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// 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 a configuration change. Like any proposal, the
// configuration change may be dropped with or without an error being
// returned. In particular, configuration changes are dropped unless the
// leader has certainty that there is no prior unapplied configuration
// change in its log.
//
// The method accepts either a pb.ConfChange (deprecated) or pb.ConfChangeV2
// message. The latter allows arbitrary configuration changes via joint
// consensus, notably including replacing a voter. Passing a ConfChangeV2
// message is only allowed if all Nodes participating in the cluster run a
// version of this library aware of the V2 API. See pb.ConfChangeV2 for
// usage details and semantics.
ProposeConfChange(ctx context.Context, cc pb.ConfChangeI) 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 a config change (previously passed to
// ProposeConfChange) to the node. This must be called whenever a config
// change is observed in Ready.CommittedEntries.
//
// Returns an opaque non-nil ConfState protobuf which must be recorded in
// snapshots.
ApplyConfChange(cc pb.ConfChangeI) *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.
//
// Peers must not be zero length; call RestartNode in that case.
func StartNode(c *Config, peers []Peer) Node {
if len(peers) == 0 {
panic("no peers given; use RestartNode instead")
}
rn, err := NewRawNode(c)
if err != nil {
panic(err)
}
rn.Bootstrap(peers)
n := newNode(rn)
go n.run()
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 {
rn, err := NewRawNode(c)
if err != nil {
panic(err)
}
n := newNode(rn)
go n.run()
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.ConfChangeV2
confstatec chan pb.ConfState
readyc chan Ready
advancec chan struct{}
tickc chan struct{}
done chan struct{}
stop chan struct{}
status chan chan Status
rn *RawNode
}
func newNode(rn *RawNode) node {
return node{
propc: make(chan msgWithResult),
recvc: make(chan pb.Message),
confc: make(chan pb.ConfChangeV2),
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),
rn: rn,
}
}
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() {
var propc chan msgWithResult
var readyc chan Ready
var advancec chan struct{}
var rd Ready
r := n.rn.raft
lead := None
for {
if advancec != nil {
readyc = nil
} else if n.rn.HasReady() {
// Populate a Ready. Note that this Ready is not guaranteed to
// actually be handled. We will arm readyc, but there's no guarantee
// that we will actually send on it. It's possible that we will
// service another channel instead, loop around, and then populate
// the Ready again. We could instead force the previous Ready to be
// handled first, but it's generally good to emit larger Readys plus
// it simplifies testing (by emitting less frequently and more
// predictably).
rd = n.rn.readyWithoutAccept()
readyc = n.readyc
}
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.prs.Progress[m.From]; pr != nil || !IsResponseMsg(m.Type) {
r.Step(m)
}
case cc := <-n.confc:
_, okBefore := r.prs.Progress[r.id]
cs := r.applyConfChange(cc)
// If the node was removed, block incoming proposals. Note that we
// only do this if the node was in the config before. Nodes may be
// a member of the group without knowing this (when they're catching
// up on the log and don't have the latest config) and we don't want
// to block the proposal channel in that case.
//
// NB: propc is reset when the leader changes, which, if we learn
// about it, sort of implies that we got readded, maybe? This isn't
// very sound and likely has bugs.
if _, okAfter := r.prs.Progress[r.id]; okBefore && !okAfter {
var found bool
for _, sl := range [][]uint64{cs.Voters, cs.VotersOutgoing} {
for _, id := range sl {
if id == r.id {
found = true
}
}
}
if !found {
propc = nil
}
}
select {
case n.confstatec <- cs:
case <-n.done:
}
case <-n.tickc:
n.rn.Tick()
case readyc <- rd:
n.rn.acceptReady(rd)
advancec = n.advancec
case <-advancec:
n.rn.Advance(rd)
rd = Ready{}
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.rn.raft.logger.Warningf("%x (leader %v) A tick missed to fire. Node blocks too long!", n.rn.raft.id, n.rn.raft.id == n.rn.raft.lead)
}
}
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 confChangeToMsg(c pb.ConfChangeI) (pb.Message, error) {
typ, data, err := pb.MarshalConfChange(c)
if err != nil {
return pb.Message{}, err
}
return pb.Message{Type: pb.MsgProp, Entries: []pb.Entry{{Type: typ, Data: data}}}, nil
}
func (n *node) ProposeConfChange(ctx context.Context, cc pb.ConfChangeI) error {
msg, err := confChangeToMsg(cc)
if err != nil {
return err
}
return n.Step(ctx, msg)
}
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.ConfChangeI) *pb.ConfState {
var cs pb.ConfState
select {
case n.confc <- cc.AsV2():
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
}
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