SERVER-47097 Add ReplSetMetadata.isPrimary

11 files changed, 158 insertions, 84 deletions

diff --git a/src/mongo/db/repl/README.md b/src/mongo/db/repl/README.md
index 20085f7a6ae..a3ffc9afad9 100644
--- a/src/mongo/db/repl/README.md
+++ b/src/mongo/db/repl/README.md
@@ -320,6 +320,7 @@ It then creates a `ReplSetHeartbeatResponse` object. This includes:
 7. The state of the receiving node
 8. The receiving node's sync source
 9. The receiving node's `ReplicaSetConfig` version
+10. Whether the receiving node is primary
 
 When the sending node receives the response to the heartbeat, it first processes its
 `ReplSetMetadata` like before.
diff --git a/src/mongo/db/repl/check_quorum_for_config_change_test.cpp b/src/mongo/db/repl/check_quorum_for_config_change_test.cpp
index d592d3fbfba..e84b1881961 100644
--- a/src/mongo/db/repl/check_quorum_for_config_change_test.cpp
+++ b/src/mongo/db/repl/check_quorum_for_config_change_test.cpp
@@ -449,7 +449,8 @@ TEST_F(CheckQuorumForInitiate, QuorumCheckFailedDueToSetIdMismatch) {
                                           rsConfig.getConfigVersion(),
                                           unexpectedId,
                                           rpc::ReplSetMetadata::kNoPrimary,
-                                          -1);
+                                          -1,
+                                          false);
             BSONObjBuilder bob;
             uassertStatusOK(metadata.writeToMetadata(&bob));
 
diff --git a/src/mongo/db/repl/oplog_fetcher_test.cpp b/src/mongo/db/repl/oplog_fetcher_test.cpp
index ab6db9e2358..0309db2f51a 100644
--- a/src/mongo/db/repl/oplog_fetcher_test.cpp
+++ b/src/mongo/db/repl/oplog_fetcher_test.cpp
@@ -335,8 +335,8 @@ const Date_t OplogFetcherTest::staleWallTime = Date_t() + Seconds(staleOpTime.ge
 const rpc::OplogQueryMetadata OplogFetcherTest::staleOqMetadata = rpc::OplogQueryMetadata(
     {staleOpTime, staleWallTime}, staleOpTime, rbid, primaryIndex, syncSourceIndex);
 
-const rpc::ReplSetMetadata OplogFetcherTest::replSetMetadata =
-    rpc::ReplSetMetadata(1, OpTimeAndWallTime(), OpTime(), 1, OID(), primaryIndex, syncSourceIndex);
+const rpc::ReplSetMetadata OplogFetcherTest::replSetMetadata = rpc::ReplSetMetadata(
+    1, OpTimeAndWallTime(), OpTime(), 1, OID(), primaryIndex, syncSourceIndex, false);
 
 void OplogFetcherTest::setUp() {
     executor::ThreadPoolExecutorTest::setUp();
diff --git a/src/mongo/db/repl/replication_coordinator_impl_heartbeat_v1_test.cpp b/src/mongo/db/repl/replication_coordinator_impl_heartbeat_v1_test.cpp
index c8bb9b6c08c..b497d407730 100644
--- a/src/mongo/db/repl/replication_coordinator_impl_heartbeat_v1_test.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl_heartbeat_v1_test.cpp
@@ -786,7 +786,8 @@ TEST_F(ReplCoordHBV1Test, IgnoreTheContentsOfMetadataWhenItsReplicaSetIdDoesNotM
                                       rsConfig.getConfigVersion(),
                                       unexpectedId,
                                       1,
-                                      -1);
+                                      -1,
+                                      true);
         uassertStatusOK(metadata.writeToMetadata(&responseBuilder));
 
         heartbeatResponse = makeResponseStatus(responseBuilder.obj());
@@ -869,9 +870,10 @@ TEST_F(ReplCoordHBV1Test,
         {commitPoint, Date_t() + Seconds(commitPoint.getSecs())},  // committed OpTime
         commitPoint,                                               // visibleOpTime
         config.getConfigVersion(),
-        {},  // replset id
-        1,   // currentPrimaryIndex,
-        1);  // currentSyncSourceIndex
+        {},     // replset id
+        1,      // currentPrimaryIndex,
+        1,      // currentSyncSourceIndex
+        true);  // isPrimary
 
