SERVER-30626 Remove TopologyCoordinator interface.

16 files changed, 996 insertions, 407 deletions

diff --git a/src/mongo/db/SConscript b/src/mongo/db/SConscript
index 904c31e9092..5d466a1777e 100644
--- a/src/mongo/db/SConscript
+++ b/src/mongo/db/SConscript
@@ -840,7 +840,7 @@ env.Library(
         "repl/serveronly",
         "repl/storage_interface",
         "repl/sync_tail",
-        "repl/topology_coordinator_impl",
+        "repl/topology_coordinator",
         "rw_concern_d",
         "s/collection_metadata",
         "s/commands",
diff --git a/src/mongo/db/db.cpp b/src/mongo/db/db.cpp
index cf7acb988cf..fe0402f15e3 100644
--- a/src/mongo/db/db.cpp
+++ b/src/mongo/db/db.cpp
@@ -102,7 +102,7 @@
 #include "mongo/db/repl/replication_process.h"
 #include "mongo/db/repl/replication_recovery.h"
 #include "mongo/db/repl/storage_interface_impl.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/db/s/balancer/balancer.h"
 #include "mongo/db/s/sharded_connection_info.h"
 #include "mongo/db/s/sharding_initialization_mongod.h"
@@ -1194,7 +1194,7 @@ MONGO_INITIALIZER_WITH_PREREQUISITES(CreateReplicationManager,
         serviceContext, stdx::make_unique<repl::DropPendingCollectionReaper>(storageInterface));
     auto dropPendingCollectionReaper = repl::DropPendingCollectionReaper::get(serviceContext);
 
-    repl::TopologyCoordinatorImpl::Options topoCoordOptions;
+    repl::TopologyCoordinator::Options topoCoordOptions;
     topoCoordOptions.maxSyncSourceLagSecs = Seconds(repl::maxSyncSourceLagSecs);
     topoCoordOptions.clusterRole = serverGlobalParams.clusterRole;
 
@@ -1207,7 +1207,7 @@ MONGO_INITIALIZER_WITH_PREREQUISITES(CreateReplicationManager,
         stdx::make_unique<repl::ReplicationCoordinatorExternalStateImpl>(
             serviceContext, dropPendingCollectionReaper, storageInterface, replicationProcess),
         makeReplicationExecutor(serviceContext),
-        stdx::make_unique<repl::TopologyCoordinatorImpl>(topoCoordOptions),
+        stdx::make_unique<repl::TopologyCoordinator>(topoCoordOptions),
         replicationProcess,
         storageInterface,
         static_cast<int64_t>(curTimeMillis64()));
diff --git a/src/mongo/db/repl/SConscript b/src/mongo/db/repl/SConscript
index 1621bb65651..bf1821ce75d 100644
--- a/src/mongo/db/repl/SConscript
+++ b/src/mongo/db/repl/SConscript
@@ -732,23 +732,13 @@ env.Library('topology_coordinator',
                 'topology_coordinator.cpp',
             ],
             LIBDEPS=[
-                'repl_coordinator_interface',
-                'replica_set_messages',
-                'rslog',
-                '$BUILD_DIR/mongo/rpc/metadata',
-            ])
-
-env.Library('topology_coordinator_impl',
-            [
-                'topology_coordinator_impl.cpp',
-            ],
-            LIBDEPS=[
                 '$BUILD_DIR/mongo/db/audit',
+                '$BUILD_DIR/mongo/rpc/metadata',
+                '$BUILD_DIR/mongo/util/fail_point',
                 'replica_set_messages',
                 'repl_settings',
                 'rslog',
-                'topology_coordinator',
-                '$BUILD_DIR/mongo/util/fail_point',
+                'repl_coordinator_interface',
             ])
 
 env.CppUnitTest('repl_set_heartbeat_response_test',
@@ -756,12 +746,12 @@ env.CppUnitTest('repl_set_heartbeat_response_test',
                 LIBDEPS=['replica_set_messages'])
 
 env.CppUnitTest(
-    target='topology_coordinator_impl_test',
+    target='topology_coordinator_test',
     source=[
-        'topology_coordinator_impl_test.cpp',
+        'topology_coordinator_test.cpp',
     ],
     LIBDEPS=[
-        'topology_coordinator_impl',
+        'topology_coordinator',
         'replica_set_messages',
         '$BUILD_DIR/mongo/db/auth/authorization_manager_mock_init',
         '$BUILD_DIR/mongo/db/service_context_noop_init',
@@ -769,12 +759,12 @@ env.CppUnitTest(
 )
 
 env.CppUnitTest(
-    target='topology_coordinator_impl_v1_test',
+    target='topology_coordinator_v1_test',
     source=[
-        'topology_coordinator_impl_v1_test.cpp',
+        'topology_coordinator_v1_test.cpp',
     ],
     LIBDEPS=[
-        'topology_coordinator_impl',
+        'topology_coordinator',
         'replica_set_messages',
         '$BUILD_DIR/mongo/db/auth/authorization_manager_mock_init',
         '$BUILD_DIR/mongo/db/service_context_noop_init',
@@ -828,7 +818,7 @@ env.Library(
         'repl_coordinator_impl',
         'replmocks',
         'service_context_repl_mock_init',
-        'topology_coordinator_impl',
+        'topology_coordinator',
         '$BUILD_DIR/mongo/db/auth/authorization_manager_global',
         '$BUILD_DIR/mongo/db/auth/authorization_manager_mock_init',
         '$BUILD_DIR/mongo/executor/network_interface_mock',
diff --git a/src/mongo/db/repl/replication_coordinator_impl_elect.cpp b/src/mongo/db/repl/replication_coordinator_impl_elect.cpp
index 727d652630a..ac6e07ce3ee 100644
--- a/src/mongo/db/repl/replication_coordinator_impl_elect.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl_elect.cpp
@@ -34,7 +34,7 @@
 #include "mongo/db/repl/elect_cmd_runner.h"
 #include "mongo/db/repl/freshness_checker.h"
 #include "mongo/db/repl/replication_coordinator_impl.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/stdx/mutex.h"
 #include "mongo/util/log.h"
 #include "mongo/util/scopeguard.h"
diff --git a/src/mongo/db/repl/replication_coordinator_impl_elect_test.cpp b/src/mongo/db/repl/replication_coordinator_impl_elect_test.cpp
index e449665a483..0025f70464b 100644
--- a/src/mongo/db/repl/replication_coordinator_impl_elect_test.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl_elect_test.cpp
@@ -39,7 +39,7 @@
 #include "mongo/db/repl/replication_coordinator_external_state_mock.h"
 #include "mongo/db/repl/replication_coordinator_impl.h"
 #include "mongo/db/repl/replication_coordinator_test_fixture.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/executor/network_interface_mock.h"
 #include "mongo/unittest/unittest.h"
 #include "mongo/util/fail_point_service.h"
diff --git a/src/mongo/db/repl/replication_coordinator_impl_elect_v1.cpp b/src/mongo/db/repl/replication_coordinator_impl_elect_v1.cpp
index 75b419765b8..dac78fdd56e 100644
--- a/src/mongo/db/repl/replication_coordinator_impl_elect_v1.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl_elect_v1.cpp
@@ -34,7 +34,7 @@
 
 #include "mongo/base/disallow_copying.h"
 #include "mongo/db/repl/replication_coordinator_impl.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/db/repl/vote_requester.h"
 #include "mongo/platform/unordered_set.h"
 #include "mongo/stdx/mutex.h"
diff --git a/src/mongo/db/repl/replication_coordinator_impl_elect_v1_test.cpp b/src/mongo/db/repl/replication_coordinator_impl_elect_v1_test.cpp
index 6aff08b85dc..0fd15751fec 100644
--- a/src/mongo/db/repl/replication_coordinator_impl_elect_v1_test.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl_elect_v1_test.cpp
@@ -39,7 +39,7 @@
 #include "mongo/db/repl/replication_coordinator_external_state_mock.h"
 #include "mongo/db/repl/replication_coordinator_impl.h"
 #include "mongo/db/repl/replication_coordinator_test_fixture.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/executor/network_interface_mock.h"
 #include "mongo/unittest/unittest.h"
 #include "mongo/util/fail_point_service.h"
diff --git a/src/mongo/db/repl/replication_coordinator_impl_heartbeat_test.cpp b/src/mongo/db/repl/replication_coordinator_impl_heartbeat_test.cpp
index c83657a154e..55140bb9a23 100644
--- a/src/mongo/db/repl/replication_coordinator_impl_heartbeat_test.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl_heartbeat_test.cpp
@@ -39,7 +39,7 @@
 #include "mongo/db/repl/replication_coordinator_external_state_mock.h"
 #include "mongo/db/repl/replication_coordinator_impl.h"
 #include "mongo/db/repl/replication_coordinator_test_fixture.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/executor/network_interface_mock.h"
 #include "mongo/unittest/unittest.h"
 #include "mongo/util/log.h"
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 6e77d46c084..2637deed13a 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
@@ -39,7 +39,7 @@
 #include "mongo/db/repl/replication_coordinator_external_state_mock.h"
 #include "mongo/db/repl/replication_coordinator_impl.h"
 #include "mongo/db/repl/replication_coordinator_test_fixture.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/executor/network_interface_mock.h"
 #include "mongo/rpc/metadata/repl_set_metadata.h"
 #include "mongo/unittest/unittest.h"
diff --git a/src/mongo/db/repl/replication_coordinator_impl_test.cpp b/src/mongo/db/repl/replication_coordinator_impl_test.cpp
index 24975726a36..a7201c03d32 100644
--- a/src/mongo/db/repl/replication_coordinator_impl_test.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl_test.cpp
@@ -55,7 +55,7 @@
 #include "mongo/db/repl/replication_coordinator_impl.h"
 #include "mongo/db/repl/replication_coordinator_test_fixture.h"
 #include "mongo/db/repl/storage_interface_mock.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/db/repl/update_position_args.h"
 #include "mongo/db/server_options.h"
 #include "mongo/db/service_context_noop.h"
diff --git a/src/mongo/db/repl/replication_coordinator_test_fixture.cpp b/src/mongo/db/repl/replication_coordinator_test_fixture.cpp
index 6d1555e2ed0..c2278a72e9a 100644
--- a/src/mongo/db/repl/replication_coordinator_test_fixture.cpp
+++ b/src/mongo/db/repl/replication_coordinator_test_fixture.cpp
@@ -44,7 +44,7 @@
 #include "mongo/db/repl/replication_process.h"
 #include "mongo/db/repl/replication_recovery_mock.h"
 #include "mongo/db/repl/storage_interface_mock.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/executor/network_interface_mock.h"
 #include "mongo/executor/thread_pool_mock.h"
 #include "mongo/executor/thread_pool_task_executor.h"
@@ -138,8 +138,8 @@ void ReplCoordTest::init() {
     auto logicalClock = stdx::make_unique<LogicalClock>(service);
     LogicalClock::set(service, std::move(logicalClock));
 
-    TopologyCoordinatorImpl::Options settings;
-    auto topo = stdx::make_unique<TopologyCoordinatorImpl>(settings);
+    TopologyCoordinator::Options settings;
+    auto topo = stdx::make_unique<TopologyCoordinator>(settings);
     _topo = topo.get();
     auto net = stdx::make_unique<NetworkInterfaceMock>();
     _net = net.get();
diff --git a/src/mongo/db/repl/replication_coordinator_test_fixture.h b/src/mongo/db/repl/replication_coordinator_test_fixture.h
index 92eeea79f63..15b0027b32f 100644
--- a/src/mongo/db/repl/replication_coordinator_test_fixture.h
+++ b/src/mongo/db/repl/replication_coordinator_test_fixture.h
@@ -48,7 +48,7 @@ class ReplSetConfig;
 class ReplicationCoordinatorExternalStateMock;
 class ReplicationCoordinatorImpl;
 class StorageInterfaceMock;
-class TopologyCoordinatorImpl;
+class TopologyCoordinator;
 
 using executor::NetworkInterfaceMock;
 
@@ -120,7 +120,7 @@ protected:
     /**
      * Gets the topology coordinator used by the replication coordinator under test.
      */
-    TopologyCoordinatorImpl& getTopoCoord() {
+    TopologyCoordinator& getTopoCoord() {
         return *_topo;
     }
 
@@ -288,7 +288,7 @@ protected:
 private:
     std::unique_ptr<ReplicationCoordinatorImpl> _repl;
     // Owned by ReplicationCoordinatorImpl
-    TopologyCoordinatorImpl* _topo = nullptr;
+    TopologyCoordinator* _topo = nullptr;
     // Owned by executor
     executor::NetworkInterfaceMock* _net = nullptr;
     // Owned by ReplicationCoordinatorImpl
diff --git a/src/mongo/db/repl/topology_coordinator.cpp b/src/mongo/db/repl/topology_coordinator.cpp
index 84f4ccbedfe..ffca19a4949 100644
--- a/src/mongo/db/repl/topology_coordinator.cpp
+++ b/src/mongo/db/repl/topology_coordinator.cpp
@@ -30,9 +30,10 @@
 
 #include "mongo/platform/basic.h"
 
-#include "mongo/db/repl/topology_coordinator_impl.h"
+#include "mongo/db/repl/topology_coordinator.h"
 
 #include <limits>
+#include <string>
 
 #include "mongo/db/audit.h"
 #include "mongo/db/client.h"
@@ -50,6 +51,7 @@
 #include "mongo/db/server_parameters.h"
 #include "mongo/rpc/metadata/oplog_query_metadata.h"
 #include "mongo/rpc/metadata/repl_set_metadata.h"
+#include "mongo/util/assert_util.h"
 #include "mongo/util/fail_point_service.h"
 #include "mongo/util/hex.h"
 #include "mongo/util/log.h"
@@ -59,19 +61,61 @@
 namespace mongo {
 namespace repl {
 using std::vector;
-const Seconds TopologyCoordinatorImpl::VoteLease::leaseTime = Seconds(30);
+const Seconds TopologyCoordinator::VoteLease::leaseTime = Seconds(30);
 
 // Controls how caught up in replication a secondary with higher priority than the current primary
 // must be before it will call for a priority takeover election.
 MONGO_EXPORT_STARTUP_SERVER_PARAMETER(priorityTakeoverFreshnessWindowSeconds, int, 2);
 
-// If this fail point is enabled, TopologyCoordinatorImpl::shouldChangeSyncSource() will ignore
-// the option TopologyCoordinatorImpl::Options::maxSyncSourceLagSecs. The sync source will not be
+// If this fail point is enabled, TopologyCoordinator::shouldChangeSyncSource() will ignore
+// the option TopologyCoordinator::Options::maxSyncSourceLagSecs. The sync source will not be
 // re-evaluated if it lags behind another node by more than 'maxSyncSourceLagSecs' seconds.
 MONGO_FP_DECLARE(disableMaxSyncSourceLagSecs);
 
