feat(Skiplist): Introduce a way to hand over skiplists to Badger (#1696)

In Dgraph, we already use Raft write-ahead log. Also, when we commit transactions, we update tens of thousands of keys in one go. To optimize this write path, this PR introduces a way to directly hand over Skiplist to Badger, short circuiting Badger's Value Log and WAL.

This feature allows Dgraph to generate Skiplists while processing mutations and just hand them over to Badger during commits. It also accepts a callback which can be run when Skiplist is written to disk. This is useful for determining when to create a snapshot in Dgraph.

Author: manishrjain

backup_test.go

@@ -446,7 +446,7 @@ func TestBackupLoadIncremental(t *testing.T) {
 				if err := txn.SetEntry(entry); err != nil {
 					return err
 				}
-				updates[i] = bitDiscardEarlierVersions
+				updates[i] = BitDiscardEarlierVersions
 			}
 			return nil
 		})

db.go

@@ -763,16 +763,9 @@ var requestPool = sync.Pool{
 }
 
 func (db *DB) writeToLSM(b *request) error {
-	// We should check the length of b.Prts and b.Entries only when badger is not
-	// running in InMemory mode. In InMemory mode, we don't write anything to the
-	// value log and that's why the length of b.Ptrs will always be zero.
-	if !db.opt.InMemory && len(b.Ptrs) != len(b.Entries) {
-		return errors.Errorf("Ptrs and Entries don't match: %+v", b)
-	}
-
 	for i, entry := range b.Entries {
 		var err error
-		if entry.skipVlogAndSetThreshold(db.valueThreshold()) {
+		if db.opt.managedTxns || entry.skipVlogAndSetThreshold(db.valueThreshold()) {
 			// Will include deletion / tombstone case.
 			err = db.mt.Put(entry.Key,
 				y.ValueStruct{
@@ -818,10 +811,13 @@ func (db *DB) writeRequests(reqs []*request) error {
 		}
 	}
 	db.opt.Debugf("writeRequests called. Writing to value log")
-	err := db.vlog.write(reqs)
-	if err != nil {
-		done(err)
-		return err
+	if !db.opt.managedTxns {
+		// Don't do value log writes in managed mode.
+		err := db.vlog.write(reqs)
+		if err != nil {
+			done(err)
+			return err
+		}
 	}
 
 	db.opt.Debugf("Sending updates to subscribers")
@@ -834,6 +830,7 @@ func (db *DB) writeRequests(reqs []*request) error {
 		}
 		count += len(b.Entries)
 		var i uint64
+		var err error
 		for err = db.ensureRoomForWrite(); err == errNoRoom; err = db.ensureRoomForWrite() {
 			i++
 			if i%100 == 0 {
@@ -1010,16 +1007,61 @@ func (db *DB) ensureRoomForWrite() error {
 	}
 }
 
+func (db *DB) HandoverSkiplist(skl *skl.Skiplist, callback func()) error {
+	if !db.opt.managedTxns {
+		panic("Handover Skiplist is only available in managed mode.")
+	}
+	db.lock.Lock()
+	defer db.lock.Unlock()
+
+	// If we have some data in db.mt, we should push that first, so the ordering of writes is
+	// maintained.
+	if !db.mt.sl.Empty() {
+		sz := db.mt.sl.MemSize()
+		db.opt.Infof("Handover found %d B data in current memtable. Pushing to flushChan.", sz)
+		var err error
+		select {
+		case db.flushChan <- flushTask{mt: db.mt}:
+			db.imm = append(db.imm, db.mt)
+			db.mt, err = db.newMemTable()
+			if err != nil {
+				return y.Wrapf(err, "cannot push current memtable")
+			}
+		default:
+			return errNoRoom
+		}
+	}
+
+	mt := &memTable{sl: skl}
+	select {
+	case db.flushChan <- flushTask{mt: mt, cb: callback}:
+		db.imm = append(db.imm, mt)
+		return nil
+	default:
+		return errNoRoom
+	}
+}
+
 func arenaSize(opt Options) int64 {
 	return opt.MemTableSize + opt.maxBatchSize + opt.maxBatchCount*int64(skl.MaxNodeSize)
 }
 
