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diff --git a/src/mongo/db/s/database_sharding_state.h b/src/mongo/db/s/database_sharding_state.h
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+/**
+ *    Copyright (C) 2018 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the GNU Affero General Public License in all respects
+ *    for all of the code used other than as permitted herein. If you modify
+ *    file(s) with this exception, you may extend this exception to your
+ *    version of the file(s), but you are not obligated to do so. If you do not
+ *    wish to do so, delete this exception statement from your version. If you
+ *    delete this exception statement from all source files in the program,
+ *    then also delete it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/base/disallow_copying.h"
+#include "mongo/db/catalog/database.h"
+#include "mongo/s/database_version_gen.h"
+
+namespace mongo {
+
+class OperationContext;
+
+/**
+ * Synchronizes access to this shard server's cached database version for Database.
+ */
+class DatabaseShardingState {
+    MONGO_DISALLOW_COPYING(DatabaseShardingState);
+
+public:
+    static const Database::Decoration<DatabaseShardingState> get;
+
+    DatabaseShardingState(Database* db);
+    ~DatabaseShardingState() = default;
+
+    /**
+     * Assigns a new Notification to _critSecSignal and invalidates all yielded readers and writers
+     * on collections in Database that have a client dbVersion on their OperationContext.
+     *
+     * Invariants that _critSecSignal was null and that the caller holds the DBLock in X mode.
+     */
+    void enterCriticalSection(OperationContext* opCtx);
+
+    /**
+     * Signals and clears _critSecSignal, and sets _dbVersion to 'newDbVersion'.
+     *
+     * Invariants that _critSecSignal was not null and that the caller holds the DBLock in X mode.
+     */
+    void exitCriticalSection(OperationContext* opCtx,
+                             boost::optional<DatabaseVersion> newDbVersion);
+
+    /**
+     * Returns a shared_ptr to _critSecSignal if it's non-null, otherwise nullptr.
+     */
+    std::shared_ptr<Notification<void>> getCriticalSectionSignal() const;
+
+    /**
+     * Sets this shard server's cached dbVersion to newVersion.
+     *
+     * Invariants that the caller holds the DBLock in X mode.
+     */
+    void setDbVersion(OperationContext* opCtx, boost::optional<DatabaseVersion> newVersion);
+
+    /**
+     * If _critSecSignal is non-null, always throws StaleDbVersion.
+     * Otherwise, if there is a client dbVersion on the OperationContext, compares it with this
+     * shard server's cached dbVersion and throws StaleDbVersion if they do not match.
+     */
+    void checkDbVersion(OperationContext* opCtx) const;
+
+private:
+    // The database to which this sharding state corresponds.
+    const Database* _db;
+
+    // Modifying the state below requires holding the DBLock in X mode; holding the DBLock in any
+    // mode is acceptable for reading it. (Note: accessing this class at all requires holding the
+    // DBLock in some mode, since it requires having a pointer to the Database).
+
+    // Is non-null if this shard server is in a movePrimary critical section for the database.
+    // Stored as shared_ptr rather than boost::optional so callers can wait on it outside a DBLock.
+    std::shared_ptr<Notification<void>> _critSecSignal;
+
+    // This shard server's cached dbVersion. If boost::none, indicates this shard server does not
+    // know the dbVersion.
+    boost::optional<DatabaseVersion> _dbVersion = boost::none;
+};
+
+}  // namespace mongo