+constexpr Milliseconds TopologyCoordinator::PingStats::UninitializedPing;
+
 namespace {
+constexpr int kLeaderValue = 0;
+constexpr int kFollowerValue = 1;
+constexpr int kCandidateValue = 2;
+}  // namespace
+
+const TopologyCoordinator::Role TopologyCoordinator::Role::leader(kLeaderValue);
+const TopologyCoordinator::Role TopologyCoordinator::Role::follower(kFollowerValue);
+const TopologyCoordinator::Role TopologyCoordinator::Role::candidate(kCandidateValue);
+
+TopologyCoordinator::Role::Role(int value) : _value(value) {}
+
+std::string TopologyCoordinator::Role::toString() const {
+    switch (_value) {
+        case kLeaderValue:
+            return "leader";
+        case kFollowerValue:
+            return "follower";
+        case kCandidateValue:
+            return "candidate";
+    }
+    invariant(false);
+}
+
+TopologyCoordinator::~TopologyCoordinator() {}
 
+std::ostream& operator<<(std::ostream& os, TopologyCoordinator::Role role) {
+    return os << role.toString();
+}
+
+std::ostream& operator<<(std::ostream& os,
+                         TopologyCoordinator::PrepareFreezeResponseResult result) {
+    switch (result) {
+        case TopologyCoordinator::PrepareFreezeResponseResult::kNoAction:
+            return os << "no action";
+        case TopologyCoordinator::PrepareFreezeResponseResult::kElectSelf:
+            return os << "elect self";
+    }
+    MONGO_UNREACHABLE;
+}
+
+namespace {
 template <typename T>
 int indexOfIterator(const std::vector<T>& vec, typename std::vector<T>::const_iterator& it) {
     return static_cast<int>(it - vec.begin());
@@ -114,23 +158,23 @@ void appendOpTime(BSONObjBuilder* bob,
 }
 }  // namespace
 
-void PingStats::start(Date_t now) {
+void TopologyCoordinator::PingStats::start(Date_t now) {
     _lastHeartbeatStartDate = now;
     _numFailuresSinceLastStart = 0;
 }
 
-void PingStats::hit(Milliseconds millis) {
+void TopologyCoordinator::PingStats::hit(Milliseconds millis) {
     _numFailuresSinceLastStart = std::numeric_limits<int>::max();
     ++count;
 
     value = value == UninitializedPing ? millis : Milliseconds((value * 4 + millis) / 5);
 }
 
-void PingStats::miss() {
+void TopologyCoordinator::PingStats::miss() {
     ++_numFailuresSinceLastStart;
 }
 
-TopologyCoordinatorImpl::TopologyCoordinatorImpl(Options options)
+TopologyCoordinator::TopologyCoordinator(Options options)
     : _role(Role::follower),
       _term(OpTime::kUninitializedTerm),
       _currentPrimaryIndex(-1),
@@ -145,22 +189,22 @@ TopologyCoordinatorImpl::TopologyCoordinatorImpl(Options options)
     _memberData.back().setIsSelf(true);
 }
 
-TopologyCoordinator::Role TopologyCoordinatorImpl::getRole() const {
+TopologyCoordinator::Role TopologyCoordinator::getRole() const {
     return _role;
 }
 
-void TopologyCoordinatorImpl::setForceSyncSourceIndex(int index) {
+void TopologyCoordinator::setForceSyncSourceIndex(int index) {
     invariant(_forceSyncSourceIndex < _rsConfig.getNumMembers());
     _forceSyncSourceIndex = index;
 }
 
-HostAndPort TopologyCoordinatorImpl::getSyncSourceAddress() const {
+HostAndPort TopologyCoordinator::getSyncSourceAddress() const {
     return _syncSource;
 }
 
-HostAndPort TopologyCoordinatorImpl::chooseNewSyncSource(Date_t now,
-                                                         const OpTime& lastOpTimeFetched,
-                                                         ChainingPreference chainingPreference) {
+HostAndPort TopologyCoordinator::chooseNewSyncSource(Date_t now,
+                                                     const OpTime& lastOpTimeFetched,
+                                                     ChainingPreference chainingPreference) {
     // If we are not a member of the current replica set configuration, no sync source is valid.
     if (_selfIndex == -1) {
         LOG(1) << "Cannot sync from any members because we are not in the replica set config";
@@ -361,8 +405,7 @@ HostAndPort TopologyCoordinatorImpl::chooseNewSyncSource(Date_t now,
     return _syncSource;
 }
 
-bool TopologyCoordinatorImpl::_memberIsBlacklisted(const MemberConfig& memberConfig,
-                                                   Date_t now) const {
+bool TopologyCoordinator::_memberIsBlacklisted(const MemberConfig& memberConfig, Date_t now) const {
     std::map<HostAndPort, Date_t>::const_iterator blacklisted =
         _syncSourceBlacklist.find(memberConfig.getHostAndPort());
     if (blacklisted != _syncSourceBlacklist.end()) {
@@ -373,12 +416,12 @@ bool TopologyCoordinatorImpl::_memberIsBlacklisted(const MemberConfig& memberCon
     return false;
 }
 
-void TopologyCoordinatorImpl::blacklistSyncSource(const HostAndPort& host, Date_t until) {
+void TopologyCoordinator::blacklistSyncSource(const HostAndPort& host, Date_t until) {
     LOG(2) << "blacklisting " << host << " until " << until.toString();
     _syncSourceBlacklist[host] = until;
 }
 
-void TopologyCoordinatorImpl::unblacklistSyncSource(const HostAndPort& host, Date_t now) {
+void TopologyCoordinator::unblacklistSyncSource(const HostAndPort& host, Date_t now) {
     std::map<HostAndPort, Date_t>::iterator hostItr = _syncSourceBlacklist.find(host);
     if (hostItr != _syncSourceBlacklist.end() && now >= hostItr->second) {
         LOG(2) << "unblacklisting " << host;
@@ -386,13 +429,13 @@ void TopologyCoordinatorImpl::unblacklistSyncSource(const HostAndPort& host, Dat
     }
 }
 
-void TopologyCoordinatorImpl::clearSyncSourceBlacklist() {
+void TopologyCoordinator::clearSyncSourceBlacklist() {
     _syncSourceBlacklist.clear();
 }
 
-void TopologyCoordinatorImpl::prepareSyncFromResponse(const HostAndPort& target,
-                                                      BSONObjBuilder* response,
-                                                      Status* result) {
+void TopologyCoordinator::prepareSyncFromResponse(const HostAndPort& target,
+                                                  BSONObjBuilder* response,
+                                                  Status* result) {
     response->append("syncFromRequested", target.toString());
 
     if (_selfIndex == -1) {
@@ -483,11 +526,10 @@ void TopologyCoordinatorImpl::prepareSyncFromResponse(const HostAndPort& target,
     *result = Status::OK();
 }
 
-void TopologyCoordinatorImpl::prepareFreshResponse(
-    const ReplicationCoordinator::ReplSetFreshArgs& args,
-    const Date_t now,
-    BSONObjBuilder* response,
-    Status* result) {
+void TopologyCoordinator::prepareFreshResponse(const ReplicationCoordinator::ReplSetFreshArgs& args,
+                                               const Date_t now,
+                                               BSONObjBuilder* response,
+                                               Status* result) {
     if (_rsConfig.getProtocolVersion() != 0) {
         *result = Status(ErrorCodes::BadValue,
                          str::stream() << "replset: incompatible replset protocol version: "
@@ -543,10 +585,9 @@ void TopologyCoordinatorImpl::prepareFreshResponse(
     *result = Status::OK();
 }
 
-bool TopologyCoordinatorImpl::_shouldVetoMember(
-    const ReplicationCoordinator::ReplSetFreshArgs& args,
-    const Date_t& now,
-    std::string* errmsg) const {
+bool TopologyCoordinator::_shouldVetoMember(const ReplicationCoordinator::ReplSetFreshArgs& args,
+                                            const Date_t& now,
+                                            std::string* errmsg) const {
     if (_rsConfig.getConfigVersion() < args.cfgver) {
         // We are stale; do not veto.
         return false;
@@ -611,11 +652,10 @@ bool TopologyCoordinatorImpl::_shouldVetoMember(
 }
 
 // produce a reply to a received electCmd
-void TopologyCoordinatorImpl::prepareElectResponse(
-    const ReplicationCoordinator::ReplSetElectArgs& args,
-    const Date_t now,
-    BSONObjBuilder* response,
-    Status* result) {
+void TopologyCoordinator::prepareElectResponse(const ReplicationCoordinator::ReplSetElectArgs& args,
+                                               const Date_t now,
+                                               BSONObjBuilder* response,
+                                               Status* result) {
     if (_rsConfig.getProtocolVersion() != 0) {
         *result = Status(ErrorCodes::BadValue,
                          str::stream() << "replset: incompatible replset protocol version: "
@@ -688,10 +728,10 @@ void TopologyCoordinatorImpl::prepareElectResponse(
 }
 
 // produce a reply to a heartbeat
-Status TopologyCoordinatorImpl::prepareHeartbeatResponse(Date_t now,
-                                                         const ReplSetHeartbeatArgs& args,
-                                                         const std::string& ourSetName,
-                                                         ReplSetHeartbeatResponse* response) {
+Status TopologyCoordinator::prepareHeartbeatResponse(Date_t now,
+                                                     const ReplSetHeartbeatArgs& args,
+                                                     const std::string& ourSetName,
+                                                     ReplSetHeartbeatResponse* response) {
     if (args.getProtocolVersion() != 1) {
         return Status(ErrorCodes::BadValue,
                       str::stream() << "replset: incompatible replset protocol version: "
@@ -784,10 +824,10 @@ Status TopologyCoordinatorImpl::prepareHeartbeatResponse(Date_t now,
     return Status::OK();
 }
 
-Status TopologyCoordinatorImpl::prepareHeartbeatResponseV1(Date_t now,
-                                                           const ReplSetHeartbeatArgsV1& args,
-                                                           const std::string& ourSetName,
-                                                           ReplSetHeartbeatResponse* response) {
+Status TopologyCoordinator::prepareHeartbeatResponseV1(Date_t now,
+                                                       const ReplSetHeartbeatArgsV1& args,
+                                                       const std::string& ourSetName,
+                                                       ReplSetHeartbeatResponse* response) {
     // Verify that replica set names match
     const std::string rshb = args.getSetName();
     if (ourSetName != rshb) {
@@ -864,7 +904,7 @@ Status TopologyCoordinatorImpl::prepareHeartbeatResponseV1(Date_t now,
     return Status::OK();
 }
 
-int TopologyCoordinatorImpl::_getMemberIndex(int id) const {
+int TopologyCoordinator::_getMemberIndex(int id) const {
     int index = 0;
     for (ReplSetConfig::MemberIterator it = _rsConfig.membersBegin(); it != _rsConfig.membersEnd();
          ++it, ++index) {
@@ -875,7 +915,7 @@ int TopologyCoordinatorImpl::_getMemberIndex(int id) const {
     return -1;
 }
 
-std::pair<ReplSetHeartbeatArgs, Milliseconds> TopologyCoordinatorImpl::prepareHeartbeatRequest(
+std::pair<ReplSetHeartbeatArgs, Milliseconds> TopologyCoordinator::prepareHeartbeatRequest(
     Date_t now, const std::string& ourSetName, const HostAndPort& target) {
     PingStats& hbStats = _pings[target];
     Milliseconds alreadyElapsed = now - hbStats.getLastHeartbeatStartDate();
@@ -914,7 +954,7 @@ std::pair<ReplSetHeartbeatArgs, Milliseconds> TopologyCoordinatorImpl::prepareHe
     return std::make_pair(hbArgs, timeout);
 }
 
-std::pair<ReplSetHeartbeatArgsV1, Milliseconds> TopologyCoordinatorImpl::prepareHeartbeatRequestV1(
+std::pair<ReplSetHeartbeatArgsV1, Milliseconds> TopologyCoordinator::prepareHeartbeatRequestV1(
     Date_t now, const std::string& ourSetName, const HostAndPort& target) {
     PingStats& hbStats = _pings[target];
     Milliseconds alreadyElapsed(now.asInt64() - hbStats.getLastHeartbeatStartDate().asInt64());
@@ -954,7 +994,7 @@ std::pair<ReplSetHeartbeatArgsV1, Milliseconds> TopologyCoordinatorImpl::prepare
     return std::make_pair(hbArgs, timeout);
 }
 
-HeartbeatResponseAction TopologyCoordinatorImpl::processHeartbeatResponse(
+HeartbeatResponseAction TopologyCoordinator::processHeartbeatResponse(
     Date_t now,
     Milliseconds networkRoundTripTime,
     const HostAndPort& target,
@@ -1092,9 +1132,9 @@ HeartbeatResponseAction TopologyCoordinatorImpl::processHeartbeatResponse(
     return nextAction;
 }
 
-bool TopologyCoordinatorImpl::haveNumNodesReachedOpTime(const OpTime& targetOpTime,
-                                                        int numNodes,
-                                                        bool durablyWritten) {
+bool TopologyCoordinator::haveNumNodesReachedOpTime(const OpTime& targetOpTime,
+                                                    int numNodes,
+                                                    bool durablyWritten) {
     // Replication progress that is for some reason ahead of us should not allow us to
     // satisfy a write concern if we aren't caught up ourselves.
     OpTime myOpTime = durablyWritten ? getMyLastDurableOpTime() : getMyLastAppliedOpTime();
@@ -1116,9 +1156,9 @@ bool TopologyCoordinatorImpl::haveNumNodesReachedOpTime(const OpTime& targetOpTi
     return false;
 }
 
-bool TopologyCoordinatorImpl::haveTaggedNodesReachedOpTime(const OpTime& opTime,
-                                                           const ReplSetTagPattern& tagPattern,
-                                                           bool durablyWritten) {
+bool TopologyCoordinator::haveTaggedNodesReachedOpTime(const OpTime& opTime,
+                                                       const ReplSetTagPattern& tagPattern,
+                                                       bool durablyWritten) {
     ReplSetTagMatch matcher(tagPattern);
     for (auto&& memberData : _memberData) {
         const OpTime& memberOpTime =
@@ -1141,7 +1181,7 @@ bool TopologyCoordinatorImpl::haveTaggedNodesReachedOpTime(const OpTime& opTime,
     return false;
 }
 
