_extractReadPref(query.obj, queryOptions));
     if (_isSecondaryQuery(ns, query.obj, *readPref)) {
         LOG(3) << "dbclient_rs findOne using secondary or tagged node selection in "
-               << _getMonitor()->getName() << ", read pref is " << readPref->toBSON()
+               << _getMonitor()->getName() << ", read pref is " << readPref->toString()
                << " (primary : "
                << (_master.get() != NULL ? _master->getServerAddress() : "[not cached]")
                << ", lastTagged : " << (_lastSlaveOkConn.get() != NULL
@@ -761,7 +752,7 @@ void DBClientReplicaSet::say(Message& toSend, bool isRetry, string* actualServer
         shared_ptr<ReadPreferenceSetting> readPref(_extractReadPref(qm.query, qm.queryOptions));
         if (_isSecondaryQuery(qm.ns, qm.query, *readPref)) {
             LOG(3) << "dbclient_rs say using secondary or tagged node selection in "
-                   << _getMonitor()->getName() << ", read pref is " << readPref->toBSON()
+                   << _getMonitor()->getName() << ", read pref is " << readPref->toString()
                    << " (primary : "
                    << (_master.get() != NULL ? _master->getServerAddress() : "[not cached]")
                    << ", lastTagged : " << (_lastSlaveOkConn.get() != NULL
@@ -932,16 +923,7 @@ DBClientReplicaSet::runCommandWithMetadataAndTarget(StringData database,
     // so we don't have to re-parse it, however, that will come with its own set of
     // complications (e.g. some kind of base class or concept for MetadataSerializable
     // objects). For now we do it the stupid way.
-    auto ssm = uassertStatusOK(rpc::ServerSelectionMetadata::readFromMetadata(
-        metadata.getField(rpc::ServerSelectionMetadata::fieldName())));
-
-    // If we didn't get a readPref with this query, we assume SecondaryPreferred if secondaryOk
-    // is true, and PrimaryOnly otherwise. This logic is replicated from _extractReadPref.
-    auto defaultReadPref = ssm.isSecondaryOk()
-        ? ReadPreferenceSetting(ReadPreference::SecondaryPreferred, TagSet())
-        : ReadPreferenceSetting(ReadPreference::PrimaryOnly, TagSet::primaryOnly());
-
-    auto readPref = ssm.getReadPreference().get_value_or(defaultReadPref);
+    auto readPref = uassertStatusOK(ReadPreferenceSetting::fromContainingBSON(metadata));
 
     if (readPref.pref == ReadPreference::PrimaryOnly ||
         // If the command is not runnable on a secondary, we run it on the primary
@@ -970,8 +952,8 @@ DBClientReplicaSet::runCommandWithMetadataAndTarget(StringData database,
     }
 
     uasserted(ErrorCodes::NodeNotFound,
-              str::stream() << "Could not satisfy $readPreference of '" << readPref.toBSON() << "' "
-                            << "while attempting to run command "
+              str::stream() << "Could not satisfy $readPreference of '" << readPref.toString()
+                            << "' while attempting to run command "
                             << command);
 }
 
@@ -990,7 +972,7 @@ bool DBClientReplicaSet::call(Message& toSend,
         shared_ptr<ReadPreferenceSetting> readPref(_extractReadPref(qm.query, qm.queryOptions));
         if (_isSecondaryQuery(ns, qm.query, *readPref)) {
             LOG(3) << "dbclient_rs call using secondary or tagged node selection in "
-                   << _getMonitor()->getName() << ", read pref is " << readPref->toBSON()
+                   << _getMonitor()->getName() << ", read pref is " << readPref->toString()
                    << " (primary : "
                    << (_master.get() != NULL ? _master->getServerAddress() : "[not cached]")
                    << ", lastTagged : " << (_lastSlaveOkConn.get() != NULL
diff --git a/src/mongo/client/dbclient_rs_test.cpp b/src/mongo/client/dbclient_rs_test.cpp
index 15967c07677..15bc9f24a58 100644
--- a/src/mongo/client/dbclient_rs_test.cpp
+++ b/src/mongo/client/dbclient_rs_test.cpp
@@ -44,7 +44,6 @@
 #include "mongo/db/jsobj.h"
 #include "mongo/dbtests/mock/mock_conn_registry.h"
 #include "mongo/dbtests/mock/mock_replica_set.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/stdx/unordered_set.h"
 #include "mongo/unittest/unittest.h"
 #include "mongo/util/assert_util.h"
@@ -62,11 +61,8 @@ using std::vector;
 /**
  * Constructs a metadata object containing the passed server selection metadata.
  */
-BSONObj makeMetadata(ReadPreference rp, TagSet tagSet, bool secondaryOk) {
-    BSONObjBuilder metadataBob;
-    rpc::ServerSelectionMetadata ssm(secondaryOk, ReadPreferenceSetting(rp, tagSet));
-    uassertStatusOK(ssm.writeToMetadata(&metadataBob));
-    return metadataBob.obj();
+BSONObj makeMetadata(ReadPreference rp, TagSet tagSet) {
+    return ReadPreferenceSetting(rp, std::move(tagSet)).toContainingBSON();
 }
 
 /**
@@ -105,7 +101,7 @@ void assertOneOfNodesSelected(MockReplicaSet* replSet,
     auto tagSet = secondaryOk ? TagSet() : TagSet::primaryOnly();
     // We need the command to be a "SecOk command"
     auto res = replConn.runCommandWithMetadata(
-        "foo", "dbStats", makeMetadata(rp, tagSet, secondaryOk), BSON("dbStats" << 1));
+        "foo", "dbStats", makeMetadata(rp, tagSet), BSON("dbStats" << 1));
     stdx::unordered_set<HostAndPort> hostSet;
     for (const auto& hostName : hostNames) {
         hostSet.emplace(hostName);
@@ -228,13 +224,11 @@ private:
 
 void assertRunCommandWithReadPrefThrows(MockReplicaSet* replSet, ReadPreference rp) {
     bool isPrimaryOnly = (rp == ReadPreference::PrimaryOnly);
-
-    bool secondaryOk = !isPrimaryOnly;
     TagSet ts = isPrimaryOnly ? TagSet::primaryOnly() : TagSet();
 
     DBClientReplicaSet replConn(replSet->getSetName(), replSet->getHosts(), StringData());
     ASSERT_THROWS(replConn.runCommandWithMetadata(
-                      "foo", "whoami", makeMetadata(rp, ts, secondaryOk), BSON("dbStats" << 1)),
+                      "foo", "whoami", makeMetadata(rp, ts), BSON("dbStats" << 1)),
                   AssertionException);
 }
 
diff --git a/src/mongo/client/query.cpp b/src/mongo/client/query.cpp
index 419f5b485e0..349ed5ff96d 100644
--- a/src/mongo/client/query.cpp
+++ b/src/mongo/client/query.cpp
@@ -115,7 +115,8 @@ bool Query::isComplex(const BSONObj& obj, bool* hasDollar) {
 }
 
 Query& Query::readPref(ReadPreference pref, const BSONArray& tags) {
-    appendComplex(ReadPrefField.name().c_str(), ReadPreferenceSetting(pref, TagSet(tags)).toBSON());
+    appendComplex(ReadPrefField.name().c_str(),
+                  ReadPreferenceSetting(pref, TagSet(tags)).toInnerBSON());
     return *this;
 }
 
diff --git a/src/mongo/client/read_preference.cpp b/src/mongo/client/read_preference.cpp
index acdd83c1771..a8790ae9f3c 100644
--- a/src/mongo/client/read_preference.cpp
+++ b/src/mongo/client/read_preference.cpp
@@ -119,10 +119,14 @@ TagSet defaultTagSetForMode(ReadPreference mode) {
  */
 const Seconds ReadPreferenceSetting::kMinimalMaxStalenessValue(90);
 
+const OperationContext::Decoration<ReadPreferenceSetting> ReadPreferenceSetting::get =
+    OperationContext::declareDecoration<ReadPreferenceSetting>();
+
 const BSONObj& ReadPreferenceSetting::secondaryPreferredMetadata() {
     // This is a static method rather than a static member only because it is used by another TU
     // during dynamic init.
-    static const auto bson = BSON("$ssm" << BSON("$secondaryOk" << true));
+    static const auto bson =
+        ReadPreferenceSetting(ReadPreference::SecondaryPreferred).toContainingBSON();
     return bson;
 }
 
@@ -148,7 +152,7 @@ ReadPreferenceSetting::ReadPreferenceSetting(ReadPreference pref, TagSet tags)
 ReadPreferenceSetting::ReadPreferenceSetting(ReadPreference pref)
     : ReadPreferenceSetting(pref, defaultTagSetForMode(pref)) {}
 
-StatusWith<ReadPreferenceSetting> ReadPreferenceSetting::fromBSON(const BSONObj& readPrefObj) {
+StatusWith<ReadPreferenceSetting> ReadPreferenceSetting::fromInnerBSON(const BSONObj& readPrefObj) {
     std::string modeStr;
     auto modeExtractStatus = bsonExtractStringField(readPrefObj, kModeFieldName, &modeStr);
     if (!modeExtractStatus.isOK()) {
@@ -227,20 +231,37 @@ StatusWith<ReadPreferenceSetting> ReadPreferenceSetting::fromBSON(const BSONObj&
     return ReadPreferenceSetting(mode, tags, Seconds(maxStalenessSecondsValue));
 }
 
