stream.go - external/github.com/dgraph-io/badger - Git at Google

 /*
  * SPDX-FileCopyrightText: © Hypermode Inc. <hello@hypermode.com>
  * SPDX-License-Identifier: Apache-2.0
  */

 package badger

 import (
 	"bytes"
 	"context"
 	"sort"
 	"sync"
 	"sync/atomic"
 	"time"

 	humanize "github.com/dustin/go-humanize"
 	"google.golang.org/protobuf/proto"

 	"github.com/dgraph-io/badger/v4/pb"
 	"github.com/dgraph-io/badger/v4/y"
 	"github.com/dgraph-io/ristretto/v2/z"
 )

 const batchSize = 16 << 20 // 16 MB

 // maxStreamSize is the maximum allowed size of a stream batch. This is a soft limit
 // as a single list that is still over the limit will have to be sent as is since it
 // cannot be split further. This limit prevents the framework from creating batches
 // so big that sending them causes issues (e.g running into the max size gRPC limit).
 var maxStreamSize = uint64(100 << 20) // 100MB

 // Stream provides a framework to concurrently iterate over a snapshot of Badger, pick up
 // key-values, batch them up and call Send. Stream does concurrent iteration over many smaller key
 // ranges. It does NOT send keys in lexicographical sorted order. To get keys in sorted
 // order, use Iterator.
 type Stream struct {
 	// Prefix to only iterate over certain range of keys. If set to nil (default), Stream would
 	// iterate over the entire DB.
 	Prefix []byte

 	// Number of goroutines to use for iterating over key ranges. Defaults to 8.
 	NumGo int

 	// Badger would produce log entries in Infof to indicate the progress of Stream. LogPrefix can
 	// be used to help differentiate them from other activities. Default is "Badger.Stream".
 	LogPrefix string

 	// ChooseKey is invoked each time a new key is encountered. Note that this is not called
 	// on every version of the value, only the first encountered version (i.e. the highest version
 	// of the value a key has). ChooseKey can be left nil to select all keys.
 	//
 	// Note: Calls to ChooseKey are concurrent.
 	ChooseKey func(item *Item) bool

 	// MaxSize is the maximum allowed size of a stream batch. This is a soft limit
 	// as a single list that is still over the limit will have to be sent as is since it
 	// cannot be split further. This limit prevents the framework from creating batches
 	// so big that sending them causes issues (e.g running into the max size gRPC limit).
 	// If necessary, set it up before the Stream starts synchronisation
 	// This is not a concurrency-safe setting
 	MaxSize uint64

 	// KeyToList, similar to ChooseKey, is only invoked on the highest version of the value. It
 	// is upto the caller to iterate over the versions and generate zero, one or more KVs. It
 	// is expected that the user would advance the iterator to go through the versions of the
 	// values. However, the user MUST immediately return from this function on the first encounter
 	// with a mismatching key. See example usage in ToList function. Can be left nil to use ToList
 	// function by default.
 	//
 	// KeyToList has access to z.Allocator accessible via stream.Allocator(itr.ThreadId). This
 	// allocator can be used to allocate KVs, to decrease the memory pressure on Go GC. Stream
 	// framework takes care of releasing those resources after calling Send. AllocRef does
 	// NOT need to be set in the returned KVList, as Stream framework would ignore that field,
 	// instead using the allocator assigned to that thread id.
 	//
 	// Note: Calls to KeyToList are concurrent.
 	KeyToList func(key []byte, itr *Iterator) (*pb.KVList, error)
 	// UseKeyToListWithThreadId is used to indicate that KeyToListWithThreadId should be used
 	// instead of KeyToList. This is a new api that can be used to figure out parallelism
 	// of the stream. Each threadId would be run serially. KeyToList being concurrent makes you
 	// take care of concurrency in KeyToList. Here threadId could be used to do some things serially.
 	// Once a thread finishes FinishThread() would be called.
 	UseKeyToListWithThreadId bool
 	KeyToListWithThreadId    func(key []byte, itr *Iterator, threadId int) (*pb.KVList, error)
 	FinishThread             func(threadId int) (*pb.KVList, error)

 	// This is the method where Stream sends the final output. All calls to Send are done by a
 	// single goroutine, i.e. logic within Send method can expect single threaded execution.
 	Send func(buf *z.Buffer) error

 	// Read data above the sinceTs. All keys with version =< sinceTs will be ignored.
 	SinceTs      uint64
 	readTs       uint64
 	db           *DB
 	rangeCh      chan keyRange
 	kvChan       chan *z.Buffer
 	nextStreamId atomic.Uint32
 	doneMarkers  bool
 	scanned      atomic.Uint64 // used to estimate the ETA for data scan.
 	numProducers atomic.Int32
 }

