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diff --git a/src/mongo/client/server_is_master_monitor.h b/src/mongo/client/server_is_master_monitor.h
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+
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+#include "mongo/client/mongo_uri.h"
+#include "mongo/client/sdam/sdam.h"
+#include "mongo/executor/task_executor.h"
+#include "mongo/stdx/unordered_map.h"
+#include "mongo/util/net/hostandport.h"
+
+namespace mongo {
+using namespace sdam;
+
+class SingleServerIsMasterMonitor
+    : public std::enable_shared_from_this<SingleServerIsMasterMonitor> {
+public:
+    explicit SingleServerIsMasterMonitor(const MongoURI& setUri,
+                                         const ServerAddress& host,
+                                         Milliseconds heartbeatFrequencyMS,
+                                         TopologyEventsPublisherPtr eventListener,
+                                         std::shared_ptr<executor::TaskExecutor> executor);
+
+    void init();
+    void shutdown();
+
+    /**
+     * Request an immediate check. The server will be checked immediately if we haven't completed
+     * an isMaster less than SdamConfiguration::kMinHeartbeatFrequencyMS ago. Otherwise,
+     * we schedule a check that runs after SdamConfiguration::kMinHeartbeatFrequencyMS since
+     * the last isMaster.
+     */
+    void requestImmediateCheck();
+    void disableExpeditedChecking();
+
+private:
+    void _scheduleNextIsMaster(WithLock, Milliseconds delay);
+    void _doRemoteCommand();
+
+    void _onIsMasterSuccess(IsMasterRTT latency, const BSONObj bson);
+    void _onIsMasterFailure(IsMasterRTT latency, const Status& status, const BSONObj bson);
+
+    Milliseconds _overrideRefreshPeriod(Milliseconds original);
+    Milliseconds _currentRefreshPeriod(WithLock);
+    void _cancelOutstandingRequest(WithLock);
+
+    static inline const logger::LogSeverity kLogLevel = logger::LogSeverity::Debug(1);
+
+    Mutex _mutex;
+    ServerAddress _host;
+    TopologyEventsPublisherPtr _eventListener;
+    std::shared_ptr<executor::TaskExecutor> _executor;
+    Milliseconds _heartbeatFrequencyMS;
+    Milliseconds _timeoutMS = SdamConfiguration::kDefaultConnectTimeoutMS;
+
+    boost::optional<Date_t> _lastIsMasterAt;
+    bool _isMasterOutstanding = false;
+    bool _isExpedited = false;
+    executor::TaskExecutor::CallbackHandle _nextIsMasterHandle;
+    executor::TaskExecutor::CallbackHandle _remoteCommandHandle;
+
+    bool _isShutdown;
+    MongoURI _setUri;
+};
+using SingleServerIsMasterMonitorPtr = std::shared_ptr<SingleServerIsMasterMonitor>;
+
+
+class ServerIsMasterMonitor : public TopologyListener {
+public:
+    ServerIsMasterMonitor(const MongoURI& setUri,
+                          const SdamConfiguration& sdamConfiguration,
+                          TopologyEventsPublisherPtr eventsPublisher,
+                          TopologyDescriptionPtr initialTopologyDescription,
+                          std::shared_ptr<executor::TaskExecutor> executor = nullptr);
+
+    virtual ~ServerIsMasterMonitor() {}
+
+    void shutdown();
+
+    /**
+     * Request an immediate check of each member in the replica set.
+     */
+    void requestImmediateCheck();
+
+    /**
+     * Add/Remove Single Monitors based on the current topology membership.
+     */
+    void onTopologyDescriptionChangedEvent(UUID topologyId,
+                                           TopologyDescriptionPtr previousDescription,
+                                           TopologyDescriptionPtr newDescription) override;
+
+private:
+    /**
+     * If the provided executor exists, use that one (for testing). Otherwise create a new one.
+     */
+    std::shared_ptr<executor::TaskExecutor> _setupExecutor(
+        const std::shared_ptr<executor::TaskExecutor>& executor);
+    void _disableExpeditedChecking(WithLock);
+
+    static inline const logger::LogSeverity kLogLevel = logger::LogSeverity::Debug(0);
+
+    Mutex _mutex;
+    SdamConfiguration _sdamConfiguration;
+    TopologyEventsPublisherPtr _eventPublisher;
+    std::shared_ptr<executor::TaskExecutor> _executor;
+    std::unordered_map<ServerAddress, SingleServerIsMasterMonitorPtr> _singleMonitors;
+    bool _isShutdown;
+    MongoURI _setUri;
+};
+using ServerIsMasterMonitorPtr = std::shared_ptr<ServerIsMasterMonitor>;
+}  // namespace mongo