-BSONObj ReadPreferenceSetting::toBSON() const {
-    BSONObjBuilder bob;
-    bob.append(kModeFieldName, readPreferenceName(pref));
+StatusWith<ReadPreferenceSetting> ReadPreferenceSetting::fromInnerBSON(const BSONElement& elem) {
+    if (elem.type() != mongo::Object) {
+        return Status(ErrorCodes::TypeMismatch,
+                      str::stream() << "$readPreference has incorrect type: expected "
+                                    << mongo::Object
+                                    << " but got "
+                                    << elem.type());
+    }
+    return fromInnerBSON(elem.Obj());
+}
+
+StatusWith<ReadPreferenceSetting> ReadPreferenceSetting::fromContainingBSON(
+    const BSONObj& obj, ReadPreference defaultReadPref) {
+    if (auto elem = obj["$readPreference"]) {
+        return fromInnerBSON(elem);
+    }
+    return ReadPreferenceSetting(defaultReadPref);
+}
+
+void ReadPreferenceSetting::toInnerBSON(BSONObjBuilder* bob) const {
+    bob->append(kModeFieldName, readPreferenceName(pref));
     if (tags != defaultTagSetForMode(pref)) {
-        bob.append(kTagsFieldName, tags.getTagBSON());
+        bob->append(kTagsFieldName, tags.getTagBSON());
     }
     if (maxStalenessSeconds.count() > 0) {
-        bob.append(kMaxStalenessSecondsFieldName, maxStalenessSeconds.count());
+        bob->append(kMaxStalenessSecondsFieldName, maxStalenessSeconds.count());
     }
-    return bob.obj();
 }
 
 std::string ReadPreferenceSetting::toString() const {
-    return toBSON().toString();
+    return toInnerBSON().toString();
 }
 
 }  // namespace mongo
diff --git a/src/mongo/client/read_preference.h b/src/mongo/client/read_preference.h
index 35ef1c9067b..c1f09765be5 100644
--- a/src/mongo/client/read_preference.h
+++ b/src/mongo/client/read_preference.h
@@ -30,6 +30,7 @@
 
 #include "mongo/bson/simple_bsonobj_comparator.h"
 #include "mongo/db/jsobj.h"
+#include "mongo/db/operation_context.h"
 #include "mongo/db/repl/optime.h"
 #include "mongo/util/duration.h"
 
@@ -115,6 +116,8 @@ private:
 };
 
 struct ReadPreferenceSetting {
+    static const OperationContext::Decoration<ReadPreferenceSetting> get;
+
     /**
      * The minimal value maxStalenessSeconds can have.
      */
@@ -122,7 +125,7 @@ struct ReadPreferenceSetting {
 
     /**
      * An object representing the metadata generated for a SecondaryPreferred read preference:
-     * {$ssm: {$secondaryOk: true}}
+     * {$readPreference: {mode: "secondaryPreferred"}}
      */
     static const BSONObj& secondaryPreferredMetadata();
 
@@ -137,6 +140,7 @@ struct ReadPreferenceSetting {
     ReadPreferenceSetting(ReadPreference pref, Seconds maxStalenessSeconds);
     ReadPreferenceSetting(ReadPreference pref, TagSet tags);
     explicit ReadPreferenceSetting(ReadPreference pref);
+    ReadPreferenceSetting() : ReadPreferenceSetting(ReadPreference::PrimaryOnly) {}
 
     inline bool equals(const ReadPreferenceSetting& other) const {
         return (pref == other.pref) && (tags == other.tags) &&
@@ -144,14 +148,33 @@ struct ReadPreferenceSetting {
     }
 
     /**
-     * Serializes this ReadPreferenceSetting as a BSON document.
+     * Serializes this ReadPreferenceSetting as an inner BSON document. (The document inside a
+     * $readPreference element)
      */
-    BSONObj toBSON() const;
+    void toInnerBSON(BSONObjBuilder* builder) const;
+    BSONObj toInnerBSON() const {
+        BSONObjBuilder bob;
+        toInnerBSON(&bob);
+        return bob.obj();
+    }
 
     /**
-     * Describes this ReadPreferenceSetting as a string.
+     * Serializes this ReadPreferenceSetting as a containing BSON document. (The document containing
+     * a $readPreference element)
+     *
+     * Will not add the $readPreference element if the read preference is PrimaryOnly.
      */
-    std::string toString() const;
+    void toContainingBSON(BSONObjBuilder* builder) const {
+        if (!canRunOnSecondary())
+            return;  // Write nothing since default is fine.
+        BSONObjBuilder inner(builder->subobjStart("$readPreference"));
+        toInnerBSON(&inner);
+    }
+    BSONObj toContainingBSON() const {
+        BSONObjBuilder bob;
+        toContainingBSON(&bob);
+        return bob.obj();
+    }
 
     /**
      * Parses a ReadPreferenceSetting from a BSON document of the form:
@@ -161,7 +184,25 @@ struct ReadPreferenceSetting {
      * further validation is performed on it other than checking that it is an array, and that it is
      * empty if 'mode' is 'primary'.
      */
-    static StatusWith<ReadPreferenceSetting> fromBSON(const BSONObj& readPrefSettingObj);
+    static StatusWith<ReadPreferenceSetting> fromInnerBSON(const BSONObj& readPrefSettingObj);
+    static StatusWith<ReadPreferenceSetting> fromInnerBSON(const BSONElement& readPrefSettingObj);
+
+    /**
+     * Parses a ReadPreference setting from an object that may contain a $readPreference object
+     * field with the contents described in fromInnerObject(). If the field is missing, returns the
+     * default read preference.
+     */
+    static StatusWith<ReadPreferenceSetting> fromContainingBSON(
+        const BSONObj& obj, ReadPreference defaultReadPref = ReadPreference::PrimaryOnly);
+
+    /**
+     * Describes this ReadPreferenceSetting as a string.
+     */
+    std::string toString() const;
+
+    bool canRunOnSecondary() const {
+        return pref != ReadPreference::PrimaryOnly;
+    }
 
     ReadPreference pref;
     TagSet tags;
diff --git a/src/mongo/client/read_preference_test.cpp b/src/mongo/client/read_preference_test.cpp
index dcf20107433..fda208ef45f 100644
--- a/src/mongo/client/read_preference_test.cpp
+++ b/src/mongo/client/read_preference_test.cpp
@@ -39,7 +39,7 @@ using namespace mongo;
 static const Seconds kMinMaxStaleness = ReadPreferenceSetting::kMinimalMaxStalenessValue;
 
 void checkParse(const BSONObj& rpsObj, const ReadPreferenceSetting& expected) {
-    const auto swRps = ReadPreferenceSetting::fromBSON(rpsObj);
+    const auto swRps = ReadPreferenceSetting::fromInnerBSON(rpsObj);
     ASSERT_OK(swRps.getStatus());
     const auto rps = swRps.getValue();
     ASSERT_TRUE(rps.equals(expected));
@@ -92,12 +92,18 @@ TEST(ReadPreferenceSetting, ParseValid) {
 }
 
 void checkParseFails(const BSONObj& rpsObj) {
-    auto swRps = ReadPreferenceSetting::fromBSON(rpsObj);
+    auto swRps = ReadPreferenceSetting::fromInnerBSON(rpsObj);
     ASSERT_NOT_OK(swRps.getStatus());
 }
 
 void checkParseFailsWithError(const BSONObj& rpsObj, ErrorCodes::Error error) {
-    auto swRps = ReadPreferenceSetting::fromBSON(rpsObj);
+    auto swRps = ReadPreferenceSetting::fromInnerBSON(rpsObj);
+    ASSERT_NOT_OK(swRps.getStatus());
+    ASSERT_EQUALS(swRps.getStatus().code(), error);
+}
+
+void checkParseContainerFailsWithError(const BSONObj& rpsObj, ErrorCodes::Error error) {
+    auto swRps = ReadPreferenceSetting::fromContainingBSON(rpsObj);
     ASSERT_NOT_OK(swRps.getStatus());
     ASSERT_EQUALS(swRps.getStatus().code(), error);
 }
@@ -169,10 +175,16 @@ TEST(ReadPreferenceSetting, ParseInvalid) {
                          << "secondary"
                          << "maxStalenessSeconds"
                          << Seconds::max().count()));
+
+    checkParseContainerFailsWithError(BSON("$query" << BSON("pang"
+                                                            << "pong")
+                                                    << "$readPreference"
+                                                    << 2),
+                                      ErrorCodes::TypeMismatch);
 }
 
 void checkRoundtrip(const ReadPreferenceSetting& rps) {
-    auto parsed = ReadPreferenceSetting::fromBSON(rps.toBSON());
+    auto parsed = ReadPreferenceSetting::fromInnerBSON(rps.toInnerBSON());
     ASSERT_OK(parsed.getStatus());
     ASSERT_TRUE(parsed.getValue().equals(rps));
 }
diff --git a/src/mongo/db/commands.h b/src/mongo/db/commands.h
index a6a53116bac..780af5827cc 100644
--- a/src/mongo/db/commands.h
+++ b/src/mongo/db/commands.h
@@ -63,10 +63,6 @@ namespace mutablebson {
 class Document;
 }  // namespace mutablebson
 
-namespace rpc {
-class ServerSelectionMetadata;
-}  // namespace rpc
-
 /**
  * Serves as a base for server commands. See the constructor for more details.
  */
@@ -483,8 +479,8 @@ public:
             arg == "$gleStats" ||           //
             arg == "$oplogQueryData" ||     //
             arg == "$queryOptions" ||       //
+            arg == "$readPreference" ||     //
             arg == "$replData" ||           //
-            arg == "$ssm" ||                //
             arg == "logicalTime" ||         //
             arg == "maxTimeMS" ||           //
             arg == "readConcern" ||         //
diff --git a/src/mongo/db/commands/dbcommands.cpp b/src/mongo/db/commands/dbcommands.cpp
index 05600734aa7..8751e871a82 100644
--- a/src/mongo/db/commands/dbcommands.cpp
+++ b/src/mongo/db/commands/dbcommands.cpp
@@ -102,7 +102,6 @@
 #include "mongo/rpc/metadata/logical_time_metadata.h"
 #include "mongo/rpc/metadata/oplog_query_metadata.h"
 #include "mongo/rpc/metadata/repl_set_metadata.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/rpc/metadata/sharding_metadata.h"
 #include "mongo/rpc/metadata/tracking_metadata.h"
 #include "mongo/rpc/reply_builder_interface.h"
@@ -1602,8 +1601,8 @@ void mongo::execCommandDatabase(OperationContext* opCtx,
         {
             bool commandCanRunOnSecondary = command->slaveOk();
 