 // SendDoneMarkers when true would send out done markers on the stream. False by default.
 func (st *Stream) SendDoneMarkers(done bool) {
 	st.doneMarkers = done
 }

 // ToList is a default implementation of KeyToList. It picks up all valid versions of the key,
 // skipping over deleted or expired keys.
 func (st *Stream) ToList(key []byte, itr *Iterator) (*pb.KVList, error) {
 	a := itr.Alloc
 	ka := a.Copy(key)

 	list := &pb.KVList{}
 	for ; itr.Valid(); itr.Next() {
 		item := itr.Item()
 		if item.IsDeletedOrExpired() {
 			break
 		}
 		if !bytes.Equal(key, item.Key()) {
 			// Break out on the first encounter with another key.
 			break
 		}

 		kv := y.NewKV(a)
 		kv.Key = ka

 		if err := item.Value(func(val []byte) error {
 			kv.Value = a.Copy(val)
 			return nil

 		}); err != nil {
 			return nil, err
 		}
 		kv.Version = item.Version()
 		kv.ExpiresAt = item.ExpiresAt()
 		kv.UserMeta = a.Copy([]byte{item.UserMeta()})

 		list.Kv = append(list.Kv, kv)
 		if st.db.opt.NumVersionsToKeep == 1 {
 			break
 		}

 		if item.DiscardEarlierVersions() {
 			break
 		}
 	}
 	return list, nil
 }

 // keyRange is [start, end), including start, excluding end. Do ensure that the start,
 // end byte slices are owned by keyRange struct.
 func (st *Stream) produceRanges(ctx context.Context) {
 	ranges := st.db.Ranges(st.Prefix, st.NumGo)
 	y.AssertTrue(len(ranges) > 0)
 	y.AssertTrue(ranges[0].left == nil)
 	y.AssertTrue(ranges[len(ranges)-1].right == nil)
 	st.db.opt.Infof("Number of ranges found: %d\n", len(ranges))

 	// Sort in descending order of size.
 	sort.Slice(ranges, func(i, j int) bool {
 		return ranges[i].size > ranges[j].size
 	})
 	for i, r := range ranges {
 		st.rangeCh <- *r
 		st.db.opt.Infof("Sent range %d for iteration: [%x, %x) of size: %s\n",
 			i, r.left, r.right, humanize.IBytes(uint64(r.size)))
 	}
 	close(st.rangeCh)
 }

 // produceKVs picks up ranges from rangeCh, generates KV lists and sends them to kvChan.
 func (st *Stream) produceKVs(ctx context.Context, threadId int) error {
 	st.numProducers.Add(1)
 	defer st.numProducers.Add(-1)

 	var txn *Txn
 	if st.readTs > 0 {
 		txn = st.db.NewTransactionAt(st.readTs, false)
 	} else {
 		txn = st.db.NewTransaction(false)
 	}
 	defer txn.Discard()

 	// produceKVs is running iterate serially. So, we can define the outList here.
 	outList := z.NewBuffer(2*batchSize, "Stream.ProduceKVs")
 	defer func() {
 		// The outList variable changes. So, we need to evaluate the variable in the defer. DO NOT
 		// call `defer outList.Release()`.
 		_ = outList.Release()
 	}()

 	iterate := func(kr keyRange) error {
 		iterOpts := DefaultIteratorOptions
 		iterOpts.AllVersions = true
 		iterOpts.Prefix = st.Prefix
 		iterOpts.PrefetchValues = true
 		iterOpts.SinceTs = st.SinceTs
 		itr := txn.NewIterator(iterOpts)
 		itr.ThreadId = threadId
 		defer itr.Close()

 		itr.Alloc = z.NewAllocator(1<<20, "Stream.Iterate")
 		defer itr.Alloc.Release()

 		// This unique stream id is used to identify all the keys from this iteration.
 		streamId := st.nextStreamId.Add(1)
 		var scanned int

 		sendIt := func() error {
 			select {
 			case st.kvChan <- outList:
 				outList = z.NewBuffer(2*batchSize, "Stream.ProduceKVs")
 				st.scanned.Add(uint64(itr.scanned - scanned))
 				scanned = itr.scanned
 			case <-ctx.Done():
 				return ctx.Err()
 			}
 			return nil
 		}

 		var prevKey []byte
 		for itr.Seek(kr.left); itr.Valid(); {
 			// it.Valid would only return true for keys with the provided Prefix in iterOpts.
 			item := itr.Item()
 			if bytes.Equal(item.Key(), prevKey) {
 				itr.Next()
 				continue
 			}
 			prevKey = append(prevKey[:0], item.Key()...)