     auto net = getNet();
     BSONObjBuilder responseBuilder;
@@ -944,9 +946,10 @@ TEST_F(ReplCoordHBV1Test, LastCommittedOpTimeOnlyUpdatesFromHeartbeatIfNotInStar
         {commitPoint, Date_t() + Seconds(commitPoint.getSecs())},  // committed OpTime
         commitPoint,                                               // visibleOpTime
         config.getConfigVersion(),
-        {},  // replset id
-        1,   // currentPrimaryIndex,
-        1);  // currentSyncSourceIndex
+        {},     // replset id
+        1,      // currentPrimaryIndex,
+        1,      // currentSyncSourceIndex
+        true);  // isPrimary
 
     auto net = getNet();
     BSONObjBuilder responseBuilder;
diff --git a/src/mongo/db/repl/replication_coordinator_impl_test.cpp b/src/mongo/db/repl/replication_coordinator_impl_test.cpp
index f2f79e2c07b..aff27217a52 100644
--- a/src/mongo/db/repl/replication_coordinator_impl_test.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl_test.cpp
@@ -5645,12 +5645,12 @@ TEST_F(ReplCoordTest, DoNotIgnoreTheContentsOfMetadataWhenItsConfigVersionDoesNo
     // lower configVersion
     auto lowerConfigVersion = 1;
     StatusWith<rpc::ReplSetMetadata> metadata = rpc::ReplSetMetadata::readFromMetadata(BSON(
-        rpc::kReplSetMetadataFieldName
-        << BSON("lastOpCommitted" << BSON("ts" << Timestamp(10, 0) << "t" << 2)
-                                  << "lastCommittedWall" << Date_t() + Seconds(100)
-                                  << "lastOpVisible" << BSON("ts" << Timestamp(10, 0) << "t" << 2)
-                                  << "configVersion" << lowerConfigVersion << "primaryIndex" << 2
-                                  << "term" << 2 << "syncSourceIndex" << 1)));
+        rpc::kReplSetMetadataFieldName << BSON(
+            "lastOpCommitted" << BSON("ts" << Timestamp(10, 0) << "t" << 2) << "lastCommittedWall"
+                              << Date_t() + Seconds(100) << "lastOpVisible"
+                              << BSON("ts" << Timestamp(10, 0) << "t" << 2) << "configVersion"
+                              << lowerConfigVersion << "primaryIndex" << 2 << "term" << 2
+                              << "syncSourceIndex" << 1 << "isPrimary" << true)));
     getReplCoord()->processReplSetMetadata(metadata.getValue());
     // term should advance
     ASSERT_EQUALS(2, getReplCoord()->getTerm());
@@ -5658,12 +5658,12 @@ TEST_F(ReplCoordTest, DoNotIgnoreTheContentsOfMetadataWhenItsConfigVersionDoesNo
     // higher configVersion
     auto higherConfigVersion = 100;
     StatusWith<rpc::ReplSetMetadata> metadata2 = rpc::ReplSetMetadata::readFromMetadata(BSON(
-        rpc::kReplSetMetadataFieldName
-        << BSON("lastOpCommitted" << BSON("ts" << Timestamp(10, 0) << "t" << 2)
-                                  << "lastCommittedWall" << Date_t() + Seconds(100)
-                                  << "lastOpVisible" << BSON("ts" << Timestamp(10, 0) << "t" << 2)
-                                  << "configVersion" << higherConfigVersion << "primaryIndex" << 2
-                                  << "term" << 2 << "syncSourceIndex" << 1)));
+        rpc::kReplSetMetadataFieldName << BSON(
+            "lastOpCommitted" << BSON("ts" << Timestamp(10, 0) << "t" << 2) << "lastCommittedWall"
+                              << Date_t() + Seconds(100) << "lastOpVisible"
+                              << BSON("ts" << Timestamp(10, 0) << "t" << 2) << "configVersion"
+                              << higherConfigVersion << "primaryIndex" << 2 << "term" << 2
+                              << "syncSourceIndex" << 1 << "isPrimary" << true)));
     getReplCoord()->processReplSetMetadata(metadata2.getValue());
     // term should advance
     ASSERT_EQUALS(2, getReplCoord()->getTerm());
@@ -5733,11 +5733,12 @@ TEST_F(ReplCoordTest, UpdateTermWhenTheTermFromMetadataIsNewerButNeverUpdateCurr
 