-HeartbeatResponseAction TopologyCoordinatorImpl::checkMemberTimeouts(Date_t now) {
+HeartbeatResponseAction TopologyCoordinator::checkMemberTimeouts(Date_t now) {
     bool stepdown = false;
     for (int memberIndex = 0; memberIndex < static_cast<int>(_memberData.size()); memberIndex++) {
         auto& memberData = _memberData[memberIndex];
@@ -1160,9 +1200,9 @@ HeartbeatResponseAction TopologyCoordinatorImpl::checkMemberTimeouts(Date_t now)
     return HeartbeatResponseAction::makeNoAction();
 }
 
-std::vector<HostAndPort> TopologyCoordinatorImpl::getHostsWrittenTo(const OpTime& op,
-                                                                    bool durablyWritten,
-                                                                    bool skipSelf) {
+std::vector<HostAndPort> TopologyCoordinator::getHostsWrittenTo(const OpTime& op,
+                                                                bool durablyWritten,
+                                                                bool skipSelf) {
     std::vector<HostAndPort> hosts;
     for (const auto& memberData : _memberData) {
         if (skipSelf && memberData.isSelf()) {
@@ -1182,7 +1222,7 @@ std::vector<HostAndPort> TopologyCoordinatorImpl::getHostsWrittenTo(const OpTime
     return hosts;
 }
 
-bool TopologyCoordinatorImpl::setMemberAsDown(Date_t now, const int memberIndex) {
+bool TopologyCoordinator::setMemberAsDown(Date_t now, const int memberIndex) {
     invariant(memberIndex != _selfIndex);
     invariant(memberIndex != -1);
     invariant(_currentPrimaryIndex == _selfIndex);
@@ -1196,7 +1236,7 @@ bool TopologyCoordinatorImpl::setMemberAsDown(Date_t now, const int memberIndex)
     return false;
 }
 
-std::pair<int, Date_t> TopologyCoordinatorImpl::getStalestLiveMember() const {
+std::pair<int, Date_t> TopologyCoordinator::getStalestLiveMember() const {
     Date_t earliestDate = Date_t::max();
     int earliestMemberId = -1;
     for (const auto& memberData : _memberData) {
@@ -1217,39 +1257,39 @@ std::pair<int, Date_t> TopologyCoordinatorImpl::getStalestLiveMember() const {
     return std::make_pair(earliestMemberId, earliestDate);
 }
 
-void TopologyCoordinatorImpl::resetAllMemberTimeouts(Date_t now) {
+void TopologyCoordinator::resetAllMemberTimeouts(Date_t now) {
     for (auto&& memberData : _memberData)
         memberData.updateLiveness(now);
 }
 
-void TopologyCoordinatorImpl::resetMemberTimeouts(
-    Date_t now, const stdx::unordered_set<HostAndPort>& member_set) {
+void TopologyCoordinator::resetMemberTimeouts(Date_t now,
+                                              const stdx::unordered_set<HostAndPort>& member_set) {
     for (auto&& memberData : _memberData) {
         if (member_set.count(memberData.getHostAndPort()))
             memberData.updateLiveness(now);
     }
 }
 
-OpTime TopologyCoordinatorImpl::getMyLastAppliedOpTime() const {
+OpTime TopologyCoordinator::getMyLastAppliedOpTime() const {
     return _selfMemberData().getLastAppliedOpTime();
 }
 
-OpTime TopologyCoordinatorImpl::getMyLastDurableOpTime() const {
+OpTime TopologyCoordinator::getMyLastDurableOpTime() const {
     return _selfMemberData().getLastDurableOpTime();
 }
 
-MemberData* TopologyCoordinatorImpl::getMyMemberData() {
+MemberData* TopologyCoordinator::getMyMemberData() {
     return &_memberData[_selfMemberDataIndex()];
 }
 
-MemberData* TopologyCoordinatorImpl::findMemberDataByMemberId(const int memberId) {
+MemberData* TopologyCoordinator::findMemberDataByMemberId(const int memberId) {
     const int memberIndex = _getMemberIndex(memberId);
     if (memberIndex >= 0)
         return &_memberData[memberIndex];
     return nullptr;
 }
 
-MemberData* TopologyCoordinatorImpl::findMemberDataByRid(const OID rid) {
+MemberData* TopologyCoordinator::findMemberDataByRid(const OID rid) {
     for (auto& memberData : _memberData) {
         if (memberData.getRid() == rid)
             return &memberData;
@@ -1257,7 +1297,7 @@ MemberData* TopologyCoordinatorImpl::findMemberDataByRid(const OID rid) {
     return nullptr;
 }
 
-MemberData* TopologyCoordinatorImpl::addSlaveMemberData(const OID rid) {
+MemberData* TopologyCoordinator::addSlaveMemberData(const OID rid) {
     invariant(!_memberData.empty());        // Must always have our own entry first.
     invariant(!_rsConfig.isInitialized());  // Used only for master-slave.
     _memberData.emplace_back();
@@ -1266,7 +1306,7 @@ MemberData* TopologyCoordinatorImpl::addSlaveMemberData(const OID rid) {
     return result;
 }
 
-HeartbeatResponseAction TopologyCoordinatorImpl::_updatePrimaryFromHBDataV1(
+HeartbeatResponseAction TopologyCoordinator::_updatePrimaryFromHBDataV1(
     int updatedConfigIndex, const MemberState& originalState, Date_t now) {
     //
     // Updates the local notion of which remote node, if any is primary.
@@ -1339,7 +1379,7 @@ HeartbeatResponseAction TopologyCoordinatorImpl::_updatePrimaryFromHBDataV1(
     return HeartbeatResponseAction::makeNoAction();
 }
 
-HeartbeatResponseAction TopologyCoordinatorImpl::_updatePrimaryFromHBData(
+HeartbeatResponseAction TopologyCoordinator::_updatePrimaryFromHBData(
     int updatedConfigIndex, const MemberState& originalState, Date_t now) {
     // This method has two interrelated responsibilities, performed in two phases.
     //
@@ -1519,14 +1559,14 @@ HeartbeatResponseAction TopologyCoordinatorImpl::_updatePrimaryFromHBData(
     const auto status = checkShouldStandForElection(now);
     if (!status.isOK()) {
         // NOTE: This log line is checked in unit test(s).
-        LOG(2) << "TopologyCoordinatorImpl::_updatePrimaryFromHBData - " << status.reason();
+        LOG(2) << "TopologyCoordinator::_updatePrimaryFromHBData - " << status.reason();
         return HeartbeatResponseAction::makeNoAction();
     }
     fassertStatusOK(28816, becomeCandidateIfElectable(now, StartElectionReason::kElectionTimeout));
     return HeartbeatResponseAction::makeElectAction();
 }
 
-Status TopologyCoordinatorImpl::checkShouldStandForElection(Date_t now) const {
+Status TopologyCoordinator::checkShouldStandForElection(Date_t now) const {
     if (_currentPrimaryIndex != -1) {
         return {ErrorCodes::NodeNotElectable, "Not standing for election since there is a Primary"};
     }
@@ -1570,7 +1610,7 @@ Status TopologyCoordinatorImpl::checkShouldStandForElection(Date_t now) const {
     return Status::OK();
 }
 
-bool TopologyCoordinatorImpl::_aMajoritySeemsToBeUp() const {
+bool TopologyCoordinator::_aMajoritySeemsToBeUp() const {
     int vUp = 0;
     for (std::vector<MemberData>::const_iterator it = _memberData.begin(); it != _memberData.end();
          ++it) {
@@ -1583,7 +1623,7 @@ bool TopologyCoordinatorImpl::_aMajoritySeemsToBeUp() const {
     return vUp * 2 > _rsConfig.getTotalVotingMembers();
 }
 
-int TopologyCoordinatorImpl::_findHealthyPrimaryOfEqualOrGreaterPriority(
+int TopologyCoordinator::_findHealthyPrimaryOfEqualOrGreaterPriority(
     const int candidateIndex) const {
     const double candidatePriority = _rsConfig.getMemberAt(candidateIndex).getPriority();
     for (auto it = _memberData.begin(); it != _memberData.end(); ++it) {
@@ -1600,13 +1640,13 @@ int TopologyCoordinatorImpl::_findHealthyPrimaryOfEqualOrGreaterPriority(
     return -1;
 }
 
-bool TopologyCoordinatorImpl::_isOpTimeCloseEnoughToLatestToElect(const OpTime& otherOpTime) const {
+bool TopologyCoordinator::_isOpTimeCloseEnoughToLatestToElect(const OpTime& otherOpTime) const {
     const OpTime latestKnownOpTime = _latestKnownOpTime();
     // Use addition instead of subtraction to avoid overflow.
     return otherOpTime.getSecs() + 10 >= (latestKnownOpTime.getSecs());
 }
 
-bool TopologyCoordinatorImpl::_amIFreshEnoughForPriorityTakeover() const {
+bool TopologyCoordinator::_amIFreshEnoughForPriorityTakeover() const {
     const OpTime latestKnownOpTime = _latestKnownOpTime();
 
     // Rules are:
@@ -1634,7 +1674,7 @@ bool TopologyCoordinatorImpl::_amIFreshEnoughForPriorityTakeover() const {
     }
 }
 
-bool TopologyCoordinatorImpl::_amIFreshEnoughForCatchupTakeover() const {
+bool TopologyCoordinator::_amIFreshEnoughForCatchupTakeover() const {
 
     const OpTime latestKnownOpTime = _latestKnownOpTime();
 
@@ -1668,7 +1708,7 @@ bool TopologyCoordinatorImpl::_amIFreshEnoughForCatchupTakeover() const {
     return ourLastOpApplied.getTerm() < _term;
 }
 
-bool TopologyCoordinatorImpl::_iAmPrimary() const {
+bool TopologyCoordinator::_iAmPrimary() const {
     if (_role == Role::leader) {
         invariant(_currentPrimaryIndex == _selfIndex);
         invariant(_leaderMode != LeaderMode::kNotLeader);
@@ -1677,7 +1717,7 @@ bool TopologyCoordinatorImpl::_iAmPrimary() const {
     return false;
 }
 
-OpTime TopologyCoordinatorImpl::_latestKnownOpTime() const {
+OpTime TopologyCoordinator::_latestKnownOpTime() const {
     OpTime latest = getMyLastAppliedOpTime();
     for (std::vector<MemberData>::const_iterator it = _memberData.begin(); it != _memberData.end();
          ++it) {
@@ -1705,8 +1745,7 @@ OpTime TopologyCoordinatorImpl::_latestKnownOpTime() const {
     return latest;
 }
 
-bool TopologyCoordinatorImpl::_isMemberHigherPriority(int memberOneIndex,
-                                                      int memberTwoIndex) const {
+bool TopologyCoordinator::_isMemberHigherPriority(int memberOneIndex, int memberTwoIndex) const {
     if (memberOneIndex == -1)
         return false;
 
@@ -1717,7 +1756,7 @@ bool TopologyCoordinatorImpl::_isMemberHigherPriority(int memberOneIndex,
         _rsConfig.getMemberAt(memberTwoIndex).getPriority();
 }
 
-int TopologyCoordinatorImpl::_getHighestPriorityElectableIndex(Date_t now) const {
+int TopologyCoordinator::_getHighestPriorityElectableIndex(Date_t now) const {
     int maxIndex = -1;
     for (int currentIndex = 0; currentIndex < _rsConfig.getNumMembers(); currentIndex++) {
         UnelectableReasonMask reason = currentIndex == _selfIndex
@@ -1731,7 +1770,7 @@ int TopologyCoordinatorImpl::_getHighestPriorityElectableIndex(Date_t now) const
     return maxIndex;
 }
 
-bool TopologyCoordinatorImpl::prepareForUnconditionalStepDown() {
+bool TopologyCoordinator::prepareForUnconditionalStepDown() {
     if (_leaderMode == LeaderMode::kSteppingDown) {
         // Can only be processing one required stepdown at a time.
         return false;
@@ -1742,7 +1781,7 @@ bool TopologyCoordinatorImpl::prepareForUnconditionalStepDown() {
     return true;
 }
 
-Status TopologyCoordinatorImpl::prepareForStepDownAttempt() {
+Status TopologyCoordinator::prepareForStepDownAttempt() {
     if (_leaderMode == LeaderMode::kSteppingDown ||
         _leaderMode == LeaderMode::kAttemptingStepDown) {
         return Status{ErrorCodes::ConflictingOperationInProgress,
@@ -1752,14 +1791,14 @@ Status TopologyCoordinatorImpl::prepareForStepDownAttempt() {
     return Status::OK();
 }
 
-void TopologyCoordinatorImpl::abortAttemptedStepDownIfNeeded() {
+void TopologyCoordinator::abortAttemptedStepDownIfNeeded() {
     if (_leaderMode == TopologyCoordinator::LeaderMode::kAttemptingStepDown) {
         _setLeaderMode(TopologyCoordinator::LeaderMode::kMaster);
     }
 }
 
-void TopologyCoordinatorImpl::changeMemberState_forTest(const MemberState& newMemberState,
-                                                        const Timestamp& electionTime) {
+void TopologyCoordinator::changeMemberState_forTest(const MemberState& newMemberState,
+                                                    const Timestamp& electionTime) {
     invariant(_selfIndex != -1);
     if (newMemberState == getMemberState())
         return;
@@ -1795,7 +1834,7 @@ void TopologyCoordinatorImpl::changeMemberState_forTest(const MemberState& newMe
     log() << newMemberState;
 }
 
-void TopologyCoordinatorImpl::_setCurrentPrimaryForTest(int primaryIndex) {
+void TopologyCoordinator::_setCurrentPrimaryForTest(int primaryIndex) {
     if (primaryIndex == _selfIndex) {
         changeMemberState_forTest(MemberState::RS_PRIMARY);
     } else {
@@ -1817,16 +1856,16 @@ void TopologyCoordinatorImpl::_setCurrentPrimaryForTest(int primaryIndex) {
     }
 }
 