 			// Check if we reached the end of the key range.
 			if len(kr.right) > 0 && bytes.Compare(item.Key(), kr.right) >= 0 {
 				break
 			}

 			// Check if we should pick this key.
 			if st.ChooseKey != nil && !st.ChooseKey(item) {
 				continue
 			}

 			// Now convert to key value.
 			itr.Alloc.Reset()
 			var list *pb.KVList
 			var err error
 			if st.UseKeyToListWithThreadId {
 				list, err = st.KeyToListWithThreadId(item.KeyCopy(nil), itr, threadId)
 			} else {
 				list, err = st.KeyToList(item.KeyCopy(nil), itr)
 			}
 			if err != nil {
 				st.db.opt.Warningf("While reading key: %x, got error: %v", item.Key(), err)
 				continue
 			}
 			if list == nil || len(list.Kv) == 0 {
 				continue
 			}
 			for _, kv := range list.Kv {
 				kv.StreamId = streamId
 				KVToBuffer(kv, outList)
 				if outList.LenNoPadding() < batchSize {
 					continue
 				}
 				if err := sendIt(); err != nil {
 					return err
 				}
 			}
 		}

 		if st.UseKeyToListWithThreadId {
 			if kvs, err := st.FinishThread(threadId); err != nil {
 				return err
 			} else {
 				for _, kv := range kvs.Kv {
 					kv.StreamId = streamId
 					KVToBuffer(kv, outList)
 					if outList.LenNoPadding() < batchSize {
 						continue
 					}
 					if err := sendIt(); err != nil {
 						return err
 					}
 				}
 			}
 		}
 		// Mark the stream as done.
 		if st.doneMarkers {
 			kv := &pb.KV{
 				StreamId:   streamId,
 				StreamDone: true,
 			}
 			KVToBuffer(kv, outList)
 		}
 		return sendIt()
 	}

 	for {
 		select {
 		case kr, ok := <-st.rangeCh:
 			if !ok {
 				// Done with the keys.
 				return nil
 			}
 			if err := iterate(kr); err != nil {
 				return err
 			}
 		case <-ctx.Done():
 			return ctx.Err()
 		}
 	}
 }

 func (st *Stream) streamKVs(ctx context.Context) error {
 	onDiskSize, uncompressedSize := st.db.EstimateSize(st.Prefix)
 	// Manish has seen uncompressed size to be in 20% error margin.
 	uncompressedSize = uint64(float64(uncompressedSize) * 1.2)
 	st.db.opt.Infof("%s Streaming about %s of uncompressed data (%s on disk)\n",
 		st.LogPrefix, humanize.IBytes(uncompressedSize), humanize.IBytes(onDiskSize))

 	tickerDur := 5 * time.Second
 	var bytesSent uint64
 	t := time.NewTicker(tickerDur)
 	defer t.Stop()
 	now := time.Now()

 	sendBatch := func(batch *z.Buffer) error {
 		defer func() { _ = batch.Release() }()
 		sz := uint64(batch.LenNoPadding())
 		if sz == 0 {
 			return nil
 		}
 		bytesSent += sz
 		// st.db.opt.Infof("%s Sending batch of size: %s.\n", st.LogPrefix, humanize.IBytes(sz))
 		if err := st.Send(batch); err != nil {
 			st.db.opt.Warningf("Error while sending: %v\n", err)
 			return err
 		}
 		return nil
 	}

 	slurp := func(batch *z.Buffer) error {
 	loop:
 		for {
 			// Send the batch immediately if it already exceeds the maximum allowed size.
 			// If the size of the batch exceeds maxStreamSize, break from the loop to
 			// avoid creating a batch that is so big that certain limits are reached.
 			if uint64(batch.LenNoPadding()) > st.MaxSize {
 				break loop
 			}
 			select {
 			case kvs, ok := <-st.kvChan:
 				if !ok {
 					break loop
 				}
 				y.AssertTrue(kvs != nil)
 				y.Check2(batch.Write(kvs.Bytes()))
 				y.Check(kvs.Release())

 			default:
 				break loop
 			}
 		}
 		return sendBatch(batch)
 	} // end of slurp.

