vendor/go.etcd.io/etcd/raft/node.go - voltha-go - Gitiles

 // 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 raft

 import (
 	"context"
 	"errors"

 	pb "go.etcd.io/etcd/raft/raftpb"
 )

 type SnapshotStatus int

 const (
 	SnapshotFinish  SnapshotStatus = 1
 	SnapshotFailure SnapshotStatus = 2
 )

 var (
 	emptyState = pb.HardState{}

 	// ErrStopped is returned by methods on Nodes that have been stopped.
 	ErrStopped = errors.New("raft: stopped")
 )

 // SoftState provides state that is useful for logging and debugging.
 // The state is volatile and does not need to be persisted to the WAL.
 type SoftState struct {
 	Lead      uint64 // must use atomic operations to access; keep 64-bit aligned.
 	RaftState StateType
 }

 func (a *SoftState) equal(b *SoftState) bool {
 	return a.Lead == b.Lead && a.RaftState == b.RaftState
 }

 // Ready encapsulates the entries and messages that are ready to read,
 // be saved to stable storage, committed or sent to other peers.
 // All fields in Ready are read-only.
 type Ready struct {
 	// The current volatile state of a Node.
 	// SoftState will be nil if there is no update.
 	// It is not required to consume or store SoftState.
 	*SoftState

 	// The current state of a Node to be saved to stable storage BEFORE
 	// Messages are sent.
 	// HardState will be equal to empty state if there is no update.
 	pb.HardState

 	// ReadStates can be used for node to serve linearizable read requests locally
 	// when its applied index is greater than the index in ReadState.
 	// Note that the readState will be returned when raft receives msgReadIndex.
 	// The returned is only valid for the request that requested to read.
 	ReadStates []ReadState

 	// Entries specifies entries to be saved to stable storage BEFORE
 	// Messages are sent.
 	Entries []pb.Entry

 	// Snapshot specifies the snapshot to be saved to stable storage.
 	Snapshot pb.Snapshot

 	// CommittedEntries specifies entries to be committed to a
 	// store/state-machine. These have previously been committed to stable
 	// store.
 	CommittedEntries []pb.Entry

 	// Messages specifies outbound messages to be sent AFTER Entries are
 	// committed to stable storage.
 	// If it contains a MsgSnap message, the application MUST report back to raft
 	// when the snapshot has been received or has failed by calling ReportSnapshot.
 	Messages []pb.Message

 	// MustSync indicates whether the HardState and Entries must be synchronously
 	// written to disk or if an asynchronous write is permissible.
 	MustSync bool
 }

 func isHardStateEqual(a, b pb.HardState) bool {
 	return a.Term == b.Term && a.Vote == b.Vote && a.Commit == b.Commit
 }

 // IsEmptyHardState returns true if the given HardState is empty.
 func IsEmptyHardState(st pb.HardState) bool {
 	return isHardStateEqual(st, emptyState)
 }

 // IsEmptySnap returns true if the given Snapshot is empty.
 func IsEmptySnap(sp pb.Snapshot) bool {
 	return sp.Metadata.Index == 0
 }

 func (rd Ready) containsUpdates() bool {
 	return rd.SoftState != nil || !IsEmptyHardState(rd.HardState) ||
 		!IsEmptySnap(rd.Snapshot) || len(rd.Entries) > 0 ||
 		len(rd.CommittedEntries) > 0 || len(rd.Messages) > 0 || len(rd.ReadStates) != 0
 }

 // appliedCursor extracts from the Ready the highest index the client has
 // applied (once the Ready is confirmed via Advance). If no information is
 // contained in the Ready, returns zero.
 func (rd Ready) appliedCursor() uint64 {
 	if n := len(rd.CommittedEntries); n > 0 {
 		return rd.CommittedEntries[n-1].Index
 	}
 	if index := rd.Snapshot.Metadata.Index; index > 0 {
 		return index
 	}
 	return 0
 }

 // Node represents a node in a raft cluster.
 type Node interface {
 	// Tick increments the internal logical clock for the Node by a single tick. Election
 	// timeouts and heartbeat timeouts are in units of ticks.
 	Tick()
 	// Campaign causes the Node to transition to candidate state and start campaigning to become leader.
 	Campaign(ctx context.Context) error
 	// Propose proposes that data be appended to the log. Note that proposals can be lost without
 	// notice, therefore it is user's job to ensure proposal retries.
 	Propose(ctx context.Context, data []byte) error
 	// ProposeConfChange proposes config change.
 	// At most one ConfChange can be in the process of going through consensus.
 	// Application needs to call ApplyConfChange when applying EntryConfChange type entry.
 	ProposeConfChange(ctx context.Context, cc pb.ConfChange) error
 	// Step advances the state machine using the given message. ctx.Err() will be returned, if any.
 	Step(ctx context.Context, msg pb.Message) error

 	// Ready returns a channel that returns the current point-in-time state.
 	// Users of the Node must call Advance after retrieving the state returned by Ready.
 	//
 	// NOTE: No committed entries from the next Ready may be applied until all committed entries
 	// and snapshots from the previous one have finished.
 	Ready() <-chan Ready

