1// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file. See the AUTHORS file for names of contributors.
4
5#ifndef STORAGE_LEVELDB_DB_DBFORMAT_H_
6#define STORAGE_LEVELDB_DB_DBFORMAT_H_
7
8#include <cstddef>
9#include <cstdint>
10#include <string>
11
12#include "leveldb/comparator.h"
13#include "leveldb/db.h"
14#include "leveldb/filter_policy.h"
15#include "leveldb/slice.h"
16#include "leveldb/table_builder.h"
17#include "util/coding.h"
18#include "util/logging.h"
19
20namespace leveldb {
21
22// Grouping of constants. We may want to make some of these
23// parameters set via options.
24namespace config {
25static const int kNumLevels = 7;
26
27// Level-0 compaction is started when we hit this many files.
28static const int kL0_CompactionTrigger = 4;
29
30// Soft limit on number of level-0 files. We slow down writes at this point.
31static const int kL0_SlowdownWritesTrigger = 8;
32
33// Maximum number of level-0 files. We stop writes at this point.
34static const int kL0_StopWritesTrigger = 12;
35
36// Maximum level to which a new compacted memtable is pushed if it
37// does not create overlap. We try to push to level 2 to avoid the
38// relatively expensive level 0=>1 compactions and to avoid some
39// expensive manifest file operations. We do not push all the way to
40// the largest level since that can generate a lot of wasted disk
41// space if the same key space is being repeatedly overwritten.
42static const int kMaxMemCompactLevel = 2;
43
44// Approximate gap in bytes between samples of data read during iteration.
45static const int kReadBytesPeriod = 1048576;
46
47} // namespace config
48
49class InternalKey;
50
51// Value types encoded as the last component of internal keys.
52// DO NOT CHANGE THESE ENUM VALUES: they are embedded in the on-disk
53// data structures.
54enum ValueType { kTypeDeletion = 0x0, kTypeValue = 0x1 };
55// kValueTypeForSeek defines the ValueType that should be passed when
56// constructing a ParsedInternalKey object for seeking to a particular
57// sequence number (since we sort sequence numbers in decreasing order
58// and the value type is embedded as the low 8 bits in the sequence
59// number in internal keys, we need to use the highest-numbered
60// ValueType, not the lowest).
61static const ValueType kValueTypeForSeek = kTypeValue;
62
63typedef uint64_t SequenceNumber;
64
65// We leave eight bits empty at the bottom so a type and sequence#
66// can be packed together into 64-bits.
67static const SequenceNumber kMaxSequenceNumber = ((0x1ull << 56) - 1);
68
69struct ParsedInternalKey {
70 Slice user_key;
71 SequenceNumber sequence;
72 ValueType type;
73
74 ParsedInternalKey() {} // Intentionally left uninitialized (for speed)
75 ParsedInternalKey(const Slice& u, const SequenceNumber& seq, ValueType t)
76 : user_key(u), sequence(seq), type(t) {}
77 std::string DebugString() const;
78};
79
80// Return the length of the encoding of "key".
81inline size_t InternalKeyEncodingLength(const ParsedInternalKey& key) {
82 return key.user_key.size() + 8;
83}
84
85// Append the serialization of "key" to *result.
86void AppendInternalKey(std::string* result, const ParsedInternalKey& key);
87
88// Attempt to parse an internal key from "internal_key". On success,
89// stores the parsed data in "*result", and returns true.
90//
91// On error, returns false, leaves "*result" in an undefined state.
92bool ParseInternalKey(const Slice& internal_key, ParsedInternalKey* result);
93
94// Returns the user key portion of an internal key.
95inline Slice ExtractUserKey(const Slice& internal_key) {
96 assert(internal_key.size() >= 8);
97 return Slice(internal_key.data(), internal_key.size() - 8);
98}
99
100// A comparator for internal keys that uses a specified comparator for
101// the user key portion and breaks ties by decreasing sequence number.
