diff --git a/vendor/go.etcd.io/etcd/mvcc/key_index.go b/vendor/go.etcd.io/etcd/mvcc/key_index.go
new file mode 100644
index 0000000..cf77cb4
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/key_index.go
@@ -0,0 +1,402 @@
+// Copyright 2015 The etcd Authors
+//
+// 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 mvcc
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+
+	"github.com/google/btree"
+	"go.uber.org/zap"
+)
+
+var (
+	ErrRevisionNotFound = errors.New("mvcc: revision not found")
+)
+
+// keyIndex stores the revisions of a key in the backend.
+// Each keyIndex has at least one key generation.
+// Each generation might have several key versions.
+// Tombstone on a key appends an tombstone version at the end
+// of the current generation and creates a new empty generation.
+// Each version of a key has an index pointing to the backend.
+//
+// For example: put(1.0);put(2.0);tombstone(3.0);put(4.0);tombstone(5.0) on key "foo"
+// generate a keyIndex:
+// key:     "foo"
+// rev: 5
+// generations:
+//    {empty}
+//    {4.0, 5.0(t)}
+//    {1.0, 2.0, 3.0(t)}
+//
+// Compact a keyIndex removes the versions with smaller or equal to
+// rev except the largest one. If the generation becomes empty
+// during compaction, it will be removed. if all the generations get
+// removed, the keyIndex should be removed.
+//
+// For example:
+// compact(2) on the previous example
+// generations:
+//    {empty}
+//    {4.0, 5.0(t)}
+//    {2.0, 3.0(t)}
+//
+// compact(4)
+// generations:
+//    {empty}
+//    {4.0, 5.0(t)}
+//
+// compact(5):
+// generations:
+//    {empty} -> key SHOULD be removed.
+//
+// compact(6):
+// generations:
+//    {empty} -> key SHOULD be removed.
+type keyIndex struct {
+	key         []byte
+	modified    revision // the main rev of the last modification
+	generations []generation
+}
+
+// put puts a revision to the keyIndex.
+func (ki *keyIndex) put(lg *zap.Logger, main int64, sub int64) {
+	rev := revision{main: main, sub: sub}
+
+	if !rev.GreaterThan(ki.modified) {
+		if lg != nil {
+			lg.Panic(
+				"'put' with an unexpected smaller revision",
+				zap.Int64("given-revision-main", rev.main),
+				zap.Int64("given-revision-sub", rev.sub),
+				zap.Int64("modified-revision-main", ki.modified.main),
+				zap.Int64("modified-revision-sub", ki.modified.sub),
+			)
+		} else {
+			plog.Panicf("store.keyindex: put with unexpected smaller revision [%v / %v]", rev, ki.modified)
+		}
+	}
+	if len(ki.generations) == 0 {
+		ki.generations = append(ki.generations, generation{})
+	}
+	g := &ki.generations[len(ki.generations)-1]
+	if len(g.revs) == 0 { // create a new key
+		keysGauge.Inc()
+		g.created = rev
+	}
+	g.revs = append(g.revs, rev)
+	g.ver++
+	ki.modified = rev
+}
+
+func (ki *keyIndex) restore(lg *zap.Logger, created, modified revision, ver int64) {
+	if len(ki.generations) != 0 {
+		if lg != nil {
+			lg.Panic(
+				"'restore' got an unexpected non-empty generations",
+				zap.Int("generations-size", len(ki.generations)),
+			)
+		} else {
+			plog.Panicf("store.keyindex: cannot restore non-empty keyIndex")
+		}
+	}
+
+	ki.modified = modified
+	g := generation{created: created, ver: ver, revs: []revision{modified}}
+	ki.generations = append(ki.generations, g)
+	keysGauge.Inc()
+}
+
+// tombstone puts a revision, pointing to a tombstone, to the keyIndex.
+// It also creates a new empty generation in the keyIndex.
+// It returns ErrRevisionNotFound when tombstone on an empty generation.
