khenaidoo | d948f77 | 2021-08-11 17:49:24 -0400 | [diff] [blame] | 1 | // Copyright 2015 The etcd Authors |
| 2 | // |
| 3 | // Licensed under the Apache License, Version 2.0 (the "License"); |
| 4 | // you may not use this file except in compliance with the License. |
| 5 | // You may obtain a copy of the License at |
| 6 | // |
| 7 | // http://www.apache.org/licenses/LICENSE-2.0 |
| 8 | // |
| 9 | // Unless required by applicable law or agreed to in writing, software |
| 10 | // distributed under the License is distributed on an "AS IS" BASIS, |
| 11 | // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 12 | // See the License for the specific language governing permissions and |
| 13 | // limitations under the License. |
| 14 | |
| 15 | package mvcc |
| 16 | |
| 17 | import ( |
| 18 | "bytes" |
| 19 | "errors" |
| 20 | "fmt" |
| 21 | |
| 22 | "github.com/google/btree" |
| 23 | ) |
| 24 | |
| 25 | var ( |
| 26 | ErrRevisionNotFound = errors.New("mvcc: revision not found") |
| 27 | ) |
| 28 | |
| 29 | // keyIndex stores the revisions of a key in the backend. |
| 30 | // Each keyIndex has at least one key generation. |
| 31 | // Each generation might have several key versions. |
| 32 | // Tombstone on a key appends an tombstone version at the end |
| 33 | // of the current generation and creates a new empty generation. |
| 34 | // Each version of a key has an index pointing to the backend. |
| 35 | // |
| 36 | // For example: put(1.0);put(2.0);tombstone(3.0);put(4.0);tombstone(5.0) on key "foo" |
| 37 | // generate a keyIndex: |
| 38 | // key: "foo" |
| 39 | // rev: 5 |
| 40 | // generations: |
| 41 | // {empty} |
| 42 | // {4.0, 5.0(t)} |
| 43 | // {1.0, 2.0, 3.0(t)} |
| 44 | // |
| 45 | // Compact a keyIndex removes the versions with smaller or equal to |
| 46 | // rev except the largest one. If the generation becomes empty |
| 47 | // during compaction, it will be removed. if all the generations get |
| 48 | // removed, the keyIndex should be removed. |
| 49 | // |
| 50 | // For example: |
| 51 | // compact(2) on the previous example |
| 52 | // generations: |
| 53 | // {empty} |
| 54 | // {4.0, 5.0(t)} |
| 55 | // {2.0, 3.0(t)} |
| 56 | // |
| 57 | // compact(4) |
| 58 | // generations: |
| 59 | // {empty} |
| 60 | // {4.0, 5.0(t)} |
| 61 | // |
| 62 | // compact(5): |
| 63 | // generations: |
| 64 | // {empty} -> key SHOULD be removed. |
| 65 | // |
| 66 | // compact(6): |
| 67 | // generations: |
| 68 | // {empty} -> key SHOULD be removed. |
| 69 | type keyIndex struct { |
| 70 | key []byte |
| 71 | modified revision // the main rev of the last modification |
| 72 | generations []generation |
| 73 | } |
| 74 | |
| 75 | // put puts a revision to the keyIndex. |
| 76 | func (ki *keyIndex) put(main int64, sub int64) { |
| 77 | rev := revision{main: main, sub: sub} |
| 78 | |
| 79 | if !rev.GreaterThan(ki.modified) { |
| 80 | plog.Panicf("store.keyindex: put with unexpected smaller revision [%v / %v]", rev, ki.modified) |
| 81 | } |
| 82 | if len(ki.generations) == 0 { |
| 83 | ki.generations = append(ki.generations, generation{}) |
| 84 | } |
| 85 | g := &ki.generations[len(ki.generations)-1] |
| 86 | if len(g.revs) == 0 { // create a new key |
| 87 | keysGauge.Inc() |
| 88 | g.created = rev |
| 89 | } |
| 90 | g.revs = append(g.revs, rev) |
| 91 | g.ver++ |
| 92 | ki.modified = rev |
| 93 | } |
| 94 | |
| 95 | func (ki *keyIndex) restore(created, modified revision, ver int64) { |
| 96 | if len(ki.generations) != 0 { |
| 97 | plog.Panicf("store.keyindex: cannot restore non-empty keyIndex") |
| 98 | } |
| 99 | |
| 100 | ki.modified = modified |
| 101 | g := generation{created: created, ver: ver, revs: []revision{modified}} |
| 102 | ki.generations = append(ki.generations, g) |
| 103 | keysGauge.Inc() |
| 104 | } |
| 105 | |
| 106 | // tombstone puts a revision, pointing to a tombstone, to the keyIndex. |
| 107 | // It also creates a new empty generation in the keyIndex. |
| 108 | // It returns ErrRevisionNotFound when tombstone on an empty generation. |
| 109 | func (ki *keyIndex) tombstone(main int64, sub int64) error { |
| 110 | if ki.isEmpty() { |
| 111 | plog.Panicf("store.keyindex: unexpected tombstone on empty keyIndex %s", string(ki.key)) |