-const MemberConfig* TopologyCoordinatorImpl::_currentPrimaryMember() const {
+const MemberConfig* TopologyCoordinator::_currentPrimaryMember() const {
     if (_currentPrimaryIndex == -1)
         return NULL;
 
     return &(_rsConfig.getMemberAt(_currentPrimaryIndex));
 }
 
-void TopologyCoordinatorImpl::prepareStatusResponse(const ReplSetStatusArgs& rsStatusArgs,
-                                                    BSONObjBuilder* response,
-                                                    Status* result) {
+void TopologyCoordinator::prepareStatusResponse(const ReplSetStatusArgs& rsStatusArgs,
+                                                BSONObjBuilder* response,
+                                                Status* result) {
     // output for each member
     vector<BSONObj> membersOut;
     const MemberState myState = getMemberState();
@@ -1933,12 +1972,10 @@ void TopologyCoordinatorImpl::prepareStatusResponse(const ReplSetStatusArgs& rsS
             bb.appendDate("lastHeartbeat", it->getLastHeartbeat());
             bb.appendDate("lastHeartbeatRecv", it->getLastHeartbeatRecv());
             Milliseconds ping = _getPing(itConfig.getHostAndPort());
-            if (ping != UninitializedPing) {
-                bb.append("pingMs", durationCount<Milliseconds>(ping));
-                std::string s = it->getLastHeartbeatMsg();
-                if (!s.empty())
-                    bb.append("lastHeartbeatMessage", s);
-            }
+            bb.append("pingMs", durationCount<Milliseconds>(ping));
+            std::string s = it->getLastHeartbeatMsg();
+            if (!s.empty())
+                bb.append("lastHeartbeatMessage", s);
             if (it->hasAuthIssue()) {
                 bb.append("authenticated", false);
             }
@@ -1997,7 +2034,7 @@ void TopologyCoordinatorImpl::prepareStatusResponse(const ReplSetStatusArgs& rsS
     *result = Status::OK();
 }
 
-StatusWith<BSONObj> TopologyCoordinatorImpl::prepareReplSetUpdatePositionCommand(
+StatusWith<BSONObj> TopologyCoordinator::prepareReplSetUpdatePositionCommand(
     ReplicationCoordinator::ReplSetUpdatePositionCommandStyle commandStyle,
     OpTime currentCommittedSnapshotOpTime) const {
     BSONObjBuilder cmdBuilder;
@@ -2051,7 +2088,7 @@ StatusWith<BSONObj> TopologyCoordinatorImpl::prepareReplSetUpdatePositionCommand
     return cmdBuilder.obj();
 }
 
-void TopologyCoordinatorImpl::fillMemberData(BSONObjBuilder* result) {
+void TopologyCoordinator::fillMemberData(BSONObjBuilder* result) {
     BSONArrayBuilder replicationProgress(result->subarrayStart("replicationProgress"));
     {
         for (const auto& memberData : _memberData) {
@@ -2076,7 +2113,7 @@ void TopologyCoordinatorImpl::fillMemberData(BSONObjBuilder* result) {
     }
 }
 
-void TopologyCoordinatorImpl::fillIsMasterForReplSet(IsMasterResponse* response) {
+void TopologyCoordinator::fillIsMasterForReplSet(IsMasterResponse* response) {
     const MemberState myState = getMemberState();
     if (!_rsConfig.isInitialized()) {
         response->markAsNoConfig();
@@ -2146,8 +2183,8 @@ void TopologyCoordinatorImpl::fillIsMasterForReplSet(IsMasterResponse* response)
     }
 }
 
-StatusWith<TopologyCoordinatorImpl::PrepareFreezeResponseResult>
-TopologyCoordinatorImpl::prepareFreezeResponse(Date_t now, int secs, BSONObjBuilder* response) {
+StatusWith<TopologyCoordinator::PrepareFreezeResponseResult>
+TopologyCoordinator::prepareFreezeResponse(Date_t now, int secs, BSONObjBuilder* response) {
     if (_role != TopologyCoordinator::Role::follower) {
         std::string msg = str::stream()
             << "cannot freeze node when primary or running for election. state: "
@@ -2180,7 +2217,7 @@ TopologyCoordinatorImpl::prepareFreezeResponse(Date_t now, int secs, BSONObjBuil
     return PrepareFreezeResponseResult::kNoAction;
 }
 
-bool TopologyCoordinatorImpl::becomeCandidateIfStepdownPeriodOverAndSingleNodeSet(Date_t now) {
+bool TopologyCoordinator::becomeCandidateIfStepdownPeriodOverAndSingleNodeSet(Date_t now) {
     if (_stepDownUntil > now) {
         return false;
     }
@@ -2195,31 +2232,31 @@ bool TopologyCoordinatorImpl::becomeCandidateIfStepdownPeriodOverAndSingleNodeSe
     return false;
 }
 
-void TopologyCoordinatorImpl::setElectionSleepUntil(Date_t newTime) {
+void TopologyCoordinator::setElectionSleepUntil(Date_t newTime) {
     if (_electionSleepUntil < newTime) {
         _electionSleepUntil = newTime;
     }
 }
 
-Timestamp TopologyCoordinatorImpl::getElectionTime() const {
+Timestamp TopologyCoordinator::getElectionTime() const {
     return _electionTime;
 }
 
-OID TopologyCoordinatorImpl::getElectionId() const {
+OID TopologyCoordinator::getElectionId() const {
     return _electionId;
 }
 
-int TopologyCoordinatorImpl::getCurrentPrimaryIndex() const {
+int TopologyCoordinator::getCurrentPrimaryIndex() const {
     return _currentPrimaryIndex;
 }
 
-Date_t TopologyCoordinatorImpl::getStepDownTime() const {
+Date_t TopologyCoordinator::getStepDownTime() const {
     return _stepDownUntil;
 }
 
-void TopologyCoordinatorImpl::_updateHeartbeatDataForReconfig(const ReplSetConfig& newConfig,
-                                                              int selfIndex,
-                                                              Date_t now) {
+void TopologyCoordinator::_updateHeartbeatDataForReconfig(const ReplSetConfig& newConfig,
+                                                          int selfIndex,
+                                                          Date_t now) {
     std::vector<MemberData> oldHeartbeats;
     _memberData.swap(oldHeartbeats);
 
@@ -2265,9 +2302,7 @@ void TopologyCoordinatorImpl::_updateHeartbeatDataForReconfig(const ReplSetConfi
 
 // This function installs a new config object and recreates MemberData objects
 // that reflect the new config.
-void TopologyCoordinatorImpl::updateConfig(const ReplSetConfig& newConfig,
-                                           int selfIndex,
-                                           Date_t now) {
+void TopologyCoordinator::updateConfig(const ReplSetConfig& newConfig, int selfIndex, Date_t now) {
     invariant(_role != Role::candidate);
     invariant(selfIndex < newConfig.getNumMembers());
 
@@ -2312,7 +2347,7 @@ void TopologyCoordinatorImpl::updateConfig(const ReplSetConfig& newConfig,
         _role = Role::candidate;
     }
 }
-std::string TopologyCoordinatorImpl::_getHbmsg(Date_t now) const {
+std::string TopologyCoordinator::_getHbmsg(Date_t now) const {
     // ignore messages over 2 minutes old
     if ((now - _hbmsgTime) > Seconds{120}) {
         return "";
@@ -2320,20 +2355,20 @@ std::string TopologyCoordinatorImpl::_getHbmsg(Date_t now) const {
     return _hbmsg;
 }
 
-void TopologyCoordinatorImpl::setMyHeartbeatMessage(const Date_t now, const std::string& message) {
+void TopologyCoordinator::setMyHeartbeatMessage(const Date_t now, const std::string& message) {
     _hbmsgTime = now;
     _hbmsg = message;
 }
 
-const MemberConfig& TopologyCoordinatorImpl::_selfConfig() const {
+const MemberConfig& TopologyCoordinator::_selfConfig() const {
     return _rsConfig.getMemberAt(_selfIndex);
 }
 
-const MemberData& TopologyCoordinatorImpl::_selfMemberData() const {
+const MemberData& TopologyCoordinator::_selfMemberData() const {
     return _memberData[_selfMemberDataIndex()];
 }
 
-const int TopologyCoordinatorImpl::_selfMemberDataIndex() const {
+const int TopologyCoordinator::_selfMemberDataIndex() const {
     invariant(!_memberData.empty());
     if (_selfIndex >= 0)
         return _selfIndex;
@@ -2342,7 +2377,7 @@ const int TopologyCoordinatorImpl::_selfMemberDataIndex() const {
     return 0;
 }
 
-TopologyCoordinatorImpl::UnelectableReasonMask TopologyCoordinatorImpl::_getUnelectableReason(
+TopologyCoordinator::UnelectableReasonMask TopologyCoordinator::_getUnelectableReason(
     int index) const {
     invariant(index != _selfIndex);
     const MemberConfig& memberConfig = _rsConfig.getMemberAt(index);
@@ -2368,7 +2403,7 @@ TopologyCoordinatorImpl::UnelectableReasonMask TopologyCoordinatorImpl::_getUnel
     return result;
 }
 
-TopologyCoordinatorImpl::UnelectableReasonMask TopologyCoordinatorImpl::_getMyUnelectableReason(
+TopologyCoordinator::UnelectableReasonMask TopologyCoordinator::_getMyUnelectableReason(
     const Date_t now, StartElectionReason reason) const {
     UnelectableReasonMask result = None;
     const OpTime lastApplied = getMyLastAppliedOpTime();
@@ -2423,8 +2458,7 @@ TopologyCoordinatorImpl::UnelectableReasonMask TopologyCoordinatorImpl::_getMyUn
     return result;
 }
 
-std::string TopologyCoordinatorImpl::_getUnelectableReasonString(
-    const UnelectableReasonMask ur) const {
+std::string TopologyCoordinator::_getUnelectableReasonString(const UnelectableReasonMask ur) const {
     invariant(ur);
     str::stream ss;
     bool hasWrittenToStream = false;
@@ -2521,15 +2555,15 @@ std::string TopologyCoordinatorImpl::_getUnelectableReasonString(
     return ss;
 }
 
-Milliseconds TopologyCoordinatorImpl::_getPing(const HostAndPort& host) {
+Milliseconds TopologyCoordinator::_getPing(const HostAndPort& host) {
     return _pings[host].getMillis();
 }
 
-void TopologyCoordinatorImpl::_setElectionTime(const Timestamp& newElectionTime) {
+void TopologyCoordinator::_setElectionTime(const Timestamp& newElectionTime) {
     _electionTime = newElectionTime;
 }
 
-int TopologyCoordinatorImpl::_getTotalPings() {
+int TopologyCoordinator::_getTotalPings() {
     PingMap::iterator it = _pings.begin();
     PingMap::iterator end = _pings.end();
     int totalPings = 0;
@@ -2540,7 +2574,7 @@ int TopologyCoordinatorImpl::_getTotalPings() {
     return totalPings;
 }
 
-std::vector<HostAndPort> TopologyCoordinatorImpl::getMaybeUpHostAndPorts() const {
+std::vector<HostAndPort> TopologyCoordinator::getMaybeUpHostAndPorts() const {
     std::vector<HostAndPort> upHosts;
     for (std::vector<MemberData>::const_iterator it = _memberData.begin(); it != _memberData.end();
          ++it) {
@@ -2557,7 +2591,7 @@ std::vector<HostAndPort> TopologyCoordinatorImpl::getMaybeUpHostAndPorts() const
     return upHosts;
 }
 
-bool TopologyCoordinatorImpl::voteForMyself(Date_t now) {
+bool TopologyCoordinator::voteForMyself(Date_t now) {
     if (_role != Role::candidate) {
         return false;
     }
@@ -2574,12 +2608,12 @@ bool TopologyCoordinatorImpl::voteForMyself(Date_t now) {
     return true;
 }
 
-bool TopologyCoordinatorImpl::isSteppingDown() const {
+bool TopologyCoordinator::isSteppingDown() const {
     return _leaderMode == LeaderMode::kAttemptingStepDown ||
         _leaderMode == LeaderMode::kSteppingDown;
 }
 
-void TopologyCoordinatorImpl::_setLeaderMode(TopologyCoordinator::LeaderMode newMode) {
+void TopologyCoordinator::_setLeaderMode(TopologyCoordinator::LeaderMode newMode) {
     // Invariants for valid state transitions.
     switch (_leaderMode) {
         case LeaderMode::kNotLeader:
@@ -2607,7 +2641,7 @@ void TopologyCoordinatorImpl::_setLeaderMode(TopologyCoordinator::LeaderMode new
     _leaderMode = std::move(newMode);
 }
 
-MemberState TopologyCoordinatorImpl::getMemberState() const {
+MemberState TopologyCoordinator::getMemberState() const {
     if (_selfIndex == -1) {
         if (_rsConfig.isInitialized()) {
             return MemberState::RS_REMOVED;
@@ -2646,17 +2680,17 @@ MemberState TopologyCoordinatorImpl::getMemberState() const {
     return _followerMode;
 }
 
-bool TopologyCoordinatorImpl::canAcceptWrites() const {
+bool TopologyCoordinator::canAcceptWrites() const {
     return _leaderMode == LeaderMode::kMaster;
 }
 
-void TopologyCoordinatorImpl::setElectionInfo(OID electionId, Timestamp electionOpTime) {
+void TopologyCoordinator::setElectionInfo(OID electionId, Timestamp electionOpTime) {
     invariant(_role == Role::leader);
     _electionTime = electionOpTime;
     _electionId = electionId;
 }
 
-void TopologyCoordinatorImpl::processWinElection(OID electionId, Timestamp electionOpTime) {
+void TopologyCoordinator::processWinElection(OID electionId, Timestamp electionOpTime) {
     invariant(_role == Role::candidate);
     invariant(_leaderMode == LeaderMode::kNotLeader);
     _role = Role::leader;
@@ -2670,7 +2704,7 @@ void TopologyCoordinatorImpl::processWinElection(OID electionId, Timestamp elect
         OpTime(Timestamp(std::numeric_limits<int>::max(), 0), std::numeric_limits<int>::max());
 }
 
-void TopologyCoordinatorImpl::processLoseElection() {
+void TopologyCoordinator::processLoseElection() {
     invariant(_role == Role::candidate);
     invariant(_leaderMode == LeaderMode::kNotLeader);
     const HostAndPort syncSourceAddress = getSyncSourceAddress();
@@ -2685,7 +2719,7 @@ void TopologyCoordinatorImpl::processLoseElection() {
     }
 }
 
-bool TopologyCoordinatorImpl::attemptStepDown(
+bool TopologyCoordinator::attemptStepDown(
     long long termAtStart, Date_t now, Date_t waitUntil, Date_t stepDownUntil, bool force) {
 
     if (_role != Role::leader || _leaderMode == LeaderMode::kSteppingDown || _term != termAtStart) {
@@ -2724,9 +2758,7 @@ bool TopologyCoordinatorImpl::attemptStepDown(
     return true;
 }
 
-bool TopologyCoordinatorImpl::_canCompleteStepDownAttempt(Date_t now,
-                                                          Date_t waitUntil,
-                                                          bool force) {
+bool TopologyCoordinator::_canCompleteStepDownAttempt(Date_t now, Date_t waitUntil, bool force) {
     const bool forceNow = force && (now >= waitUntil);
     if (forceNow) {
         return true;
@@ -2735,7 +2767,7 @@ bool TopologyCoordinatorImpl::_canCompleteStepDownAttempt(Date_t now,
     return isSafeToStepDown();
 }
 