 	// Advance notifies the Node that the application has saved progress up to the last Ready.
 	// It prepares the node to return the next available Ready.
 	//
 	// The application should generally call Advance after it applies the entries in last Ready.
 	//
 	// However, as an optimization, the application may call Advance while it is applying the
 	// commands. For example. when the last Ready contains a snapshot, the application might take
 	// a long time to apply the snapshot data. To continue receiving Ready without blocking raft
 	// progress, it can call Advance before finishing applying the last ready.
 	Advance()
 	// ApplyConfChange applies config change to the local node.
 	// Returns an opaque ConfState protobuf which must be recorded
 	// in snapshots. Will never return nil; it returns a pointer only
 	// to match MemoryStorage.Compact.
 	ApplyConfChange(cc pb.ConfChange) *pb.ConfState

 	// TransferLeadership attempts to transfer leadership to the given transferee.
 	TransferLeadership(ctx context.Context, lead, transferee uint64)

 	// ReadIndex request a read state. The read state will be set in the ready.
 	// Read state has a read index. Once the application advances further than the read
 	// index, any linearizable read requests issued before the read request can be
 	// processed safely. The read state will have the same rctx attached.
 	ReadIndex(ctx context.Context, rctx []byte) error

 	// Status returns the current status of the raft state machine.
 	Status() Status
 	// ReportUnreachable reports the given node is not reachable for the last send.
 	ReportUnreachable(id uint64)
 	// ReportSnapshot reports the status of the sent snapshot. The id is the raft ID of the follower
 	// who is meant to receive the snapshot, and the status is SnapshotFinish or SnapshotFailure.
 	// Calling ReportSnapshot with SnapshotFinish is a no-op. But, any failure in applying a
 	// snapshot (for e.g., while streaming it from leader to follower), should be reported to the
 	// leader with SnapshotFailure. When leader sends a snapshot to a follower, it pauses any raft
 	// log probes until the follower can apply the snapshot and advance its state. If the follower
 	// can't do that, for e.g., due to a crash, it could end up in a limbo, never getting any
 	// updates from the leader. Therefore, it is crucial that the application ensures that any
 	// failure in snapshot sending is caught and reported back to the leader; so it can resume raft
 	// log probing in the follower.
 	ReportSnapshot(id uint64, status SnapshotStatus)
 	// Stop performs any necessary termination of the Node.
 	Stop()
 }

 type Peer struct {
 	ID      uint64
 	Context []byte
 }

 // StartNode returns a new Node given configuration and a list of raft peers.
 // It appends a ConfChangeAddNode entry for each given peer to the initial log.
 func StartNode(c *Config, peers []Peer) Node {
 	r := newRaft(c)
 	// become the follower at term 1 and apply initial configuration
 	// entries of term 1
 	r.becomeFollower(1, None)
 	for _, peer := range peers {
 		cc := pb.ConfChange{Type: pb.ConfChangeAddNode, NodeID: peer.ID, Context: peer.Context}
 		d, err := cc.Marshal()
 		if err != nil {
 			panic("unexpected marshal error")
 		}
 		e := pb.Entry{Type: pb.EntryConfChange, Term: 1, Index: r.raftLog.lastIndex() + 1, Data: d}
 		r.raftLog.append(e)
 	}
 	// Mark these initial entries as committed.
 	// TODO(bdarnell): These entries are still unstable; do we need to preserve
 	// the invariant that committed < unstable?
 	r.raftLog.committed = r.raftLog.lastIndex()
 	// Now apply them, mainly so that the application can call Campaign
 	// immediately after StartNode in tests. Note that these nodes will
 	// be added to raft twice: here and when the application's Ready
 	// loop calls ApplyConfChange. The calls to addNode must come after
 	// all calls to raftLog.append so progress.next is set after these
 	// bootstrapping entries (it is an error if we try to append these
 	// entries since they have already been committed).
 	// We do not set raftLog.applied so the application will be able
 	// to observe all conf changes via Ready.CommittedEntries.
 	for _, peer := range peers {
 		r.addNode(peer.ID)
 	}

 	n := newNode()
 	n.logger = c.Logger
 	go n.run(r)
 	return &n
 }

 // RestartNode is similar to StartNode but does not take a list of peers.
 // The current membership of the cluster will be restored from the Storage.
 // If the caller has an existing state machine, pass in the last log index that
 // has been applied to it; otherwise use zero.
 func RestartNode(c *Config) Node {
 	r := newRaft(c)

 	n := newNode()
 	n.logger = c.Logger
 	go n.run(r)
 	return &n
 }

 type msgWithResult struct {
 	m      pb.Message
 	result chan error
 }

 // node is the canonical implementation of the Node interface
 type node struct {
 	propc      chan msgWithResult
 	recvc      chan pb.Message
 	confc      chan pb.ConfChange
 	confstatec chan pb.ConfState
 	readyc     chan Ready
 	advancec   chan struct{}
 	tickc      chan struct{}
 	done       chan struct{}
 	stop       chan struct{}
 	status     chan chan Status

