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oracle.go
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oracle.go
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// Copyright 2023 Linka Cloud All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package db
import (
"context"
"sync"
"github.com/dgraph-io/badger/v3/y"
"github.com/dgraph-io/ristretto/z"
)
type oracle struct {
sync.Mutex // For nextTxnTs and commits.
// writeChLock lock is for ensuring that transactions go to the write
// channel in the same order as their commit timestamps.
writeChLock sync.Mutex
nextTxnTs uint64
// Used to block NewTransaction, so all previous commits are visible to a new read.
txnMark *y.WaterMark
// Used to determine which versions can be permanently
// discarded during compaction.
discardTs uint64 // Used by ManagedDB.
readMark *y.WaterMark // Used by DB.
// committedTxns contains all committed writes (contains fingerprints
// of keys written and their latest commit counter).
committedTxns []committedTxn
lastCleanupTs uint64
// closer is used to stop watermarks.
closer *z.Closer
}
type committedTxn struct {
ts uint64
// ConflictKeys Keeps track of the entries written at timestamp ts.
conflictKeys map[uint64]struct{}
}
func newOracle() *oracle {
orc := &oracle{
// We're not initializing nextTxnTs and readOnlyTs. It would be done after replay in Open.
//
// WaterMarks must be 64-bit aligned for atomic package, hence we must use pointers here.
// See https://golang.org/pkg/sync/atomic/#pkg-note-BUG.
readMark: &y.WaterMark{Name: "badger.PendingReads"},
txnMark: &y.WaterMark{Name: "badger.TxnTimestamp"},
closer: z.NewCloser(2),
}
orc.readMark.Init(orc.closer)
orc.txnMark.Init(orc.closer)
return orc
}
func (o *oracle) Stop() {
o.closer.SignalAndWait()
}
func (o *oracle) readTs() uint64 {
var readTs uint64
o.Lock()
readTs = o.nextTxnTs - 1
o.readMark.Begin(readTs)
o.Unlock()
// Wait for all txns which have no conflicts, have been assigned a commit
// timestamp and are going through the write to value log and LSM tree
// process. Not waiting here could mean that some txns which have been
// committed would not be read.
y.Check(o.txnMark.WaitForMark(context.Background(), readTs))
return readTs
}
func (o *oracle) nextTs() uint64 {
o.Lock()
defer o.Unlock()
return o.nextTxnTs
}
func (o *oracle) incrementNextTs() {
o.Lock()
defer o.Unlock()
o.nextTxnTs++
}
// Any deleted or invalid versions at or below ts would be discarded during
// compaction to reclaim disk space in LSM tree and thence value log.
func (o *oracle) setDiscardTs(ts uint64) {
o.Lock()
defer o.Unlock()
o.discardTs = ts
o.cleanupCommittedTransactions()
}
func (o *oracle) discardAtOrBelow() uint64 {
return o.readMark.DoneUntil()
}
// hasConflict must be called while having a lock.
func (o *oracle) hasConflict(txn *tx) bool {
if len(txn.reads) == 0 {
return false
}
for _, committedTxn := range o.committedTxns {
// If the committedTxn.ts is less than txn.readTs that implies that the
// committedTxn finished before the current transaction started.
// We don't need to check for conflict in that case.
// This change assumes linearizability. Lack of linearizability could
// cause the read ts of a new txn to be lower than the commit ts of
// a txn before it (@mrjn).
if committedTxn.ts <= txn.readTs {
continue
}
for _, ro := range txn.reads {
if _, has := committedTxn.conflictKeys[ro]; has {
return true
}
}
}
return false
}
func (o *oracle) newCommitTs(txn *tx) (uint64, bool) {
o.Lock()
defer o.Unlock()
if o.hasConflict(txn) {
return 0, true
}
o.doneRead(txn)
o.cleanupCommittedTransactions()
// This is the general case, when user doesn't specify the read and commit ts.
ts := o.nextTxnTs
o.nextTxnTs++
o.txnMark.Begin(ts)
y.AssertTrue(ts >= o.lastCleanupTs)
// We should ensure that txns are not added to o.committedTxns slice when
// conflict detection is disabled otherwise this slice would keep growing.
o.committedTxns = append(o.committedTxns, committedTxn{
ts: ts,
conflictKeys: txn.conflictKeys,
})
return ts, false
}
func (o *oracle) doneRead(txn *tx) {
if !txn.doneRead {
txn.doneRead = true
o.readMark.Done(txn.readTs)
}
}
func (o *oracle) cleanupCommittedTransactions() { // Must be called under o.Lock
// Same logic as discardAtOrBelow but unlocked
maxReadTs := o.readMark.DoneUntil()
y.AssertTrue(maxReadTs >= o.lastCleanupTs)
// do not run clean up if the maxReadTs (read timestamp of the
// oldest transaction that is still in flight) has not increased
if maxReadTs == o.lastCleanupTs {
return
}
o.lastCleanupTs = maxReadTs
tmp := o.committedTxns[:0]
for _, txn := range o.committedTxns {
if txn.ts <= maxReadTs {
continue
}
tmp = append(tmp, txn)
}
o.committedTxns = tmp
}
func (o *oracle) doneCommit(cts uint64) {
o.txnMark.Done(cts)
}