     // higher term, should change
     StatusWith<rpc::ReplSetMetadata> metadata = rpc::ReplSetMetadata::readFromMetadata(BSON(
-        rpc::kReplSetMetadataFieldName << BSON(
-            "lastOpCommitted" << BSON("ts" << Timestamp(10, 0) << "t" << 3) << "lastCommittedWall"
-                              << Date_t() + Seconds(100) << "lastOpVisible"
-                              << BSON("ts" << Timestamp(10, 0) << "t" << 3) << "configVersion" << 2
-                              << "primaryIndex" << 2 << "term" << 3 << "syncSourceIndex" << 1)));
+        rpc::kReplSetMetadataFieldName
+        << BSON("lastOpCommitted" << BSON("ts" << Timestamp(10, 0) << "t" << 3)
+                                  << "lastCommittedWall" << Date_t() + Seconds(100)
+                                  << "lastOpVisible" << BSON("ts" << Timestamp(10, 0) << "t" << 3)
+                                  << "configVersion" << 2 << "primaryIndex" << 2 << "term" << 3
+                                  << "syncSourceIndex" << 1 << "isPrimary" << true)));
     getReplCoord()->processReplSetMetadata(metadata.getValue());
     ASSERT_EQUALS(3, getReplCoord()->getTerm());
     ASSERT_EQUALS(-1, getTopoCoord().getCurrentPrimaryIndex());
@@ -5745,11 +5746,12 @@ TEST_F(ReplCoordTest, UpdateTermWhenTheTermFromMetadataIsNewerButNeverUpdateCurr
 
     // lower term, should not change
     StatusWith<rpc::ReplSetMetadata> metadata2 = rpc::ReplSetMetadata::readFromMetadata(BSON(
-        rpc::kReplSetMetadataFieldName << BSON(
-            "lastOpCommitted" << BSON("ts" << Timestamp(11, 0) << "t" << 3) << "lastCommittedWall"
-                              << Date_t() + Seconds(100) << "lastOpVisible"
-                              << BSON("ts" << Timestamp(11, 0) << "t" << 3) << "configVersion" << 2
-                              << "primaryIndex" << 1 << "term" << 2 << "syncSourceIndex" << 1)));
+        rpc::kReplSetMetadataFieldName
+        << BSON("lastOpCommitted" << BSON("ts" << Timestamp(11, 0) << "t" << 3)
+                                  << "lastCommittedWall" << Date_t() + Seconds(100)
+                                  << "lastOpVisible" << BSON("ts" << Timestamp(11, 0) << "t" << 3)
+                                  << "configVersion" << 2 << "primaryIndex" << 1 << "term" << 2
+                                  << "syncSourceIndex" << 1 << "isPrimary" << true)));
     getReplCoord()->processReplSetMetadata(metadata2.getValue());
     ASSERT_EQUALS(3, getReplCoord()->getTerm());
     ASSERT_EQUALS(-1, getTopoCoord().getCurrentPrimaryIndex());
@@ -5757,11 +5759,12 @@ TEST_F(ReplCoordTest, UpdateTermWhenTheTermFromMetadataIsNewerButNeverUpdateCurr
 
     // same term, should not change
     StatusWith<rpc::ReplSetMetadata> metadata3 = rpc::ReplSetMetadata::readFromMetadata(BSON(
-        rpc::kReplSetMetadataFieldName << BSON(
-            "lastOpCommitted" << BSON("ts" << Timestamp(11, 0) << "t" << 3) << "lastCommittedWall"
-                              << Date_t() + Seconds(100) << "lastOpVisible"
-                              << BSON("ts" << Timestamp(11, 0) << "t" << 3) << "configVersion" << 2
-                              << "primaryIndex" << 1 << "term" << 3 << "syncSourceIndex" << 1)));
+        rpc::kReplSetMetadataFieldName
+        << BSON("lastOpCommitted" << BSON("ts" << Timestamp(11, 0) << "t" << 3)
+                                  << "lastCommittedWall" << Date_t() + Seconds(100)
+                                  << "lastOpVisible" << BSON("ts" << Timestamp(11, 0) << "t" << 3)
+                                  << "configVersion" << 2 << "primaryIndex" << 1 << "term" << 3
+                                  << "syncSourceIndex" << 1 << "isPrimary" << true)));
     getReplCoord()->processReplSetMetadata(metadata3.getValue());
     ASSERT_EQUALS(3, getReplCoord()->getTerm());
     ASSERT_EQUALS(-1, getTopoCoord().getCurrentPrimaryIndex());
@@ -5792,12 +5795,12 @@ TEST_F(ReplCoordTest,
 