 	writeRate := y.NewRateMonitor(20)
 	scanRate := y.NewRateMonitor(20)
 outer:
 	for {
 		var batch *z.Buffer
 		select {
 		case <-ctx.Done():
 			return ctx.Err()

 		case <-t.C:
 			// Instead of calculating speed over the entire lifetime, we average the speed over
 			// ticker duration.
 			writeRate.Capture(bytesSent)
 			scanned := st.scanned.Load()
 			scanRate.Capture(scanned)
 			numProducers := st.numProducers.Load()

 			st.db.opt.Infof("%s [%s] Scan (%d): ~%s/%s at %s/sec. Sent: %s at %s/sec."+
 				" jemalloc: %s\n",
 				st.LogPrefix, y.FixedDuration(time.Since(now)), numProducers,
 				y.IBytesToString(scanned, 1), humanize.IBytes(uncompressedSize),
 				humanize.IBytes(scanRate.Rate()),
 				y.IBytesToString(bytesSent, 1), humanize.IBytes(writeRate.Rate()),
 				humanize.IBytes(uint64(z.NumAllocBytes())))

 		case kvs, ok := <-st.kvChan:
 			if !ok {
 				break outer
 			}
 			y.AssertTrue(kvs != nil)
 			batch = kvs

 			// Otherwise, slurp more keys into this batch.
 			if err := slurp(batch); err != nil {
 				return err
 			}
 		}
 	}

 	st.db.opt.Infof("%s Sent data of size %s\n", st.LogPrefix, humanize.IBytes(bytesSent))
 	return nil
 }

 // Orchestrate runs Stream. It picks up ranges from the SSTables, then runs NumGo number of
 // goroutines to iterate over these ranges and batch up KVs in lists. It concurrently runs a single
 // goroutine to pick these lists, batch them up further and send to Output.Send. Orchestrate also
 // spits logs out to Infof, using provided LogPrefix. Note that all calls to Output.Send
 // are serial. In case any of these steps encounter an error, Orchestrate would stop execution and
 // return that error. Orchestrate can be called multiple times, but in serial order.
 func (st *Stream) Orchestrate(ctx context.Context) error {
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 	st.rangeCh = make(chan keyRange, 3) // Contains keys for posting lists.

 	// kvChan should only have a small capacity to ensure that we don't buffer up too much data if
 	// sending is slow. Page size is set to 4MB, which is used to lazily cap the size of each
 	// KVList. To get 128MB buffer, we can set the channel size to 32.
 	st.kvChan = make(chan *z.Buffer, 32)

 	if st.KeyToList == nil {
 		st.KeyToList = st.ToList
 	}

 	// Picks up ranges from Badger, and sends them to rangeCh.
 	go st.produceRanges(ctx)

 	errCh := make(chan error, st.NumGo) // Stores error by consumeKeys.
 	var wg sync.WaitGroup
 	for i := 0; i < st.NumGo; i++ {
 		wg.Add(1)

 		go func(threadId int) {
 			defer wg.Done()
 			// Picks up ranges from rangeCh, generates KV lists, and sends them to kvChan.
 			if err := st.produceKVs(ctx, threadId); err != nil {
 				select {
 				case errCh <- err:
 				default:
 				}
 			}
 		}(i)
 	}

 	// Pick up key-values from kvChan and send to stream.
 	kvErr := make(chan error, 1)
 	go func() {
 		// Picks up KV lists from kvChan, and sends them to Output.
 		err := st.streamKVs(ctx)
 		if err != nil {
 			cancel() // Stop all the go routines.
 		}
 		kvErr <- err
 	}()
 	wg.Wait()        // Wait for produceKVs to be over.
 	close(st.kvChan) // Now we can close kvChan.
 	defer func() {
 		// If due to some error, we have buffers left in kvChan, we should release them.
 		for buf := range st.kvChan {
 			_ = buf.Release()
 		}
 	}()

 	select {
 	case err := <-errCh: // Check error from produceKVs.
 		return err
 	default:
 	}

 	// Wait for key streaming to be over.
 	err := <-kvErr
 	return err
 }

 func (db *DB) newStream() *Stream {
 	return &Stream{
 		db:        db,
 		NumGo:     db.opt.NumGoroutines,
 		LogPrefix: "Badger.Stream",
 		MaxSize:   maxStreamSize,
 	}
 }

 // NewStream creates a new Stream.
 func (db *DB) NewStream() *Stream {
 	if db.opt.managedTxns {
 		panic("This API can not be called in managed mode.")
 	}
 	return db.newStream()
 }