-            bool commandIsOverriddenToRunOnSecondary = command->slaveOverrideOk() &&
-                rpc::ServerSelectionMetadata::get(opCtx).canRunOnSecondary();
+            bool commandIsOverriddenToRunOnSecondary =
+                command->slaveOverrideOk() && ReadPreferenceSetting::get(opCtx).canRunOnSecondary();
 
             bool iAmStandalone = !opCtx->writesAreReplicated();
             bool canRunHere = iAmPrimary || commandCanRunOnSecondary ||
diff --git a/src/mongo/db/commands/explain_cmd.cpp b/src/mongo/db/commands/explain_cmd.cpp
index 59004e87e12..b62fe416ac7 100644
--- a/src/mongo/db/commands/explain_cmd.cpp
+++ b/src/mongo/db/commands/explain_cmd.cpp
@@ -31,7 +31,6 @@
 #include "mongo/db/commands.h"
 #include "mongo/db/query/explain.h"
 #include "mongo/db/repl/replication_coordinator_global.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/util/mongoutils/str.h"
 
 namespace mongo {
@@ -150,7 +149,7 @@ public:
         bool commandCanRunOnSecondary = commToExplain->slaveOk();
 
         bool commandIsOverriddenToRunOnSecondary = commToExplain->slaveOverrideOk() &&
-            rpc::ServerSelectionMetadata::get(opCtx).canRunOnSecondary();
+            ReadPreferenceSetting::get(opCtx).canRunOnSecondary();
         bool iAmStandalone = !opCtx->writesAreReplicated();
 
         const bool canRunHere = iAmPrimary || commandCanRunOnSecondary ||
diff --git a/src/mongo/rpc/SConscript b/src/mongo/rpc/SConscript
index 3d7982e6a34..47e3af75eee 100644
--- a/src/mongo/rpc/SConscript
+++ b/src/mongo/rpc/SConscript
@@ -134,7 +134,6 @@ env.Clone().InjectModule("enterprise").Library(
         'metadata/config_server_metadata.cpp',
         'metadata/egress_metadata_hook_list.cpp',
         'metadata/logical_time_metadata.cpp',
-        'metadata/server_selection_metadata.cpp',
         'metadata/sharding_metadata.cpp',
         'metadata/repl_set_metadata.cpp',
         'metadata/oplog_query_metadata.cpp',
@@ -158,9 +157,9 @@ env.CppUnitTest(
         'rpc_metadata_test',
     ],
     source=[
+        'metadata_test.cpp',
         'metadata/egress_metadata_hook_list_test.cpp',
         'metadata/logical_time_metadata_test.cpp',
-        'metadata/server_selection_metadata_test.cpp',
         'metadata/sharding_metadata_test.cpp',
         'metadata/tracking_metadata_test.cpp',
     ],
diff --git a/src/mongo/rpc/command_request.cpp b/src/mongo/rpc/command_request.cpp
index 08bb54e91fe..54cb58a4313 100644
--- a/src/mongo/rpc/command_request.cpp
+++ b/src/mongo/rpc/command_request.cpp
@@ -93,13 +93,24 @@ CommandRequest::CommandRequest(const Message* message) : _message(message) {
             _commandArgs.firstElementFieldName() == _commandName);
 
     // OP_COMMAND is only used when communicating with 3.4 nodes and they serialize their metadata
-    // fields differently. We do all up- and down-conversion here so that the rest of the code only
-    // has to deal with the current format.
+    // fields differently. We do all up-conversion here so that the rest of the code only has to
+    // deal with the current format.
     uassertStatusOK(cur.readAndAdvance<>(&obj));
     BSONObjBuilder metadataBuilder;
     for (auto elem : obj.val) {
         if (elem.fieldNameStringData() == "configsvr") {
             metadataBuilder.appendAs(elem, "$configServerState");
+        } else if (elem.fieldNameStringData() == "$ssm") {
+            auto ssmObj = elem.Obj();
+            if (auto readPrefElem = ssmObj["$readPreference"]) {
+                // Promote the read preference to the top level.
+                metadataBuilder.append(readPrefElem);
+            } else if (ssmObj["$secondaryOk"].trueValue()) {
+                // Convert secondaryOk to equivalent read preference if none was explicitly
+                // provided.
+                ReadPreferenceSetting(ReadPreference::SecondaryPreferred)
+                    .toContainingBSON(&metadataBuilder);
+            }
         } else {
             metadataBuilder.append(elem);
         }
diff --git a/src/mongo/rpc/command_request_builder.cpp b/src/mongo/rpc/command_request_builder.cpp
index 34f6daa0888..9b784db1841 100644
--- a/src/mongo/rpc/command_request_builder.cpp
+++ b/src/mongo/rpc/command_request_builder.cpp
@@ -32,6 +32,7 @@
 
 #include <utility>
 
+#include "mongo/client/read_preference.h"
 #include "mongo/stdx/memory.h"
 #include "mongo/util/assert_util.h"
 #include "mongo/util/net/message.h"
@@ -71,12 +72,18 @@ CommandRequestBuilder& CommandRequestBuilder::setCommandArgs(BSONObj commandArgs
 CommandRequestBuilder& CommandRequestBuilder::setMetadata(BSONObj metadata) {
     invariant(_state == State::kMetadata);
     // OP_COMMAND is only used when communicating with 3.4 nodes and they serialize their metadata
-    // fields differently. We do all up- and down-conversion here so that the rest of the code only
-    // has to deal with the current format.
+    // fields differently. We do all down-conversion here so that the rest of the code only has to
+    // deal with the current format.
     BSONObjBuilder bob(_builder);
     for (auto elem : metadata) {
         if (elem.fieldNameStringData() == "$configServerState") {
             bob.appendAs(elem, "configsvr");
+        } else if (elem.fieldNameStringData() == "$readPreference") {
+            BSONObjBuilder ssmBuilder(bob.subobjStart("$ssm"));
+            ssmBuilder.append(elem);
+            ssmBuilder.append(
+                "$secondaryOk",
+                uassertStatusOK(ReadPreferenceSetting::fromInnerBSON(elem)).canRunOnSecondary());
         } else {
             bob.append(elem);
         }
diff --git a/src/mongo/rpc/metadata.cpp b/src/mongo/rpc/metadata.cpp
index 5dc5f65d169..77545303ff7 100644
--- a/src/mongo/rpc/metadata.cpp
+++ b/src/mongo/rpc/metadata.cpp
@@ -43,7 +43,6 @@
 #include "mongo/rpc/metadata/client_metadata_ismaster.h"
 #include "mongo/rpc/metadata/config_server_metadata.h"
 #include "mongo/rpc/metadata/logical_time_metadata.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/rpc/metadata/sharding_metadata.h"
 #include "mongo/rpc/metadata/tracking_metadata.h"
 
@@ -76,7 +75,7 @@ BSONObj makeEmptyMetadata() {
 }
 
 void readRequestMetadata(OperationContext* opCtx, const BSONObj& metadataObj) {
-    BSONElement ssmElem;
+    BSONElement readPreferenceElem;
     BSONElement auditElem;
     BSONElement configSvrElem;
     BSONElement trackingElem;
@@ -85,8 +84,8 @@ void readRequestMetadata(OperationContext* opCtx, const BSONObj& metadataObj) {
 
     for (const auto& metadataElem : metadataObj) {
         auto fieldName = metadataElem.fieldNameStringData();
-        if (fieldName == ServerSelectionMetadata::fieldName()) {
-            ssmElem = metadataElem;
+        if (fieldName == "$readPreference") {
+            readPreferenceElem = metadataElem;
         } else if (fieldName == AuditMetadata::fieldName()) {
             auditElem = metadataElem;
         } else if (fieldName == ConfigServerMetadata::fieldName()) {
@@ -100,8 +99,10 @@ void readRequestMetadata(OperationContext* opCtx, const BSONObj& metadataObj) {
         }
     }
 
-    ServerSelectionMetadata::get(opCtx) =
-        uassertStatusOK(ServerSelectionMetadata::readFromMetadata(ssmElem));
+    if (readPreferenceElem) {
+        ReadPreferenceSetting::get(opCtx) =
+            uassertStatusOK(ReadPreferenceSetting::fromInnerBSON(readPreferenceElem));
+    }
 
     AuditMetadata::get(opCtx) = uassertStatusOK(AuditMetadata::readFromMetadata(auditElem));
 
@@ -143,20 +144,44 @@ void readRequestMetadata(OperationContext* opCtx, const BSONObj& metadataObj) {
 CommandAndMetadata upconvertRequestMetadata(BSONObj legacyCmdObj, int queryFlags) {
     // We can reuse the same metadata BOB for every upconvert call, but we need to keep
     // making new command BOBs as each metadata bob will need to remove fields. We can not use
-    // mutablebson here because the ServerSelectionMetadata upconvert routine performs
+    // mutablebson here because the ReadPreference upconvert routine performs
     // manipulations (replacing a root with its child) that mutablebson doesn't
     // support.
-    BSONObjBuilder metadataBob;
 
-    // Ordering is important here - ServerSelectionMetadata must be upconverted
-    // first, then AuditMetadata.
-    BSONObjBuilder ssmCommandBob;
-    uassertStatusOK(
-        ServerSelectionMetadata::upconvert(legacyCmdObj, queryFlags, &ssmCommandBob, &metadataBob));
+    auto readPrefContainer = BSONObj();
+    const StringData firstFieldName = legacyCmdObj.firstElementFieldName();
+    if (firstFieldName == "$query" || firstFieldName == "query") {
+        // Commands sent over OP_QUERY specify read preference by putting it at the top level and
+        // putting the command in a nested field called either query or $query.
+
+        // Check if legacyCommand has an invalid $maxTimeMS option.
+        uassert(ErrorCodes::InvalidOptions,
+                "cannot use $maxTimeMS query option with commands; use maxTimeMS command option "
+                "instead",
+                !legacyCmdObj.hasField("$maxTimeMS"));
+        readPrefContainer = legacyCmdObj;
+        legacyCmdObj = legacyCmdObj.firstElement().Obj().getOwned();
+    } else if (auto queryOptions = legacyCmdObj["$queryOptions"]) {
+        // Mongos rewrites commands with $readPreference to put it in a field nested inside of
+        // $queryOptions. Its command implementations often forward commands in that format to
+        // shards. This function is responsible for rewriting it to a format that the shards
+        // understand.
+        readPrefContainer = queryOptions.Obj().getOwned();
+        legacyCmdObj = legacyCmdObj.removeField("$queryOptions");
+    }
 