     // Higher term - should update term but not last committed optime.
     StatusWith<rpc::ReplSetMetadata> metadata = rpc::ReplSetMetadata::readFromMetadata(BSON(
-        rpc::kReplSetMetadataFieldName
-        << BSON("lastOpCommitted" << BSON("ts" << Timestamp(10, 0) << "t" << 3)
-                                  << "lastCommittedWall" << Date_t() + Seconds(100)
-                                  << "lastOpVisible" << BSON("ts" << Timestamp(10, 0) << "t" << 3)
-                                  << "configVersion" << config.getConfigVersion() << "primaryIndex"
-                                  << 1 << "term" << 3 << "syncSourceIndex" << 1)));
+        rpc::kReplSetMetadataFieldName << BSON(
+            "lastOpCommitted" << BSON("ts" << Timestamp(10, 0) << "t" << 3) << "lastCommittedWall"
+                              << Date_t() + Seconds(100) << "lastOpVisible"
+                              << BSON("ts" << Timestamp(10, 0) << "t" << 3) << "configVersion"
+                              << config.getConfigVersion() << "primaryIndex" << 1 << "term" << 3
+                              << "syncSourceIndex" << 1 << "isPrimary" << true)));
     BSONObjBuilder responseBuilder;
     ASSERT_OK(metadata.getValue().writeToMetadata(&responseBuilder));
 
@@ -5904,6 +5907,7 @@ TEST_F(ReplCoordTest, PrepareOplogQueryMetadata) {
     ASSERT_EQ(replMetadata.getValue().getTerm(), 0);
     ASSERT_EQ(replMetadata.getValue().getSyncSourceIndex(), -1);
     ASSERT_EQ(replMetadata.getValue().getPrimaryIndex(), -1);
+    ASSERT_EQ(replMetadata.getValue().getIsPrimary(), false);
 }
 
 TEST_F(ReplCoordTest, TermAndLastCommittedOpTimeUpdatedFromHeartbeatWhenArbiter) {
@@ -5930,12 +5934,12 @@ TEST_F(ReplCoordTest, TermAndLastCommittedOpTimeUpdatedFromHeartbeatWhenArbiter)
     // Higher term - should update term and lastCommittedOpTime since arbiters learn of the
     // commit point via heartbeats.
     StatusWith<rpc::ReplSetMetadata> metadata = rpc::ReplSetMetadata::readFromMetadata(BSON(
-        rpc::kReplSetMetadataFieldName
-        << BSON("lastOpCommitted" << BSON("ts" << Timestamp(10, 1) << "t" << 3)
-                                  << "lastCommittedWall" << Date_t() + Seconds(100)
-                                  << "lastOpVisible" << BSON("ts" << Timestamp(10, 1) << "t" << 3)
-                                  << "configVersion" << config.getConfigVersion() << "primaryIndex"
-                                  << 1 << "term" << 3 << "syncSourceIndex" << 1)));
+        rpc::kReplSetMetadataFieldName << BSON(
+            "lastOpCommitted" << BSON("ts" << Timestamp(10, 1) << "t" << 3) << "lastCommittedWall"
+                              << Date_t() + Seconds(100) << "lastOpVisible"
+                              << BSON("ts" << Timestamp(10, 1) << "t" << 3) << "configVersion"
+                              << config.getConfigVersion() << "primaryIndex" << 1 << "term" << 3
+                              << "syncSourceIndex" << 1 << "isPrimary" << true)));
     BSONObjBuilder responseBuilder;
     ASSERT_OK(metadata.getValue().writeToMetadata(&responseBuilder));
 