102class InternalKeyComparator : public Comparator {
103 private:
104 const Comparator* user_comparator_;
105
106 public:
107 explicit InternalKeyComparator(const Comparator* c) : user_comparator_(c) {}
108 const char* Name() const override;
109 int Compare(const Slice& a, const Slice& b) const override;
110 void FindShortestSeparator(std::string* start,
111 const Slice& limit) const override;
112 void FindShortSuccessor(std::string* key) const override;
113
114 const Comparator* user_comparator() const { return user_comparator_; }
115
116 int Compare(const InternalKey& a, const InternalKey& b) const;
117};
118
119// Filter policy wrapper that converts from internal keys to user keys
120class InternalFilterPolicy : public FilterPolicy {
121 private:
122 const FilterPolicy* const user_policy_;
123
124 public:
125 explicit InternalFilterPolicy(const FilterPolicy* p) : user_policy_(p) {}
126 const char* Name() const override;
127 void CreateFilter(const Slice* keys, int n, std::string* dst) const override;
128 bool KeyMayMatch(const Slice& key, const Slice& filter) const override;
129};
130
131// Modules in this directory should keep internal keys wrapped inside
132// the following class instead of plain strings so that we do not
133// incorrectly use string comparisons instead of an InternalKeyComparator.
134class InternalKey {
135 private:
136 std::string rep_;
137
138 public:
139 InternalKey() {} // Leave rep_ as empty to indicate it is invalid
140 InternalKey(const Slice& user_key, SequenceNumber s, ValueType t) {
141 AppendInternalKey(&rep_, ParsedInternalKey(user_key, s, t));
142 }
143
144 bool DecodeFrom(const Slice& s) {
145 rep_.assign(s.data(), s.size());
146 return !rep_.empty();
147 }
148
149 Slice Encode() const {
150 assert(!rep_.empty());
151 return rep_;
152 }
153
154 Slice user_key() const { return ExtractUserKey(rep_); }
155
156 void SetFrom(const ParsedInternalKey& p) {
157 rep_.clear();
158 AppendInternalKey(&rep_, p);
159 }
160
161 void Clear() { rep_.clear(); }
162
163 std::string DebugString() const;
164};
165
166inline int InternalKeyComparator::Compare(const InternalKey& a,
167 const InternalKey& b) const {
168 return Compare(a.Encode(), b.Encode());
169}
170
171inline bool ParseInternalKey(const Slice& internal_key,
172 ParsedInternalKey* result) {
173 const size_t n = internal_key.size();
174 if (n < 8) return false;
175 uint64_t num = DecodeFixed64(internal_key.data() + n - 8);
176 uint8_t c = num & 0xff;
177 result->sequence = num >> 8;
178 result->type = static_cast<ValueType>(c);
179 result->user_key = Slice(internal_key.data(), n - 8);
180 return (c <= static_cast<uint8_t>(kTypeValue));
181}
182
183// A helper class useful for DBImpl::Get()
184class LookupKey {
185 public:
186 // Initialize *this for looking up user_key at a snapshot with
187 // the specified sequence number.
188 LookupKey(const Slice& user_key, SequenceNumber sequence);
189
190 LookupKey(const LookupKey&) = delete;
191 LookupKey& operator=(const LookupKey&) = delete;
192
193 ~LookupKey();
194
195 // Return a key suitable for lookup in a MemTable.
196 Slice memtable_key() const { return Slice(start_, end_ - start_); }
197
198 // Return an internal key (suitable for passing to an internal iterator)
199 Slice internal_key() const { return Slice(kstart_, end_ - kstart_); }
200
201 // Return the user key
202 Slice user_key() const { return Slice(kstart_, end_ - kstart_ - 8); }
203
204 private:
205 // We construct a char array of the form:
206 // klength varint32 <-- start_
207 // userkey char[klength] <-- kstart_
208 // tag uint64
209 // <-- end_
210 // The array is a suitable MemTable key.
211 // The suffix starting with "userkey" can be used as an InternalKey.
212 const char* start_;
213 const char* kstart_;
214 const char* end_;
215 char space_[200]; // Avoid allocation for short keys
216};
217
218inline LookupKey::~LookupKey() {
219 if (start_ != space_) delete[] start_;
220}
221
222} // namespace leveldb
223
224#endif // STORAGE_LEVELDB_DB_DBFORMAT_H_
225