+    BSONObjBuilder metadataBob;
+    if (auto readPref = readPrefContainer["$readPreference"]) {
+        metadataBob.append(readPref);
+    } else if (queryFlags & QueryOption_SlaveOk) {
+        ReadPreferenceSetting(ReadPreference::SecondaryPreferred).toContainingBSON(&metadataBob);
+    }
+
+    // Ordering is important here - AuditMetadata::upconvert() expects the above up-conversion to
+    // already be done.
     BSONObjBuilder auditCommandBob;
     uassertStatusOK(
-        AuditMetadata::upconvert(ssmCommandBob.done(), queryFlags, &auditCommandBob, &metadataBob));
+        AuditMetadata::upconvert(legacyCmdObj, queryFlags, &auditCommandBob, &metadataBob));
 
     return std::make_tuple(auditCommandBob.obj(), metadataBob.obj());
 }
@@ -165,16 +190,28 @@ LegacyCommandAndFlags downconvertRequestMetadata(BSONObj cmdObj, BSONObj metadat
     int legacyQueryFlags = 0;
     BSONObjBuilder auditCommandBob;
     // Ordering is important here - AuditingMetadata must be downconverted first,
-    // then ServerSelectionMetadata.
+    // then ReadPreference.
     uassertStatusOK(
         AuditMetadata::downconvert(cmdObj, metadata, &auditCommandBob, &legacyQueryFlags));
 
 
-    BSONObjBuilder ssmCommandBob;
-    uassertStatusOK(ServerSelectionMetadata::downconvert(
-        auditCommandBob.done(), metadata, &ssmCommandBob, &legacyQueryFlags));
+    auto readPref = metadata["$readPreference"];
+    if (!readPref)
+        readPref = cmdObj["$readPreference"];
+
+    if (readPref) {
+        BSONObjBuilder bob;
+        bob.append("$query", cmdObj);
+        bob.append(readPref);
+        cmdObj = bob.obj();
+
+        auto parsed = ReadPreferenceSetting::fromInnerBSON(readPref);
+        if (parsed.isOK() && parsed.getValue().canRunOnSecondary()) {
+            legacyQueryFlags |= QueryOption_SlaveOk;
+        }
+    }
 
-    return std::make_tuple(ssmCommandBob.obj(), std::move(legacyQueryFlags));
+    return std::make_tuple(cmdObj, std::move(legacyQueryFlags));
 }
 