 	logger Logger
 }

 func newNode() node {
 	return node{
 		propc:      make(chan msgWithResult),
 		recvc:      make(chan pb.Message),
 		confc:      make(chan pb.ConfChange),
 		confstatec: make(chan pb.ConfState),
 		readyc:     make(chan Ready),
 		advancec:   make(chan struct{}),
 		// make tickc a buffered chan, so raft node can buffer some ticks when the node
 		// is busy processing raft messages. Raft node will resume process buffered
 		// ticks when it becomes idle.
 		tickc:  make(chan struct{}, 128),
 		done:   make(chan struct{}),
 		stop:   make(chan struct{}),
 		status: make(chan chan Status),
 	}
 }

 func (n *node) Stop() {
 	select {
 	case n.stop <- struct{}{}:
 		// Not already stopped, so trigger it
 	case <-n.done:
 		// Node has already been stopped - no need to do anything
 		return
 	}
 	// Block until the stop has been acknowledged by run()
 	<-n.done
 }

 func (n *node) run(r *raft) {
 	var propc chan msgWithResult
 	var readyc chan Ready
 	var advancec chan struct{}
 	var prevLastUnstablei, prevLastUnstablet uint64
 	var havePrevLastUnstablei bool
 	var prevSnapi uint64
 	var applyingToI uint64
 	var rd Ready

 	lead := None
 	prevSoftSt := r.softState()
 	prevHardSt := emptyState

 	for {
 		if advancec != nil {
 			readyc = nil
 		} else {
 			rd = newReady(r, prevSoftSt, prevHardSt)
 			if rd.containsUpdates() {
 				readyc = n.readyc
 			} else {
 				readyc = nil
 			}
 		}

 		if lead != r.lead {
 			if r.hasLeader() {
 				if lead == None {
 					r.logger.Infof("raft.node: %x elected leader %x at term %d", r.id, r.lead, r.Term)
 				} else {
 					r.logger.Infof("raft.node: %x changed leader from %x to %x at term %d", r.id, lead, r.lead, r.Term)
 				}
 				propc = n.propc
 			} else {
 				r.logger.Infof("raft.node: %x lost leader %x at term %d", r.id, lead, r.Term)
 				propc = nil
 			}
 			lead = r.lead
 		}

 		select {
 		// TODO: maybe buffer the config propose if there exists one (the way
 		// described in raft dissertation)
 		// Currently it is dropped in Step silently.
 		case pm := <-propc:
 			m := pm.m
 			m.From = r.id
 			err := r.Step(m)
 			if pm.result != nil {
 				pm.result <- err
 				close(pm.result)
 			}
 		case m := <-n.recvc:
 			// filter out response message from unknown From.
 			if pr := r.getProgress(m.From); pr != nil || !IsResponseMsg(m.Type) {
 				r.Step(m)
 			}
 		case cc := <-n.confc:
 			if cc.NodeID == None {
 				select {
 				case n.confstatec <- pb.ConfState{
 					Nodes:    r.nodes(),
 					Learners: r.learnerNodes()}:
 				case <-n.done:
 				}
 				break
 			}
 			switch cc.Type {
 			case pb.ConfChangeAddNode:
 				r.addNode(cc.NodeID)
 			case pb.ConfChangeAddLearnerNode:
 				r.addLearner(cc.NodeID)
 			case pb.ConfChangeRemoveNode:
 				// block incoming proposal when local node is
 				// removed
 				if cc.NodeID == r.id {
 					propc = nil
 				}
 				r.removeNode(cc.NodeID)
 			case pb.ConfChangeUpdateNode:
 			default:
 				panic("unexpected conf type")
 			}
 			select {
 			case n.confstatec <- pb.ConfState{
 				Nodes:    r.nodes(),
 				Learners: r.learnerNodes()}:
 			case <-n.done:
 			}
 		case <-n.tickc:
 			r.tick()
 		case readyc <- rd:
 			if rd.SoftState != nil {
 				prevSoftSt = rd.SoftState
 			}
 			if len(rd.Entries) > 0 {
 				prevLastUnstablei = rd.Entries[len(rd.Entries)-1].Index
 				prevLastUnstablet = rd.Entries[len(rd.Entries)-1].Term
 				havePrevLastUnstablei = true
 			}
 			if !IsEmptyHardState(rd.HardState) {
 				prevHardSt = rd.HardState
 			}
 			if !IsEmptySnap(rd.Snapshot) {
 				prevSnapi = rd.Snapshot.Metadata.Index
 			}
 			if index := rd.appliedCursor(); index != 0 {
 				applyingToI = index
 			}

 			r.msgs = nil
 			r.readStates = nil
 			r.reduceUncommittedSize(rd.CommittedEntries)
 			advancec = n.advancec
 		case <-advancec:
 			if applyingToI != 0 {
 				r.raftLog.appliedTo(applyingToI)
 				applyingToI = 0
 			}
 			if havePrevLastUnstablei {
 				r.raftLog.stableTo(prevLastUnstablei, prevLastUnstablet)
 				havePrevLastUnstablei = false
 			}
 			r.raftLog.stableSnapTo(prevSnapi)
 			advancec = nil
 		case c := <-n.status:
 			c <- getStatus(r)
 		case <-n.stop:
 			close(n.done)
 			return
 		}
 	}
 }