-bool TopologyCoordinatorImpl::isSafeToStepDown() {
+bool TopologyCoordinator::isSafeToStepDown() {
     if (!_rsConfig.isInitialized() || _selfIndex < 0) {
         return false;
     }
@@ -2767,7 +2799,7 @@ bool TopologyCoordinatorImpl::isSafeToStepDown() {
     return false;
 }
 
-void TopologyCoordinatorImpl::setFollowerMode(MemberState::MS newMode) {
+void TopologyCoordinator::setFollowerMode(MemberState::MS newMode) {
     invariant(_role == Role::follower);
     switch (newMode) {
         case MemberState::RS_RECOVERING:
@@ -2793,13 +2825,13 @@ void TopologyCoordinatorImpl::setFollowerMode(MemberState::MS newMode) {
     }
 }
 
-bool TopologyCoordinatorImpl::_isElectableNodeInSingleNodeReplicaSet() const {
+bool TopologyCoordinator::_isElectableNodeInSingleNodeReplicaSet() const {
     return _followerMode == MemberState::RS_SECONDARY && _rsConfig.getNumMembers() == 1 &&
         _selfIndex == 0 && _rsConfig.getMemberAt(_selfIndex).isElectable() &&
         _maintenanceModeCalls == 0;
 }
 
-void TopologyCoordinatorImpl::finishUnconditionalStepDown() {
+void TopologyCoordinator::finishUnconditionalStepDown() {
     invariant(_leaderMode == LeaderMode::kSteppingDown);
 
     int remotePrimaryIndex = -1;
@@ -2824,7 +2856,7 @@ void TopologyCoordinatorImpl::finishUnconditionalStepDown() {
     _stepDownSelfAndReplaceWith(remotePrimaryIndex);
 }
 
-void TopologyCoordinatorImpl::_stepDownSelfAndReplaceWith(int newPrimary) {
+void TopologyCoordinator::_stepDownSelfAndReplaceWith(int newPrimary) {
     invariant(_role == Role::leader);
     invariant(_selfIndex != -1);
     invariant(_selfIndex != newPrimary);
@@ -2834,7 +2866,7 @@ void TopologyCoordinatorImpl::_stepDownSelfAndReplaceWith(int newPrimary) {
     _setLeaderMode(LeaderMode::kNotLeader);
 }
 
-bool TopologyCoordinatorImpl::updateLastCommittedOpTime() {
+bool TopologyCoordinator::updateLastCommittedOpTime() {
     // If we're not primary or we're stepping down due to learning of a new term then  we must not
     // advance the commit point.  If we are stepping down due to a user request, however, then it
     // is safe to advance the commit point, and in fact we must since the stepdown request may be
@@ -2870,7 +2902,7 @@ bool TopologyCoordinatorImpl::updateLastCommittedOpTime() {
     return advanceLastCommittedOpTime(committedOpTime);
 }
 
-bool TopologyCoordinatorImpl::advanceLastCommittedOpTime(const OpTime& committedOpTime) {
+bool TopologyCoordinator::advanceLastCommittedOpTime(const OpTime& committedOpTime) {
     if (committedOpTime == _lastCommittedOpTime) {
         return false;  // Hasn't changed, so ignore it.
     } else if (committedOpTime < _lastCommittedOpTime) {
@@ -2891,12 +2923,11 @@ bool TopologyCoordinatorImpl::advanceLastCommittedOpTime(const OpTime& committed
     return true;
 }
 
-OpTime TopologyCoordinatorImpl::getLastCommittedOpTime() const {
+OpTime TopologyCoordinator::getLastCommittedOpTime() const {
     return _lastCommittedOpTime;
 }
 
-bool TopologyCoordinatorImpl::canCompleteTransitionToPrimary(
-    long long termWhenDrainCompleted) const {
+bool TopologyCoordinator::canCompleteTransitionToPrimary(long long termWhenDrainCompleted) const {
 
     if (termWhenDrainCompleted != _term) {
         return false;
@@ -2910,7 +2941,7 @@ bool TopologyCoordinatorImpl::canCompleteTransitionToPrimary(
     return true;
 }
 
-Status TopologyCoordinatorImpl::completeTransitionToPrimary(const OpTime& firstOpTimeOfTerm) {
+Status TopologyCoordinator::completeTransitionToPrimary(const OpTime& firstOpTimeOfTerm) {
     if (!canCompleteTransitionToPrimary(firstOpTimeOfTerm.getTerm())) {
         return Status(ErrorCodes::PrimarySteppedDown,
                       "By the time this node was ready to complete its transition to PRIMARY it "
@@ -2923,18 +2954,17 @@ Status TopologyCoordinatorImpl::completeTransitionToPrimary(const OpTime& firstO
     return Status::OK();
 }
 
-void TopologyCoordinatorImpl::adjustMaintenanceCountBy(int inc) {
+void TopologyCoordinator::adjustMaintenanceCountBy(int inc) {
     invariant(_role == Role::follower);
     _maintenanceModeCalls += inc;
     invariant(_maintenanceModeCalls >= 0);
 }
 
-int TopologyCoordinatorImpl::getMaintenanceCount() const {
+int TopologyCoordinator::getMaintenanceCount() const {
     return _maintenanceModeCalls;
 }
 
-TopologyCoordinator::UpdateTermResult TopologyCoordinatorImpl::updateTerm(long long term,
-                                                                          Date_t now) {
+TopologyCoordinator::UpdateTermResult TopologyCoordinator::updateTerm(long long term, Date_t now) {
     if (term <= _term) {
         return TopologyCoordinator::UpdateTermResult::kAlreadyUpToDate;
     }
@@ -2952,13 +2982,13 @@ TopologyCoordinator::UpdateTermResult TopologyCoordinatorImpl::updateTerm(long l
 }
 
 
-long long TopologyCoordinatorImpl::getTerm() {
+long long TopologyCoordinator::getTerm() const {
     return _term;
 }
 
 // TODO(siyuan): Merge _hddata into _slaveInfo, so that we have a single view of the
 // replset. Passing metadata is unnecessary.
-bool TopologyCoordinatorImpl::shouldChangeSyncSource(
+bool TopologyCoordinator::shouldChangeSyncSource(
     const HostAndPort& currentSource,
     const rpc::ReplSetMetadata& replMetadata,
     boost::optional<rpc::OplogQueryMetadata> oqMetadata,
@@ -3079,7 +3109,7 @@ bool TopologyCoordinatorImpl::shouldChangeSyncSource(
     return false;
 }
 
-rpc::ReplSetMetadata TopologyCoordinatorImpl::prepareReplSetMetadata(
+rpc::ReplSetMetadata TopologyCoordinator::prepareReplSetMetadata(
     const OpTime& lastVisibleOpTime) const {
     return rpc::ReplSetMetadata(_term,
                                 _lastCommittedOpTime,
@@ -3090,7 +3120,7 @@ rpc::ReplSetMetadata TopologyCoordinatorImpl::prepareReplSetMetadata(
                                 _rsConfig.findMemberIndexByHostAndPort(getSyncSourceAddress()));
 }
 
-rpc::OplogQueryMetadata TopologyCoordinatorImpl::prepareOplogQueryMetadata(int rbid) const {
+rpc::OplogQueryMetadata TopologyCoordinator::prepareOplogQueryMetadata(int rbid) const {
     return rpc::OplogQueryMetadata(_lastCommittedOpTime,
                                    getMyLastAppliedOpTime(),
                                    rbid,
@@ -3098,7 +3128,7 @@ rpc::OplogQueryMetadata TopologyCoordinatorImpl::prepareOplogQueryMetadata(int r
                                    _rsConfig.findMemberIndexByHostAndPort(getSyncSourceAddress()));
 }
 
-void TopologyCoordinatorImpl::summarizeAsHtml(ReplSetHtmlSummary* output) {
+void TopologyCoordinator::summarizeAsHtml(ReplSetHtmlSummary* output) {
     output->setConfig(_rsConfig);
     output->setHBData(_memberData);
     output->setSelfIndex(_selfIndex);
@@ -3107,8 +3137,8 @@ void TopologyCoordinatorImpl::summarizeAsHtml(ReplSetHtmlSummary* output) {
     output->setSelfHeartbeatMessage(_hbmsg);
 }
 
-void TopologyCoordinatorImpl::processReplSetRequestVotes(const ReplSetRequestVotesArgs& args,
-                                                         ReplSetRequestVotesResponse* response) {
+void TopologyCoordinator::processReplSetRequestVotes(const ReplSetRequestVotesArgs& args,
+                                                     ReplSetRequestVotesResponse* response) {
     response->setTerm(_term);
 
     if (args.getTerm() < _term) {
@@ -3164,21 +3194,21 @@ void TopologyCoordinatorImpl::processReplSetRequestVotes(const ReplSetRequestVot
     }
 }
 
-void TopologyCoordinatorImpl::loadLastVote(const LastVote& lastVote) {
+void TopologyCoordinator::loadLastVote(const LastVote& lastVote) {
     _lastVote = lastVote;
 }
 
-void TopologyCoordinatorImpl::voteForMyselfV1() {
+void TopologyCoordinator::voteForMyselfV1() {
     _lastVote.setTerm(_term);
     _lastVote.setCandidateIndex(_selfIndex);
 }
 
-void TopologyCoordinatorImpl::setPrimaryIndex(long long primaryIndex) {
+void TopologyCoordinator::setPrimaryIndex(long long primaryIndex) {
     _currentPrimaryIndex = primaryIndex;
 }
 
-Status TopologyCoordinatorImpl::becomeCandidateIfElectable(const Date_t now,
-                                                           StartElectionReason reason) {
+Status TopologyCoordinator::becomeCandidateIfElectable(const Date_t now,
+                                                       StartElectionReason reason) {
     if (_role == Role::leader) {
         return {ErrorCodes::NodeNotElectable, "Not standing for election again; already primary"};
     }
@@ -3200,18 +3230,18 @@ Status TopologyCoordinatorImpl::becomeCandidateIfElectable(const Date_t now,
     return Status::OK();
 }
 
-void TopologyCoordinatorImpl::setStorageEngineSupportsReadCommitted(bool supported) {
+void TopologyCoordinator::setStorageEngineSupportsReadCommitted(bool supported) {
     _storageEngineSupportsReadCommitted =
         supported ? ReadCommittedSupport::kYes : ReadCommittedSupport::kNo;
 }
 
-void TopologyCoordinatorImpl::restartHeartbeats() {
+void TopologyCoordinator::restartHeartbeats() {
     for (auto& hb : _memberData) {
         hb.restart();
     }
 }
 
-boost::optional<OpTime> TopologyCoordinatorImpl::latestKnownOpTimeSinceHeartbeatRestart() const {
+boost::optional<OpTime> TopologyCoordinator::latestKnownOpTimeSinceHeartbeatRestart() const {
     // The smallest OpTime in PV1.
     OpTime latest(Timestamp(0, 0), 0);
     for (size_t i = 0; i < _memberData.size(); i++) {
diff --git a/src/mongo/db/repl/topology_coordinator.h b/src/mongo/db/repl/topology_coordinator.h
index 046ff2217a6..eca071e590a 100644
--- a/src/mongo/db/repl/topology_coordinator.h
+++ b/src/mongo/db/repl/topology_coordinator.h
@@ -28,241 +28,735 @@
 
 #pragma once
 
+#include <iosfwd>
 #include <string>
-#include <vector>
 
-#include "mongo/bson/timestamp.h"
+#include "mongo/base/disallow_copying.h"
 #include "mongo/db/repl/last_vote.h"
-#include "mongo/db/repl/member_data.h"
-#include "mongo/db/repl/member_state.h"
-#include "mongo/db/repl/optime.h"
-#include "mongo/db/repl/repl_set_config.h"
+#include "mongo/db/repl/repl_set_heartbeat_response.h"
 #include "mongo/db/repl/replication_coordinator.h"
-#include "mongo/db/repl/topology_coordinator.h"
 #include "mongo/db/server_options.h"
+#include "mongo/stdx/functional.h"
+#include "mongo/util/net/hostandport.h"
 #include "mongo/util/time_support.h"
 
 namespace mongo {
-
-class OperationContext;
-
-namespace rpc {
-class ReplSetMetadata;
-}  // namespace rpc
+class Timestamp;
 
 namespace repl {
-
-static const Milliseconds UninitializedPing{-1};
+class HeartbeatResponseAction;
+class MemberData;
+class OpTime;
+class ReplSetHeartbeatArgs;
+class ReplSetConfig;
+class TagSubgroup;
+struct MemberState;
 
 /**
- * Represents a latency measurement for each replica set member based on heartbeat requests.
- * The measurement is an average weighted 80% to the old value, and 20% to the new value.
+ * Replication Topology Coordinator
  *
- * Also stores information about heartbeat progress and retries.
+ * This object is responsible for managing the topology of the cluster.
+ * Tasks include consensus and leader election, chaining, and configuration management.
+ * Methods of this class should be non-blocking.
  */
-class PingStats {
+class TopologyCoordinator {
+    MONGO_DISALLOW_COPYING(TopologyCoordinator);
+
 public:
     /**
-     * Records that a new heartbeat request started at "now".
+     * Type that denotes the role of a node in the replication protocol.
      *
-     * This resets the failure count used in determining whether the next request to a target
-     * should be a retry or a regularly scheduled heartbeat message.
+     * The role is distinct from MemberState, in that it only deals with the
+     * roles a node plays in the basic protocol -- leader, follower and candidate.
+     * The mapping between MemberState and Role is complex -- several MemberStates
+     * map to the follower role, and MemberState::RS_SECONDARY maps to either
+     * follower or candidate roles, e.g.
      */
-    void start(Date_t now);
+    class Role {
+    public:
+        /**
+         * Constant indicating leader role.
+         */
+        static const Role leader;
+
+        /**
+         * Constant indicating follower role.
+         */
+        static const Role follower;
+
+        /**
+         * Constant indicating candidate role
+         */
+        static const Role candidate;
+
+        Role() {}
+
+        bool operator==(Role other) const {
+            return _value == other._value;
+        }
+        bool operator!=(Role other) const {
+            return _value != other._value;
+        }
+
+        std::string toString() const;
+
+    private:
+        explicit Role(int value);
+
+        int _value;
+    };
+
+    struct Options {
+        // A sync source is re-evaluated after it lags behind further than this amount.
+        Seconds maxSyncSourceLagSecs{0};
+
+        // Whether or not this node is running as a config server.
+        ClusterRole clusterRole{ClusterRole::None};
+    };
 