+func (ki *keyIndex) tombstone(lg *zap.Logger, main int64, sub int64) error {
+	if ki.isEmpty() {
+		if lg != nil {
+			lg.Panic(
+				"'tombstone' got an unexpected empty keyIndex",
+				zap.String("key", string(ki.key)),
+			)
+		} else {
+			plog.Panicf("store.keyindex: unexpected tombstone on empty keyIndex %s", string(ki.key))
+		}
+	}
+	if ki.generations[len(ki.generations)-1].isEmpty() {
+		return ErrRevisionNotFound
+	}
+	ki.put(lg, main, sub)
+	ki.generations = append(ki.generations, generation{})
+	keysGauge.Dec()
+	return nil
+}
+
+// get gets the modified, created revision and version of the key that satisfies the given atRev.
+// Rev must be higher than or equal to the given atRev.
+func (ki *keyIndex) get(lg *zap.Logger, atRev int64) (modified, created revision, ver int64, err error) {
+	if ki.isEmpty() {
+		if lg != nil {
+			lg.Panic(
+				"'get' got an unexpected empty keyIndex",
+				zap.String("key", string(ki.key)),
+			)
+		} else {
+			plog.Panicf("store.keyindex: unexpected get on empty keyIndex %s", string(ki.key))
+		}
+	}
+	g := ki.findGeneration(atRev)
+	if g.isEmpty() {
+		return revision{}, revision{}, 0, ErrRevisionNotFound
+	}
+
+	n := g.walk(func(rev revision) bool { return rev.main > atRev })
+	if n != -1 {
+		return g.revs[n], g.created, g.ver - int64(len(g.revs)-n-1), nil
+	}
+
+	return revision{}, revision{}, 0, ErrRevisionNotFound
+}
+
+// since returns revisions since the given rev. Only the revision with the
+// largest sub revision will be returned if multiple revisions have the same
+// main revision.
+func (ki *keyIndex) since(lg *zap.Logger, rev int64) []revision {
+	if ki.isEmpty() {
+		if lg != nil {
+			lg.Panic(
+				"'since' got an unexpected empty keyIndex",
+				zap.String("key", string(ki.key)),
+			)
+		} else {
+			plog.Panicf("store.keyindex: unexpected get on empty keyIndex %s", string(ki.key))
+		}
+	}
+	since := revision{rev, 0}
+	var gi int
+	// find the generations to start checking
+	for gi = len(ki.generations) - 1; gi > 0; gi-- {
+		g := ki.generations[gi]
+		if g.isEmpty() {
+			continue
+		}
+		if since.GreaterThan(g.created) {
+			break
+		}
+	}
+
+	var revs []revision
+	var last int64
+	for ; gi < len(ki.generations); gi++ {
+		for _, r := range ki.generations[gi].revs {
+			if since.GreaterThan(r) {
+				continue
+			}
+			if r.main == last {
+				// replace the revision with a new one that has higher sub value,
+				// because the original one should not be seen by external
+				revs[len(revs)-1] = r
+				continue
+			}
+			revs = append(revs, r)
+			last = r.main
+		}
+	}
+	return revs
+}
+
+// compact compacts a keyIndex by removing the versions with smaller or equal
+// revision than the given atRev except the largest one (If the largest one is
+// a tombstone, it will not be kept).
+// If a generation becomes empty during compaction, it will be removed.
+func (ki *keyIndex) compact(lg *zap.Logger, atRev int64, available map[revision]struct{}) {
+	if ki.isEmpty() {
+		if lg != nil {
+			lg.Panic(
+				"'compact' got an unexpected empty keyIndex",
+				zap.String("key", string(ki.key)),
+			)
+		} else {
+			plog.Panicf("store.keyindex: unexpected compact on empty keyIndex %s", string(ki.key))
+		}
+	}
+
+	genIdx, revIndex := ki.doCompact(atRev, available)
+
+	g := &ki.generations[genIdx]
+	if !g.isEmpty() {
+		// remove the previous contents.
+		if revIndex != -1 {
+			g.revs = g.revs[revIndex:]
+		}
+		// remove any tombstone
+		if len(g.revs) == 1 && genIdx != len(ki.generations)-1 {
+			delete(available, g.revs[0])
+			genIdx++
+		}
+	}
+
+	// remove the previous generations.
+	ki.generations = ki.generations[genIdx:]
+}
+
+// keep finds the revision to be kept if compact is called at given atRev.