+func (db *DB) NewSkiplist() *skl.Skiplist {
+	return skl.NewSkiplist(arenaSize(db.opt))
+}
+
 // buildL0Table builds a new table from the memtable.
 func buildL0Table(ft flushTask, bopts table.Options) *table.Builder {
-	iter := ft.mt.sl.NewIterator()
+	var iter y.Iterator
+	if ft.itr != nil {
+		iter = ft.itr
+	} else {
+		iter = ft.mt.sl.NewUniIterator(false)
+	}
 	defer iter.Close()
+
 	b := table.NewTableBuilder(bopts)
-	for iter.SeekToFirst(); iter.Valid(); iter.Next() {
+	for iter.Rewind(); iter.Valid(); iter.Next() {
 		if len(ft.dropPrefixes) > 0 && hasAnyPrefixes(iter.Key(), ft.dropPrefixes) {
 			continue
 		}
@@ -1035,16 +1077,14 @@ func buildL0Table(ft flushTask, bopts table.Options) *table.Builder {
 
 type flushTask struct {
 	mt           *memTable
+	cb           func()
+	itr          y.Iterator
 	dropPrefixes [][]byte
 }
 
 // handleFlushTask must be run serially.
 func (db *DB) handleFlushTask(ft flushTask) error {
-	// There can be a scenario, when empty memtable is flushed.
-	if ft.mt.sl.Empty() {
-		return nil
-	}
-
+	// ft.mt could be nil with ft.itr being the valid field.
 	bopts := buildTableOptions(db)
 	builder := buildL0Table(ft, bopts)
 	defer builder.Close()
@@ -1080,11 +1120,52 @@ func (db *DB) handleFlushTask(ft flushTask) error {
 func (db *DB) flushMemtable(lc *z.Closer) error {
 	defer lc.Done()
 
+	var sz int64
+	var itrs []y.Iterator
+	var mts []*memTable
+	var cbs []func()
+	slurp := func() {
+		for {
+			select {
+			case more := <-db.flushChan:
+				if more.mt == nil {
+					return
+				}
+				sl := more.mt.sl
+				itrs = append(itrs, sl.NewUniIterator(false))
+				mts = append(mts, more.mt)
+				cbs = append(cbs, more.cb)
+
+				sz += sl.MemSize()
+				if sz > db.opt.MemTableSize {
+					return
+				}
+			default:
+				return
+			}
+		}
+	}
+
 	for ft := range db.flushChan {
 		if ft.mt == nil {
 			// We close db.flushChan now, instead of sending a nil ft.mt.
 			continue
 		}
+		sz = ft.mt.sl.MemSize()
+		// Reset of itrs, mts etc. is being done below.
+		y.AssertTrue(len(itrs) == 0 && len(mts) == 0 && len(cbs) == 0)
+		itrs = append(itrs, ft.mt.sl.NewUniIterator(false))
+		mts = append(mts, ft.mt)
+		cbs = append(cbs, ft.cb)
+
+		// Pick more memtables, so we can really fill up the L0 table.
+		slurp()
+
+		// db.opt.Infof("Picked %d memtables. Size: %d\n", len(itrs), sz)
+		ft.mt = nil
+		ft.itr = table.NewMergeIterator(itrs, false)
+		ft.cb = nil
+
 		for {
 			err := db.handleFlushTask(ft)
 			if err == nil {
@@ -1095,17 +1176,26 @@ func (db *DB) flushMemtable(lc *z.Closer) error {
 				// which would arrive here would match db.imm[0], because we acquire a
 				// lock over DB when pushing to flushChan.
 				// TODO: This logic is dirty AF. Any change and this could easily break.
-				y.AssertTrue(ft.mt == db.imm[0])
-				db.imm = db.imm[1:]
-				ft.mt.DecrRef() // Return memory.
+				for _, mt := range mts {
+					y.AssertTrue(mt == db.imm[0])
+					db.imm = db.imm[1:]
+					mt.DecrRef() // Return memory.
+				}
 				db.lock.Unlock()
 
+				for _, cb := range cbs {
+					if cb != nil {
+						cb()
+					}
+				}
 				break
 			}
 			// Encountered error. Retry indefinitely.
 			db.opt.Errorf("Failure while flushing memtable to disk: %v. Retrying...\n", err)
 			time.Sleep(time.Second)
 		}
+		// Reset everything.
+		itrs, mts, cbs, sz = itrs[:0], mts[:0], cbs[:0], 0
 	}
 	return nil
 }

db_test.go

@@ -2088,7 +2088,7 @@ func TestVerifyChecksum(t *testing.T) {
 			y.Check2(rand.Read(value))
 			st := 0
 
