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/**
* Copyright (C) 2020-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/storage/wiredtiger/wiredtiger_record_store.h"
#include "mongo/platform/mutex.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/stdx/thread.h"
#include "mongo/util/concurrency/with_lock.h"
namespace mongo {
class WiredTigerRecordStore;
class WiredTigerSessionCache;
/**
* Manages oplog visibility.
*
* On demand, queries WiredTiger's all_durable timestamp value and updates the oplog read timestamp.
* This is done asynchronously on a thread that startVisibilityThread() will set up.
*
* The WT all_durable timestamp is the in-memory timestamp behind which there are no oplog holes
* in-memory. Note, all_durable is the timestamp that has no holes in-memory, which may NOT be
* the case on disk, despite 'durable' in the name.
*
* The oplog read timestamp is used to read from the oplog with forward cursors, in order to ensure
* readers never see 'holes' in the oplog and thereby miss data that was not yet committed when
* scanning passed. Out-of-order primary writes allow writes with later timestamps to be committed
* before writes assigned earlier timestamps, creating oplog 'holes'.
*/
class WiredTigerOplogManager {
WiredTigerOplogManager(const WiredTigerOplogManager&) = delete;
WiredTigerOplogManager& operator=(const WiredTigerOplogManager&) = delete;
public:
WiredTigerOplogManager() {}
~WiredTigerOplogManager() {}
/*
* Initializes the oplog read timestamp and start the update visibility thread.
*/
void startVisibilityThread(OperationContext* opCtx, WiredTigerRecordStore* oplogRecordStore);
void haltVisibilityThread();
bool isRunning() {
stdx::lock_guard<Latch> lk(_oplogVisibilityStateMutex);
return _isRunning && !_shuttingDown;
}
/**
* Signals the oplog visibility thread to update the oplog read timestamp.
*/
void triggerOplogVisibilityUpdate();
/**
* Waits for all committed writes at this time to become visible (that is, until no holes exist
* in the oplog up to the time we start waiting.)
*/
void waitForAllEarlierOplogWritesToBeVisible(const WiredTigerRecordStore* oplogRecordStore,
OperationContext* opCtx);
/**
* The oplogReadTimestamp is the read timestamp used for forward cursor oplog reads to prevent
* such readers from missing any entries in the oplog that may not yet have committed ('holes')
* when the scan passes along the data. The 'oplogReadTimestamp' is a guaranteed 'no holes'
* point in the oplog.
*
* Holes in the oplog occur due to out-of-order primary writes, where a write with a later
* assigned timestamp can commit before a write assigned an earlier timestamp.
*/
std::uint64_t getOplogReadTimestamp() const;
void setOplogReadTimestamp(Timestamp ts);
private:
/**
* Runs the oplog visibility updates when signaled by triggerOplogVisibilityUpdate() until
* _shuttingDown is set to true.
*/
void _updateOplogVisibilityLoop(WiredTigerSessionCache* sessionCache,
WiredTigerRecordStore* oplogRecordStore);
void _setOplogReadTimestamp(WithLock, uint64_t newTimestamp);
AtomicWord<unsigned long long> _oplogReadTimestamp{0};
stdx::thread _oplogVisibilityThread;
// Signaled to trigger the oplog visibility thread to run.
mutable stdx::condition_variable _oplogVisibilityThreadCV;
// Signaled when oplog visibility has been updated.
mutable stdx::condition_variable _oplogEntriesBecameVisibleCV;
// Protects the state below.
mutable Mutex _oplogVisibilityStateMutex =
MONGO_MAKE_LATCH("WiredTigerOplogManager::_oplogVisibilityStateMutex");
bool _isRunning = false;
bool _shuttingDown = false;
// Triggers an oplog visibility update -- can be delayed if no callers are waiting for an
// update, per the _opsWaitingForOplogVisibility counter.
bool _triggerOplogVisibilityUpdate = false;
// Incremented when a caller is waiting for more of the oplog to become visible, to avoid update
// delays for batching.
int64_t _opsWaitingForOplogVisibilityUpdate = 0;
};
} // namespace mongo
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