@@ -6648,8 +6652,14 @@ TEST_F(ReplCoordTest, UpdatePositionCmdHasMetadata) {
 
     // Set last committed optime via metadata. Pass dummy Date_t to avoid advanceCommitPoint
     // invariant.
-    rpc::ReplSetMetadata syncSourceMetadata(
-        optime.getTerm(), {optime, Date_t() + Seconds(optime.getSecs())}, optime, 1, OID(), -1, 1);
+    rpc::ReplSetMetadata syncSourceMetadata(optime.getTerm(),
+                                            {optime, Date_t() + Seconds(optime.getSecs())},
+                                            optime,
+                                            1,
+                                            OID(),
+                                            -1,
+                                            1,
+                                            false);
     getReplCoord()->processReplSetMetadata(syncSourceMetadata);
     // Pass dummy Date_t to avoid advanceCommitPoint invariant.
     getReplCoord()->advanceCommitPoint({optime, Date_t() + Seconds(optime.getSecs())}, true);
diff --git a/src/mongo/db/repl/topology_coordinator.cpp b/src/mongo/db/repl/topology_coordinator.cpp
index e1ffa9a6339..3116173a018 100644
--- a/src/mongo/db/repl/topology_coordinator.cpp
+++ b/src/mongo/db/repl/topology_coordinator.cpp
@@ -3022,7 +3022,8 @@ rpc::ReplSetMetadata TopologyCoordinator::prepareReplSetMetadata(
                                 _rsConfig.getConfigVersion(),
                                 _rsConfig.getReplicaSetId(),
                                 _currentPrimaryIndex,
-                                _rsConfig.findMemberIndexByHostAndPort(getSyncSourceAddress()));
+                                _rsConfig.findMemberIndexByHostAndPort(getSyncSourceAddress()),
+                                _role == Role::kLeader /* isPrimary */);
 }
 
 rpc::OplogQueryMetadata TopologyCoordinator::prepareOplogQueryMetadata(int rbid) const {
diff --git a/src/mongo/db/repl/topology_coordinator_v1_test.cpp b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
index 06c47afbfa0..01d658c6cb8 100644
--- a/src/mongo/db/repl/topology_coordinator_v1_test.cpp
+++ b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
@@ -221,13 +221,15 @@ protected:
                                         int syncSourceIndex = -1,
                                         long long configVersion = -1) {
         auto configIn = (configVersion != -1) ? configVersion : _currentConfig.getConfigVersion();
+        auto isPrimary = primaryIndex != -1 && _topo->getCurrentPrimaryIndex() == primaryIndex;
         return ReplSetMetadata(_topo->getTerm(),
                                OpTimeAndWallTime(),
                                visibleOpTime,
                                configIn,
                                OID(),
                                primaryIndex,
-                               syncSourceIndex);
+                               syncSourceIndex,
+                               isPrimary);
     }
 
     // Make the OplogQueryMetadata coming from sync source.
@@ -4297,7 +4299,7 @@ TEST_F(HeartbeatResponseTestV1, ShouldNotChangeSyncSourceWhenMemberHasYetToHeart
 TEST_F(HeartbeatResponseTestV1, ShouldNotChangeSyncSourceWhenMemberNotInConfig) {
     // In this test, the TopologyCoordinator should tell us to change sync sources away from
     // "host4" since "host4" is absent from the config of version 10.
-    ReplSetMetadata replMetadata(0, {OpTime(), Date_t()}, OpTime(), 10, OID(), -1, -1);
+    ReplSetMetadata replMetadata(0, {OpTime(), Date_t()}, OpTime(), 10, OID(), -1, -1, false);
     ASSERT_TRUE(getTopoCoord().shouldChangeSyncSource(
         HostAndPort("host4"), replMetadata, makeOplogQueryMetadata(), now()));
 }
diff --git a/src/mongo/rpc/metadata/repl_set_metadata.cpp b/src/mongo/rpc/metadata/repl_set_metadata.cpp
index 210412eb4c6..9698ded3eb5 100644
--- a/src/mongo/rpc/metadata/repl_set_metadata.cpp
+++ b/src/mongo/rpc/metadata/repl_set_metadata.cpp
@@ -53,6 +53,7 @@ const char kReplicaSetIdFieldName[] = "replicaSetId";
 const char kPrimaryIndexFieldName[] = "primaryIndex";
 const char kSyncSourceIndexFieldName[] = "syncSourceIndex";
 const char kTermFieldName[] = "term";
+const char kIsPrimaryFieldName[] = "isPrimary";
 