     /**
-     * Records that a heartbeat request completed successfully, and that "millis" milliseconds
-     * were spent for a single network roundtrip plus remote processing time.
+     * Constructs a Topology Coordinator object.
+     **/
+    TopologyCoordinator(Options options);
+
+
+    ~TopologyCoordinator();
+
+    /**
+     * Different modes a node can be in while still reporting itself as in state PRIMARY.
+     *
+     * Valid transitions:
+     *
+     *       kNotLeader <----------------------------------
+     *          |                                         |
+     *          |                                         |
+     *          |                                         |
+     *          v                                         |
+     *       kLeaderElect-----                            |
+     *          |            |                            |
+     *          |            |                            |
+     *          v            |                            |
+     *       kMaster -------------------------            |
+     *        |  ^           |                |           |
+     *        |  |     -------------------    |           |
+     *        |  |     |                 |    |           |
+     *        v  |     v                 v    v           |
+     *  kAttemptingStepDown----------->kSteppingDown      |
+     *        |                              |            |
+     *        |                              |            |
+     *        |                              |            |
+     *        ---------------------------------------------
+     *
      */
-    void hit(Milliseconds millis);
+    enum class LeaderMode {
+        kNotLeader,           // This node is not currently a leader.
+        kLeaderElect,         // This node has been elected leader, but can't yet accept writes.
+        kMaster,              // This node reports ismaster:true and can accept writes.
+        kSteppingDown,        // This node is in the middle of a (hb) stepdown that must complete.
+        kAttemptingStepDown,  // This node is in the middle of a stepdown (cmd) that might fail.
+    };
+
+    ////////////////////////////////////////////////////////////
+    //
+    // State inspection methods.
+    //
+    ////////////////////////////////////////////////////////////
 
     /**
-     * Records that a heartbeat request failed.
+     * Gets the role of this member in the replication protocol.
      */
-    void miss();
+    Role getRole() const;
 
     /**
-     * Gets the number of hit() calls.
+     * Gets the MemberState of this member in the replica set.
      */
-    unsigned int getCount() const {
-        return count;
-    }
+    MemberState getMemberState() const;
 
     /**
-     * Gets the weighted average round trip time for heartbeat messages to the target.
-     * Returns 0 if there have been no pings recorded yet.
+     * Returns whether this node should be allowed to accept writes.
      */
-    Milliseconds getMillis() const {
-        return value == UninitializedPing ? Milliseconds(0) : value;
-    }
+    bool canAcceptWrites() const;
 
     /**
-     * Gets the date at which start() was last called, which is used to determine if
-     * a heartbeat should be retried or if the time limit has expired.
+     * Returns true if this node is in the process of stepping down.  Note that this can be
+     * due to an unconditional stepdown that must succeed (for instance from learning about a new
+     * term) or due to a stepdown attempt that could fail (for instance from a stepdown cmd that
+     * could fail if not enough nodes are caught up).
      */
-    Date_t getLastHeartbeatStartDate() const {
-        return _lastHeartbeatStartDate;
-    }
+    bool isSteppingDown() const;
 
     /**
-     * Gets the number of failures since start() was last called.
+     * Returns the address of the current sync source, or an empty HostAndPort if there is no
+     * current sync source.
+     */
+    HostAndPort getSyncSourceAddress() const;
+
+    /**
+     * Retrieves a vector of HostAndPorts containing all nodes that are neither DOWN nor
+     * ourself.
+     */
+    std::vector<HostAndPort> getMaybeUpHostAndPorts() const;
+
+    /**
+     * Gets the earliest time the current node will stand for election.
+     */
+    Date_t getStepDownTime() const;
+
+    /**
+     * Gets the current value of the maintenance mode counter.
+     */
+    int getMaintenanceCount() const;
+
+    /**
+     * Gets the latest term this member is aware of. If this member is the primary,
+     * it's the current term of the replica set.
+     */
+    long long getTerm() const;
+
+    enum class UpdateTermResult { kAlreadyUpToDate, kTriggerStepDown, kUpdatedTerm };
+
+    ////////////////////////////////////////////////////////////
+    //
+    // Basic state manipulation methods.
+    //
+    ////////////////////////////////////////////////////////////
+
+    /**
+     * Sets the latest term this member is aware of to the higher of its current value and
+     * the value passed in as "term".
+     * Returns the result of setting the term value, or if a stepdown should be triggered.
+     */
+    UpdateTermResult updateTerm(long long term, Date_t now);
+
+    /**
+     * Sets the index into the config used when we next choose a sync source
+     */
+    void setForceSyncSourceIndex(int index);
+
+    enum class ChainingPreference { kAllowChaining, kUseConfiguration };
+
+    /**
+     * Chooses and sets a new sync source, based on our current knowledge of the world.
+     */
+    HostAndPort chooseNewSyncSource(Date_t now,
+                                    const OpTime& lastOpTimeFetched,
+                                    ChainingPreference chainingPreference);
+
+    /**
+     * Suppresses selecting "host" as sync source until "until".
+     */
+    void blacklistSyncSource(const HostAndPort& host, Date_t until);
+
+    /**
+     * Removes a single entry "host" from the list of potential sync sources which we
+     * have blacklisted, if it is supposed to be unblacklisted by "now".
+     */
+    void unblacklistSyncSource(const HostAndPort& host, Date_t now);
+
+    /**
+     * Clears the list of potential sync sources we have blacklisted.
+     */
+    void clearSyncSourceBlacklist();
+
+    /**
+     * Determines if a new sync source should be chosen, if a better candidate sync source is
+     * available.  If the current sync source's last optime ("syncSourceLastOpTime" under
+     * protocolVersion 1, but pulled from the MemberData in protocolVersion 0) is more than
+     * _maxSyncSourceLagSecs behind any syncable source, this function returns true. If we are
+     * running in ProtocolVersion 1, our current sync source is not primary, has no sync source
+     * ("syncSourceHasSyncSource" is false), and only has data up to "myLastOpTime", returns true.
      *
-     * This value is incremented by calls to miss(), cleared by calls to start() and
-     * set to the maximum possible value by calls to hit().
+     * "now" is used to skip over currently blacklisted sync sources.
+     *
+     * TODO (SERVER-27668): Make OplogQueryMetadata non-optional in mongodb 3.8.
      */
-    int getNumFailuresSinceLastStart() const {
-        return _numFailuresSinceLastStart;
-    }
+    bool shouldChangeSyncSource(const HostAndPort& currentSource,
+                                const rpc::ReplSetMetadata& replMetadata,
+                                boost::optional<rpc::OplogQueryMetadata> oqMetadata,
+                                Date_t now) const;
 
-private:
-    unsigned int count = 0;
-    Milliseconds value = UninitializedPing;
-    Date_t _lastHeartbeatStartDate;
-    int _numFailuresSinceLastStart = std::numeric_limits<int>::max();
-};
+    /**
+     * Checks whether we are a single node set and we are not in a stepdown period.  If so,
+     * puts us into candidate mode, otherwise does nothing.  This is used to ensure that
+     * nodes in a single node replset become primary again when their stepdown period ends.
+     */
+    bool becomeCandidateIfStepdownPeriodOverAndSingleNodeSet(Date_t now);
 
-class TopologyCoordinatorImpl : public TopologyCoordinator {
-public:
-    struct Options {
-        // A sync source is re-evaluated after it lags behind further than this amount.
-        Seconds maxSyncSourceLagSecs{0};
+    /**
+     * Sets the earliest time the current node will stand for election to "newTime".
+     *
+     * Until this time, while the node may report itself as electable, it will not stand
+     * for election.
+     */
+    void setElectionSleepUntil(Date_t newTime);
 
-        // Whether or not this node is running as a config server.
-        ClusterRole clusterRole{ClusterRole::None};
-    };
+    /**
+     * Sets the reported mode of this node to one of RS_SECONDARY, RS_STARTUP2, RS_ROLLBACK or
+     * RS_RECOVERING, when getRole() == Role::follower.  This is the interface by which the
+     * applier changes the reported member state of the current node, and enables or suppresses
+     * electability of the current node.  All modes but RS_SECONDARY indicate an unelectable
+     * follower state (one that cannot transition to candidate).
+     */
+    void setFollowerMode(MemberState::MS newMode);
 
     /**
-     * Constructs a Topology Coordinator object.
-     **/
-    TopologyCoordinatorImpl(Options options);
+     * Scan the memberData and determine the highest last applied or last
+     * durable optime present on a majority of servers; set _lastCommittedOpTime to this
+     * new entry.
+     * Whether the last applied or last durable op time is used depends on whether
+     * the config getWriteConcernMajorityShouldJournal is set.
+     * Returns true if the _lastCommittedOpTime was changed.
+     */
+    bool updateLastCommittedOpTime();
+
+    /**
+     * Updates _lastCommittedOpTime to be "committedOpTime" if it is more recent than the
+     * current last committed OpTime.  Returns true if _lastCommittedOpTime is changed.
+     */
+    bool advanceLastCommittedOpTime(const OpTime& committedOpTime);
+
+    /**
+     * Returns the OpTime of the latest majority-committed op known to this server.
+     */
+    OpTime getLastCommittedOpTime() const;
+
+    /**
+     * Returns true if it's safe to transition to LeaderMode::kMaster.
+     */
+    bool canCompleteTransitionToPrimary(long long termWhenDrainCompleted) const;
+
+    /**
+     * Called by the ReplicationCoordinator to signal that we have finished catchup and drain modes
+     * and are ready to fully become primary and start accepting writes.
+     * "firstOpTimeOfTerm" is a floor on the OpTimes this node will be allowed to consider committed
+     * for this tenure as primary. This prevents entries from before our election from counting as
+     * committed in our view, until our election (the "firstOpTimeOfTerm" op) has been committed.
+     * Returns PrimarySteppedDown if this node is no longer eligible to begin accepting writes.
+     */
+    Status completeTransitionToPrimary(const OpTime& firstOpTimeOfTerm);
+
+    /**
+     * Adjusts the maintenance mode count by "inc".
+     *
+     * It is an error to call this method if getRole() does not return Role::follower.
+     * It is an error to allow the maintenance count to go negative.
+     */
+    void adjustMaintenanceCountBy(int inc);
 
     ////////////////////////////////////////////////////////////
     //
-    // Implementation of TopologyCoordinator interface
+    // Methods that prepare responses to command requests.
     //
     ////////////////////////////////////////////////////////////
 
-    virtual Role getRole() const;
-    virtual MemberState getMemberState() const;
-    virtual bool canAcceptWrites() const override;
-    virtual bool isSteppingDown() const override;
-    virtual HostAndPort getSyncSourceAddress() const;
-    virtual std::vector<HostAndPort> getMaybeUpHostAndPorts() const;
-    virtual int getMaintenanceCount() const;
-    virtual long long getTerm();
-    virtual UpdateTermResult updateTerm(long long term, Date_t now);
-    virtual void setForceSyncSourceIndex(int index);
-    virtual HostAndPort chooseNewSyncSource(Date_t now,
-                                            const OpTime& lastOpTimeFetched,
-                                            ChainingPreference chainingPreference) override;
-    virtual void blacklistSyncSource(const HostAndPort& host, Date_t until);
-    virtual void unblacklistSyncSource(const HostAndPort& host, Date_t now);
-    virtual void clearSyncSourceBlacklist();
-    virtual bool shouldChangeSyncSource(const HostAndPort& currentSource,
-                                        const rpc::ReplSetMetadata& replMetadata,
-                                        boost::optional<rpc::OplogQueryMetadata> oqMetadata,
-                                        Date_t now) const;
-    virtual bool becomeCandidateIfStepdownPeriodOverAndSingleNodeSet(Date_t now);
-    virtual void setElectionSleepUntil(Date_t newTime);
-    virtual void setFollowerMode(MemberState::MS newMode);
-    virtual bool updateLastCommittedOpTime();
-    virtual bool advanceLastCommittedOpTime(const OpTime& committedOpTime);
-    virtual OpTime getLastCommittedOpTime() const;
-    virtual bool canCompleteTransitionToPrimary(long long termWhenDrainCompleted) const override;
-    virtual Status completeTransitionToPrimary(const OpTime& firstOpTimeOfTerm) override;
-    virtual void adjustMaintenanceCountBy(int inc);
-    virtual void prepareSyncFromResponse(const HostAndPort& target,
-                                         BSONObjBuilder* response,
-                                         Status* result);
-    virtual void prepareFreshResponse(const ReplicationCoordinator::ReplSetFreshArgs& args,
-                                      Date_t now,
-                                      BSONObjBuilder* response,
-                                      Status* result);
-    virtual void prepareElectResponse(const ReplicationCoordinator::ReplSetElectArgs& args,
-                                      Date_t now,
-                                      BSONObjBuilder* response,
-                                      Status* result);
-    virtual Status prepareHeartbeatResponse(Date_t now,
-                                            const ReplSetHeartbeatArgs& args,
-                                            const std::string& ourSetName,
-                                            ReplSetHeartbeatResponse* response);
-    virtual Status prepareHeartbeatResponseV1(Date_t now,
-                                              const ReplSetHeartbeatArgsV1& args,
-                                              const std::string& ourSetName,
-                                              ReplSetHeartbeatResponse* response);
-    virtual void prepareStatusResponse(const ReplSetStatusArgs& rsStatusArgs,
-                                       BSONObjBuilder* response,
-                                       Status* result);
-    virtual StatusWith<BSONObj> prepareReplSetUpdatePositionCommand(
+    // produces a reply to a replSetSyncFrom command
+    void prepareSyncFromResponse(const HostAndPort& target,
+                                 BSONObjBuilder* response,
+                                 Status* result);
+
+    // produce a reply to a replSetFresh command
+    void prepareFreshResponse(const ReplicationCoordinator::ReplSetFreshArgs& args,
+                              Date_t now,
+                              BSONObjBuilder* response,
+                              Status* result);
+
+    // produce a reply to a received electCmd
+    void prepareElectResponse(const ReplicationCoordinator::ReplSetElectArgs& args,
+                              Date_t now,
+                              BSONObjBuilder* response,
+                              Status* result);
+
+    // produce a reply to a heartbeat
+    Status prepareHeartbeatResponse(Date_t now,
+                                    const ReplSetHeartbeatArgs& args,
+                                    const std::string& ourSetName,
+                                    ReplSetHeartbeatResponse* response);
+
+    // produce a reply to a V1 heartbeat
+    Status prepareHeartbeatResponseV1(Date_t now,
+                                      const ReplSetHeartbeatArgsV1& args,
+                                      const std::string& ourSetName,
+                                      ReplSetHeartbeatResponse* response);
+
+    struct ReplSetStatusArgs {
+        Date_t now;
+        unsigned selfUptime;
+        const OpTime& readConcernMajorityOpTime;
+        const BSONObj& initialSyncStatus;
+    };
+
+    // produce a reply to a status request
+    void prepareStatusResponse(const ReplSetStatusArgs& rsStatusArgs,
+                               BSONObjBuilder* response,
+                               Status* result);
+
+    // Produce a replSetUpdatePosition command to be sent to the node's sync source.
+    StatusWith<BSONObj> prepareReplSetUpdatePositionCommand(
         ReplicationCoordinator::ReplSetUpdatePositionCommandStyle commandStyle,
         OpTime currentCommittedSnapshotOpTime) const;
 
