diff --git a/vendor/go.etcd.io/etcd/raft/read_only.go b/vendor/go.etcd.io/etcd/raft/read_only.go
new file mode 100644
index 0000000..6987f1b
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/raft/read_only.go
@@ -0,0 +1,121 @@
+// Copyright 2016 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 raft
+
+import pb "go.etcd.io/etcd/raft/raftpb"
+
+// ReadState provides state for read only query.
+// It's caller's responsibility to call ReadIndex first before getting
+// this state from ready, it's also caller's duty to differentiate if this
+// state is what it requests through RequestCtx, eg. given a unique id as
+// RequestCtx
+type ReadState struct {
+	Index      uint64
+	RequestCtx []byte
+}
+
+type readIndexStatus struct {
+	req   pb.Message
+	index uint64
+	// NB: this never records 'false', but it's more convenient to use this
+	// instead of a map[uint64]struct{} due to the API of quorum.VoteResult. If
+	// this becomes performance sensitive enough (doubtful), quorum.VoteResult
+	// can change to an API that is closer to that of CommittedIndex.
+	acks map[uint64]bool
+}
+
+type readOnly struct {
+	option           ReadOnlyOption
+	pendingReadIndex map[string]*readIndexStatus
+	readIndexQueue   []string
+}
+
+func newReadOnly(option ReadOnlyOption) *readOnly {
+	return &readOnly{
+		option:           option,
+		pendingReadIndex: make(map[string]*readIndexStatus),
+	}
+}
+
+// addRequest adds a read only reuqest into readonly struct.
+// `index` is the commit index of the raft state machine when it received
+// the read only request.
+// `m` is the original read only request message from the local or remote node.
+func (ro *readOnly) addRequest(index uint64, m pb.Message) {
+	s := string(m.Entries[0].Data)
+	if _, ok := ro.pendingReadIndex[s]; ok {
+		return
+	}
+	ro.pendingReadIndex[s] = &readIndexStatus{index: index, req: m, acks: make(map[uint64]bool)}
+	ro.readIndexQueue = append(ro.readIndexQueue, s)
+}
+
+// recvAck notifies the readonly struct that the raft state machine received
+// an acknowledgment of the heartbeat that attached with the read only request
+// context.
+func (ro *readOnly) recvAck(id uint64, context []byte) map[uint64]bool {
+	rs, ok := ro.pendingReadIndex[string(context)]
+	if !ok {
+		return nil
+	}
+
+	rs.acks[id] = true
+	return rs.acks
+}
+
+// advance advances the read only request queue kept by the readonly struct.
+// It dequeues the requests until it finds the read only request that has
+// the same context as the given `m`.
+func (ro *readOnly) advance(m pb.Message) []*readIndexStatus {
+	var (
+		i     int
+		found bool
+	)
+
+	ctx := string(m.Context)
+	rss := []*readIndexStatus{}
+
+	for _, okctx := range ro.readIndexQueue {
+		i++
+		rs, ok := ro.pendingReadIndex[okctx]
+		if !ok {
+			panic("cannot find corresponding read state from pending map")
+		}
+		rss = append(rss, rs)
+		if okctx == ctx {
+			found = true
+			break
+		}
+	}
+
+	if found {
+		ro.readIndexQueue = ro.readIndexQueue[i:]
+		for _, rs := range rss {
+			delete(ro.pendingReadIndex, string(rs.req.Entries[0].Data))
+		}
+		return rss
+	}
+
+	return nil
+}
+
+// lastPendingRequestCtx returns the context of the last pending read only
+// request in readonly struct.
+func (ro *readOnly) lastPendingRequestCtx() string {
+	if len(ro.readIndexQueue) == 0 {
+		return ""
+	}
+	return ro.readIndexQueue[len(ro.readIndexQueue)-1]
+}