 }  // unnamed namespace
 
@@ -64,14 +65,16 @@ ReplSetMetadata::ReplSetMetadata(long long term,
                                  long long configVersion,
                                  OID id,
                                  int currentPrimaryIndex,
-                                 int currentSyncSourceIndex)
+                                 int currentSyncSourceIndex,
+                                 boost::optional<bool> isPrimary)
     : _lastOpCommitted(std::move(committedOpTime)),
       _lastOpVisible(std::move(visibleOpTime)),
       _currentTerm(term),
       _configVersion(configVersion),
       _replicaSetId(id),
       _currentPrimaryIndex(currentPrimaryIndex),
-      _currentSyncSourceIndex(currentSyncSourceIndex) {}
+      _currentSyncSourceIndex(currentSyncSourceIndex),
+      _isPrimary(isPrimary) {}
 
 StatusWith<ReplSetMetadata> ReplSetMetadata::readFromMetadata(const BSONObj& metadataObj) {
     BSONElement replMetadataElement;
@@ -105,6 +108,17 @@ StatusWith<ReplSetMetadata> ReplSetMetadata::readFromMetadata(const BSONObj& met
     if (!status.isOK())
         return status;
 
+    // TODO(SERVER-47125): require the isPrimary field.
+    boost::optional<bool> isPrimary;
+    if (replMetadataObj.hasField(kIsPrimaryFieldName)) {
+        bool isPrimaryBool;
+        status = bsonExtractBooleanField(replMetadataObj, kIsPrimaryFieldName, &isPrimaryBool);
+        if (!status.isOK())
+            return status;
+
+        isPrimary = isPrimaryBool;
+    }
+
     long long term;
     status = bsonExtractIntegerField(replMetadataObj, kTermFieldName, &term);
     if (!status.isOK())
@@ -131,8 +145,14 @@ StatusWith<ReplSetMetadata> ReplSetMetadata::readFromMetadata(const BSONObj& met
     }
 
     lastOpCommitted.wallTime = wallClockTimeElement.Date();
-    return ReplSetMetadata(
-        term, lastOpCommitted, lastOpVisible, configVersion, id, primaryIndex, syncSourceIndex);
+    return ReplSetMetadata(term,
+                           lastOpCommitted,
+                           lastOpVisible,
+                           configVersion,
+                           id,
+                           primaryIndex,
+                           syncSourceIndex,
+                           isPrimary);
 }
 
 Status ReplSetMetadata::writeToMetadata(BSONObjBuilder* builder) const {
@@ -145,6 +165,7 @@ Status ReplSetMetadata::writeToMetadata(BSONObjBuilder* builder) const {
     replMetadataBuilder.append(kReplicaSetIdFieldName, _replicaSetId);
     replMetadataBuilder.append(kPrimaryIndexFieldName, _currentPrimaryIndex);
     replMetadataBuilder.append(kSyncSourceIndexFieldName, _currentSyncSourceIndex);
+    replMetadataBuilder.append(kIsPrimaryFieldName, _isPrimary.get());
     replMetadataBuilder.doneFast();
 
     return Status::OK();
@@ -158,6 +179,7 @@ std::string ReplSetMetadata::toString() const {
     output << " Term: " << _currentTerm;
     output << " Primary Index: " << _currentPrimaryIndex;
     output << " Sync Source Index: " << _currentSyncSourceIndex;
+    output << " Is Primary: " << _isPrimary;
     output << " Last Op Committed: " << _lastOpCommitted.toString();
     output << " Last Op Visible: " << _lastOpVisible.toString();
     return output;
diff --git a/src/mongo/rpc/metadata/repl_set_metadata.h b/src/mongo/rpc/metadata/repl_set_metadata.h
index 48b71f8cb69..df2e2ae96b2 100644
--- a/src/mongo/rpc/metadata/repl_set_metadata.h
+++ b/src/mongo/rpc/metadata/repl_set_metadata.h
@@ -59,7 +59,8 @@ public:
                     long long configVersion,
                     OID replicaSetId,
                     int currentPrimaryIndex,
-                    int currentSyncSourceIndex);
+                    int currentSyncSourceIndex,
+                    boost::optional<bool> isPrimary);
 
     /**
      * format:
@@ -70,7 +71,8 @@ public:
      *     configVersion: 0,
      *     replicaSetId: ObjectId("..."), // Only present in certain versions and above.
      *     primaryIndex: 0,
-     *     syncSourceIndex: 0
+     *     syncSourceIndex: 0,
+     *     isPrimary: false // 4.4 and later
      * }
      */
     static StatusWith<ReplSetMetadata> readFromMetadata(const BSONObj& doc);
@@ -128,6 +130,15 @@ public:
     }
 