| 112 | } |
| 113 | if ki.generations[len(ki.generations)-1].isEmpty() { |
| 114 | return ErrRevisionNotFound |
| 115 | } |
| 116 | ki.put(main, sub) |
| 117 | ki.generations = append(ki.generations, generation{}) |
| 118 | keysGauge.Dec() |
| 119 | return nil |
| 120 | } |
| 121 | |
| 122 | // get gets the modified, created revision and version of the key that satisfies the given atRev. |
| 123 | // Rev must be higher than or equal to the given atRev. |
| 124 | func (ki *keyIndex) get(atRev int64) (modified, created revision, ver int64, err error) { |
| 125 | if ki.isEmpty() { |
| 126 | plog.Panicf("store.keyindex: unexpected get on empty keyIndex %s", string(ki.key)) |
| 127 | } |
| 128 | g := ki.findGeneration(atRev) |
| 129 | if g.isEmpty() { |
| 130 | return revision{}, revision{}, 0, ErrRevisionNotFound |
| 131 | } |
| 132 | |
| 133 | n := g.walk(func(rev revision) bool { return rev.main > atRev }) |
| 134 | if n != -1 { |
| 135 | return g.revs[n], g.created, g.ver - int64(len(g.revs)-n-1), nil |
| 136 | } |
| 137 | |
| 138 | return revision{}, revision{}, 0, ErrRevisionNotFound |
| 139 | } |
| 140 | |
| 141 | // since returns revisions since the given rev. Only the revision with the |
| 142 | // largest sub revision will be returned if multiple revisions have the same |
| 143 | // main revision. |
| 144 | func (ki *keyIndex) since(rev int64) []revision { |
| 145 | if ki.isEmpty() { |
| 146 | plog.Panicf("store.keyindex: unexpected get on empty keyIndex %s", string(ki.key)) |
| 147 | } |
| 148 | since := revision{rev, 0} |
| 149 | var gi int |
| 150 | // find the generations to start checking |
| 151 | for gi = len(ki.generations) - 1; gi > 0; gi-- { |
| 152 | g := ki.generations[gi] |
| 153 | if g.isEmpty() { |
| 154 | continue |
| 155 | } |
| 156 | if since.GreaterThan(g.created) { |
| 157 | break |
| 158 | } |
| 159 | } |
| 160 | |
| 161 | var revs []revision |
| 162 | var last int64 |
| 163 | for ; gi < len(ki.generations); gi++ { |
| 164 | for _, r := range ki.generations[gi].revs { |
| 165 | if since.GreaterThan(r) { |
| 166 | continue |
| 167 | } |
| 168 | if r.main == last { |
| 169 | // replace the revision with a new one that has higher sub value, |
| 170 | // because the original one should not be seen by external |
| 171 | revs[len(revs)-1] = r |
| 172 | continue |
| 173 | } |
| 174 | revs = append(revs, r) |
| 175 | last = r.main |
| 176 | } |
| 177 | } |
| 178 | return revs |
| 179 | } |
| 180 | |
| 181 | // compact compacts a keyIndex by removing the versions with smaller or equal |
| 182 | // revision than the given atRev except the largest one (If the largest one is |
| 183 | // a tombstone, it will not be kept). |
| 184 | // If a generation becomes empty during compaction, it will be removed. |
| 185 | func (ki *keyIndex) compact(atRev int64, available map[revision]struct{}) { |
| 186 | if ki.isEmpty() { |
| 187 | plog.Panicf("store.keyindex: unexpected compact on empty keyIndex %s", string(ki.key)) |
| 188 | } |
| 189 | |
| 190 | genIdx, revIndex := ki.doCompact(atRev, available) |
| 191 | |
| 192 | g := &ki.generations[genIdx] |
| 193 | if !g.isEmpty() { |
| 194 | // remove the previous contents. |
| 195 | if revIndex != -1 { |
| 196 | g.revs = g.revs[revIndex:] |
| 197 | } |
| 198 | // remove any tombstone |
| 199 | if len(g.revs) == 1 && genIdx != len(ki.generations)-1 { |
| 200 | delete(available, g.revs[0]) |
| 201 | genIdx++ |
| 202 | } |
| 203 | } |
| 204 | |
| 205 | // remove the previous generations. |
| 206 | ki.generations = ki.generations[genIdx:] |
| 207 | } |
| 208 | |
| 209 | // keep finds the revision to be kept if compact is called at given atRev. |
| 210 | func (ki *keyIndex) keep(atRev int64, available map[revision]struct{}) { |
| 211 | if ki.isEmpty() { |
| 212 | return |
| 213 | } |
| 214 | |
| 215 | genIdx, revIndex := ki.doCompact(atRev, available) |
| 216 | g := &ki.generations[genIdx] |
| 217 | if !g.isEmpty() { |
| 218 | // remove any tombstone |
| 219 | if revIndex == len(g.revs)-1 && genIdx != len(ki.generations)-1 { |
| 220 | delete(available, g.revs[revIndex]) |
| 221 | } |
| 222 | } |
| 223 | } |
| 224 | |
