1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
|
/**
* 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/exec/collection_scan.h"
#include "mongo/db/exec/plan_stage.h"
#include "mongo/db/matcher/expression.h"
#include "mongo/db/record_id.h"
#include "mongo/util/timer.h"
namespace mongo {
class RecordCursor;
/**
* 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 RecordId
* invalidations. :(
*/
class OplogStart final : public PlanStage {
public:
// Does not take ownership.
OplogStart(OperationContext* txn,
const Collection* collection,
MatchExpression* filter,
WorkingSet* ws);
StageState doWork(WorkingSetID* out) final;
bool isEOF() final;
void doInvalidate(OperationContext* txn, const RecordId& dl, InvalidationType type) final;
void doSaveState() final;
void doRestoreState() final;
void doDetachFromOperationContext() final;
void doReattachToOperationContext() final;
// Returns empty PlanStageStats object
std::unique_ptr<PlanStageStats> getStats() final;
//
// Exec stats -- do not call for the oplog start stage.
//
const SpecificStats* getSpecificStats() const final {
return NULL;
}
StageType stageType() const final {
return STAGE_OPLOG_START;
}
// For testing only.
void setBackwardsScanTime(int newTime) {
_backwardsScanTime = newTime;
}
bool isExtentHopping() {
return _extentHopping;
}
bool isBackwardsScanning() {
return _backwardsScanning;
}
static const char* kStageType;
private:
StageState workBackwardsScan(WorkingSetID* out);
void switchToExtentHopping();
StageState workExtentHopping(WorkingSetID* out);
// This is only used for the extent hopping scan.
std::vector<std::unique_ptr<RecordCursor>> _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
|