     /**
+     * Returns true if the sender is primary, false if it isn't, and boost::none if this metadata
+     * is from a pre-4.4 node that doesn't send isPrimary.
+     */
+    // TODO(SERVER-47125): make this a regular bool post-4.4.
+    boost::optional<bool> getIsPrimary() const {
+        return _isPrimary;
+    }
+
+    /**
      * Returns the current term from the perspective of the sender.
      */
     long long getTerm() const {
@@ -147,6 +158,7 @@ private:
     OID _replicaSetId;
     int _currentPrimaryIndex = kNoPrimary;
     int _currentSyncSourceIndex = -1;
+    boost::optional<bool> _isPrimary;
 };
 
 }  // namespace rpc
diff --git a/src/mongo/rpc/metadata/repl_set_metadata_test.cpp b/src/mongo/rpc/metadata/repl_set_metadata_test.cpp
index 9775bbf3724..ca89b0f7e64 100644
--- a/src/mongo/rpc/metadata/repl_set_metadata_test.cpp
+++ b/src/mongo/rpc/metadata/repl_set_metadata_test.cpp
@@ -40,17 +40,18 @@ namespace {
 using repl::OpTime;
 using repl::OpTimeAndWallTime;
 
+static const OpTime opTime(Timestamp(1234, 100), 5);
+static const OpTime opTime2(Timestamp(7777, 100), 6);
+static const Date_t committedWallTime = Date_t() + Seconds(opTime.getSecs());
+static const ReplSetMetadata metadata(
+    3, {opTime, committedWallTime}, opTime2, 6, OID("abcdefabcdefabcdefabcdef"), 12, -1, false);
+
 TEST(ReplResponseMetadataTest, ReplicaSetIdNotSet) {
-    ASSERT_FALSE(
-        ReplSetMetadata(3, OpTimeAndWallTime(), OpTime(), 6, OID(), 12, -1).hasReplicaSetId());
+    ASSERT_FALSE(ReplSetMetadata(3, OpTimeAndWallTime(), OpTime(), 6, OID(), 12, -1, false)
+                     .hasReplicaSetId());
 }
 
 TEST(ReplResponseMetadataTest, Roundtrip) {
-    OpTime opTime(Timestamp(1234, 100), 5);
-    OpTime opTime2(Timestamp(7777, 100), 6);
-    Date_t committedWallTime = Date_t() + Seconds(opTime.getSecs());
-    ReplSetMetadata metadata(3, {opTime, committedWallTime}, opTime2, 6, OID::gen(), 12, -1);
-
     ASSERT_EQ(opTime, metadata.getLastOpCommitted().opTime);
     ASSERT_EQ(committedWallTime, metadata.getLastOpCommitted().wallTime);
     ASSERT_EQ(opTime2, metadata.getLastOpVisible());
@@ -59,14 +60,14 @@ TEST(ReplResponseMetadataTest, Roundtrip) {
     BSONObjBuilder builder;
     metadata.writeToMetadata(&builder).transitional_ignore();
 
-    BSONObj expectedObj(
-        BSON(kReplSetMetadataFieldName
-             << BSON("term" << 3 << "lastOpCommitted"
-                            << BSON("ts" << opTime.getTimestamp() << "t" << opTime.getTerm())
-                            << "lastCommittedWall" << committedWallTime << "lastOpVisible"
-                            << BSON("ts" << opTime2.getTimestamp() << "t" << opTime2.getTerm())
-                            << "configVersion" << 6 << "replicaSetId" << metadata.getReplicaSetId()
-                            << "primaryIndex" << 12 << "syncSourceIndex" << -1)));
+    BSONObj expectedObj(BSON(
+        kReplSetMetadataFieldName << BSON(
+            "term" << 3 << "lastOpCommitted"
+                   << BSON("ts" << opTime.getTimestamp() << "t" << opTime.getTerm())
+                   << "lastCommittedWall" << committedWallTime << "lastOpVisible"
+                   << BSON("ts" << opTime2.getTimestamp() << "t" << opTime2.getTerm())
+                   << "configVersion" << 6 << "replicaSetId" << metadata.getReplicaSetId()
+                   << "primaryIndex" << 12 << "syncSourceIndex" << -1 << "isPrimary" << false)));
 