 // Tick increments the internal logical clock for this Node. Election timeouts
 // and heartbeat timeouts are in units of ticks.
 func (n *node) Tick() {
 	select {
 	case n.tickc <- struct{}{}:
 	case <-n.done:
 	default:
 		n.logger.Warningf("A tick missed to fire. Node blocks too long!")
 	}
 }

 func (n *node) Campaign(ctx context.Context) error { return n.step(ctx, pb.Message{Type: pb.MsgHup}) }

 func (n *node) Propose(ctx context.Context, data []byte) error {
 	return n.stepWait(ctx, pb.Message{Type: pb.MsgProp, Entries: []pb.Entry{{Data: data}}})
 }

 func (n *node) Step(ctx context.Context, m pb.Message) error {
 	// ignore unexpected local messages receiving over network
 	if IsLocalMsg(m.Type) {
 		// TODO: return an error?
 		return nil
 	}
 	return n.step(ctx, m)
 }

 func (n *node) ProposeConfChange(ctx context.Context, cc pb.ConfChange) error {
 	data, err := cc.Marshal()
 	if err != nil {
 		return err
 	}
 	return n.Step(ctx, pb.Message{Type: pb.MsgProp, Entries: []pb.Entry{{Type: pb.EntryConfChange, Data: data}}})
 }

 func (n *node) step(ctx context.Context, m pb.Message) error {
 	return n.stepWithWaitOption(ctx, m, false)
 }

 func (n *node) stepWait(ctx context.Context, m pb.Message) error {
 	return n.stepWithWaitOption(ctx, m, true)
 }

 // Step advances the state machine using msgs. The ctx.Err() will be returned,
 // if any.
 func (n *node) stepWithWaitOption(ctx context.Context, m pb.Message, wait bool) error {
 	if m.Type != pb.MsgProp {
 		select {
 		case n.recvc <- m:
 			return nil
 		case <-ctx.Done():
 			return ctx.Err()
 		case <-n.done:
 			return ErrStopped
 		}
 	}
 	ch := n.propc
 	pm := msgWithResult{m: m}
 	if wait {
 		pm.result = make(chan error, 1)
 	}
 	select {
 	case ch <- pm:
 		if !wait {
 			return nil
 		}
 	case <-ctx.Done():
 		return ctx.Err()
 	case <-n.done:
 		return ErrStopped
 	}
 	select {
 	case err := <-pm.result:
 		if err != nil {
 			return err
 		}
 	case <-ctx.Done():
 		return ctx.Err()
 	case <-n.done:
 		return ErrStopped
 	}
 	return nil
 }

 func (n *node) Ready() <-chan Ready { return n.readyc }

 func (n *node) Advance() {
 	select {
 	case n.advancec <- struct{}{}:
 	case <-n.done:
 	}
 }

 func (n *node) ApplyConfChange(cc pb.ConfChange) *pb.ConfState {
 	var cs pb.ConfState
 	select {
 	case n.confc <- cc:
 	case <-n.done:
 	}
 	select {
 	case cs = <-n.confstatec:
 	case <-n.done:
 	}
 	return &cs
 }

 func (n *node) Status() Status {
 	c := make(chan Status)
 	select {
 	case n.status <- c:
 		return <-c
 	case <-n.done:
 		return Status{}
 	}
 }

 func (n *node) ReportUnreachable(id uint64) {
 	select {
 	case n.recvc <- pb.Message{Type: pb.MsgUnreachable, From: id}:
 	case <-n.done:
 	}
 }

 func (n *node) ReportSnapshot(id uint64, status SnapshotStatus) {
 	rej := status == SnapshotFailure

 	select {
 	case n.recvc <- pb.Message{Type: pb.MsgSnapStatus, From: id, Reject: rej}:
 	case <-n.done:
 	}
 }

 func (n *node) TransferLeadership(ctx context.Context, lead, transferee uint64) {
 	select {
 	// manually set 'from' and 'to', so that leader can voluntarily transfers its leadership
 	case n.recvc <- pb.Message{Type: pb.MsgTransferLeader, From: transferee, To: lead}:
 	case <-n.done:
 	case <-ctx.Done():
 	}
 }

 func (n *node) ReadIndex(ctx context.Context, rctx []byte) error {
 	return n.step(ctx, pb.Message{Type: pb.MsgReadIndex, Entries: []pb.Entry{{Data: rctx}}})
 }

 func newReady(r *raft, prevSoftSt *SoftState, prevHardSt pb.HardState) Ready {
 	rd := Ready{
 		Entries:          r.raftLog.unstableEntries(),
 		CommittedEntries: r.raftLog.nextEnts(),
 		Messages:         r.msgs,
 	}
 	if softSt := r.softState(); !softSt.equal(prevSoftSt) {
 		rd.SoftState = softSt
 	}
 	if hardSt := r.hardState(); !isHardStateEqual(hardSt, prevHardSt) {
 		rd.HardState = hardSt
 	}
 	if r.raftLog.unstable.snapshot != nil {
 		rd.Snapshot = *r.raftLog.unstable.snapshot
 	}
 	if len(r.readStates) != 0 {
 		rd.ReadStates = r.readStates
 	}
 	rd.MustSync = MustSync(r.hardState(), prevHardSt, len(rd.Entries))
 	return rd
 }