-    virtual void fillIsMasterForReplSet(IsMasterResponse* response);
-    virtual void fillMemberData(BSONObjBuilder* result);
-    virtual StatusWith<PrepareFreezeResponseResult> prepareFreezeResponse(Date_t now,
-                                                                          int secs,
-                                                                          BSONObjBuilder* response);
-    virtual void updateConfig(const ReplSetConfig& newConfig, int selfIndex, Date_t now);
-    virtual std::pair<ReplSetHeartbeatArgs, Milliseconds> prepareHeartbeatRequest(
+    // produce a reply to an ismaster request.  It is only valid to call this if we are a
+    // replset.
+    void fillIsMasterForReplSet(IsMasterResponse* response);
+
+    // Produce member data for the serverStatus command and diagnostic logging.
+    void fillMemberData(BSONObjBuilder* result);
+
+    enum class PrepareFreezeResponseResult { kNoAction, kElectSelf };
+
+    /**
+     * Produce a reply to a freeze request. Returns a PostMemberStateUpdateAction on success that
+     * may trigger state changes in the caller.
+     */
+    StatusWith<PrepareFreezeResponseResult> prepareFreezeResponse(Date_t now,
+                                                                  int secs,
+                                                                  BSONObjBuilder* response);
+
+    ////////////////////////////////////////////////////////////
+    //
+    // Methods for sending and receiving heartbeats,
+    // reconfiguring and handling the results of standing for
+    // election.
+    //
+    ////////////////////////////////////////////////////////////
+
+    /**
+     * Updates the topology coordinator's notion of the replica set configuration.
+     *
+     * "newConfig" is the new configuration, and "selfIndex" is the index of this
+     * node's configuration information in "newConfig", or "selfIndex" is -1 to
+     * indicate that this node is not a member of "newConfig".
+     *
+     * newConfig.isInitialized() should be true, though implementations may accept
+     * configurations where this is not true, for testing purposes.
+     */
+    void updateConfig(const ReplSetConfig& newConfig, int selfIndex, Date_t now);
+
+    /**
+     * Prepares a heartbeat request appropriate for sending to "target", assuming the
+     * current time is "now".  "ourSetName" is used as the name for our replica set if
+     * the topology coordinator does not have a valid configuration installed.
+     *
+     * The returned pair contains proper arguments for a replSetHeartbeat command, and
+     * an amount of time to wait for the response.
+     *
+     * This call should be paired (with intervening network communication) with a call to
+     * processHeartbeatResponse for the same "target".
+     */
+    std::pair<ReplSetHeartbeatArgs, Milliseconds> prepareHeartbeatRequest(
         Date_t now, const std::string& ourSetName, const HostAndPort& target);
-    virtual std::pair<ReplSetHeartbeatArgsV1, Milliseconds> prepareHeartbeatRequestV1(
+    std::pair<ReplSetHeartbeatArgsV1, Milliseconds> prepareHeartbeatRequestV1(
         Date_t now, const std::string& ourSetName, const HostAndPort& target);
-    virtual HeartbeatResponseAction processHeartbeatResponse(
+
+    /**
+     * Processes a heartbeat response from "target" that arrived around "now", having
+     * spent "networkRoundTripTime" millis on the network.
+     *
+     * Updates internal topology coordinator state, and returns instructions about what action
+     * to take next.
+     *
+     * If the next action indicates StartElection, the topology coordinator has transitioned to
+     * the "candidate" role, and will remain there until processWinElection or
+     * processLoseElection are called.
+     *
+     * If the next action indicates "StepDownSelf", the topology coordinator has transitioned
+     * to the "follower" role from "leader", and the caller should take any necessary actions
+     * to become a follower.
+     *
+     * If the next action indicates "StepDownRemotePrimary", the caller should take steps to
+     * cause the specified remote host to step down from primary to secondary.
+     *
+     * If the next action indicates "Reconfig", the caller should verify the configuration in
+     * hbResponse is acceptable, perform any other reconfiguration actions it must, and call
+     * updateConfig with the new configuration and the appropriate value for "selfIndex".  It
+     * must also wrap up any outstanding elections (by calling processLoseElection or
+     * processWinElection) before calling updateConfig.
+     *
+     * This call should be paired (with intervening network communication) with a call to
+     * prepareHeartbeatRequest for the same "target".
+     */
+    HeartbeatResponseAction processHeartbeatResponse(
         Date_t now,
         Milliseconds networkRoundTripTime,
         const HostAndPort& target,
         const StatusWith<ReplSetHeartbeatResponse>& hbResponse);
-    virtual bool voteForMyself(Date_t now);
-    virtual void setElectionInfo(OID electionId, Timestamp electionOpTime);
-    virtual void processWinElection(OID electionId, Timestamp electionOpTime);
-    virtual void processLoseElection();
-    virtual Status checkShouldStandForElection(Date_t now) const;
-    virtual void setMyHeartbeatMessage(const Date_t now, const std::string& message);
-    virtual bool attemptStepDown(long long termAtStart,
-                                 Date_t now,
-                                 Date_t waitUntil,
-                                 Date_t stepDownUntil,
-                                 bool force) override;
-    virtual bool isSafeToStepDown() override;
-    virtual bool prepareForUnconditionalStepDown() override;
-    virtual Status prepareForStepDownAttempt() override;
-    virtual void abortAttemptedStepDownIfNeeded() override;
-    virtual void finishUnconditionalStepDown() override;
-    virtual Date_t getStepDownTime() const;
-    virtual rpc::ReplSetMetadata prepareReplSetMetadata(const OpTime& lastVisibleOpTime) const;
-    virtual rpc::OplogQueryMetadata prepareOplogQueryMetadata(int rbid) const;
-    virtual void processReplSetRequestVotes(const ReplSetRequestVotesArgs& args,
-                                            ReplSetRequestVotesResponse* response);
-    virtual void summarizeAsHtml(ReplSetHtmlSummary* output);
-    virtual void loadLastVote(const LastVote& lastVote);
-    virtual void voteForMyselfV1();
-    virtual void setPrimaryIndex(long long primaryIndex);
-    virtual int getCurrentPrimaryIndex() const;
-    virtual bool haveNumNodesReachedOpTime(const OpTime& opTime, int numNodes, bool durablyWritten);
-    virtual bool haveTaggedNodesReachedOpTime(const OpTime& opTime,
-                                              const ReplSetTagPattern& tagPattern,
-                                              bool durablyWritten);
-    virtual std::vector<HostAndPort> getHostsWrittenTo(const OpTime& op,
-                                                       bool durablyWritten,
-                                                       bool skipSelf);
-    virtual bool setMemberAsDown(Date_t now, const int memberIndex) override;
-    virtual std::pair<int, Date_t> getStalestLiveMember() const;
-    virtual HeartbeatResponseAction checkMemberTimeouts(Date_t now);
-    virtual void resetAllMemberTimeouts(Date_t now);
-    virtual void resetMemberTimeouts(Date_t now,
-                                     const stdx::unordered_set<HostAndPort>& member_set);
-    virtual OpTime getMyLastAppliedOpTime() const;
-    virtual OpTime getMyLastDurableOpTime() const;
-    virtual MemberData* getMyMemberData();
-    virtual MemberData* findMemberDataByMemberId(const int memberId);
-    virtual MemberData* findMemberDataByRid(const OID rid);
-    virtual MemberData* addSlaveMemberData(const OID rid);
-    virtual Status becomeCandidateIfElectable(const Date_t now, StartElectionReason reason);
-    virtual void setStorageEngineSupportsReadCommitted(bool supported);
-
-    virtual void restartHeartbeats();
-
-    virtual boost::optional<OpTime> latestKnownOpTimeSinceHeartbeatRestart() const;
+
+    /**
+     *  Returns whether or not at least 'numNodes' have reached the given opTime.
+     * "durablyWritten" indicates whether the operation has to be durably applied.
+     */
+    bool haveNumNodesReachedOpTime(const OpTime& opTime, int numNodes, bool durablyWritten);
+
+    /**
+     * Returns whether or not at least one node matching the tagPattern has reached
+     * the given opTime.
+     * "durablyWritten" indicates whether the operation has to be durably applied.
+     */
+    bool haveTaggedNodesReachedOpTime(const OpTime& opTime,
+                                      const ReplSetTagPattern& tagPattern,
+                                      bool durablyWritten);
+
+    /**
+     * Returns a vector of members that have applied the operation with OpTime 'op'.
+     * "durablyWritten" indicates whether the operation has to be durably applied.
+     * "skipSelf" means to exclude this node whether or not the op has been applied.
+     */
+    std::vector<HostAndPort> getHostsWrittenTo(const OpTime& op,
+                                               bool durablyWritten,
+                                               bool skipSelf);
+
+    /**
+     * Marks a member as down from our perspective and returns a bool which indicates if we can no
+     * longer see a majority of the nodes and thus should step down.
+     */
+    bool setMemberAsDown(Date_t now, const int memberIndex);
+
+    /**
+     * Goes through the memberData and determines which member that is currently live
+     * has the stalest (earliest) last update time.  Returns (-1, Date_t::max()) if there are
+     * no other members.
+     */
+    std::pair<int, Date_t> getStalestLiveMember() const;
+
+    /**
+     * Go through the memberData, and mark nodes which haven't been updated
+     * recently (within an election timeout) as "down".  Returns a HeartbeatResponseAction, which
+     * will be StepDownSelf if we can no longer see a majority of the nodes, otherwise NoAction.
+     */
+    HeartbeatResponseAction checkMemberTimeouts(Date_t now);
+
+    /**
+     * Set all nodes in memberData to not stale with a lastUpdate of "now".
+     */
+    void resetAllMemberTimeouts(Date_t now);
+
+    /**
+     * Set all nodes in memberData that are present in member_set
+     * to not stale with a lastUpdate of "now".
+     */
+    void resetMemberTimeouts(Date_t now, const stdx::unordered_set<HostAndPort>& member_set);
+
+    /*
+     * Returns the last optime that this node has applied, whether or not it has been journaled.
+     */
+    OpTime getMyLastAppliedOpTime() const;
+
+    /*
+     * Returns the last optime that this node has applied and journaled.
+     */
+    OpTime getMyLastDurableOpTime() const;
+
+    /*
+     * Returns information we have on the state of this node.
+     */
+    MemberData* getMyMemberData();
+
+    /*
+     * Returns information we have on the state of the node identified by memberId.  Returns
+     * nullptr if memberId is not found in the configuration.
+     */
+    MemberData* findMemberDataByMemberId(const int memberId);
+
+    /*
+     * Returns information we have on the state of the node identified by rid.  Returns
+     * nullptr if rid is not found in the heartbeat data.  This method is used only for
+     * master/slave replication.
+     */
+    MemberData* findMemberDataByRid(const OID rid);
+
+    /*
+     * Adds and returns a memberData entry for the given RID.
+     * Used only in master/slave mode.
+     */
+    MemberData* addSlaveMemberData(const OID rid);
+
+    /**
+     * If getRole() == Role::candidate and this node has not voted too recently, updates the
+     * lastVote tracker and returns true.  Otherwise, returns false.
+     */
+    bool voteForMyself(Date_t now);
+
+    /**
+     * Sets lastVote to be for ourself in this term.
+     */
+    void voteForMyselfV1();
+
+    /**
+     * Sets election id and election optime.
+     */
+    void setElectionInfo(OID electionId, Timestamp electionOpTime);
+
+    /**
+     * Performs state updates associated with winning an election.
+     *
+     * It is an error to call this if the topology coordinator is not in candidate mode.
+     *
+     * Exactly one of either processWinElection or processLoseElection must be called if
+     * processHeartbeatResponse returns StartElection, to exit candidate mode.
+     */
+    void processWinElection(OID electionId, Timestamp electionOpTime);
+
+    /**
+     * Performs state updates associated with losing an election.
+     *
+     * It is an error to call this if the topology coordinator is not in candidate mode.
+     *
+     * Exactly one of either processWinElection or processLoseElection must be called if
+     * processHeartbeatResponse returns StartElection, to exit candidate mode.
+     */
+    void processLoseElection();
+
+    /**
+     * Readies the TopologyCoordinator for an attempt to stepdown that may fail.  This is used
+     * when we receive a stepdown command (which can fail if not enough secondaries are caught up)
+     * to ensure that we never process more than one stepdown request at a time.
+     * Returns OK if it is safe to continue with the stepdown attempt, or returns
+     * ConflictingOperationInProgess if this node is already processing a stepdown request of any
+     * kind.
+     */
+    Status prepareForStepDownAttempt();
+
+    /**
+     * If this node is still attempting to process a stepdown attempt, aborts the attempt and
+     * returns this node to normal primary/master state.  If this node has already completed
+     * stepping down or is now in the process of handling an unconditional stepdown, then this
+     * method does nothing.
+     */
+    void abortAttemptedStepDownIfNeeded();
+
+    /**
+     * Tries to transition the coordinator from the leader role to the follower role.
+     *
+     * A step down succeeds based on the following conditions:
+     *
+     *      C1. 'force' is true and now > waitUntil
+     *
+     *      C2. A majority set of nodes, M, in the replica set have optimes greater than or
+     *      equal to the last applied optime of the primary.
+     *
+     *      C3. There exists at least one electable secondary node in the majority set M.
+     *
+     *
+     * If C1 is true, or if both C2 and C3 are true, then the stepdown occurs and this method
+     * returns true. If the conditions for successful stepdown aren't met yet, but waiting for more
+     * time to pass could make it succeed, returns false.  If the whole stepdown attempt should be
+     * abandoned (for example because the time limit expired or because we've already stepped down),
+     * throws an exception.
+     * TODO(spencer): Unify with the finishUnconditionalStepDown() method.
+     */
+    bool attemptStepDown(
+        long long termAtStart, Date_t now, Date_t waitUntil, Date_t stepDownUntil, bool force);
+
+    /**
+     * Returns whether it is safe for a stepdown attempt to complete, ignoring the 'force' argument.
+     * This is essentially checking conditions C2 and C3 as described in the comment to
+     * attemptStepDown().
+     */
+    bool isSafeToStepDown();
+
+    /**
+     * Readies the TopologyCoordinator for stepdown.  Returns false if we're already in the process
+     * of an unconditional step down.  If we are in the middle of a stepdown command attempt when
+     * this is called then this unconditional stepdown will supersede the stepdown attempt, which
+     * will cause the stepdown to fail.  When this returns true it must be followed by a call to
+     * finishUnconditionalStepDown() that is called when holding the global X lock.
+     */
+    bool prepareForUnconditionalStepDown();
+
+    /**
+     * Sometimes a request to step down comes in (like via a heartbeat), but we don't have the
+     * global exclusive lock so we can't actually stepdown at that moment. When that happens
+     * we record that a stepdown request is pending (by calling prepareForUnconditionalStepDown())
+     * and schedule work to stepdown in the global X lock.  This method is called after holding the
+     * global lock to perform the actual stepdown.
+     * TODO(spencer): Unify with the finishAttemptedStepDown() method.
+     */
+    void finishUnconditionalStepDown();
+
+    /**
+     * Considers whether or not this node should stand for election, and returns true
+     * if the node has transitioned to candidate role as a result of the call.
+     */
+    Status checkShouldStandForElection(Date_t now) const;
+
+    /**
+     * Set the outgoing heartbeat message from self
+     */
+    void setMyHeartbeatMessage(const Date_t now, const std::string& s);
+
+    /**
+     * Prepares a ReplSetMetadata object describing the current term, primary, and lastOp
+     * information.
+     */
+    rpc::ReplSetMetadata prepareReplSetMetadata(const OpTime& lastVisibleOpTime) const;
+
+    /**
+     * Prepares an OplogQueryMetadata object describing the current sync source, rbid, primary,
+     * lastOpApplied, and lastOpCommitted.
+     */
+    rpc::OplogQueryMetadata prepareOplogQueryMetadata(int rbid) const;
+
+    /**
+     * Writes into 'output' all the information needed to generate a summary of the current
+     * replication state for use by the web interface.
+     */
+    void summarizeAsHtml(ReplSetHtmlSummary* output);
+
+    /**
+     * Prepares a ReplSetRequestVotesResponse.
+     */
+    void processReplSetRequestVotes(const ReplSetRequestVotesArgs& args,
+                                    ReplSetRequestVotesResponse* response);
+
+    /**
+     * Loads an initial LastVote document, which was read from local storage.
+     *
+     * Called only during replication startup. All other updates are done internally.
+     */
+    void loadLastVote(const LastVote& lastVote);
+
+    /**
+     * Updates the current primary index.
+     */
+    void setPrimaryIndex(long long primaryIndex);
+
+    /**
+     * Returns the current primary index.
+     */
+    int getCurrentPrimaryIndex() const;
+
+    enum StartElectionReason {
+        kElectionTimeout,
+        kPriorityTakeover,
+        kStepUpRequest,
+        kCatchupTakeover
+    };
+
+    /**
+     * Transitions to the candidate role if the node is electable.
+     */
+    Status becomeCandidateIfElectable(const Date_t now, StartElectionReason reason);
+
+    /**
+     * Updates the storage engine read committed support in the TopologyCoordinator options after
+     * creation.
+     */
+    void setStorageEngineSupportsReadCommitted(bool supported);
+
+    /**
+     * Reset the booleans to record the last heartbeat restart.
+     */
+    void restartHeartbeats();
+
+    /**
+     * Scans through all members that are 'up' and return the latest known optime, if we have
+     * received (successful or failed) heartbeats from all nodes since heartbeat restart.
+     *
+     * Returns boost::none if any node hasn't responded to a heartbeat since we last restarted
+     * heartbeats.
+     * Returns OpTime(Timestamp(0, 0), 0), the smallest OpTime in PV1, if other nodes are all down.
+     */
+    boost::optional<OpTime> latestKnownOpTimeSinceHeartbeatRestart() const;
 