 // NewStreamAt creates a new Stream at a particular timestamp. Should only be used with managed DB.
 func (db *DB) NewStreamAt(readTs uint64) *Stream {
 	if !db.opt.managedTxns {
 		panic("This API can only be called in managed mode.")
 	}
 	stream := db.newStream()
 	stream.readTs = readTs
 	return stream
 }

 func BufferToKVList(buf *z.Buffer) (*pb.KVList, error) {
 	var list pb.KVList
 	err := buf.SliceIterate(func(s []byte) error {
 		kv := new(pb.KV)
 		if err := proto.Unmarshal(s, kv); err != nil {
 			return err
 		}
 		list.Kv = append(list.Kv, kv)
 		return nil
 	})
 	return &list, err
 }

 func KVToBuffer(kv *pb.KV, buf *z.Buffer) {
 	in := buf.SliceAllocate(proto.Size(kv))[:0]
 	_, err := proto.MarshalOptions{}.MarshalAppend(in, kv)
 	y.AssertTrue(err == nil)
 }
	/*
	* SPDX-FileCopyrightText: © Hypermode Inc. <hello@hypermode.com>
	* SPDX-License-Identifier: Apache-2.0
	*/

	package badger

	import (
	"bytes"
	"context"
	"sort"
	"sync"
	"sync/atomic"
	"time"

	humanize "github.com/dustin/go-humanize"
	"google.golang.org/protobuf/proto"

	"github.com/dgraph-io/badger/v4/pb"
	"github.com/dgraph-io/badger/v4/y"
	"github.com/dgraph-io/ristretto/v2/z"
	)

	const batchSize = 16 << 20 // 16 MB

	// maxStreamSize is the maximum allowed size of a stream batch. This is a soft limit
	// as a single list that is still over the limit will have to be sent as is since it
	// cannot be split further. This limit prevents the framework from creating batches
	// so big that sending them causes issues (e.g running into the max size gRPC limit).
	var maxStreamSize = uint64(100 << 20) // 100MB

	// Stream provides a framework to concurrently iterate over a snapshot of Badger, pick up
	// key-values, batch them up and call Send. Stream does concurrent iteration over many smaller key
	// ranges. It does NOT send keys in lexicographical sorted order. To get keys in sorted
	// order, use Iterator.
	type Stream struct {
	// Prefix to only iterate over certain range of keys. If set to nil (default), Stream would
	// iterate over the entire DB.
	Prefix []byte

	// Number of goroutines to use for iterating over key ranges. Defaults to 8.
	NumGo int

	// Badger would produce log entries in Infof to indicate the progress of Stream. LogPrefix can
	// be used to help differentiate them from other activities. Default is "Badger.Stream".
	LogPrefix string

	// ChooseKey is invoked each time a new key is encountered. Note that this is not called
	// on every version of the value, only the first encountered version (i.e. the highest version
	// of the value a key has). ChooseKey can be left nil to select all keys.
	//
	// Note: Calls to ChooseKey are concurrent.
	ChooseKey func(item *Item) bool

	// MaxSize is the maximum allowed size of a stream batch. This is a soft limit
	// as a single list that is still over the limit will have to be sent as is since it
	// cannot be split further. This limit prevents the framework from creating batches
	// so big that sending them causes issues (e.g running into the max size gRPC limit).
	// If necessary, set it up before the Stream starts synchronisation
	// This is not a concurrency-safe setting
	MaxSize uint64

	// KeyToList, similar to ChooseKey, is only invoked on the highest version of the value. It
	// is upto the caller to iterate over the versions and generate zero, one or more KVs. It
	// is expected that the user would advance the iterator to go through the versions of the
	// values. However, the user MUST immediately return from this function on the first encounter
	// with a mismatching key. See example usage in ToList function. Can be left nil to use ToList
	// function by default.
	//
	// KeyToList has access to z.Allocator accessible via stream.Allocator(itr.ThreadId). This
	// allocator can be used to allocate KVs, to decrease the memory pressure on Go GC. Stream
	// framework takes care of releasing those resources after calling Send. AllocRef does
	// NOT need to be set in the returned KVList, as Stream framework would ignore that field,
	// instead using the allocator assigned to that thread id.
	//
	// Note: Calls to KeyToList are concurrent.
	KeyToList func(key []byte, itr Iterator) (pb.KVList, error)
	// UseKeyToListWithThreadId is used to indicate that KeyToListWithThreadId should be used
	// instead of KeyToList. This is a new api that can be used to figure out parallelism
	// of the stream. Each threadId would be run serially. KeyToList being concurrent makes you
	// take care of concurrency in KeyToList. Here threadId could be used to do some things serially.
	// Once a thread finishes FinishThread() would be called.
	UseKeyToListWithThreadId bool
	KeyToListWithThreadId func(key []byte, itr Iterator, threadId int) (pb.KVList, error)
	FinishThread func(threadId int) (*pb.KVList, error)

	// This is the method where Stream sends the final output. All calls to Send are done by a
	// single goroutine, i.e. logic within Send method can expect single threaded execution.
	Send func(buf *z.Buffer) error

