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// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package raft
import pb ""
// 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
acks map[uint64]struct{}
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) {
ctx := string(m.Entries[0].Data)
if _, ok := ro.pendingReadIndex[ctx]; ok {
ro.pendingReadIndex[ctx] = &readIndexStatus{index: index, req: m, acks: make(map[uint64]struct{})}
ro.readIndexQueue = append(ro.readIndexQueue, ctx)
// 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(m pb.Message) int {
rs, ok := ro.pendingReadIndex[string(m.Context)]
if !ok {
return 0
rs.acks[m.From] = struct{}{}
// add one to include an ack from local node
return len(rs.acks) + 1
// 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 {
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
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]