     ////////////////////////////////////////////////////////////
     //
@@ -289,6 +783,9 @@ public:
     OID getElectionId() const;
 
 private:
+    typedef int UnelectableReasonMask;
+    class PingStats;
+
     enum UnelectableReason {
         None = 0,
         CannotSeeMajority = 1 << 0,
@@ -304,8 +801,6 @@ private:
         NotCloseEnoughToLatestForPriorityTakeover = 1 << 10,
         NotFreshEnoughForCatchupTakeover = 1 << 11,
     };
-    typedef int UnelectableReasonMask;
-
 
     // Set what type of PRIMARY this node currently is.
     void _setLeaderMode(LeaderMode mode);
@@ -525,5 +1020,81 @@ private:
     ReadCommittedSupport _storageEngineSupportsReadCommitted{ReadCommittedSupport::kUnknown};
 };
 
+/**
+ * Represents a latency measurement for each replica set member based on heartbeat requests.
+ * The measurement is an average weighted 80% to the old value, and 20% to the new value.
+ *
+ * Also stores information about heartbeat progress and retries.
+ */
+class TopologyCoordinator::PingStats {
+public:
+    /**
+     * Records that a new heartbeat request started at "now".
+     *
+     * This resets the failure count used in determining whether the next request to a target
+     * should be a retry or a regularly scheduled heartbeat message.
+     */
+    void start(Date_t now);
+
+    /**
+     * Records that a heartbeat request completed successfully, and that "millis" milliseconds
+     * were spent for a single network roundtrip plus remote processing time.
+     */
+    void hit(Milliseconds millis);
+
+    /**
+     * Records that a heartbeat request failed.
+     */
+    void miss();
+
+    /**
+     * Gets the number of hit() calls.
+     */
+    unsigned int getCount() const {
+        return count;
+    }
+
+    /**
+     * Gets the weighted average round trip time for heartbeat messages to the target.
+     * Returns 0 if there have been no pings recorded yet.
+     */
+    Milliseconds getMillis() const {
+        return value == UninitializedPing ? Milliseconds(0) : value;
+    }
+
+    /**
+     * Gets the date at which start() was last called, which is used to determine if
+     * a heartbeat should be retried or if the time limit has expired.
+     */
+    Date_t getLastHeartbeatStartDate() const {
+        return _lastHeartbeatStartDate;
+    }
+
+    /**
+     * Gets the number of failures since start() was last called.
+     *
+     * This value is incremented by calls to miss(), cleared by calls to start() and
+     * set to the maximum possible value by calls to hit().
+     */
+    int getNumFailuresSinceLastStart() const {
+        return _numFailuresSinceLastStart;
+    }
+
+private:
+    static constexpr Milliseconds UninitializedPing{-1};
+
+    unsigned int count = 0;
+    Milliseconds value = UninitializedPing;
+    Date_t _lastHeartbeatStartDate;
+    int _numFailuresSinceLastStart = std::numeric_limits<int>::max();
+};
+
+//
+// Convenience method for unittest code. Please use accessors otherwise.
+//
+
+std::ostream& operator<<(std::ostream& os, TopologyCoordinator::Role role);
+std::ostream& operator<<(std::ostream& os, TopologyCoordinator::PrepareFreezeResponseResult result);
+
 }  // namespace repl
 }  // namespace mongo
diff --git a/src/mongo/db/repl/topology_coordinator_test.cpp b/src/mongo/db/repl/topology_coordinator_test.cpp
index a22aec9f72f..034aaabda89 100644
--- a/src/mongo/db/repl/topology_coordinator_test.cpp
+++ b/src/mongo/db/repl/topology_coordinator_test.cpp
@@ -37,7 +37,6 @@
 #include "mongo/db/repl/repl_set_heartbeat_response.h"
 #include "mongo/db/repl/repl_set_request_votes_args.h"
 #include "mongo/db/repl/topology_coordinator.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
 #include "mongo/db/server_options.h"
 #include "mongo/executor/task_executor.h"
 #include "mongo/logger/logger.h"
@@ -73,9 +72,9 @@ bool stringContains(const std::string& haystack, const std::string& needle) {
 class TopoCoordTest : public mongo::unittest::Test {
 public:
     virtual void setUp() {
-        _options = TopologyCoordinatorImpl::Options{};
+        _options = TopologyCoordinator::Options{};
         _options.maxSyncSourceLagSecs = Seconds{100};
-        _topo.reset(new TopologyCoordinatorImpl(_options));
+        _topo.reset(new TopologyCoordinator(_options));
         _now = Date_t();
         _selfIndex = -1;
         _cbData.reset(new executor::TaskExecutor::CallbackArgs(
@@ -88,7 +87,7 @@ public:
     }
 
 protected:
-    TopologyCoordinatorImpl& getTopoCoord() {
+    TopologyCoordinator& getTopoCoord() {
         return *_topo;
     }
     executor::TaskExecutor::CallbackArgs cbData() {
@@ -98,9 +97,9 @@ protected:
         return _now;
     }
 
-    void setOptions(const TopologyCoordinatorImpl::Options& options) {
+    void setOptions(const TopologyCoordinator::Options& options) {
         _options = options;
-        _topo.reset(new TopologyCoordinatorImpl(_options));
+        _topo.reset(new TopologyCoordinator(_options));
     }
 
     int64_t countLogLinesContaining(const std::string& needle) {
@@ -239,12 +238,12 @@ private:
     }
 
 private:
-    unique_ptr<TopologyCoordinatorImpl> _topo;
+    unique_ptr<TopologyCoordinator> _topo;
     unique_ptr<executor::TaskExecutor::CallbackArgs> _cbData;
     ReplSetConfig _currentConfig;
     Date_t _now;
     int _selfIndex;
-    TopologyCoordinatorImpl::Options _options;
+    TopologyCoordinator::Options _options;
 };
 
 TEST_F(TopoCoordTest, NodeReturnsSecondaryWithMostRecentDataAsSyncSource) {
@@ -6263,7 +6262,7 @@ TEST_F(TopoCoordTest, DoNotGrantDryRunVoteWhenOpTimeIsStale) {
 TEST_F(TopoCoordTest, CSRSConfigServerRejectsPV0Config) {
     ON_BLOCK_EXIT([]() { serverGlobalParams.clusterRole = ClusterRole::None; });
     serverGlobalParams.clusterRole = ClusterRole::ConfigServer;
-    TopologyCoordinatorImpl::Options options;
+    TopologyCoordinator::Options options;
     options.clusterRole = ClusterRole::ConfigServer;
     setOptions(options);
     getTopoCoord().setStorageEngineSupportsReadCommitted(false);
diff --git a/src/mongo/db/repl/topology_coordinator_v1_test.cpp b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
index 048bb17b573..c43c0b53066 100644
--- a/src/mongo/db/repl/topology_coordinator_v1_test.cpp
+++ b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
@@ -38,7 +38,6 @@
 #include "mongo/db/repl/repl_set_heartbeat_response.h"
 #include "mongo/db/repl/repl_set_request_votes_args.h"
 #include "mongo/db/repl/topology_coordinator.h"
-#include "mongo/db/repl/topology_coordinator_impl.h"
 #include "mongo/db/server_options.h"
 #include "mongo/executor/task_executor.h"
 #include "mongo/logger/logger.h"
@@ -74,9 +73,9 @@ bool stringContains(const std::string& haystack, const std::string& needle) {
 class TopoCoordTest : public mongo::unittest::Test {
 public:
     virtual void setUp() {
-        _options = TopologyCoordinatorImpl::Options{};
+        _options = TopologyCoordinator::Options{};
         _options.maxSyncSourceLagSecs = Seconds{100};
-        _topo.reset(new TopologyCoordinatorImpl(_options));
+        _topo.reset(new TopologyCoordinator(_options));
         _now = Date_t();
         _selfIndex = -1;
         _cbData.reset(new executor::TaskExecutor::CallbackArgs(
@@ -89,7 +88,7 @@ public:
     }
 
 protected:
-    TopologyCoordinatorImpl& getTopoCoord() {
+    TopologyCoordinator& getTopoCoord() {
         return *_topo;
     }
     executor::TaskExecutor::CallbackArgs cbData() {
@@ -99,9 +98,9 @@ protected:
         return _now;
     }
 
-    void setOptions(const TopologyCoordinatorImpl::Options& options) {
+    void setOptions(const TopologyCoordinator::Options& options) {
         _options = options;
-        _topo.reset(new TopologyCoordinatorImpl(_options));
+        _topo.reset(new TopologyCoordinator(_options));
     }
 
     int64_t countLogLinesContaining(const std::string& needle) {
@@ -252,12 +251,12 @@ private:
     }
 
 private:
-    unique_ptr<TopologyCoordinatorImpl> _topo;
+    unique_ptr<TopologyCoordinator> _topo;
     unique_ptr<executor::TaskExecutor::CallbackArgs> _cbData;
     ReplSetConfig _currentConfig;
     Date_t _now;
     int _selfIndex;
-    TopologyCoordinatorImpl::Options _options;
+    TopologyCoordinator::Options _options;
 };
 
 TEST_F(TopoCoordTest, NodeReturnsSecondaryWithMostRecentDataAsSyncSource) {
@@ -3179,7 +3178,7 @@ TEST_F(TopoCoordTest, DoNotGrantDryRunVoteWhenOpTimeIsStale) {
 TEST_F(TopoCoordTest, NodeTransitionsToRemovedIfCSRSButHaveNoReadCommittedSupport) {
     ON_BLOCK_EXIT([]() { serverGlobalParams.clusterRole = ClusterRole::None; });
     serverGlobalParams.clusterRole = ClusterRole::ConfigServer;
-    TopologyCoordinatorImpl::Options options;
+    TopologyCoordinator::Options options;
     options.clusterRole = ClusterRole::ConfigServer;
     setOptions(options);
     getTopoCoord().setStorageEngineSupportsReadCommitted(false);
@@ -3206,7 +3205,7 @@ TEST_F(TopoCoordTest, NodeTransitionsToRemovedIfCSRSButHaveNoReadCommittedSuppor
 TEST_F(TopoCoordTest, NodeBecomesSecondaryAsNormalWhenReadCommittedSupportedAndCSRS) {
     ON_BLOCK_EXIT([]() { serverGlobalParams.clusterRole = ClusterRole::None; });
     serverGlobalParams.clusterRole = ClusterRole::ConfigServer;
-    TopologyCoordinatorImpl::Options options;
+    TopologyCoordinator::Options options;
     options.clusterRole = ClusterRole::ConfigServer;
     setOptions(options);
     getTopoCoord().setStorageEngineSupportsReadCommitted(true);