diff --git a/pkg/depth/buffer.go b/pkg/depth/buffer.go
new file mode 100644
index 000000000..30a72a869
--- /dev/null
+++ b/pkg/depth/buffer.go
@@ -0,0 +1,174 @@
+package depth
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/c9s/bbgo/pkg/types"
+	"github.com/c9s/bbgo/pkg/util"
+	log "github.com/sirupsen/logrus"
+)
+
+type SnapshotFetcher func() (snapshot types.SliceOrderBook, finalUpdateID int64, err error)
+
+type Update struct {
+	FirstUpdateID, FinalUpdateID int64
+
+	// Object is the update object
+	Object types.SliceOrderBook
+}
+
+//go:generate callbackgen -type Buffer
+type Buffer struct {
+	Symbol string
+	buffer []Update
+
+	finalUpdateID int64
+	fetcher       SnapshotFetcher
+	snapshot      *types.SliceOrderBook
+
+	resetCallbacks []func()
+	readyCallbacks []func(snapshot types.SliceOrderBook, updates []Update)
+	pushCallbacks  []func(update Update)
+
+	resetC chan struct{}
+	mu     sync.Mutex
+	once   util.Reonce
+}
+
+func NewDepthBuffer(symbol string, fetcher SnapshotFetcher) *Buffer {
+	return &Buffer{
+		Symbol:  symbol,
+		fetcher: fetcher,
+		resetC:  make(chan struct{}, 1),
+	}
+}
+
+func (b *Buffer) resetSnapshot() {
+	b.snapshot = nil
+	b.finalUpdateID = 0
+	b.EmitReset()
+}
+
+func (b *Buffer) emitReset() {
+	select {
+	case b.resetC <- struct{}{}:
+	default:
+	}
+}
+
+// AddUpdate adds the update to the buffer or push the update to the subscriber
+func (b *Buffer) AddUpdate(o types.SliceOrderBook, firstUpdateID int64, finalArgs ...int64) error {
+	finalUpdateID := firstUpdateID
+	if len(finalArgs) > 0 {
+		finalUpdateID = finalArgs[0]
+	}
+
+	u := Update{
+		FirstUpdateID: firstUpdateID,
+		FinalUpdateID: finalUpdateID,
+		Object:        o,
+	}
+
+	// we lock here because there might be 2+ calls to the AddUpdate method
+	// we don't want to reset sync.Once 2 times here
+	b.mu.Lock()
+	select {
+	case <-b.resetC:
+		log.Warnf("received depth reset signal, resetting...")
+
+		// if the once goroutine is still running, overwriting this once might cause "unlock of unlocked mutex" panic.
+		b.once.Reset()
+	default:
+	}
+
+	// if the snapshot is set to nil, we need to buffer the message
+	if b.snapshot == nil {
+		b.buffer = append(b.buffer, u)
+		b.once.Do(func() {
+			go b.tryFetch()
+		})
+		b.mu.Unlock()
+		return nil
+	}
+
+	// if there is a missing update, we should reset the snapshot and re-fetch the snapshot
+	if u.FirstUpdateID > b.finalUpdateID+1 {
+		// emitReset will reset the once outside the mutex lock section
+		b.resetSnapshot()
+		b.buffer = nil
+		b.buffer = append(b.buffer, u)
+		b.emitReset()
+		b.mu.Unlock()
+		return fmt.Errorf("there is a missing update between %d and %d", u.FirstUpdateID, b.finalUpdateID+1)
+	}
+
+	b.finalUpdateID = u.FinalUpdateID
+	b.EmitPush(u)
+	b.mu.Unlock()
+	return nil
+}
+
+func (b *Buffer) fetchAndPush() error {
+	log.Info("fetching depth snapshot...")