 CommandReplyWithMetadata upconvertReplyMetadata(const BSONObj& legacyReply) {
diff --git a/src/mongo/rpc/metadata/server_selection_metadata.cpp b/src/mongo/rpc/metadata/server_selection_metadata.cpp
deleted file mode 100644
index 533ebd650e1..00000000000
--- a/src/mongo/rpc/metadata/server_selection_metadata.cpp
+++ /dev/null
@@ -1,344 +0,0 @@
-/*
- *    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/server_selection_metadata.h"
-
-#include <tuple>
-#include <utility>
-
-#include "mongo/base/status_with.h"
-#include "mongo/bson/util/bson_extract.h"
-#include "mongo/client/dbclientinterface.h"
-#include "mongo/db/jsobj.h"
-#include "mongo/db/operation_context.h"
-#include "mongo/util/assert_util.h"
-
-namespace mongo {
-namespace rpc {
-
-
-namespace {
-
-// Symbolic constant for the "$readPreference" metadata field. The field should be of Object type
-// when present.
-const char kReadPreferenceFieldName[] = "$readPreference";
-
-const char kQueryOptionsFieldName[] = "$queryOptions";
-
-const char kDollarQueryWrapper[] = "$query";
-const char kQueryWrapper[] = "query";
-
-/**
- * Utility to unwrap a '$query' or 'query' wrapped command object. The first element of the
- * returned tuple indicates whether the command was unwrapped, and the second element is either
- * the unwrapped command (if it was wrapped), or the original command if it was not.
- */
-StatusWith<std::tuple<bool, BSONObj>> unwrapCommand(const BSONObj& maybeWrapped) {
-    const auto firstElFieldName = maybeWrapped.firstElementFieldName();
-
-    if ((firstElFieldName != StringData(kDollarQueryWrapper)) &&
-        (firstElFieldName != StringData(kQueryWrapper))) {
-        return std::make_tuple(false, maybeWrapped);
-    }
-
-    BSONElement inner;
-    auto extractStatus =
-        bsonExtractTypedField(maybeWrapped, firstElFieldName, mongo::Object, &inner);
-
-    if (!extractStatus.isOK()) {
-        return extractStatus;
-    }
-
-    return std::make_tuple(true, inner.Obj());
-}
-
-/**
- * Reads a top-level $readPreference field from a wrapped command.
- */
-Status extractWrappedReadPreference(const BSONObj& wrappedCommand, BSONObjBuilder* metadataBob) {
-    BSONElement readPrefEl;
-    auto rpExtractStatus =
-        bsonExtractTypedField(wrappedCommand, kReadPreferenceFieldName, mongo::Object, &readPrefEl);
-    if (rpExtractStatus.isOK()) {
-        metadataBob->append(readPrefEl);
-    } else if (rpExtractStatus != ErrorCodes::NoSuchKey) {
-        return rpExtractStatus;
-    }
-
-    return Status::OK();
-}
-
-/**
- * Reads a $readPreference from a $queryOptions subobject, if it exists, and writes it to
- * metadataBob. Writes out the original command excluding the $queryOptions subobject.
- */
-Status extractUnwrappedReadPreference(const BSONObj& unwrappedCommand,
-                                      BSONObjBuilder* commandBob,
-                                      BSONObjBuilder* metadataBob) {
-    BSONElement queryOptionsEl;
-    BSONElement readPrefEl;
-
-    auto queryOptionsExtractStatus = bsonExtractTypedField(
-        unwrappedCommand, kQueryOptionsFieldName, mongo::Object, &queryOptionsEl);
-
-    // If there is no queryOptions subobject, we write out the command and return.
-    if (queryOptionsExtractStatus == ErrorCodes::NoSuchKey) {
-        commandBob->appendElements(unwrappedCommand);
-        return Status::OK();
-    } else if (!queryOptionsExtractStatus.isOK()) {
-        return queryOptionsExtractStatus;
-    }
-
-    // Write out the command excluding the $queryOptions field.
-    for (const auto& elem : unwrappedCommand) {
-        if (elem.fieldNameStringData() != kQueryOptionsFieldName) {
-            commandBob->append(elem);
-        }
-    }
-
-    auto rpExtractStatus = bsonExtractTypedField(
-        queryOptionsEl.embeddedObject(), kReadPreferenceFieldName, mongo::Object, &readPrefEl);
-
-    // If there is a $queryOptions field, we expect there to be a $readPreference.
-    if (!rpExtractStatus.isOK()) {
-        return rpExtractStatus;
-    }
-
-    metadataBob->append(readPrefEl);
-    return Status::OK();
-}
-
-}  // namespace
-
-// Symbolic constant for the "$secondaryOk" metadata field. This field should be of boolean or
-// numeric type, and is treated as a boolean.
-const char ServerSelectionMetadata::kSecondaryOkFieldName[] = "$secondaryOk";
-
-const OperationContext::Decoration<ServerSelectionMetadata> ServerSelectionMetadata::get =
-    OperationContext::declareDecoration<ServerSelectionMetadata>();
-
-ServerSelectionMetadata::ServerSelectionMetadata(
-    bool secondaryOk, boost::optional<ReadPreferenceSetting> readPreference)
-    : _secondaryOk(secondaryOk), _readPreference(std::move(readPreference)) {}
-
-StatusWith<ServerSelectionMetadata> ServerSelectionMetadata::readFromMetadata(
-    const BSONObj& metadataObj) {
-    return readFromMetadata(metadataObj.getField(fieldName()));
-}
-
-StatusWith<ServerSelectionMetadata> ServerSelectionMetadata::readFromMetadata(
-    const BSONElement& metadataElem) {
-    if (metadataElem.eoo()) {
-        return ServerSelectionMetadata{};
-    } else if (metadataElem.type() != mongo::Object) {
-        return {ErrorCodes::TypeMismatch,
-                str::stream() << "ServerSelectionMetadata element has incorrect type: expected"
-                              << mongo::Object
-                              << " but got "
-                              << metadataElem.type()};
-    }
-
-    bool secondaryOk = false;
-    boost::optional<ReadPreferenceSetting> readPreference;
-    BSONElement rpElem;
-    for (const auto& ssmElem : metadataElem.Obj()) {
-        auto ssmElemFieldName = ssmElem.fieldNameStringData();
-        if (ssmElemFieldName == kSecondaryOkFieldName) {
-            secondaryOk = ssmElem.trueValue();
-        } else if (ssmElemFieldName == kReadPreferenceFieldName) {
-            if (ssmElem.type() != mongo::Object) {
-                return Status(ErrorCodes::TypeMismatch,
-                              str::stream() << "ReadPreference has incorrect type: expected"
-                                            << mongo::Object
-                                            << "but got"
-                                            << metadataElem.type());
-            }
-            auto parsedRps = ReadPreferenceSetting::fromBSON(ssmElem.Obj());
-            if (!parsedRps.isOK()) {
-                return parsedRps.getStatus();
-            }
-            readPreference.emplace(std::move(parsedRps.getValue()));
-        }
-    }
-
-    return ServerSelectionMetadata(secondaryOk, std::move(readPreference));
-}
-
-Status ServerSelectionMetadata::writeToMetadata(BSONObjBuilder* metadataBob) const {
-    BSONObjBuilder ssmBob;
-    if (isSecondaryOk()) {
-        ssmBob.append(kSecondaryOkFieldName, 1);
-    }
-
-    if (getReadPreference()) {
-        ssmBob.append(kReadPreferenceFieldName, getReadPreference()->toBSON());
-    }
-
-    auto ssm = ssmBob.done();
-    if (!ssm.isEmpty()) {
-        metadataBob->append(fieldName(), ssm);
-    }
-
-    return Status::OK();
-}
-
-BSONObj ServerSelectionMetadata::toBSON() const {
-    BSONObjBuilder bob;
-    writeToMetadata(&bob);
-    return bob.obj();
-}
-
-Status ServerSelectionMetadata::downconvert(const BSONObj& command,
-                                            const BSONObj& metadata,
-                                            BSONObjBuilder* legacyCommand,
-                                            int* legacyQueryFlags) {
-    auto ssmElem = metadata.getField(fieldName());
-    if (ssmElem.eoo()) {
-        // slaveOk is false by default.
-        *legacyQueryFlags &= ~mongo::QueryOption_SlaveOk;
-        legacyCommand->appendElements(command);
-        return Status::OK();
-    } else if (ssmElem.type() != mongo::Object) {
-        return {
-            ErrorCodes::TypeMismatch,
-            str::stream() << "ServerSelectionMetadata metadata element must be an object, but got "
-                          << typeName(ssmElem.type())};
-    }
-
-    auto ssmObj = ssmElem.Obj();
-    BSONElement secondaryOkElem;
-    BSONElement readPreferenceElem;
-
-    for (auto&& el : ssmObj) {
-        auto fname = el.fieldNameStringData();
-        if (fname == kSecondaryOkFieldName) {
-            secondaryOkElem = std::move(el);
-        } else if (fname == kReadPreferenceFieldName) {
-            readPreferenceElem = std::move(el);
-        }
-    }
-
-    if (!secondaryOkElem.eoo() && secondaryOkElem.trueValue()) {
-        *legacyQueryFlags |= mongo::QueryOption_SlaveOk;
-    } else {
-        *legacyQueryFlags &= ~mongo::QueryOption_SlaveOk;
-    }
-
-    if (!readPreferenceElem.eoo()) {
-        // Use 'query' to wrap query, then append read preference.
-
-        // NOTE(amidvidy): Oddly, the _isSecondaryQuery implementation in dbclient_rs does
-        // not unwrap the query properly - it only checks for 'query', and not
-        // '$query'. We should probably standardize on one - drivers use '$query',
-        // and the shell uses 'query'. See SERVER-18705 for details.
-
-        // TODO: this may need to use the $queryOptions hack on mongos.
-        legacyCommand->append(kQueryWrapper, command);
-        legacyCommand->append(readPreferenceElem);
-    } else {
-        legacyCommand->appendElements(command);
-    }
-
-    return Status::OK();
-}
-
-Status ServerSelectionMetadata::upconvert(const BSONObj& legacyCommand,
-                                          const int legacyQueryFlags,
-                                          BSONObjBuilder* commandBob,
-                                          BSONObjBuilder* metadataBob) {
-    // The secondaryOK option is equivalent to the slaveOk bit being set on legacy commands.
-    BSONObjBuilder ssmBob;
-    if (legacyQueryFlags & QueryOption_SlaveOk) {
-        ssmBob.append(kSecondaryOkFieldName, 1);
-    }
-
-    // First we need to check if we have a wrapped command. That is, a command of the form
-    // {'$query': { 'commandName': 1, ...}, '$someOption': 5, ....}. Curiously, the field name
-    // of the wrapped query can be either '$query', or 'query'.
-    auto swUnwrapped = unwrapCommand(legacyCommand);
-    if (!swUnwrapped.isOK()) {
-        return swUnwrapped.getStatus();
-    }
-
-    BSONObj maybeUnwrapped;
-    bool wasWrapped;
-    std::tie(wasWrapped, maybeUnwrapped) = swUnwrapped.getValue();
-
-    if (wasWrapped) {
-        // Check if legacyCommand has an invalid $maxTimeMS option.
-        // TODO: Move this check elsewhere when we handle upconverting/downconverting maxTimeMS.
-        if (legacyCommand.hasField("$maxTimeMS")) {
-            return Status(ErrorCodes::InvalidOptions,
-                          "cannot use $maxTimeMS query option with "
-                          "commands; use maxTimeMS command option "
-                          "instead");
-        }
-
-        // If the command was wrapped, we can write out the upconverted command now, as there
-        // is nothing else we need to remove from it.
-        commandBob->appendElements(maybeUnwrapped);
-
-        auto status = extractWrappedReadPreference(legacyCommand, &ssmBob);
-        if (!status.isOK()) {
-            return status;
-        }
-    } else {
-        // If the command was not wrapped, we need to check for a readPreference sent by mongos
-        // on the $queryOptions field of the command. If it is set, we remove it from the
-        // upconverted command, so we need to pass the command builder along.
-
-        auto status = extractUnwrappedReadPreference(maybeUnwrapped, commandBob, &ssmBob);
-        if (!status.isOK()) {
-            return status;
-        }
-    }
-
-    auto ssm = ssmBob.done();
-    if (!ssm.isEmpty()) {
-        metadataBob->append(fieldName(), ssm);
-    }
-    return Status::OK();
-}
-
-bool ServerSelectionMetadata::isSecondaryOk() const {
-    return _secondaryOk;
-}
-
-const boost::optional<ReadPreferenceSetting>& ServerSelectionMetadata::getReadPreference() const {
-    return _readPreference;
-}
-
-bool ServerSelectionMetadata::canRunOnSecondary() const {
-    return _secondaryOk ||
-        (_readPreference && (_readPreference->pref != ReadPreference::PrimaryOnly));
-}
-
-}  // rpc
-}  // mongo
diff --git a/src/mongo/rpc/metadata/server_selection_metadata.h b/src/mongo/rpc/metadata/server_selection_metadata.h
deleted file mode 100644
index 26983bf7533..00000000000
--- a/src/mongo/rpc/metadata/server_selection_metadata.h
+++ /dev/null
@@ -1,126 +0,0 @@
-/*
- *    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 <boost/optional.hpp>
-
-#include "mongo/base/disallow_copying.h"
-#include "mongo/client/read_preference.h"
-#include "mongo/db/operation_context.h"
-
-namespace mongo {
-class BSONObj;
-class BSONObjBuilder;
-class Status;
-template <typename T>
-class StatusWith;
-
-namespace rpc {
-
-/**
- * This class comprises the request metadata fields that concern server selection, that is,
- * the conditions on which servers can execute this operation.
- */
-class ServerSelectionMetadata {
-    MONGO_DISALLOW_COPYING(ServerSelectionMetadata);
-
-public:
-    static const char kSecondaryOkFieldName[];
-    static const OperationContext::Decoration<ServerSelectionMetadata> get;
-
-    ServerSelectionMetadata() = default;
-
-    ServerSelectionMetadata(ServerSelectionMetadata&&) = default;
-
-    ServerSelectionMetadata& operator=(ServerSelectionMetadata&&) = default;
-
-    /**
-     * Loads ServerSelectionMetadata from a metadata object.
-     */
-    static StatusWith<ServerSelectionMetadata> readFromMetadata(const BSONObj& metadataObj);
-
-    static StatusWith<ServerSelectionMetadata> readFromMetadata(const BSONElement& metadataElem);
-
-    /**
-     * Writes this operation's ServerSelectionMetadata to a metadata object.
-     */
-    Status writeToMetadata(BSONObjBuilder* metadataBob) const;
-
-    BSONObj toBSON() const;
-
-    /**
-     * Rewrites the ServerSelectionMetadata from the metadata object format to the legacy OP_QUERY
-     * format. In particular, if secondaryOk is set, this will set QueryOption_SlaveOk
-     * on the legacyQueryFlags. If a readPreference is set, the legacy command will be wrapped
-     * in a 'query' element and a top-level $readPreference field will be set on the command.
-     */
-    static Status downconvert(const BSONObj& command,
-                              const BSONObj& metadata,
-                              BSONObjBuilder* legacyCommand,
-                              int* legacyQueryFlags);
-
-    /**
-     * Rewrites the ServerSelectionMetadata from the legacy OP_QUERY format to the metadata
-     * object format.
-     */
-    static Status upconvert(const BSONObj& legacyCommand,
-                            const int legacyQueryFlags,
-                            BSONObjBuilder* commandBob,
-                            BSONObjBuilder* metadataBob);
-    /**
-     * Returns true if this operation has been explicitly overridden to run on a secondary.