	// Read data above the sinceTs. All keys with version =< sinceTs will be ignored.
	SinceTs uint64
	readTs uint64
	db *DB
	rangeCh chan keyRange
	kvChan chan *z.Buffer
	nextStreamId atomic.Uint32
	doneMarkers bool
	scanned atomic.Uint64 // used to estimate the ETA for data scan.
	numProducers atomic.Int32
	}

	// SendDoneMarkers when true would send out done markers on the stream. False by default.
	func (st *Stream) SendDoneMarkers(done bool) {
	st.doneMarkers = done
	}

	// ToList is a default implementation of KeyToList. It picks up all valid versions of the key,
	// skipping over deleted or expired keys.
	func (st Stream) ToList(key []byte, itr Iterator) (*pb.KVList, error) {
	a := itr.Alloc
	ka := a.Copy(key)

	list := &pb.KVList{}
	for ; itr.Valid(); itr.Next() {
	item := itr.Item()
	if item.IsDeletedOrExpired() {
	break
	}
	if !bytes.Equal(key, item.Key()) {
	// Break out on the first encounter with another key.
	break
	}

	kv := y.NewKV(a)
	kv.Key = ka

	if err := item.Value(func(val []byte) error {
	kv.Value = a.Copy(val)
	return nil

	}); err != nil {
	return nil, err
	}
	kv.Version = item.Version()
	kv.ExpiresAt = item.ExpiresAt()
	kv.UserMeta = a.Copy([]byte{item.UserMeta()})

	list.Kv = append(list.Kv, kv)
	if st.db.opt.NumVersionsToKeep == 1 {
	break
	}

	if item.DiscardEarlierVersions() {
	break
	}
	}
	return list, nil
	}

	// keyRange is [start, end), including start, excluding end. Do ensure that the start,
	// end byte slices are owned by keyRange struct.
	func (st *Stream) produceRanges(ctx context.Context) {
	ranges := st.db.Ranges(st.Prefix, st.NumGo)
	y.AssertTrue(len(ranges) > 0)
	y.AssertTrue(ranges[0].left == nil)
	y.AssertTrue(ranges[len(ranges)-1].right == nil)
	st.db.opt.Infof("Number of ranges found: %d\n", len(ranges))

	// Sort in descending order of size.
	sort.Slice(ranges, func(i, j int) bool {
	return ranges[i].size > ranges[j].size
	})
	for i, r := range ranges {
	st.rangeCh <- *r
	st.db.opt.Infof("Sent range %d for iteration: [%x, %x) of size: %s\n",
	i, r.left, r.right, humanize.IBytes(uint64(r.size)))
	}
	close(st.rangeCh)
	}

	// produceKVs picks up ranges from rangeCh, generates KV lists and sends them to kvChan.
	func (st *Stream) produceKVs(ctx context.Context, threadId int) error {
	st.numProducers.Add(1)
	defer st.numProducers.Add(-1)

	var txn *Txn
	if st.readTs > 0 {
	txn = st.db.NewTransactionAt(st.readTs, false)
	} else {
	txn = st.db.NewTransaction(false)
	}
	defer txn.Discard()

	// produceKVs is running iterate serially. So, we can define the outList here.
	outList := z.NewBuffer(2*batchSize, "Stream.ProduceKVs")
	defer func() {
	// The outList variable changes. So, we need to evaluate the variable in the defer. DO NOT
	// call `defer outList.Release()`.
	_ = outList.Release()
	}()

	iterate := func(kr keyRange) error {
	iterOpts := DefaultIteratorOptions
	iterOpts.AllVersions = true
	iterOpts.Prefix = st.Prefix
	iterOpts.PrefetchValues = true
	iterOpts.SinceTs = st.SinceTs
	itr := txn.NewIterator(iterOpts)
	itr.ThreadId = threadId
	defer itr.Close()

	itr.Alloc = z.NewAllocator(1<<20, "Stream.Iterate")
	defer itr.Alloc.Release()

	// This unique stream id is used to identify all the keys from this iteration.
	streamId := st.nextStreamId.Add(1)
	var scanned int

	sendIt := func() error {
	select {
	case st.kvChan <- outList:
	outList = z.NewBuffer(2*batchSize, "Stream.ProduceKVs")
	st.scanned.Add(uint64(itr.scanned - scanned))
	scanned = itr.scanned
	case <-ctx.Done():
	return ctx.Err()
	}
	return nil
	}

	var prevKey []byte
	for itr.Seek(kr.left); itr.Valid(); {
	// it.Valid would only return true for keys with the provided Prefix in iterOpts.
	item := itr.Item()
	if bytes.Equal(item.Key(), prevKey) {
	itr.Next()
	continue
	}
	prevKey = append(prevKey[:0], item.Key()...)