 // MustSync returns true if the hard state and count of Raft entries indicate
 // that a synchronous write to persistent storage is required.
 func MustSync(st, prevst pb.HardState, entsnum int) bool {
 	// Persistent state on all servers:
 	// (Updated on stable storage before responding to RPCs)
 	// currentTerm
 	// votedFor
 	// log entries[]
 	return entsnum != 0 || st.Vote != prevst.Vote || st.Term != prevst.Term
 }
	// 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 raft

	import (
	"context"
	"errors"

	pb "go.etcd.io/etcd/raft/raftpb"
	)

	type SnapshotStatus int

	const (
	SnapshotFinish SnapshotStatus = 1
	SnapshotFailure SnapshotStatus = 2
	)

	var (
	emptyState = pb.HardState{}

	// ErrStopped is returned by methods on Nodes that have been stopped.
	ErrStopped = errors.New("raft: stopped")
	)

	// SoftState provides state that is useful for logging and debugging.
	// The state is volatile and does not need to be persisted to the WAL.
	type SoftState struct {
	Lead uint64 // must use atomic operations to access; keep 64-bit aligned.
	RaftState StateType
	}

	func (a SoftState) equal(b SoftState) bool {
	return a.Lead == b.Lead && a.RaftState == b.RaftState
	}

	// Ready encapsulates the entries and messages that are ready to read,
	// be saved to stable storage, committed or sent to other peers.
	// All fields in Ready are read-only.
	type Ready struct {
	// The current volatile state of a Node.
	// SoftState will be nil if there is no update.
	// It is not required to consume or store SoftState.
	*SoftState

	// The current state of a Node to be saved to stable storage BEFORE
	// Messages are sent.
	// HardState will be equal to empty state if there is no update.
	pb.HardState

	// ReadStates can be used for node to serve linearizable read requests locally
	// when its applied index is greater than the index in ReadState.
	// Note that the readState will be returned when raft receives msgReadIndex.
	// The returned is only valid for the request that requested to read.
	ReadStates []ReadState

	// Entries specifies entries to be saved to stable storage BEFORE
	// Messages are sent.
	Entries []pb.Entry

	// Snapshot specifies the snapshot to be saved to stable storage.
	Snapshot pb.Snapshot

	// CommittedEntries specifies entries to be committed to a
	// store/state-machine. These have previously been committed to stable
	// store.
	CommittedEntries []pb.Entry

	// Messages specifies outbound messages to be sent AFTER Entries are
	// committed to stable storage.
	// If it contains a MsgSnap message, the application MUST report back to raft
	// when the snapshot has been received or has failed by calling ReportSnapshot.
	Messages []pb.Message

	// MustSync indicates whether the HardState and Entries must be synchronously
	// written to disk or if an asynchronous write is permissible.
	MustSync bool
	}

	func isHardStateEqual(a, b pb.HardState) bool {
	return a.Term == b.Term && a.Vote == b.Vote && a.Commit == b.Commit
	}

	// IsEmptyHardState returns true if the given HardState is empty.
	func IsEmptyHardState(st pb.HardState) bool {
	return isHardStateEqual(st, emptyState)
	}

	// IsEmptySnap returns true if the given Snapshot is empty.
	func IsEmptySnap(sp pb.Snapshot) bool {
	return sp.Metadata.Index == 0
	}

	func (rd Ready) containsUpdates() bool {
	return rd.SoftState != nil \|\| !IsEmptyHardState(rd.HardState) \|\|
	!IsEmptySnap(rd.Snapshot) \|\| len(rd.Entries) > 0 \|\|
	len(rd.CommittedEntries) > 0 \|\| len(rd.Messages) > 0 \|\| len(rd.ReadStates) != 0
	}

	// appliedCursor extracts from the Ready the highest index the client has
	// applied (once the Ready is confirmed via Advance). If no information is
	// contained in the Ready, returns zero.
	func (rd Ready) appliedCursor() uint64 {
	if n := len(rd.CommittedEntries); n > 0 {
	return rd.CommittedEntries[n-1].Index
	}
	if index := rd.Snapshot.Metadata.Index; index > 0 {
	return index
	}
	return 0
	}

	// Node represents a node in a raft cluster.
	type Node interface {
	// Tick increments the internal logical clock for the Node by a single tick. Election
	// timeouts and heartbeat timeouts are in units of ticks.
	Tick()
	// Campaign causes the Node to transition to candidate state and start campaigning to become leader.
	Campaign(ctx context.Context) error
	// Propose proposes that data be appended to the log. Note that proposals can be lost without
	// notice, therefore it is user's job to ensure proposal retries.
	Propose(ctx context.Context, data []byte) error
	// ProposeConfChange proposes config change.
	// At most one ConfChange can be in the process of going through consensus.
	// Application needs to call ApplyConfChange when applying EntryConfChange type entry.
	ProposeConfChange(ctx context.Context, cc pb.ConfChange) error
	// Step advances the state machine using the given message. ctx.Err() will be returned, if any.
	Step(ctx context.Context, msg pb.Message) error

