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+/**
+ *    Copyright (C) 2018-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/repl/optime.h"
+#include "mongo/platform/mutex.h"
+#include "mongo/s/client/shard.h"
+#include "mongo/util/hierarchical_acquisition.h"
+
+namespace mongo {
+
+/**
+ * Implements the support for "read your own write" when run on a node of a replica set. Can be used
+ * in the scenarios where causal consistency is not available yet.
+ */
+class RSLocalClient {
+    RSLocalClient(const RSLocalClient&) = delete;
+    RSLocalClient& operator=(const RSLocalClient&) = delete;
+
+public:
+    explicit RSLocalClient() = default;
+
+    ~RSLocalClient() = default;
+
+    /**
+     * Runs the specified command returns the BSON command response plus parsed out Status of this
+     * response and write concern error (if present).
+     */
+    StatusWith<Shard::CommandResponse> runCommandOnce(OperationContext* opCtx,
+                                                      StringData dbName,
+                                                      const BSONObj& cmdObj);
+
+    /**
+     * Warning: This method exhausts the cursor and pulls all data into memory.
+     * Do not use other than for very small (i.e., admin or metadata) collections.
+     */
+    StatusWith<Shard::QueryResponse> queryOnce(OperationContext* opCtx,
+                                               const ReadPreferenceSetting& readPref,
+                                               const repl::ReadConcernLevel& readConcernLevel,
+                                               const NamespaceString& nss,
+                                               const BSONObj& query,
+                                               const BSONObj& sort,
+                                               boost::optional<long long> limit);
+
+private:
+    /**
+     * Checks if an OpTime was set on the current Client (ie if the current operation performed a
+     * write) and if so updates _lastOpTime to the OpTime from the write that was just performed.
+     * The 'previousOpTimeOnClient' parameter is the optime that *was* the optime on this client
+     * before we ran this command through this RSLocalClient. By the time this method is called,
+     * if the optime set on the Client is different than 'previousOpTimeOnClient' then that means
+     * the command just run did a write and we should update _lastOpTime to capture the optime of
+     * the write we just did.  If the current optime on the client is the same as
+     * 'previousOpTimeOnClient' then the command we just ran didn't do a write, and we should leave
+     * _lastOpTime alone.
+     */
+    void _updateLastOpTimeFromClient(OperationContext* opCtx,
+                                     const repl::OpTime& previousOpTimeOnClient);
+
+    repl::OpTime _getLastOpTime();
+
+    // Guards _lastOpTime below.
+    Mutex _mutex = MONGO_MAKE_LATCH(HierarchicalAcquisitionLevel(0), "RSLocalClient::_mutex");
+
+    // Stores the optime that was generated by the last operation to perform a write that was run
+    // through _runCommand.  Used in _exhaustiveFindOnConfig for waiting for that optime to be
+    // committed so that readConcern majority reads will read the writes that were performed without
+    // a w:majority write concern.
+    repl::OpTime _lastOpTime{};
+};
+
+}  // namespace mongo