-			buf := z.NewBuffer(10 << 20, "test")
+			buf := z.NewBuffer(10<<20, "test")
 			defer buf.Release()
 			for i := 0; i < 1000; i++ {
 				key := make([]byte, 8)

iterator.go

@@ -146,7 +146,7 @@ func (item *Item) IsDeletedOrExpired() bool {
 // DiscardEarlierVersions returns whether the item was created with the
 // option to discard earlier versions of a key when multiple are available.
 func (item *Item) DiscardEarlierVersions() bool {
-	return item.meta&bitDiscardEarlierVersions > 0
+	return item.meta&BitDiscardEarlierVersions > 0
 }
 
 func (item *Item) yieldItemValue() ([]byte, func(), error) {

levels.go

@@ -716,7 +716,7 @@ func (s *levelsController) subcompact(it y.Iterator, kr keyRange, cd compactDef,
 				}
 				lastKey = y.SafeCopy(lastKey, it.Key())
 				numVersions = 0
-				firstKeyHasDiscardSet = it.Value().Meta&bitDiscardEarlierVersions > 0
+				firstKeyHasDiscardSet = it.Value().Meta&BitDiscardEarlierVersions > 0
 
 				if len(tableKr.left) == 0 {
 					tableKr.left = y.SafeCopy(tableKr.left, it.Key())
@@ -753,7 +753,7 @@ func (s *levelsController) subcompact(it y.Iterator, kr keyRange, cd compactDef,
 				// - The `discardEarlierVersions` bit is set OR
 				// - We've already processed `NumVersionsToKeep` number of versions
 				// (including the current item being processed)
-				lastValidVersion := vs.Meta&bitDiscardEarlierVersions > 0 ||
+				lastValidVersion := vs.Meta&BitDiscardEarlierVersions > 0 ||
 					numVersions == s.kv.opt.NumVersionsToKeep
 
 				if isExpired || lastValidVersion {

levels_test.go

@@ -707,11 +707,11 @@ func TestDiscardFirstVersion(t *testing.T) {
 
 	runBadgerTest(t, &opt, func(t *testing.T, db *DB) {
 		l0 := []keyValVersion{{"foo", "bar", 1, 0}}
-		l01 := []keyValVersion{{"foo", "bar", 2, bitDiscardEarlierVersions}}
+		l01 := []keyValVersion{{"foo", "bar", 2, BitDiscardEarlierVersions}}
 		l02 := []keyValVersion{{"foo", "bar", 3, 0}}
 		l03 := []keyValVersion{{"foo", "bar", 4, 0}}
 		l04 := []keyValVersion{{"foo", "bar", 9, 0}}
-		l05 := []keyValVersion{{"foo", "bar", 10, bitDiscardEarlierVersions}}
+		l05 := []keyValVersion{{"foo", "bar", 10, BitDiscardEarlierVersions}}
 
 		// Level 0 has all the tables.
 		createAndOpen(db, l0, 0)
@@ -742,11 +742,11 @@ func TestDiscardFirstVersion(t *testing.T) {
 		// - Version 1 is below DiscardTS and below the first "bitDiscardEarlierVersions"
 		//   marker so IT WILL BE REMOVED.
 		ExpectedKeys := []keyValVersion{
-			{"foo", "bar", 10, bitDiscardEarlierVersions},
+			{"foo", "bar", 10, BitDiscardEarlierVersions},
 			{"foo", "bar", 9, 0},
 			{"foo", "bar", 4, 0},
 			{"foo", "bar", 3, 0},
-			{"foo", "bar", 2, bitDiscardEarlierVersions}}
+			{"foo", "bar", 2, BitDiscardEarlierVersions}}
 
 		getAllAndCheck(t, db, ExpectedKeys)
 	})
@@ -1060,15 +1060,15 @@ func TestSameLevel(t *testing.T) {
 	opt.LmaxCompaction = true
 	runBadgerTest(t, &opt, func(t *testing.T, db *DB) {
 		l6 := []keyValVersion{
-			{"A", "bar", 4, bitDiscardEarlierVersions}, {"A", "bar", 3, 0},
+			{"A", "bar", 4, BitDiscardEarlierVersions}, {"A", "bar", 3, 0},
 			{"A", "bar", 2, 0}, {"Afoo", "baz", 2, 0},
 		}
 		l61 := []keyValVersion{
-			{"B", "bar", 4, bitDiscardEarlierVersions}, {"B", "bar", 3, 0},
+			{"B", "bar", 4, BitDiscardEarlierVersions}, {"B", "bar", 3, 0},
 			{"B", "bar", 2, 0}, {"Bfoo", "baz", 2, 0},
 		}
 		l62 := []keyValVersion{
-			{"C", "bar", 4, bitDiscardEarlierVersions}, {"C", "bar", 3, 0},
+			{"C", "bar", 4, BitDiscardEarlierVersions}, {"C", "bar", 3, 0},
 			{"C", "bar", 2, 0}, {"Cfoo", "baz", 2, 0},
 		}
 		createAndOpen(db, l6, 6)
@@ -1077,11 +1077,11 @@ func TestSameLevel(t *testing.T) {
 		require.NoError(t, db.lc.validate())
 