	// Check if we reached the end of the key range.
	if len(kr.right) > 0 && bytes.Compare(item.Key(), kr.right) >= 0 {
	break
	}

	// Check if we should pick this key.
	if st.ChooseKey != nil && !st.ChooseKey(item) {
	continue
	}

	// Now convert to key value.
	itr.Alloc.Reset()
	var list *pb.KVList
	var err error
	if st.UseKeyToListWithThreadId {
	list, err = st.KeyToListWithThreadId(item.KeyCopy(nil), itr, threadId)
	} else {
	list, err = st.KeyToList(item.KeyCopy(nil), itr)
	}
	if err != nil {
	st.db.opt.Warningf("While reading key: %x, got error: %v", item.Key(), err)
	continue
	}
	if list == nil \|\| len(list.Kv) == 0 {
	continue
	}
	for _, kv := range list.Kv {
	kv.StreamId = streamId
	KVToBuffer(kv, outList)
	if outList.LenNoPadding() < batchSize {
	continue
	}
	if err := sendIt(); err != nil {
	return err
	}
	}
	}

	if st.UseKeyToListWithThreadId {
	if kvs, err := st.FinishThread(threadId); err != nil {
	return err
	} else {
	for _, kv := range kvs.Kv {
	kv.StreamId = streamId
	KVToBuffer(kv, outList)
	if outList.LenNoPadding() < batchSize {
	continue
	}
	if err := sendIt(); err != nil {
	return err
	}
	}
	}
	}
	// Mark the stream as done.
	if st.doneMarkers {
	kv := &pb.KV{
	StreamId: streamId,
	StreamDone: true,
	}
	KVToBuffer(kv, outList)
	}
	return sendIt()
	}

	for {
	select {
	case kr, ok := <-st.rangeCh:
	if !ok {
	// Done with the keys.
	return nil
	}
	if err := iterate(kr); err != nil {
	return err
	}
	case <-ctx.Done():
	return ctx.Err()
	}
	}
	}

	func (st *Stream) streamKVs(ctx context.Context) error {
	onDiskSize, uncompressedSize := st.db.EstimateSize(st.Prefix)
	// Manish has seen uncompressed size to be in 20% error margin.
	uncompressedSize = uint64(float64(uncompressedSize) * 1.2)
	st.db.opt.Infof("%s Streaming about %s of uncompressed data (%s on disk)\n",
	st.LogPrefix, humanize.IBytes(uncompressedSize), humanize.IBytes(onDiskSize))

	tickerDur := 5 * time.Second
	var bytesSent uint64
	t := time.NewTicker(tickerDur)
	defer t.Stop()
	now := time.Now()

	sendBatch := func(batch *z.Buffer) error {
	defer func() { _ = batch.Release() }()
	sz := uint64(batch.LenNoPadding())
	if sz == 0 {
	return nil
	}
	bytesSent += sz
	// st.db.opt.Infof("%s Sending batch of size: %s.\n", st.LogPrefix, humanize.IBytes(sz))
	if err := st.Send(batch); err != nil {
	st.db.opt.Warningf("Error while sending: %v\n", err)
	return err
	}
	return nil
	}

	slurp := func(batch *z.Buffer) error {
	loop:
	for {
	// Send the batch immediately if it already exceeds the maximum allowed size.
	// If the size of the batch exceeds maxStreamSize, break from the loop to
	// avoid creating a batch that is so big that certain limits are reached.
	if uint64(batch.LenNoPadding()) > st.MaxSize {
	break loop
	}
	select {
	case kvs, ok := <-st.kvChan:
	if !ok {
	break loop
	}
	y.AssertTrue(kvs != nil)
	y.Check2(batch.Write(kvs.Bytes()))
	y.Check(kvs.Release())

	default:
	break loop
	}
	}
	return sendBatch(batch)
	} // end of slurp.

	writeRate := y.NewRateMonitor(20)
	scanRate := y.NewRateMonitor(20)
	outer:
	for {
	var batch *z.Buffer
	select {
	case <-ctx.Done():
	return ctx.Err()

	case <-t.C:
	// Instead of calculating speed over the entire lifetime, we average the speed over
	// ticker duration.
	writeRate.Capture(bytesSent)
	scanned := st.scanned.Load()
	scanRate.Capture(scanned)
	numProducers := st.numProducers.Load()

	st.db.opt.Infof("%s [%s] Scan (%d): ~%s/%s at %s/sec. Sent: %s at %s/sec."+
	" jemalloc: %s\n",
	st.LogPrefix, y.FixedDuration(time.Since(now)), numProducers,
	y.IBytesToString(scanned, 1), humanize.IBytes(uncompressedSize),
	humanize.IBytes(scanRate.Rate()),
	y.IBytesToString(bytesSent, 1), humanize.IBytes(writeRate.Rate()),
	humanize.IBytes(uint64(z.NumAllocBytes())))

	case kvs, ok := <-st.kvChan:
	if !ok {
	break outer
	}
	y.AssertTrue(kvs != nil)
	batch = kvs