-     * This replaces previous usage of QueryOption_SlaveOk.
-     */
-    bool isSecondaryOk() const;
-
-    /**
-     * Returns the ReadPreference associated with this operation. See
-     * mongo/client/read_preference.h for further details.
-     */
-    const boost::optional<ReadPreferenceSetting>& getReadPreference() const;
-
-    /**
-     * Returns true if this operation can run on secondary.
-     */
-    bool canRunOnSecondary() const;
-
-    ServerSelectionMetadata(bool secondaryOk,
-                            boost::optional<ReadPreferenceSetting> readPreference);
-
-    static StringData fieldName() {
-        return "$ssm";
-    }
-
-private:
-    bool _secondaryOk{false};
-    boost::optional<ReadPreferenceSetting> _readPreference{};
-};
-
-}  // namespace rpc
-}  // namespace mongo
diff --git a/src/mongo/rpc/metadata/server_selection_metadata_test.cpp b/src/mongo/rpc/metadata/server_selection_metadata_test.cpp
deleted file mode 100644
index 3775f489a06..00000000000
--- a/src/mongo/rpc/metadata/server_selection_metadata_test.cpp
+++ /dev/null
@@ -1,206 +0,0 @@
-/*
- *    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/base/status.h"
-#include "mongo/client/dbclientinterface.h"
-#include "mongo/client/read_preference.h"
-#include "mongo/db/jsobj.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
-#include "mongo/unittest/unittest.h"
-
-namespace {
-using namespace mongo;
-using namespace mongo::rpc;
-using mongo::unittest::assertGet;
-
-ServerSelectionMetadata checkParse(const BSONObj& metadata) {
-    return assertGet(ServerSelectionMetadata::readFromMetadata(metadata));
-}
-
-TEST(ServerSelectionMetadata, ReadFromMetadata) {
-    {
-        // Empty object - should work just fine.
-        auto ss = checkParse(BSONObj());
-        ASSERT_FALSE(ss.isSecondaryOk());
-        ASSERT_FALSE(ss.getReadPreference().is_initialized());
-    }
-    {
-        // Set secondaryOk but not readPreference.
-        auto ss = checkParse(BSON("$ssm" << BSON("$secondaryOk" << 1)));
-        ASSERT_TRUE(ss.isSecondaryOk());
-        ASSERT_FALSE(ss.getReadPreference().is_initialized());
-    }
-    {
-        // Set readPreference but not secondaryOk.
-        auto ss = checkParse(BSON("$ssm" << BSON("$readPreference" << BSON("mode"
-                                                                           << "primary"))));
-        ASSERT_FALSE(ss.isSecondaryOk());
-        ASSERT_TRUE(ss.getReadPreference().is_initialized());
-        ASSERT_TRUE(ss.getReadPreference()->pref == ReadPreference::PrimaryOnly);
-    }
-    {
-        // Set both.
-        auto ss = checkParse(BSON("$ssm" << BSON("$secondaryOk" << 1 << "$readPreference"
-                                                                << BSON("mode"
-                                                                        << "secondaryPreferred"))));
-        ASSERT_TRUE(ss.isSecondaryOk());
-        ASSERT_TRUE(ss.getReadPreference()->pref == ReadPreference::SecondaryPreferred);
-    }
-}
-
-void checkUpconvert(const BSONObj& legacyCommand,
-                    const int legacyQueryFlags,
-                    const BSONObj& upconvertedCommand,
-                    const BSONObj& upconvertedMetadata) {
-    BSONObjBuilder upconvertedCommandBob;
-    BSONObjBuilder upconvertedMetadataBob;
-    auto convertStatus = ServerSelectionMetadata::upconvert(
-        legacyCommand, legacyQueryFlags, &upconvertedCommandBob, &upconvertedMetadataBob);
-    ASSERT_OK(convertStatus);
-    // We don't care about the order of the fields in the metadata object
-    const auto sorted = [](const BSONObj& obj) {
-        BSONObjIteratorSorted iter(obj);
-        BSONObjBuilder bob;
-        while (iter.more()) {
-            bob.append(iter.next());
-        }
-        return bob.obj();
-    };
-
-    ASSERT_BSONOBJ_EQ(upconvertedCommand, upconvertedCommandBob.done());
-    ASSERT_BSONOBJ_EQ(sorted(upconvertedMetadata), sorted(upconvertedMetadataBob.done()));
-}
-
-TEST(ServerSelectionMetadata, UpconvertValidMetadata) {
-    // Wrapped in $query, with readPref and slaveOk bit set.
-    checkUpconvert(
-        BSON("$query" << BSON("ping" << 1) << "$readPreference" << BSON("mode"
-                                                                        << "secondary")),
-        mongo::QueryOption_SlaveOk,
-        BSON("ping" << 1),
-        BSON("$ssm" << BSON("$secondaryOk" << 1 << "$readPreference" << BSON("mode"
-                                                                             << "secondary"))));
-
-    // Wrapped in 'query', with readPref.
-    checkUpconvert(BSON("query" << BSON("pong" << 1 << "foo"
-                                               << "bar")
-                                << "$readPreference"
-                                << BSON("mode"
-                                        << "primary"
-                                        << "tags"
-                                        << BSON("dc"
-                                                << "ny"))),
-                   0,
-                   BSON("pong" << 1 << "foo"
-                               << "bar"),
-                   BSON("$ssm" << BSON("$readPreference" << BSON("mode"
-                                                                 << "primary"
-                                                                 << "tags"
-                                                                 << BSON("dc"
-                                                                         << "ny")))));
-    // Unwrapped, no readPref, no slaveOk
-    checkUpconvert(BSON("ping" << 1), 0, BSON("ping" << 1), BSONObj());
-
-    // Readpref wrapped in $queryOptions
-    checkUpconvert(BSON("pang"
-                        << "pong"
-                        << "$queryOptions"
-                        << BSON("$readPreference" << BSON("mode"
-                                                          << "nearest"
-                                                          << "tags"
-                                                          << BSON("rack"
-                                                                  << "city")))),
-                   0,
-                   BSON("pang"
-                        << "pong"),
-                   BSON("$ssm" << BSON("$readPreference" << BSON("mode"
-                                                                 << "nearest"
-                                                                 << "tags"
-                                                                 << BSON("rack"
-                                                                         << "city")))));
-}
-
-void checkUpconvertFails(const BSONObj& legacyCommand, ErrorCodes::Error error) {
-    BSONObjBuilder upconvertedCommandBob;
-    BSONObjBuilder upconvertedMetadataBob;
-    auto upconvertStatus = ServerSelectionMetadata::upconvert(
-        legacyCommand, 0, &upconvertedCommandBob, &upconvertedMetadataBob);
-    ASSERT_NOT_OK(upconvertStatus);
-    ASSERT_EQ(upconvertStatus.code(), error);
-}
-
-TEST(ServerSelectionMetadata, UpconvertInvalidMetadata) {
-    // $readPreference not an object.
-    checkUpconvertFails(BSON("$query" << BSON("pang"
-                                              << "pong")
-                                      << "$readPreference"
-                                      << 2),
-                        ErrorCodes::TypeMismatch);
-
-    // has $maxTimeMS option
-    checkUpconvertFails(BSON("query" << BSON("foo"
-                                             << "bar")
-                                     << "$maxTimeMS"
-                                     << 200),
-                        ErrorCodes::InvalidOptions);
-    checkUpconvertFails(BSON("$query" << BSON("foo"
-                                              << "bar")
-                                      << "$maxTimeMS"
-                                      << 200),
-                        ErrorCodes::InvalidOptions);
-
-    // has $queryOptions field, but invalid $readPreference
-    checkUpconvertFails(BSON("ping"
-                             << "pong"
-                             << "$queryOptions"
-                             << BSON("$readPreference" << 1.2)),
-                        ErrorCodes::TypeMismatch);
-
-    // has $queryOptions field, but no $readPreference
-    checkUpconvertFails(BSON("ping"
-                             << "pong"
-                             << "$queryOptions"
-                             << BSONObj()),
-                        ErrorCodes::NoSuchKey);
-
-    // invalid wrapped query
-    checkUpconvertFails(BSON("$query" << 1), ErrorCodes::TypeMismatch);
-    checkUpconvertFails(BSON("$query"
-                             << ""),
-                        ErrorCodes::TypeMismatch);
-    checkUpconvertFails(BSON("query" << 1), ErrorCodes::TypeMismatch);
-    checkUpconvertFails(BSON("query"
-                             << ""),
-                        ErrorCodes::TypeMismatch);
-}
-
-}  // namespace
diff --git a/src/mongo/rpc/metadata_test.cpp b/src/mongo/rpc/metadata_test.cpp
new file mode 100644
index 00000000000..1b538d4717d
--- /dev/null
+++ b/src/mongo/rpc/metadata_test.cpp
@@ -0,0 +1,143 @@
+/*
+ *    Copyright (C) 2017 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/client/dbclientinterface.h"
+#include "mongo/db/jsobj.h"
+#include "mongo/rpc/metadata.h"
+#include "mongo/unittest/unittest.h"
+
+namespace {
+using namespace mongo;
+using namespace mongo::rpc;
+using mongo::unittest::assertGet;
+
+void checkUpconvert(const BSONObj& legacyCommand,
+                    const int legacyQueryFlags,
+                    const BSONObj& upconvertedCommand,
+                    const BSONObj& upconvertedMetadata) {
+
+    auto converted = upconvertRequestMetadata(legacyCommand, legacyQueryFlags);
+    // We don't care about the order of the fields in the metadata object
+    const auto sorted = [](const BSONObj& obj) {
+        BSONObjIteratorSorted iter(obj);
+        BSONObjBuilder bob;
+        while (iter.more()) {
+            bob.append(iter.next());
+        }
+        return bob.obj();
+    };
+
+    ASSERT_BSONOBJ_EQ(upconvertedCommand, std::get<0>(converted));
+    ASSERT_BSONOBJ_EQ(sorted(upconvertedMetadata), sorted(std::get<1>(converted)));
+}
+
+TEST(Metadata, UpconvertValidMetadata) {
+    // Wrapped in $query, with readPref and slaveOk bit set.
+    checkUpconvert(BSON("$query" << BSON("ping" << 1) <<  //
+                        "$readPreference"
+                                 << BSON("mode"
+                                         << "secondary")),
+                   mongo::QueryOption_SlaveOk,
+                   BSON("ping" << 1),
+                   BSON("$readPreference" << BSON("mode"
+                                                  << "secondary")));
+
+    // Wrapped in 'query', with readPref.
+    checkUpconvert(BSON("query" << BSON("pong" << 1 << "foo"
+                                               << "bar")
+                                << "$readPreference"
+                                << BSON("mode"
+                                        << "primary"
+                                        << "tags"
+                                        << BSON("dc"
+                                                << "ny"))),
+                   0,
+                   BSON("pong" << 1 << "foo"
+                               << "bar"),
+                   BSON("$readPreference" << BSON("mode"
+                                                  << "primary"
+                                                  << "tags"
+                                                  << BSON("dc"
+                                                          << "ny"))));
+    // Unwrapped, no readPref, no slaveOk
+    checkUpconvert(BSON("ping" << 1), 0, BSON("ping" << 1), BSONObj());
+
+    // Readpref wrapped in $queryOptions
+    checkUpconvert(BSON("pang"
+                        << "pong"
+                        << "$queryOptions"
+                        << BSON("$readPreference" << BSON("mode"
+                                                          << "nearest"
+                                                          << "tags"
+                                                          << BSON("rack"
+                                                                  << "city")))),
+                   0,
+                   BSON("pang"
+                        << "pong"),
+                   BSON("$readPreference" << BSON("mode"
+                                                  << "nearest"
+                                                  << "tags"
+                                                  << BSON("rack"
+                                                          << "city"))));
+}
+
+TEST(Metadata, UpconvertInvalidMetadata) {
+    // has $maxTimeMS option
+    ASSERT_THROWS_CODE(upconvertRequestMetadata(BSON("query" << BSON("foo"
+                                                                     << "bar")
+                                                             << "$maxTimeMS"
+                                                             << 200),
+                                                0),
+                       UserException,
+                       ErrorCodes::InvalidOptions);
+    ASSERT_THROWS_CODE(upconvertRequestMetadata(BSON("$query" << BSON("foo"
+                                                                      << "bar")
+                                                              << "$maxTimeMS"
+                                                              << 200),
+                                                0),
+                       UserException,
+                       ErrorCodes::InvalidOptions);
+
+    // invalid wrapped query
+    ASSERT_THROWS(upconvertRequestMetadata(BSON("$query" << 1), 0), UserException);
+    ASSERT_THROWS(upconvertRequestMetadata(BSON("$query"
+                                                << ""),
+                                           0),
+                  UserException);
+    ASSERT_THROWS(upconvertRequestMetadata(BSON("query" << 1), 0), UserException);
+    ASSERT_THROWS(upconvertRequestMetadata(BSON("query"
+                                                << ""),
+                                           0),
+                  UserException);
+}
+
+}  // namespace
diff --git a/src/mongo/s/async_requests_sender.cpp b/src/mongo/s/async_requests_sender.cpp
index eb512b1a281..55f63010a13 100644
--- a/src/mongo/s/async_requests_sender.cpp
+++ b/src/mongo/s/async_requests_sender.cpp
@@ -35,7 +35,6 @@
 #include "mongo/client/remote_command_targeter.h"
 #include "mongo/executor/remote_command_request.h"
 #include "mongo/rpc/get_status_from_command_result.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/client/shard_registry.h"
 #include "mongo/s/grid.h"
 #include "mongo/stdx/memory.h"
@@ -62,11 +61,7 @@ AsyncRequestsSender::AsyncRequestsSender(OperationContext* opCtx,
     }
 