| 225 | func (ki *keyIndex) doCompact(atRev int64, available map[revision]struct{}) (genIdx int, revIndex int) { |
| 226 | // walk until reaching the first revision smaller or equal to "atRev", |
| 227 | // and add the revision to the available map |
| 228 | f := func(rev revision) bool { |
| 229 | if rev.main <= atRev { |
| 230 | available[rev] = struct{}{} |
| 231 | return false |
| 232 | } |
| 233 | return true |
| 234 | } |
| 235 | |
| 236 | genIdx, g := 0, &ki.generations[0] |
| 237 | // find first generation includes atRev or created after atRev |
| 238 | for genIdx < len(ki.generations)-1 { |
| 239 | if tomb := g.revs[len(g.revs)-1].main; tomb > atRev { |
| 240 | break |
| 241 | } |
| 242 | genIdx++ |
| 243 | g = &ki.generations[genIdx] |
| 244 | } |
| 245 | |
| 246 | revIndex = g.walk(f) |
| 247 | |
| 248 | return genIdx, revIndex |
| 249 | } |
| 250 | |
| 251 | func (ki *keyIndex) isEmpty() bool { |
| 252 | return len(ki.generations) == 1 && ki.generations[0].isEmpty() |
| 253 | } |
| 254 | |
| 255 | // findGeneration finds out the generation of the keyIndex that the |
| 256 | // given rev belongs to. If the given rev is at the gap of two generations, |
| 257 | // which means that the key does not exist at the given rev, it returns nil. |
| 258 | func (ki *keyIndex) findGeneration(rev int64) *generation { |
| 259 | lastg := len(ki.generations) - 1 |
| 260 | cg := lastg |
| 261 | |
| 262 | for cg >= 0 { |
| 263 | if len(ki.generations[cg].revs) == 0 { |
| 264 | cg-- |
| 265 | continue |
| 266 | } |
| 267 | g := ki.generations[cg] |
| 268 | if cg != lastg { |
| 269 | if tomb := g.revs[len(g.revs)-1].main; tomb <= rev { |
| 270 | return nil |
| 271 | } |
| 272 | } |
| 273 | if g.revs[0].main <= rev { |
| 274 | return &ki.generations[cg] |
| 275 | } |
| 276 | cg-- |
| 277 | } |
| 278 | return nil |
| 279 | } |
| 280 | |
| 281 | func (a *keyIndex) Less(b btree.Item) bool { |
| 282 | return bytes.Compare(a.key, b.(*keyIndex).key) == -1 |
| 283 | } |
| 284 | |
| 285 | func (a *keyIndex) equal(b *keyIndex) bool { |
| 286 | if !bytes.Equal(a.key, b.key) { |
| 287 | return false |
| 288 | } |
| 289 | if a.modified != b.modified { |
| 290 | return false |
| 291 | } |
| 292 | if len(a.generations) != len(b.generations) { |
| 293 | return false |
| 294 | } |
| 295 | for i := range a.generations { |
| 296 | ag, bg := a.generations[i], b.generations[i] |
| 297 | if !ag.equal(bg) { |
| 298 | return false |
| 299 | } |
| 300 | } |
| 301 | return true |
| 302 | } |
| 303 | |
| 304 | func (ki *keyIndex) String() string { |
| 305 | var s string |
| 306 | for _, g := range ki.generations { |
| 307 | s += g.String() |
| 308 | } |
| 309 | return s |
| 310 | } |
| 311 | |
| 312 | // generation contains multiple revisions of a key. |
| 313 | type generation struct { |
| 314 | ver int64 |
| 315 | created revision // when the generation is created (put in first revision). |
| 316 | revs []revision |
| 317 | } |
| 318 | |
| 319 | func (g *generation) isEmpty() bool { return g == nil || len(g.revs) == 0 } |
| 320 | |
| 321 | // walk walks through the revisions in the generation in descending order. |
| 322 | // It passes the revision to the given function. |
| 323 | // walk returns until: 1. it finishes walking all pairs 2. the function returns false. |
| 324 | // walk returns the position at where it stopped. If it stopped after |
| 325 | // finishing walking, -1 will be returned. |
| 326 | func (g *generation) walk(f func(rev revision) bool) int { |
| 327 | l := len(g.revs) |
| 328 | for i := range g.revs { |
| 329 | ok := f(g.revs[l-i-1]) |
| 330 | if !ok { |
| 331 | return l - i - 1 |
| 332 | } |
| 333 | } |
| 334 | return -1 |
| 335 | } |
| 336 | |
| 337 | func (g *generation) String() string { |
| 338 | return fmt.Sprintf("g: created[%d] ver[%d], revs %#v\n", g.created, g.ver, g.revs) |
| 339 | } |
| 340 | |
| 341 | func (a generation) equal(b generation) bool { |
| 342 | if a.ver != b.ver { |
| 343 | return false |
| 344 | } |
| 345 | if len(a.revs) != len(b.revs) { |
| 346 | return false |
| 347 | } |
| 348 | |
| 349 | for i := range a.revs { |
| 350 | ar, br := a.revs[i], b.revs[i] |
| 351 | if ar != br { |
| 352 | return false |
| 353 | } |
| 354 | } |
| 355 | return true |
| 356 | } |