	// Otherwise, slurp more keys into this batch.
	if err := slurp(batch); err != nil {
	return err
	}
	}
	}

	st.db.opt.Infof("%s Sent data of size %s\n", st.LogPrefix, humanize.IBytes(bytesSent))
	return nil
	}

	// Orchestrate runs Stream. It picks up ranges from the SSTables, then runs NumGo number of
	// goroutines to iterate over these ranges and batch up KVs in lists. It concurrently runs a single
	// goroutine to pick these lists, batch them up further and send to Output.Send. Orchestrate also
	// spits logs out to Infof, using provided LogPrefix. Note that all calls to Output.Send
	// are serial. In case any of these steps encounter an error, Orchestrate would stop execution and
	// return that error. Orchestrate can be called multiple times, but in serial order.
	func (st *Stream) Orchestrate(ctx context.Context) error {
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()
	st.rangeCh = make(chan keyRange, 3) // Contains keys for posting lists.

	// kvChan should only have a small capacity to ensure that we don't buffer up too much data if
	// sending is slow. Page size is set to 4MB, which is used to lazily cap the size of each
	// KVList. To get 128MB buffer, we can set the channel size to 32.
	st.kvChan = make(chan *z.Buffer, 32)

	if st.KeyToList == nil {
	st.KeyToList = st.ToList
	}

	// Picks up ranges from Badger, and sends them to rangeCh.
	go st.produceRanges(ctx)

	errCh := make(chan error, st.NumGo) // Stores error by consumeKeys.
	var wg sync.WaitGroup
	for i := 0; i < st.NumGo; i++ {
	wg.Add(1)

	go func(threadId int) {
	defer wg.Done()
	// Picks up ranges from rangeCh, generates KV lists, and sends them to kvChan.
	if err := st.produceKVs(ctx, threadId); err != nil {
	select {
	case errCh <- err:
	default:
	}
	}
	}(i)
	}

	// Pick up key-values from kvChan and send to stream.
	kvErr := make(chan error, 1)
	go func() {
	// Picks up KV lists from kvChan, and sends them to Output.
	err := st.streamKVs(ctx)
	if err != nil {
	cancel() // Stop all the go routines.
	}
	kvErr <- err
	}()
	wg.Wait() // Wait for produceKVs to be over.
	close(st.kvChan) // Now we can close kvChan.
	defer func() {
	// If due to some error, we have buffers left in kvChan, we should release them.
	for buf := range st.kvChan {
	_ = buf.Release()
	}
	}()

	select {
	case err := <-errCh: // Check error from produceKVs.
	return err
	default:
	}

	// Wait for key streaming to be over.
	err := <-kvErr
	return err
	}

	func (db DB) newStream() Stream {
	return &Stream{
	db: db,
	NumGo: db.opt.NumGoroutines,
	LogPrefix: "Badger.Stream",
	MaxSize: maxStreamSize,
	}
	}

	// NewStream creates a new Stream.
	func (db DB) NewStream() Stream {
	if db.opt.managedTxns {
	panic("This API can not be called in managed mode.")
	}
	return db.newStream()
	}

	// NewStreamAt creates a new Stream at a particular timestamp. Should only be used with managed DB.
	func (db DB) NewStreamAt(readTs uint64) Stream {
	if !db.opt.managedTxns {
	panic("This API can only be called in managed mode.")
	}
	stream := db.newStream()
	stream.readTs = readTs
	return stream
	}

	func BufferToKVList(buf z.Buffer) (pb.KVList, error) {
	var list pb.KVList
	err := buf.SliceIterate(func(s []byte) error {
	kv := new(pb.KV)
	if err := proto.Unmarshal(s, kv); err != nil {
	return err
	}
	list.Kv = append(list.Kv, kv)
	return nil
	})
	return &list, err
	}

	func KVToBuffer(kv pb.KV, buf z.Buffer) {
	in := buf.SliceAllocate(proto.Size(kv))[:0]
	_, err := proto.MarshalOptions{}.MarshalAppend(in, kv)
	y.AssertTrue(err == nil)
	}