     // Initialize command metadata to handle the read preference.
-    BSONObjBuilder metadataBuilder;
-    rpc::ServerSelectionMetadata metadata(_readPreference.pref != ReadPreference::PrimaryOnly,
-                                          boost::none);
-    uassertStatusOK(metadata.writeToMetadata(&metadataBuilder));
-    _metadataObj = metadataBuilder.obj();
+    _metadataObj = readPreference.toContainingBSON();
 
     // Schedule the requests immediately.
 
diff --git a/src/mongo/s/commands/cluster_commands_common.cpp b/src/mongo/s/commands/cluster_commands_common.cpp
index 24cfe617e98..a5e961eda2b 100644
--- a/src/mongo/s/commands/cluster_commands_common.cpp
+++ b/src/mongo/s/commands/cluster_commands_common.cpp
@@ -37,7 +37,6 @@
 #include "mongo/db/query/cursor_response.h"
 #include "mongo/executor/task_executor_pool.h"
 #include "mongo/rpc/get_status_from_command_result.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/catalog/type_collection.h"
 #include "mongo/s/catalog_cache.h"
 #include "mongo/s/client/parallel.h"
@@ -208,16 +207,7 @@ BSONObj appendShardVersion(const BSONObj& cmdObj, ChunkVersion version) {
 
 ReadPreferenceSetting getReadPref(const BSONObj& cmdObj) {
     const auto queryOptionsObj = cmdObj.getObjectField(QueryRequest::kUnwrappedReadPrefField);
-    const auto readPrefObj = queryOptionsObj.getObjectField(QueryRequest::kWrappedReadPrefField);
-    return readPrefObj.isEmpty() ? ReadPreferenceSetting(ReadPreference::PrimaryOnly, TagSet())
-                                 : uassertStatusOK(ReadPreferenceSetting::fromBSON(readPrefObj));
-}
-
-ReadPreferenceSetting getReadPref(const rpc::ServerSelectionMetadata& ssm) {
-    return ssm.getReadPreference()
-        ? *ssm.getReadPreference()
-        : (ssm.isSecondaryOk() ? ReadPreferenceSetting(ReadPreference::SecondaryPreferred, TagSet())
-                               : ReadPreferenceSetting(ReadPreference::PrimaryOnly, TagSet()));
+    return uassertStatusOK(ReadPreferenceSetting::fromContainingBSON(queryOptionsObj));
 }
 
 StatusWith<std::vector<AsyncRequestsSender::Response>> scatterGather(
diff --git a/src/mongo/s/commands/cluster_commands_common.h b/src/mongo/s/commands/cluster_commands_common.h
index 5b999272807..56bac9b8e6a 100644
--- a/src/mongo/s/commands/cluster_commands_common.h
+++ b/src/mongo/s/commands/cluster_commands_common.h
@@ -57,19 +57,6 @@ BSONObj appendShardVersion(const BSONObj& cmdObj, ChunkVersion version);
 ReadPreferenceSetting getReadPref(const BSONObj& cmdObj);
 
 /**
- * Returns the read preference from the ServerSelectionMetadata if set or defaults to PrimaryOnly
- * and an empty TagSet.
- *
- * This is used by explain commands, where the read preference is extracted and placed into
- * ServerSelectionMetadata before the explain command is run.
- *
- * This is because only the value of the 'explain' field in the cmdObj is passed to the Command's
- * explain() method, and the readPreference is at the same level as the 'explain' field:
- * { explain: { find: "foo" }, $queryOptions: { $readPreference: { ... } } }
- */
-ReadPreferenceSetting getReadPref(const rpc::ServerSelectionMetadata& ssm);
-
-/**
  * Broadcasts 'cmdObj' to all shards and returns the responses as a vector.
  *
  * Returns a non-OK status if a failure occurs on *this* node during execution.
diff --git a/src/mongo/s/commands/cluster_find_cmd.cpp b/src/mongo/s/commands/cluster_find_cmd.cpp
index 83598d92401..80e9508ae55 100644
--- a/src/mongo/s/commands/cluster_find_cmd.cpp
+++ b/src/mongo/s/commands/cluster_find_cmd.cpp
@@ -38,7 +38,6 @@
 #include "mongo/db/stats/counters.h"
 #include "mongo/db/views/resolved_view.h"
 #include "mongo/rpc/get_status_from_command_result.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/commands/cluster_aggregate.h"
 #include "mongo/s/commands/strategy.h"
 #include "mongo/s/query/cluster_find.h"
@@ -115,12 +114,8 @@ public:
             return qr.getStatus();
         }
 
-        auto result = Strategy::explainFind(opCtx,
-                                            cmdObj,
-                                            *qr.getValue(),
-                                            verbosity,
-                                            rpc::ServerSelectionMetadata::get(opCtx),
-                                            out);
+        auto result = Strategy::explainFind(
+            opCtx, cmdObj, *qr.getValue(), verbosity, ReadPreferenceSetting::get(opCtx), out);
 
         if (result == ErrorCodes::CommandOnShardedViewNotSupportedOnMongod) {
             auto resolvedView = ResolvedView::fromBSON(out->asTempObj());
diff --git a/src/mongo/s/commands/cluster_pipeline_cmd.cpp b/src/mongo/s/commands/cluster_pipeline_cmd.cpp
index 9841f87d6a4..010bed8f14b 100644
--- a/src/mongo/s/commands/cluster_pipeline_cmd.cpp
+++ b/src/mongo/s/commands/cluster_pipeline_cmd.cpp
@@ -33,7 +33,6 @@
 #include "mongo/base/status.h"
 #include "mongo/db/auth/authorization_session.h"
 #include "mongo/db/commands.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/commands/cluster_aggregate.h"
 
 namespace mongo {
@@ -81,13 +80,15 @@ public:
                    const BSONObj& cmdObj,
                    ExplainOptions::Verbosity verbosity,
                    BSONObjBuilder* out) const override {
-        // Add the server selection metadata to the aggregate command in the "unwrapped" format that
+        // Add the read preference to the aggregate command in the "unwrapped" format that
         // runAggregate() expects: {aggregate: ..., $queryOptions: {$readPreference: ...}}.
+        const auto& readPref = ReadPreferenceSetting::get(opCtx);
         BSONObjBuilder aggCmdBuilder;
         aggCmdBuilder.appendElements(cmdObj);
-        if (auto readPref = rpc::ServerSelectionMetadata::get(opCtx).getReadPreference()) {
-            aggCmdBuilder.append(QueryRequest::kUnwrappedReadPrefField,
-                                 BSON("$readPreference" << readPref->toBSON()));
+        if (readPref.canRunOnSecondary()) {
+            auto queryOptionsBuilder =
+                BSONObjBuilder(aggCmdBuilder.subobjStart(QueryRequest::kUnwrappedReadPrefField));
+            readPref.toContainingBSON(&queryOptionsBuilder);
         }
         BSONObj aggCmd = aggCmdBuilder.obj();
 
diff --git a/src/mongo/s/commands/strategy.cpp b/src/mongo/s/commands/strategy.cpp
index 7500509f366..11cd2ef1381 100644
--- a/src/mongo/s/commands/strategy.cpp
+++ b/src/mongo/s/commands/strategy.cpp
@@ -55,7 +55,6 @@
 #include "mongo/db/views/resolved_view.h"
 #include "mongo/rpc/get_status_from_command_result.h"
 #include "mongo/rpc/metadata/logical_time_metadata.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/rpc/metadata/tracking_metadata.h"
 #include "mongo/s/catalog_cache.h"
 #include "mongo/s/client/parallel.h"
@@ -224,20 +223,11 @@ DbResponse Strategy::queryOp(OperationContext* opCtx, const NamespaceString& nss
     }
 
