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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
namespace mongo {
class OperationContext;
/**
* Function helpers to manipulate parameters affecting the snapshot time window size that the
* storage engine maintains of available snapshots for point-in-time reads.
*/
namespace SnapshotWindowUtil {
/**
* Attempts to increase the setting that controls the window of time between stable_timestamp and
* oldest_timestamp, in order to provide a greater range of available snapshots for point-in-time
* operations. The window will not be increased, however, if the cache pressure is currently too
* high. This function will be called when server requests return SnapshotTooOld (or similar)
* errors. Note that this will not immediately affect the oldest_timestamp. Rather, it affects
* actions taken next time oldest_timestamp is updated, usually when the stable timestamp is
* advanced.
*
* Implements an additive increase algorithm.
*
* Calling many times all at once has the same effect as calling once. The last update time is
* tracked and attempts to increase the window are limited to once in
* minMillisBetweenSnapshotWindowInc. This is to protect against a sudden wave of function calls due
* to simultaneous SnapshotTooOld errors. Some time must be allowed for the increased target
* snapshot window size to have an effect. The target size can also never exceed
* maxTargetSnapshotHistoryWindowInSeconds.
*/
void increaseTargetSnapshotWindowSize(OperationContext* opCtx);
/**
* Attempts to decrease (if not already zero) the setting that affects the size of the window of
* time between stable_timestamp and oldest_timestamp in order to reduce storage engine cache
* pressure. The window target will not be decreased, however, if the cache is not currently under
* pressure. Pressure can occur when too much history is being maintained for point-in-time
* snapshots. Note that this will not necessarily immediately affect the actual window size; rather,
* it affects actions taken whenever oldest_timestamp is updated, usually when the stable timestamp
* is advanced.
*
* This will make one attempt to immediately adjust the window size if possible.
*
* Implements a multiplicative decrease algorithm.
*/
void decreaseTargetSnapshotWindowSize(OperationContext* opCtx);
} // namespace SnapshotWindowUtil
} // namespace mongo
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