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/**
* Copyright (C) 2013-2014 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/owned_pointer_vector.h"
#include "mongo/db/diskloc.h"
#include "mongo/db/exec/collection_scan.h"
#include "mongo/db/exec/plan_stage.h"
#include "mongo/db/matcher/expression.h"
#include "mongo/util/timer.h"
namespace mongo {
/**
* OplogStart walks a collection backwards to find the first object in the collection that
* matches the query. It's used by replication to efficiently find where the oplog should be
* replayed from.
*
* The oplog is always a capped collection. In capped collections, documents are oriented on
* disk according to insertion order. The oplog inserts documents with increasing timestamps.
* Queries on the oplog look for entries that are after a certain time. Therefore if we
* navigate backwards, the last document we encounter that satisfies our query (over the
* timestamp) is the first document we must scan from to answer the query.
*
* Why isn't this a normal reverse table scan, you may ask? We could be correct if we used a
* normal reverse collection scan. However, that's not fast enough. Since we know all
* documents are oriented on disk in insertion order, we know all documents in one extent were
* inserted before documents in a subsequent extent. As such we can skip through entire extents
* looking only at the first document.
*
* Why is this a stage? Because we want to yield, and we want to be notified of DiskLoc
* invalidations. :(
*/
class OplogStart : public PlanStage {
public:
// Does not take ownership.
OplogStart(OperationContext* txn,
const Collection* collection,
MatchExpression* filter,
WorkingSet* ws);
virtual ~OplogStart();
virtual StageState work(WorkingSetID* out);
virtual bool isEOF();
virtual void invalidate(const DiskLoc& dl, InvalidationType type);
virtual void prepareToYield();
virtual void recoverFromYield();
virtual std::vector<PlanStage*> getChildren() const;
//
// Exec stats -- do not call these for the oplog start stage.
//
virtual PlanStageStats* getStats() { return NULL; }
virtual const CommonStats* getCommonStats() { return NULL; }
virtual const SpecificStats* getSpecificStats() { return NULL; }
virtual StageType stageType() const { return STAGE_OPLOG_START; }
// For testing only.
void setBackwardsScanTime(int newTime) { _backwardsScanTime = newTime; }
bool isExtentHopping() { return _extentHopping; }
bool isBackwardsScanning() { return _backwardsScanning; }
private:
StageState workBackwardsScan(WorkingSetID* out);
void switchToExtentHopping();
StageState workExtentHopping(WorkingSetID* out);
// transactional context for read locks. Not owned by us
OperationContext* _txn;
// If we're backwards scanning we just punt to a collscan.
scoped_ptr<CollectionScan> _cs;
// This is only used for the extent hopping scan.
typedef OwnedPointerVector<RecordIterator> SubIterators;
SubIterators _subIterators;
// Have we done our heavy init yet?
bool _needInit;
// Our first state: going backwards via a collscan.
bool _backwardsScanning;
// Our second state: hopping backwards extent by extent.
bool _extentHopping;
// Our final state: done.
bool _done;
const Collection* _collection;
// We only go backwards via a collscan for a few seconds.
Timer _timer;
// WorkingSet is not owned by us.
WorkingSet* _workingSet;
std::string _ns;
MatchExpression* _filter;
static int _backwardsScanTime;
};
} // namespace mongo
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