     // Determine the default read preference mode based on the value of the slaveOk flag.
-    const ReadPreferenceSetting readPreference = [&]() {
-        BSONElement rpElem;
-        auto readPrefExtractStatus = bsonExtractTypedField(
-            q.query, QueryRequest::kWrappedReadPrefField, mongo::Object, &rpElem);
-        if (readPrefExtractStatus == ErrorCodes::NoSuchKey) {
-            return ReadPreferenceSetting(q.queryOptions & QueryOption_SlaveOk
-                                             ? ReadPreference::SecondaryPreferred
-                                             : ReadPreference::PrimaryOnly);
-        }
-
-        uassertStatusOK(readPrefExtractStatus);
-
-        return uassertStatusOK(ReadPreferenceSetting::fromBSON(rpElem.Obj()));
-    }();
+    const auto defaultReadPref = q.queryOptions & QueryOption_SlaveOk
+        ? ReadPreference::SecondaryPreferred
+        : ReadPreference::PrimaryOnly;
+    const auto readPreference =
+        uassertStatusOK(ReadPreferenceSetting::fromContainingBSON(q.query, defaultReadPref));
 
     auto canonicalQuery =
         uassertStatusOK(CanonicalQuery::canonicalize(opCtx, q, ExtensionsCallbackNoop()));
@@ -251,12 +241,9 @@ DbResponse Strategy::queryOp(OperationContext* opCtx, const NamespaceString& nss
         // We default to allPlansExecution verbosity.
         const auto verbosity = ExplainOptions::Verbosity::kExecAllPlans;
 
-        const bool secondaryOk = (readPreference.pref != ReadPreference::PrimaryOnly);
-        const rpc::ServerSelectionMetadata metadata(secondaryOk, readPreference);
-
         BSONObjBuilder explainBuilder;
         uassertStatusOK(Strategy::explainFind(
-            opCtx, findCommand, queryRequest, verbosity, metadata, &explainBuilder));
+            opCtx, findCommand, queryRequest, verbosity, readPreference, &explainBuilder));
 
         BSONObj explainObj = explainBuilder.done();
         return replyToQuery(explainObj);
@@ -357,10 +344,9 @@ DbResponse Strategy::clientCommandOp(OperationContext* opCtx,
                 // The command has a read preference setting. We don't want to lose this information
                 // so we put it on the OperationContext and copy it to a new field called
                 // $queryOptions.$readPreference
-                auto readPref =
-                    uassertStatusOK(ReadPreferenceSetting::fromBSON(readPrefElem.Obj()));
-                rpc::ServerSelectionMetadata::get(opCtx) = rpc::ServerSelectionMetadata(
-                    readPref.pref != ReadPreference::PrimaryOnly, readPref);
+                ReadPreferenceSetting::get(opCtx) =
+                    uassertStatusOK(ReadPreferenceSetting::fromInnerBSON(readPrefElem));
+
                 haveReadPref = true;
                 BSONObjBuilder finalCmdObjBuilder;
                 finalCmdObjBuilder.appendElements(e.embeddedObject());
@@ -378,16 +364,15 @@ DbResponse Strategy::clientCommandOp(OperationContext* opCtx,
         if (!haveReadPref && q.queryOptions & QueryOption_SlaveOk) {
             // If the slaveOK bit is set, behave as-if read preference secondary-preferred was
             // specified.
-            rpc::ServerSelectionMetadata::get(opCtx) = rpc::ServerSelectionMetadata(
-                true, ReadPreferenceSetting(ReadPreference::SecondaryPreferred));
+            const auto readPref = ReadPreferenceSetting(ReadPreference::SecondaryPreferred);
+            ReadPreferenceSetting::get(opCtx) = readPref;
+
             BSONObjBuilder finalCmdObjBuilder;
             finalCmdObjBuilder.appendElements(cmdObj);
 
             BSONObjBuilder queryOptionsBuilder(finalCmdObjBuilder.subobjStart("$queryOptions"));
-            queryOptionsBuilder.append(
-                Query::ReadPrefField.name(),
-                ReadPreferenceSetting(ReadPreference::SecondaryPreferred).toBSON());
-            queryOptionsBuilder.done();
+            readPref.toContainingBSON(&queryOptionsBuilder);
+            queryOptionsBuilder.doneFast();
 
             cmdObj = finalCmdObjBuilder.obj();
         }
@@ -624,7 +609,7 @@ Status Strategy::explainFind(OperationContext* opCtx,
                              const BSONObj& findCommand,
                              const QueryRequest& qr,
                              ExplainOptions::Verbosity verbosity,
-                             const rpc::ServerSelectionMetadata& ssm,
+                             const ReadPreferenceSetting& readPref,
                              BSONObjBuilder* out) {
     const auto explainCmd = ClusterExplain::wrapAsExplain(findCommand, verbosity);
 
@@ -635,7 +620,7 @@ Status Strategy::explainFind(OperationContext* opCtx,
     auto swShardResponses = scatterGatherForNamespace(opCtx,
                                                       qr.nss(),
                                                       explainCmd,
-                                                      getReadPref(ssm),
+                                                      readPref,
                                                       qr.getFilter(),
                                                       qr.getCollation(),
                                                       true,  // do shard versioning
diff --git a/src/mongo/s/commands/strategy.h b/src/mongo/s/commands/strategy.h
index f91e3d61793..9da42bae611 100644
--- a/src/mongo/s/commands/strategy.h
+++ b/src/mongo/s/commands/strategy.h
@@ -42,10 +42,6 @@ class NamespaceString;
 class OperationContext;
 class QueryRequest;
 
-namespace rpc {
-class ServerSelectionMetadata;
-}  // namespace rpc
-
 /**
  * Legacy interface for processing client read/write/cmd requests.
  */
@@ -101,7 +97,7 @@ public:
                               const BSONObj& findCommand,
                               const QueryRequest& qr,
                               ExplainOptions::Verbosity verbosity,
-                              const rpc::ServerSelectionMetadata& serverSelectionMetadata,
+                              const ReadPreferenceSetting& readPref,
                               BSONObjBuilder* out);
 
     struct CommandResult {
diff --git a/src/mongo/s/query/async_results_merger.cpp b/src/mongo/s/query/async_results_merger.cpp
index bd3c67bf44a..ced16197e98 100644
--- a/src/mongo/s/query/async_results_merger.cpp
+++ b/src/mongo/s/query/async_results_merger.cpp
@@ -38,7 +38,6 @@
 #include "mongo/db/query/killcursors_request.h"
 #include "mongo/executor/remote_command_request.h"
 #include "mongo/executor/remote_command_response.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/client/shard_registry.h"
 #include "mongo/s/grid.h"
 #include "mongo/util/assert_util.h"
@@ -72,11 +71,7 @@ AsyncResultsMerger::AsyncResultsMerger(executor::TaskExecutor* executor,
     // is primaryOnly, in which case if the remote host for one of the cursors changes roles, the
     // remote will return an error.
     if (_params->readPreference) {
-        BSONObjBuilder metadataBuilder;
-        rpc::ServerSelectionMetadata metadata(
-            _params->readPreference->pref != ReadPreference::PrimaryOnly, boost::none);
-        uassertStatusOK(metadata.writeToMetadata(&metadataBuilder));
-        _metadataObj = metadataBuilder.obj();
+        _metadataObj = _params->readPreference->toContainingBSON();
     }
 }
 
diff --git a/src/mongo/s/query/async_results_merger_test.cpp b/src/mongo/s/query/async_results_merger_test.cpp
index 3396649256c..bca910ce347 100644
--- a/src/mongo/s/query/async_results_merger_test.cpp
+++ b/src/mongo/s/query/async_results_merger_test.cpp
@@ -39,7 +39,6 @@
 #include "mongo/executor/network_interface_mock.h"
 #include "mongo/executor/task_executor.h"
 #include "mongo/executor/thread_pool_task_executor_test_fixture.h"
-#include "mongo/rpc/metadata/server_selection_metadata.h"
 #include "mongo/s/catalog/type_shard.h"
 #include "mongo/s/client/shard_registry.h"
 #include "mongo/s/sharding_test_fixture.h"
@@ -1226,7 +1225,9 @@ TEST_F(AsyncResultsMergerTest, SendsSecondaryOkAsMetadata) {
     ASSERT_FALSE(arm->ready());
 
     BSONObj cmdRequestMetadata = getFirstPendingRequest().metadata;
-    ASSERT_BSONOBJ_EQ(cmdRequestMetadata, rpc::ServerSelectionMetadata(true, boost::none).toBSON());
+    ASSERT(uassertStatusOK(ReadPreferenceSetting::fromContainingBSON(cmdRequestMetadata))
+               .canRunOnSecondary())
+        << "full metadata: " << cmdRequestMetadata;
 
     std::vector<CursorResponse> responses;
     std::vector<BSONObj> batch1 = {fromjson("{_id: 1}")};
diff --git a/src/mongo/s/query/cluster_find.cpp b/src/mongo/s/query/cluster_find.cpp
index 0dd13e834d5..c282fee7800 100644
--- a/src/mongo/s/query/cluster_find.cpp
+++ b/src/mongo/s/query/cluster_find.cpp
@@ -470,15 +470,7 @@ StatusWith<ReadPreferenceSetting> ClusterFind::extractUnwrappedReadPref(const BS
     if (status.isOK()) {
         // There must be a nested object containing the read preference if there is a queryOptions
         // field.
-        BSONObj queryOptionsObj = queryOptionsElt.Obj();
-        invariant(queryOptionsObj[QueryRequest::kWrappedReadPrefField].type() == BSONType::Object);
-        BSONObj readPrefObj = queryOptionsObj[QueryRequest::kWrappedReadPrefField].Obj();
-
-        auto readPref = ReadPreferenceSetting::fromBSON(readPrefObj);
-        if (!readPref.isOK()) {
-            return readPref.getStatus();
-        }
-        return readPref.getValue();
+        return ReadPreferenceSetting::fromContainingBSON(queryOptionsElt.Obj());
     } else if (status != ErrorCodes::NoSuchKey) {
         return status;
     }
diff --git a/src/mongo/s/query/cluster_find.h b/src/mongo/s/query/cluster_find.h
index 8cf65aac017..492c0dc81e2 100644
--- a/src/mongo/s/query/cluster_find.h
+++ b/src/mongo/s/query/cluster_find.h
@@ -44,10 +44,6 @@ class OperationContext;
 struct GetMoreRequest;
 struct ReadPreferenceSetting;
 
-namespace rpc {
-class ServerSelectionMetadata;
-}  // namespace rpc
-
 /**
  * Methods for running find and getMore operations across a sharded cluster.
  */