	// Ready returns a channel that returns the current point-in-time state.
	// Users of the Node must call Advance after retrieving the state returned by Ready.
	//
	// NOTE: No committed entries from the next Ready may be applied until all committed entries
	// and snapshots from the previous one have finished.
	Ready() <-chan Ready

	// Advance notifies the Node that the application has saved progress up to the last Ready.
	// It prepares the node to return the next available Ready.
	//
	// The application should generally call Advance after it applies the entries in last Ready.
	//
	// However, as an optimization, the application may call Advance while it is applying the
	// commands. For example. when the last Ready contains a snapshot, the application might take
	// a long time to apply the snapshot data. To continue receiving Ready without blocking raft
	// progress, it can call Advance before finishing applying the last ready.
	Advance()
	// ApplyConfChange applies config change to the local node.
	// Returns an opaque ConfState protobuf which must be recorded
	// in snapshots. Will never return nil; it returns a pointer only
	// to match MemoryStorage.Compact.
	ApplyConfChange(cc pb.ConfChange) *pb.ConfState

	// TransferLeadership attempts to transfer leadership to the given transferee.
	TransferLeadership(ctx context.Context, lead, transferee uint64)

	// ReadIndex request a read state. The read state will be set in the ready.
	// Read state has a read index. Once the application advances further than the read
	// index, any linearizable read requests issued before the read request can be
	// processed safely. The read state will have the same rctx attached.
	ReadIndex(ctx context.Context, rctx []byte) error

	// Status returns the current status of the raft state machine.
	Status() Status
	// ReportUnreachable reports the given node is not reachable for the last send.
	ReportUnreachable(id uint64)
	// ReportSnapshot reports the status of the sent snapshot. The id is the raft ID of the follower
	// who is meant to receive the snapshot, and the status is SnapshotFinish or SnapshotFailure.
	// Calling ReportSnapshot with SnapshotFinish is a no-op. But, any failure in applying a
	// snapshot (for e.g., while streaming it from leader to follower), should be reported to the
	// leader with SnapshotFailure. When leader sends a snapshot to a follower, it pauses any raft
	// log probes until the follower can apply the snapshot and advance its state. If the follower
	// can't do that, for e.g., due to a crash, it could end up in a limbo, never getting any
	// updates from the leader. Therefore, it is crucial that the application ensures that any
	// failure in snapshot sending is caught and reported back to the leader; so it can resume raft
	// log probing in the follower.
	ReportSnapshot(id uint64, status SnapshotStatus)
	// Stop performs any necessary termination of the Node.
	Stop()
	}

	type Peer struct {
	ID uint64
	Context []byte
	}

	// StartNode returns a new Node given configuration and a list of raft peers.
	// It appends a ConfChangeAddNode entry for each given peer to the initial log.
	func StartNode(c *Config, peers []Peer) Node {
	r := newRaft(c)
	// become the follower at term 1 and apply initial configuration
	// entries of term 1
	r.becomeFollower(1, None)
	for _, peer := range peers {
	cc := pb.ConfChange{Type: pb.ConfChangeAddNode, NodeID: peer.ID, Context: peer.Context}
	d, err := cc.Marshal()
	if err != nil {
	panic("unexpected marshal error")
	}
	e := pb.Entry{Type: pb.EntryConfChange, Term: 1, Index: r.raftLog.lastIndex() + 1, Data: d}
	r.raftLog.append(e)
	}
	// Mark these initial entries as committed.
	// TODO(bdarnell): These entries are still unstable; do we need to preserve
	// the invariant that committed < unstable?
	r.raftLog.committed = r.raftLog.lastIndex()
	// Now apply them, mainly so that the application can call Campaign
	// immediately after StartNode in tests. Note that these nodes will
	// be added to raft twice: here and when the application's Ready
	// loop calls ApplyConfChange. The calls to addNode must come after
	// all calls to raftLog.append so progress.next is set after these
	// bootstrapping entries (it is an error if we try to append these
	// entries since they have already been committed).
	// We do not set raftLog.applied so the application will be able
	// to observe all conf changes via Ready.CommittedEntries.
	for _, peer := range peers {
	r.addNode(peer.ID)
	}

	n := newNode()
	n.logger = c.Logger
	go n.run(r)
	return &n
	}

	// RestartNode is similar to StartNode but does not take a list of peers.
	// The current membership of the cluster will be restored from the Storage.
	// If the caller has an existing state machine, pass in the last log index that
	// has been applied to it; otherwise use zero.
	func RestartNode(c *Config) Node {
	r := newRaft(c)

	n := newNode()
	n.logger = c.Logger
	go n.run(r)
	return &n
	}

	type msgWithResult struct {
	m pb.Message
	result chan error
	}

	// node is the canonical implementation of the Node interface
	type node struct {
	propc chan msgWithResult
	recvc chan pb.Message
	confc chan pb.ConfChange
	confstatec chan pb.ConfState
	readyc chan Ready
	advancec chan struct{}
	tickc chan struct{}
	done chan struct{}
	stop chan struct{}
	status chan chan Status

