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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 <boost/optional.hpp>
#include "mongo/bson/timestamp.h"
#include "mongo/db/client.h"
namespace mongo {
class OperationContext;
namespace repl {
/**
* This structure stores information about a "speculative" majority read. Reads of this nature do
* not rely on storage engine support for reading from a historical, majority committed snapshot.
* Instead, they read from a local (uncommitted) snapshot of data, and, at the end of command
* execution, block to ensure that any results returned are majority committed.
*
* We allow commands to optionally choose what timestamp to wait on. For example, if a command
* generates a batch of data and it knows that the data doesn't reflect any operations newer than
* timestamp T, it can set the "speculative majority timestamp" to T. This is an optimization that
* can save unnecessary waiting.
*
* The basic theory of operation of this class is that once an operation has been marked as a
* speculative majority read, it cannot be "unmarked" as such. Trying to mark it as speculative
* again should have no effect. Once it has been marked as "speculative", a specific timestamp to
* wait on can also be optionally provided. This read timestamp can be set multiple times, but every
* new timestamp must be >= the previously set timestamp. This ensures that clients of this class
* utilize it sensibly. The goal is to avoid a later caller invalidating the timestamp set by an
* earlier caller, by setting the timestamp backwards. By enforcing the monotonicity of timestamps,
* we ensure that waiting on any read timestamp of this class will always guarantee that previously
* set timestamps are also majority committed. If no timestamp is ever set, then it is up to the
* user of this structure to determine what timestamp to wait on so that data read is guaranteed to
* be majority committed. This, for example, could be the timestamp of the read source chosen by the
* storage engine.
*/
class SpeculativeMajorityReadInfo {
public:
static SpeculativeMajorityReadInfo& get(OperationContext* opCtx);
/**
* Mark an operation as a speculative majority read.
*
* If this operation has already been marked as a speculative read, this method does nothing.
*/
void setIsSpeculativeRead();
/**
* Returns whether this operation is a speculative majority read.
*/
bool isSpeculativeRead() const;
/**
* Set a speculative majority timestamp for this operation to wait on.
*
* May only be called if this operation has already been marked as a speculative read.
*
* If no read timestamp has been set already, then this will set the speculative read timestamp
* to the timestamp given. If a speculative read timestamp has already been set at T, then any
* subsequent call to this method with a timestamp T' only updates the speculative read
* timestamp if T' > T. This guarantees that speculative read timestamps advance monotonically.
*/
void setSpeculativeReadTimestampForward(const Timestamp& ts);
/**
* Get the speculative read timestamp for this operation, if one exists.
*
* Only valid to call if this operation is a speculative read. Returns boost::none if there is
* no provided timestamp to wait on.
*/
boost::optional<Timestamp> getSpeculativeReadTimestamp();
private:
// Is this operation a speculative majority read.
bool _isSpeculativeRead = false;
// An optional timestamp to wait on to become majority committed, if we are doing a speculative
// majority read. This can only be non-empty if this operation is a speculative read.
boost::optional<Timestamp> _speculativeReadTimestamp;
};
} // namespace repl
} // namespace mongo
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