+func (ki *keyIndex) keep(atRev int64, available map[revision]struct{}) {
+	if ki.isEmpty() {
+		return
+	}
+
+	genIdx, revIndex := ki.doCompact(atRev, available)
+	g := &ki.generations[genIdx]
+	if !g.isEmpty() {
+		// remove any tombstone
+		if revIndex == len(g.revs)-1 && genIdx != len(ki.generations)-1 {
+			delete(available, g.revs[revIndex])
+		}
+	}
+}
+
+func (ki *keyIndex) doCompact(atRev int64, available map[revision]struct{}) (genIdx int, revIndex int) {
+	// walk until reaching the first revision smaller or equal to "atRev",
+	// and add the revision to the available map
+	f := func(rev revision) bool {
+		if rev.main <= atRev {
+			available[rev] = struct{}{}
+			return false
+		}
+		return true
+	}
+
+	genIdx, g := 0, &ki.generations[0]
+	// find first generation includes atRev or created after atRev
+	for genIdx < len(ki.generations)-1 {
+		if tomb := g.revs[len(g.revs)-1].main; tomb > atRev {
+			break
+		}
+		genIdx++
+		g = &ki.generations[genIdx]
+	}
+
+	revIndex = g.walk(f)
+
+	return genIdx, revIndex
+}
+
+func (ki *keyIndex) isEmpty() bool {
+	return len(ki.generations) == 1 && ki.generations[0].isEmpty()
+}
+
+// findGeneration finds out the generation of the keyIndex that the
+// given rev belongs to. If the given rev is at the gap of two generations,
+// which means that the key does not exist at the given rev, it returns nil.
+func (ki *keyIndex) findGeneration(rev int64) *generation {
+	lastg := len(ki.generations) - 1
+	cg := lastg
+
+	for cg >= 0 {
+		if len(ki.generations[cg].revs) == 0 {
+			cg--
+			continue
+		}
+		g := ki.generations[cg]
+		if cg != lastg {
+			if tomb := g.revs[len(g.revs)-1].main; tomb <= rev {
+				return nil
+			}
+		}
+		if g.revs[0].main <= rev {
+			return &ki.generations[cg]
+		}
+		cg--
+	}
+	return nil
+}
+
+func (ki *keyIndex) Less(b btree.Item) bool {
+	return bytes.Compare(ki.key, b.(*keyIndex).key) == -1
+}
+
+func (ki *keyIndex) equal(b *keyIndex) bool {
+	if !bytes.Equal(ki.key, b.key) {
+		return false
+	}
+	if ki.modified != b.modified {
+		return false
+	}
+	if len(ki.generations) != len(b.generations) {
+		return false
+	}
+	for i := range ki.generations {
+		ag, bg := ki.generations[i], b.generations[i]
+		if !ag.equal(bg) {
+			return false
+		}
+	}
+	return true
+}
+
+func (ki *keyIndex) String() string {
+	var s string
+	for _, g := range ki.generations {
+		s += g.String()
+	}
+	return s
+}
+
+// generation contains multiple revisions of a key.
+type generation struct {
+	ver     int64
+	created revision // when the generation is created (put in first revision).
+	revs    []revision
+}
+
+func (g *generation) isEmpty() bool { return g == nil || len(g.revs) == 0 }
+
+// walk walks through the revisions in the generation in descending order.
+// It passes the revision to the given function.
+// walk returns until: 1. it finishes walking all pairs 2. the function returns false.
+// walk returns the position at where it stopped. If it stopped after
+// finishing walking, -1 will be returned.
+func (g *generation) walk(f func(rev revision) bool) int {
+	l := len(g.revs)
+	for i := range g.revs {
+		ok := f(g.revs[l-i-1])
+		if !ok {
+			return l - i - 1
+		}
+	}
+	return -1
+}
+
+func (g *generation) String() string {
+	return fmt.Sprintf("g: created[%d] ver[%d], revs %#v\n", g.created, g.ver, g.revs)
+}
+
+func (g generation) equal(b generation) bool {
+	if g.ver != b.ver {
+		return false
+	}
+	if len(g.revs) != len(b.revs) {
+		return false
+	}
+
+	for i := range g.revs {
+		ar, br := g.revs[i], b.revs[i]
+		if ar != br {
+			return false
+		}
+	}
+	return true
+}