     BSONObj serializedObj = builder.obj();
     ASSERT_BSONOBJ_EQ(expectedObj, serializedObj);
@@ -89,21 +90,38 @@ TEST(ReplResponseMetadataTest, Roundtrip) {
 }
 
 TEST(ReplResponseMetadataTest, MetadataCanBeConstructedWhenMissingOplogQueryMetadataFields) {
-    auto id = OID::gen();
-    Date_t committedWallTime = Date_t();
     BSONObj obj(BSON(kReplSetMetadataFieldName
-                     << BSON("term" << 3 << "configVersion" << 6 << "replicaSetId" << id
-                                    << "lastCommittedWall" << committedWallTime)));
+                     << BSON("term" << 3 << "configVersion" << 6 << "replicaSetId"
+                                    << metadata.getReplicaSetId() << "lastCommittedWall"
+                                    << committedWallTime << "isPrimary" << false)));
 
     auto status = ReplSetMetadata::readFromMetadata(obj);
     ASSERT_OK(status.getStatus());
 
     const auto& metadata = status.getValue();
     ASSERT_EQ(metadata.getConfigVersion(), 6);
-    ASSERT_EQ(metadata.getReplicaSetId(), id);
+    ASSERT_EQ(metadata.getReplicaSetId(), metadata.getReplicaSetId());
     ASSERT_EQ(metadata.getTerm(), 3);
 }
 
+TEST(ReplResponseMetadataTest, MetadataCanBeConstructedWhenMissingIsPrimary) {
+    // TODO(SERVER-47125): delete this test in 4.6 when we can rely on the isPrimary field.
+    BSONObj obj(
+        BSON(kReplSetMetadataFieldName
+             << BSON("term" << 3 << "lastOpCommitted"
+                            << BSON("ts" << opTime.getTimestamp() << "t" << opTime.getTerm())
+                            << "lastCommittedWall" << committedWallTime << "lastOpVisible"
+                            << BSON("ts" << opTime2.getTimestamp() << "t" << opTime2.getTerm())
+                            << "configVersion" << 6 << "replicaSetId" << metadata.getReplicaSetId()
+                            << "primaryIndex" << 12 << "syncSourceIndex" << -1)));
+
+    auto status = ReplSetMetadata::readFromMetadata(obj);
+    ASSERT_OK(status.getStatus());
+
+    const auto& metadata = status.getValue();
+    ASSERT_FALSE(metadata.getIsPrimary().is_initialized());
+}
+
 }  // unnamed namespace
 }  // namespace rpc
 }  // namespace mongo
diff --git a/src/mongo/s/catalog/sharding_catalog_test.cpp b/src/mongo/s/catalog/sharding_catalog_test.cpp
index c0c019db581..6905cfa1e2f 100644
--- a/src/mongo/s/catalog/sharding_catalog_test.cpp
+++ b/src/mongo/s/catalog/sharding_catalog_test.cpp
@@ -123,7 +123,8 @@ TEST_F(ShardingCatalogClientTest, GetCollectionExisting) {
                                      100,
                                      OID(),
                                      30,
-                                     -1);
+                                     -1,
+                                     true);
             BSONObjBuilder builder;
             metadata.writeToMetadata(&builder).transitional_ignore();
 
@@ -196,7 +197,8 @@ TEST_F(ShardingCatalogClientTest, GetDatabaseExisting) {
                                  100,
                                  OID(),
                                  30,
-                                 -1);
+                                 -1,
+                                 true);
         BSONObjBuilder builder;
         metadata.writeToMetadata(&builder).transitional_ignore();
 
@@ -423,7 +425,8 @@ TEST_F(ShardingCatalogClientTest, GetChunksForNSWithSortAndLimit) {
                                      100,
                                      OID(),
                                      30,
-                                     -1);
+                                     -1,
+                                     true);
             BSONObjBuilder builder;
             metadata.writeToMetadata(&builder).transitional_ignore();
 
@@ -824,7 +827,8 @@ TEST_F(ShardingCatalogClientTest, GetCollectionsValidResultsNoDb) {
                                  100,
                                  OID(),
                                  30,
-                                 -1);
+                                 -1,
+                                 true);
         BSONObjBuilder builder;
         metadata.writeToMetadata(&builder).transitional_ignore();