+	book, finalUpdateID, err := b.fetcher()
+	if err != nil {
+		return err
+	}
+
+	log.Infof("fetched depth snapshot, final update id %d", finalUpdateID)
+
+	b.mu.Lock()
+	if len(b.buffer) > 0 {
+		// the snapshot is too early
+		if finalUpdateID < b.buffer[0].FirstUpdateID {
+			b.resetSnapshot()
+			b.emitReset()
+			b.mu.Unlock()
+			return fmt.Errorf("depth final update %d is < the first update id %d", finalUpdateID, b.buffer[0].FirstUpdateID)
+		}
+	}
+
+	var pushUpdates []Update
+	for _, u := range b.buffer {
+		if u.FirstUpdateID > finalUpdateID+1 {
+			b.resetSnapshot()
+			b.emitReset()
+			b.mu.Unlock()
+			return fmt.Errorf("the update id %d > final update id %d + 1", u.FirstUpdateID, finalUpdateID)
+		}
+
+		if u.FirstUpdateID < finalUpdateID+1 {
+			continue
+		}
+
+		pushUpdates = append(pushUpdates, u)
+
+		// update the final update id to the correct final update id
+		finalUpdateID = u.FinalUpdateID
+	}
+
+	// clean the buffer since we have filtered out the buffer we want
+	b.buffer = nil
+
+	// set the final update ID so that we will know if there is an update missing
+	b.finalUpdateID = finalUpdateID
+
+	// set the snapshot
+	b.snapshot = &book
+
+	b.mu.Unlock()
+	b.EmitReady(book, pushUpdates)
+	return nil
+}
+
+func (b *Buffer) tryFetch() {
+	for {
+		err := b.fetchAndPush()
+		if err != nil {
+			log.WithError(err).Errorf("snapshot fetch failed")
+			continue
+		}
+		break
+	}
+}
diff --git a/pkg/depth/buffer_callbacks.go b/pkg/depth/buffer_callbacks.go
new file mode 100644
index 000000000..bf6527166
--- /dev/null
+++ b/pkg/depth/buffer_callbacks.go
@@ -0,0 +1,37 @@
+// Code generated by "callbackgen -type Buffer"; DO NOT EDIT.
+
+package depth
+
+import (
+	"github.com/c9s/bbgo/pkg/types"
+)
+
+func (b *Buffer) OnReset(cb func()) {
+	b.resetCallbacks = append(b.resetCallbacks, cb)
+}
+
+func (b *Buffer) EmitReset() {
+	for _, cb := range b.resetCallbacks {
+		cb()
+	}
+}
+
+func (b *Buffer) OnReady(cb func(snapshot types.SliceOrderBook, updates []Update)) {
+	b.readyCallbacks = append(b.readyCallbacks, cb)
+}
+
+func (b *Buffer) EmitReady(snapshot types.SliceOrderBook, updates []Update) {
+	for _, cb := range b.readyCallbacks {
+		cb(snapshot, updates)
+	}
+}
+
+func (b *Buffer) OnPush(cb func(update Update)) {
+	b.pushCallbacks = append(b.pushCallbacks, cb)
+}
+
+func (b *Buffer) EmitPush(update Update) {
+	for _, cb := range b.pushCallbacks {
+		cb(update)
+	}
+}
diff --git a/pkg/depth/buffer_test.go b/pkg/depth/buffer_test.go
new file mode 100644
index 000000000..e538733db
--- /dev/null
+++ b/pkg/depth/buffer_test.go
@@ -0,0 +1,151 @@
+package depth
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/c9s/bbgo/pkg/fixedpoint"
+	"github.com/c9s/bbgo/pkg/types"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDepthBuffer_ReadyState(t *testing.T) {
+	buf := NewDepthBuffer("", func() (book types.SliceOrderBook, finalID int64, err error) {
+		return types.SliceOrderBook{
+			Bids: types.PriceVolumeSlice{
+				{Price: 100, Volume: 1},
+			},
+			Asks: types.PriceVolumeSlice{
+				{Price: 99, Volume: 1},
+			},
+		}, 33, nil
+	})
+
+	readyC := make(chan struct{})
+	buf.OnReady(func(snapshot types.SliceOrderBook, updates []Update) {
+		assert.Greater(t, len(updates), 33)
+		close(readyC)
+	})
+
+	var updateID int64 = 1
+	for ; updateID < 100; updateID++ {
+		buf.AddUpdate(
+			types.SliceOrderBook{
+				Bids: types.PriceVolumeSlice{
+					{Price: 100, Volume: fixedpoint.Value(updateID)},
+				},
+				Asks: types.PriceVolumeSlice{
+					{Price: 99, Volume: fixedpoint.Value(updateID)},
+				},
+			}, updateID)
+	}
+
+	<-readyC
+}
+