 		getAllAndCheck(t, db, []keyValVersion{
-			{"A", "bar", 4, bitDiscardEarlierVersions}, {"A", "bar", 3, 0},
+			{"A", "bar", 4, BitDiscardEarlierVersions}, {"A", "bar", 3, 0},
 			{"A", "bar", 2, 0}, {"Afoo", "baz", 2, 0},
-			{"B", "bar", 4, bitDiscardEarlierVersions}, {"B", "bar", 3, 0},
+			{"B", "bar", 4, BitDiscardEarlierVersions}, {"B", "bar", 3, 0},
 			{"B", "bar", 2, 0}, {"Bfoo", "baz", 2, 0},
-			{"C", "bar", 4, bitDiscardEarlierVersions}, {"C", "bar", 3, 0},
+			{"C", "bar", 4, BitDiscardEarlierVersions}, {"C", "bar", 3, 0},
 			{"C", "bar", 2, 0}, {"Cfoo", "baz", 2, 0},
 		})
 
@@ -1097,11 +1097,11 @@ func TestSameLevel(t *testing.T) {
 		db.SetDiscardTs(3)
 		require.NoError(t, db.lc.runCompactDef(-1, 6, cdef))
 		getAllAndCheck(t, db, []keyValVersion{
-			{"A", "bar", 4, bitDiscardEarlierVersions}, {"A", "bar", 3, 0},
+			{"A", "bar", 4, BitDiscardEarlierVersions}, {"A", "bar", 3, 0},
 			{"A", "bar", 2, 0}, {"Afoo", "baz", 2, 0},
-			{"B", "bar", 4, bitDiscardEarlierVersions}, {"B", "bar", 3, 0},
+			{"B", "bar", 4, BitDiscardEarlierVersions}, {"B", "bar", 3, 0},
 			{"B", "bar", 2, 0}, {"Bfoo", "baz", 2, 0},
-			{"C", "bar", 4, bitDiscardEarlierVersions}, {"C", "bar", 3, 0},
+			{"C", "bar", 4, BitDiscardEarlierVersions}, {"C", "bar", 3, 0},
 			{"C", "bar", 2, 0}, {"Cfoo", "baz", 2, 0},
 		})
 
@@ -1118,9 +1118,9 @@ func TestSameLevel(t *testing.T) {
 		cdef.t.baseLevel = 1
 		require.NoError(t, db.lc.runCompactDef(-1, 6, cdef))
 		getAllAndCheck(t, db, []keyValVersion{
-			{"A", "bar", 4, bitDiscardEarlierVersions}, {"Afoo", "baz", 2, 0},
-			{"B", "bar", 4, bitDiscardEarlierVersions}, {"Bfoo", "baz", 2, 0},
-			{"C", "bar", 4, bitDiscardEarlierVersions}, {"Cfoo", "baz", 2, 0}})
+			{"A", "bar", 4, BitDiscardEarlierVersions}, {"Afoo", "baz", 2, 0},
+			{"B", "bar", 4, BitDiscardEarlierVersions}, {"Bfoo", "baz", 2, 0},
+			{"C", "bar", 4, BitDiscardEarlierVersions}, {"Cfoo", "baz", 2, 0}})
 		require.NoError(t, db.lc.validate())
 	})
 }
@@ -1186,7 +1186,7 @@ func TestStaleDataCleanup(t *testing.T) {
 			for i := count; i > 0; i-- {
 				var meta byte
 				if i == 0 {
-					meta = bitDiscardEarlierVersions
+					meta = BitDiscardEarlierVersions
 				}
 				b.AddStaleKey(y.KeyWithTs(key, i), y.ValueStruct{Meta: meta, Value: val}, 0)
 			}