etcd/raft/node.go

/*
   Copyright 2014 CoreOS, Inc.

   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 (
	"errors"
	"log"

	"github.com/coreos/etcd/Godeps/_workspace/src/golang.org/x/net/context"
	pb "github.com/coreos/etcd/raft/raftpb"
)

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
	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

	// 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.
	Messages []pb.Message
}

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
}

// 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.
	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 applying the state returned by Ready
	Ready() <-chan Ready
	// Advance notifies the Node that the application has applied and saved progress up to the last Ready.
	// It prepares the node to return the next available 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
	// Status returns the current status of the raft state machine.
	Status() Status
	// Stop performs any necessary termination of the Node
	Stop()
}

type Peer struct {
	ID      uint64
	Context []byte
}

// StartNode returns a new Node given a unique raft id, a list of raft peers, and
// the election and heartbeat timeouts in units of ticks.
// It appends a ConfChangeAddNode entry for each given peer to the initial log.
func StartNode(id uint64, peers []Peer, election, heartbeat int, storage Storage) Node {
	n := newNode()
	r := newRaft(id, nil, election, heartbeat, storage, 0)

	// become the follower at term 1 and apply initial configuration
	// entires 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()
	r.Commit = r.raftLog.committed
	// 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)
	}

	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(id uint64, election, heartbeat int, storage Storage, applied uint64) Node {
	n := newNode()
	r := newRaft(id, nil, election, heartbeat, storage, applied)

	go n.run(r)
	return &n
}

// node is the canonical implementation of the Node interface
type node struct {
	propc      chan pb.Message
	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
}

func newNode() node {
	return node{
		propc:      make(chan pb.Message),
		recvc:      make(chan pb.Message),
		confc:      make(chan pb.ConfChange),
		confstatec: make(chan pb.ConfState),
		readyc:     make(chan Ready),
		advancec:   make(chan struct{}),
		tickc:      make(chan struct{}),
		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 pb.Message
	var readyc chan Ready
	var advancec chan struct{}
	var prevLastUnstablei, prevLastUnstablet uint64
	var havePrevLastUnstablei bool
	var prevSnapi uint64
	var rd Ready

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

	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 {
					log.Printf("raft.node: %x elected leader %x at term %d", r.id, r.lead, r.Term)
				} else {
					log.Printf("raft.node: %x changed leader from %x to %x at term %d", r.id, lead, r.lead, r.Term)
				}
				propc = n.propc
			} else {
				log.Printf("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 m := <-propc:
			m.From = r.id
			r.Step(m)
		case m := <-n.recvc:
			// filter out response message from unknow From.
			if _, ok := r.prs[m.From]; ok || !IsResponseMsg(m) {
				r.Step(m) // raft never returns an error
			}
		case cc := <-n.confc:
			if cc.NodeID == None {
				r.resetPendingConf()
				select {
				case n.confstatec <- pb.ConfState{Nodes: r.nodes()}:
				case <-n.done:
				}
				break
			}
			switch cc.Type {
			case pb.ConfChangeAddNode:
				r.addNode(cc.NodeID)
			case pb.ConfChangeRemoveNode:
				// block incoming proposal when local node is
				// removed
				if cc.NodeID == r.id {
					n.propc = nil
				}
				r.removeNode(cc.NodeID)
			case pb.ConfChangeUpdateNode:
				r.resetPendingConf()
			default:
				panic("unexpected conf type")
			}
			select {
			case n.confstatec <- pb.ConfState{Nodes: r.nodes()}:
			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
			}
			r.msgs = nil
			advancec = n.advancec
		case <-advancec:
			if prevHardSt.Commit != 0 {
				r.raftLog.appliedTo(prevHardSt.Commit)
			}
			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:
	}
}

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.step(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) {
		// 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}}})
}

// Step advances the state machine using msgs. The ctx.Err() will be returned,
// if any.
func (n *node) step(ctx context.Context, m pb.Message) error {
	ch := n.recvc
	if m.Type == pb.MsgProp {
		ch = n.propc
	}

	select {
	case ch <- m:
		return nil
	case <-ctx.Done():
		return ctx.Err()
	case <-n.done:
		return ErrStopped
	}
}

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)
	n.status <- c
	return <-c
}

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 !isHardStateEqual(r.HardState, prevHardSt) {
		rd.HardState = r.HardState
	}
	if r.raftLog.unstable.snapshot != nil {
		rd.Snapshot = *r.raftLog.unstable.snapshot
	}
	return rd
}