+func TestDepthBuffer_CorruptedUpdateAtTheBeginning(t *testing.T) {
+	// snapshot starts from 30,
+	// the first ready event should have a snapshot(30) and updates (31~50)
+	var snapshotFinalID int64 = 0
+	buf := NewDepthBuffer("", func() (types.SliceOrderBook, int64, error) {
+		snapshotFinalID += 30
+		return types.SliceOrderBook{
+			Bids: types.PriceVolumeSlice{
+				{Price: 100, Volume: 1},
+			},
+			Asks: types.PriceVolumeSlice{
+				{Price: 99, Volume: 1},
+			},
+		}, snapshotFinalID, nil
+	})
+
+	resetC := make(chan struct{}, 1)
+
+	buf.OnReset(func() {
+		resetC <- struct{}{}
+	})
+
+	var updateID int64 = 10
+	for ; updateID < 100; updateID++ {
+		if updateID == 50 {
+			updateID += 5
+		}
+
+		buf.AddUpdate(types.SliceOrderBook{
+			Bids: types.PriceVolumeSlice{
+				{Price: 100, Volume: fixedpoint.Value(updateID)},
+			},
+			Asks: types.PriceVolumeSlice{
+				{Price: 99, Volume: fixedpoint.Value(updateID)},
+			},
+		}, updateID)
+	}
+
+	<-resetC
+}
+
+func TestDepthBuffer_ConcurrentRun(t *testing.T) {
+	var snapshotFinalID int64 = 0
+	buf := NewDepthBuffer("", func() (types.SliceOrderBook, int64, error) {
+		snapshotFinalID += 30
+		time.Sleep(10 * time.Millisecond)
+		return types.SliceOrderBook{
+			Bids: types.PriceVolumeSlice{
+				{Price: 100, Volume: 1},
+			},
+			Asks: types.PriceVolumeSlice{
+				{Price: 99, Volume: 1},
+			},
+		}, snapshotFinalID, nil
+	})
+
+	readyCnt := 0
+	resetCnt := 0
+	pushCnt := 0
+
+	buf.OnPush(func(update Update) {
+		pushCnt++
+	})
+	buf.OnReady(func(snapshot types.SliceOrderBook, updates []Update) {
+		readyCnt++
+		assert.Greater(t, len(updates), 0)
+	})
+	buf.OnReset(func() {
+		resetCnt++
+	})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	ticker := time.NewTicker(time.Millisecond)
+	defer ticker.Stop()
+
+	var updateID int64 = 10
+
+	for {
+		select {
+		case <-ctx.Done():
+			assert.Greater(t, readyCnt, 1)
+			assert.Greater(t, resetCnt, 1)
+			assert.Greater(t, pushCnt, 1)
+			return
+
+		case <-ticker.C:
+			updateID++
+			if updateID%100 == 0 {
+				updateID++
+			}
+
+			buf.AddUpdate(types.SliceOrderBook{
+				Bids: types.PriceVolumeSlice{
+					{Price: 100, Volume: fixedpoint.Value(updateID)},
+				},
+				Asks: types.PriceVolumeSlice{
+					{Price: 99, Volume: fixedpoint.Value(updateID)},
+				},
+			}, updateID)
+
+		}
+	}
+}
diff --git a/pkg/util/reonce.go b/pkg/util/reonce.go
new file mode 100644
index 000000000..f1fae5dda
--- /dev/null
+++ b/pkg/util/reonce.go
@@ -0,0 +1,33 @@
+package util
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+type Reonce struct {
+	done uint32
+	m    sync.Mutex
+}
+
+func (o *Reonce) Reset() {
+	o.m.Lock()
+	atomic.StoreUint32(&o.done, 0)
+	o.m.Unlock()
+}
+
+func (o *Reonce) Do(f func()) {
+	if atomic.LoadUint32(&o.done) == 0 {
+		// Outlined slow-path to allow inlining of the fast-path.
+		o.doSlow(f)
+	}
+}
+
+func (o *Reonce) doSlow(f func()) {
+	o.m.Lock()
+	defer o.m.Unlock()
+	if o.done == 0 {
+		defer atomic.StoreUint32(&o.done, 1)
+		f()
+	}
+}
diff --git a/pkg/util/reonce_test.go b/pkg/util/reonce_test.go
new file mode 100644
index 000000000..647b3335a
--- /dev/null
+++ b/pkg/util/reonce_test.go
@@ -0,0 +1,33 @@
+package util
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestReonce_DoAndReset(t *testing.T) {
+	var cnt = 0
+	var reonce Reonce
+	go reonce.Do(func() {
+			t.Log("once #1")
+			time.Sleep(10 * time.Millisecond)
+			cnt++
+	})
+
+	// make sure it's locked
+	time.Sleep(10 * time.Millisecond)
+	t.Logf("reset")
+	reonce.Reset()
+
+	go reonce.Do(func() {
+			t.Log("once #2")
+			time.Sleep(10 * time.Millisecond)
+			cnt++
+	})
+
+	time.Sleep(time.Second)
+	assert.Equal(t, 2, cnt)
+}
+