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+/**
+ *    Copyright (C) 2019-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.
+ */
+
+#pragma once
+
+#include "mongo/db/s/server_transaction_coordinators_metrics.h"
+#include "mongo/db/s/single_transaction_coordinator_stats.h"
+#include "mongo/db/s/transaction_coordinator.h"
+
+namespace mongo {
+
+/**
+ * Updates transaction coordinator metrics (per- two-phase commit metrics and server-wide two-phase
+ * commit metrics) upon the appropriate event.
+ */
+class TransactionCoordinatorMetricsObserver {
+
+public:
+    /**
+     * Updates relevant metrics when a transaction coordinator is created.
+     */
+    void onCreate(ServerTransactionCoordinatorsMetrics* serverTransactionCoordinatorMetrics,
+                  TickSource* tickSource,
+                  Date_t curWallClockTime);
+
+    /**
+     * Updates relevant metrics when a transaction coordinator is about to write the participant
+     * list.
+     */
+    void onStartWritingParticipantList(
+        ServerTransactionCoordinatorsMetrics* serverTransactionCoordinatorMetrics,
+        TickSource* tickSource,
+        Date_t curWallClockTime);
+
+    /**
+     * Updates relevant metrics when a transaction coordinator is about to send 'prepare' and start
+     * waiting for votes (i.e., 'prepare' responses).
+     */
+    void onStartWaitingForVotes(
+        ServerTransactionCoordinatorsMetrics* serverTransactionCoordinatorMetrics,
+        TickSource* tickSource,
+        Date_t curWallClockTime);
+
+    /**
+     * Updates relevant metrics when a transaction coordinator is about to write the decision.
+     */
+    void onStartWritingDecision(
+        ServerTransactionCoordinatorsMetrics* serverTransactionCoordinatorMetrics,
+        TickSource* tickSource,
+        Date_t curWallClockTime);
+
+    /**
+     * Updates relevant metrics when a transaction coordinator is about to send the decision to
+     * participants and start waiting for acknowledgements.
+     */
+    void onStartWaitingForDecisionAcks(
+        ServerTransactionCoordinatorsMetrics* serverTransactionCoordinatorMetrics,
+        TickSource* tickSource,
+        Date_t curWallClockTime);
+
+    /**
+     * Updates relevant metrics when a transaction coordinator is about to delete its durable state.
+     */
+    void onStartDeletingCoordinatorDoc(
+        ServerTransactionCoordinatorsMetrics* serverTransactionCoordinatorMetrics,
+        TickSource* tickSource,
+        Date_t curWallClockTime);
+
+    /**
+     * Updates relevant metrics when a transaction coordinator is destroyed.
+     *
+     * The 'lastStep' parameter is needed because, unlike for the other state transitions, the
+     * coordinator can transition to the end state from any other state, for example on stepdown.
+     */
+    void onEnd(ServerTransactionCoordinatorsMetrics* serverTransactionCoordinatorMetrics,
+               TickSource* tickSource,
+               Date_t curWallClockTime,
+               TransactionCoordinator::Step lastStep);
+
+    /**
+     * Returns a read-only reference to the SingleTransactionCoordinatorStats object stored in this
+     * TransactionCoordinatorMetricsObserver instance.
+     */
+    const SingleTransactionCoordinatorStats& getSingleTransactionCoordinatorStats() const {
+        return _singleTransactionCoordinatorStats;
+    }
+
+private:
+    /**
+     * Decrements the current active in 'step'.
+     */
+    void _decrementLastStep(ServerTransactionCoordinatorsMetrics*, TransactionCoordinator::Step);
+
+    // Tracks metrics for a single commit coordination.
+    SingleTransactionCoordinatorStats _singleTransactionCoordinatorStats;
+};
+
+}  // namespace mongo