	logger Logger
	}

	func newNode() node {
	return node{
	propc: make(chan msgWithResult),
	recvc: make(chan pb.Message),
	confc: make(chan pb.ConfChange),
	confstatec: make(chan pb.ConfState),
	readyc: make(chan Ready),
	advancec: make(chan struct{}),
	// make tickc a buffered chan, so raft node can buffer some ticks when the node
	// is busy processing raft messages. Raft node will resume process buffered
	// ticks when it becomes idle.
	tickc: make(chan struct{}, 128),
	done: make(chan struct{}),
	stop: make(chan struct{}),
	status: make(chan chan Status),
	}
	}

	func (n *node) Stop() {
	select {
	case n.stop <- struct{}{}:
	// Not already stopped, so trigger it
	case <-n.done:
	// Node has already been stopped - no need to do anything
	return
	}
	// Block until the stop has been acknowledged by run()
	<-n.done
	}

	func (n node) run(r raft) {
	var propc chan msgWithResult
	var readyc chan Ready
	var advancec chan struct{}
	var prevLastUnstablei, prevLastUnstablet uint64
	var havePrevLastUnstablei bool
	var prevSnapi uint64
	var applyingToI uint64
	var rd Ready

	lead := None
	prevSoftSt := r.softState()
	prevHardSt := emptyState

	for {
	if advancec != nil {
	readyc = nil
	} else {
	rd = newReady(r, prevSoftSt, prevHardSt)
	if rd.containsUpdates() {
	readyc = n.readyc
	} else {
	readyc = nil
	}
	}

	if lead != r.lead {
	if r.hasLeader() {
	if lead == None {
	r.logger.Infof("raft.node: %x elected leader %x at term %d", r.id, r.lead, r.Term)
	} else {
	r.logger.Infof("raft.node: %x changed leader from %x to %x at term %d", r.id, lead, r.lead, r.Term)
	}
	propc = n.propc
	} else {
	r.logger.Infof("raft.node: %x lost leader %x at term %d", r.id, lead, r.Term)
	propc = nil
	}
	lead = r.lead
	}

	select {
	// TODO: maybe buffer the config propose if there exists one (the way
	// described in raft dissertation)
	// Currently it is dropped in Step silently.
	case pm := <-propc:
	m := pm.m
	m.From = r.id
	err := r.Step(m)
	if pm.result != nil {
	pm.result <- err
	close(pm.result)
	}
	case m := <-n.recvc:
	// filter out response message from unknown From.
	if pr := r.getProgress(m.From); pr != nil \|\| !IsResponseMsg(m.Type) {
	r.Step(m)
	}
	case cc := <-n.confc:
	if cc.NodeID == None {
	select {
	case n.confstatec <- pb.ConfState{
	Nodes: r.nodes(),
	Learners: r.learnerNodes()}:
	case <-n.done:
	}
	break
	}
	switch cc.Type {
	case pb.ConfChangeAddNode:
	r.addNode(cc.NodeID)
	case pb.ConfChangeAddLearnerNode:
	r.addLearner(cc.NodeID)
	case pb.ConfChangeRemoveNode:
	// block incoming proposal when local node is
	// removed
	if cc.NodeID == r.id {
	propc = nil
	}
	r.removeNode(cc.NodeID)
	case pb.ConfChangeUpdateNode:
	default:
	panic("unexpected conf type")
	}
	select {
	case n.confstatec <- pb.ConfState{
	Nodes: r.nodes(),
	Learners: r.learnerNodes()}:
	case <-n.done:
	}
	case <-n.tickc:
	r.tick()
	case readyc <- rd:
	if rd.SoftState != nil {
	prevSoftSt = rd.SoftState
	}
	if len(rd.Entries) > 0 {
	prevLastUnstablei = rd.Entries[len(rd.Entries)-1].Index
	prevLastUnstablet = rd.Entries[len(rd.Entries)-1].Term
	havePrevLastUnstablei = true
	}
	if !IsEmptyHardState(rd.HardState) {
	prevHardSt = rd.HardState
	}
	if !IsEmptySnap(rd.Snapshot) {
	prevSnapi = rd.Snapshot.Metadata.Index
	}
	if index := rd.appliedCursor(); index != 0 {
	applyingToI = index
	}

	r.msgs = nil
	r.readStates = nil
	r.reduceUncommittedSize(rd.CommittedEntries)
	advancec = n.advancec
	case <-advancec:
	if applyingToI != 0 {
	r.raftLog.appliedTo(applyingToI)
	applyingToI = 0
	}
	if havePrevLastUnstablei {
	r.raftLog.stableTo(prevLastUnstablei, prevLastUnstablet)
	havePrevLastUnstablei = false
	}
	r.raftLog.stableSnapTo(prevSnapi)
	advancec = nil
	case c := <-n.status:
	c <- getStatus(r)
	case <-n.stop:
	close(n.done)
	return
	}
	}
	}

	// Tick increments the internal logical clock for this Node. Election timeouts
	// and heartbeat timeouts are in units of ticks.
	func (n *node) Tick() {
	select {
	case n.tickc <- struct{}{}:
	case <-n.done:
	default:
	n.logger.Warningf("A tick missed to fire. Node blocks too long!")
	}
	}

	func (n *node) Campaign(ctx context.Context) error { return n.step(ctx, pb.Message{Type: pb.MsgHup}) }

	func (n *node) Propose(ctx context.Context, data []byte) error {
	return n.stepWait(ctx, pb.Message{Type: pb.MsgProp, Entries: []pb.Entry{{Data: data}}})
	}

	func (n *node) Step(ctx context.Context, m pb.Message) error {
	// ignore unexpected local messages receiving over network
	if IsLocalMsg(m.Type) {
	// TODO: return an error?
	return nil
	}
	return n.step(ctx, m)
	}

	func (n *node) ProposeConfChange(ctx context.Context, cc pb.ConfChange) error {
	data, err := cc.Marshal()
	if err != nil {
	return err
	}
	return n.Step(ctx, pb.Message{Type: pb.MsgProp, Entries: []pb.Entry{{Type: pb.EntryConfChange, Data: data}}})
	}

	func (n *node) step(ctx context.Context, m pb.Message) error {
	return n.stepWithWaitOption(ctx, m, false)
	}

	func (n *node) stepWait(ctx context.Context, m pb.Message) error {
	return n.stepWithWaitOption(ctx, m, true)
	}

	// Step advances the state machine using msgs. The ctx.Err() will be returned,
	// if any.
	func (n *node) stepWithWaitOption(ctx context.Context, m pb.Message, wait bool) error {
	if m.Type != pb.MsgProp {
	select {
	case n.recvc <- m:
	return nil
	case <-ctx.Done():
	return ctx.Err()
	case <-n.done:
	return ErrStopped
	}
	}
	ch := n.propc
	pm := msgWithResult{m: m}
	if wait {
	pm.result = make(chan error, 1)
	}
	select {
	case ch <- pm:
	if !wait {
	return nil
	}
	case <-ctx.Done():
	return ctx.Err()
	case <-n.done:
	return ErrStopped
	}
	select {
	case err := <-pm.result:
	if err != nil {
	return err
	}
	case <-ctx.Done():
	return ctx.Err()
	case <-n.done:
	return ErrStopped
	}
	return nil
	}

	func (n *node) Ready() <-chan Ready { return n.readyc }

	func (n *node) Advance() {
	select {
	case n.advancec <- struct{}{}:
	case <-n.done:
	}
	}

	func (n node) ApplyConfChange(cc pb.ConfChange) pb.ConfState {
	var cs pb.ConfState
	select {
	case n.confc <- cc:
	case <-n.done:
	}
	select {
	case cs = <-n.confstatec:
	case <-n.done:
	}
	return &cs
	}

	func (n *node) Status() Status {
	c := make(chan Status)
	select {
	case n.status <- c:
	return <-c
	case <-n.done:
	return Status{}
	}
	}

	func (n *node) ReportUnreachable(id uint64) {
	select {
	case n.recvc <- pb.Message{Type: pb.MsgUnreachable, From: id}:
	case <-n.done:
	}
	}

	func (n *node) ReportSnapshot(id uint64, status SnapshotStatus) {
	rej := status == SnapshotFailure

	select {
	case n.recvc <- pb.Message{Type: pb.MsgSnapStatus, From: id, Reject: rej}:
	case <-n.done:
	}
	}

	func (n *node) TransferLeadership(ctx context.Context, lead, transferee uint64) {
	select {
	// manually set 'from' and 'to', so that leader can voluntarily transfers its leadership
	case n.recvc <- pb.Message{Type: pb.MsgTransferLeader, From: transferee, To: lead}:
	case <-n.done:
	case <-ctx.Done():
	}
	}

	func (n *node) ReadIndex(ctx context.Context, rctx []byte) error {
	return n.step(ctx, pb.Message{Type: pb.MsgReadIndex, Entries: []pb.Entry{{Data: rctx}}})
	}

	func newReady(r raft, prevSoftSt SoftState, prevHardSt pb.HardState) Ready {
	rd := Ready{
	Entries: r.raftLog.unstableEntries(),
	CommittedEntries: r.raftLog.nextEnts(),
	Messages: r.msgs,
	}
	if softSt := r.softState(); !softSt.equal(prevSoftSt) {
	rd.SoftState = softSt
	}
	if hardSt := r.hardState(); !isHardStateEqual(hardSt, prevHardSt) {
	rd.HardState = hardSt
	}
	if r.raftLog.unstable.snapshot != nil {
	rd.Snapshot = *r.raftLog.unstable.snapshot
	}
	if len(r.readStates) != 0 {
	rd.ReadStates = r.readStates
	}
	rd.MustSync = MustSync(r.hardState(), prevHardSt, len(rd.Entries))
	return rd
	}

	// MustSync returns true if the hard state and count of Raft entries indicate
	// that a synchronous write to persistent storage is required.
	func MustSync(st, prevst pb.HardState, entsnum int) bool {
	// Persistent state on all servers:
	// (Updated on stable storage before responding to RPCs)
	// currentTerm
	// votedFor
	// log entries[]
	return entsnum != 0 \|\| st.Vote != prevst.Vote \|\| st.Term != prevst.Term
	}