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draft.go
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/*
* Copyright (C) 2017 Dgraph Labs, Inc. and Contributors
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package worker
import (
"encoding/binary"
"log"
"math/rand"
"sync"
"time"
"github.com/coreos/etcd/raft"
"github.com/coreos/etcd/raft/raftpb"
"golang.org/x/net/context"
"golang.org/x/net/trace"
"github.com/dgraph-io/badger/y"
"github.com/dgraph-io/dgraph/algo"
"github.com/dgraph-io/dgraph/conn"
"github.com/dgraph-io/dgraph/posting"
"github.com/dgraph-io/dgraph/protos"
"github.com/dgraph-io/dgraph/raftwal"
"github.com/dgraph-io/dgraph/schema"
"github.com/dgraph-io/dgraph/x"
)
type proposalCtx struct {
ch chan error
ctx context.Context
cnt int // used for reference counting
// Since each proposal consists of multiple tasks we need to store
// non-nil error returned by task
err error
index uint64
n *node
}
type proposals struct {
sync.RWMutex
ids map[uint32]*proposalCtx
}
func (p *proposals) Store(pid uint32, pctx *proposalCtx) bool {
p.Lock()
defer p.Unlock()
if _, has := p.ids[pid]; has {
return false
}
p.ids[pid] = pctx
return true
}
func (p *proposals) SetIndex(pid uint32, index uint64) {
p.Lock()
defer p.Unlock()
pd, has := p.ids[pid]
x.AssertTrue(has)
pd.index = index
pd.cnt = 1
return
}
func (p *proposals) IncRef(pid uint32, count int) {
p.Lock()
defer p.Unlock()
pd, has := p.ids[pid]
x.AssertTrue(has)
pd.cnt += count
return
}
func (p *proposals) Ctx(pid uint32) (context.Context, bool) {
p.RLock()
defer p.RUnlock()
if pd, has := p.ids[pid]; has {
return pd.ctx, true
}
return nil, false
}
func (p *proposals) Done(pid uint32, err error) {
p.Lock()
defer p.Unlock()
pd, has := p.ids[pid]
if !has {
return
}
x.AssertTrue(pd.cnt > 0 && pd.index != 0)
pd.cnt -= 1
if err != nil {
pd.err = err
}
if pd.cnt > 0 {
return
}
delete(p.ids, pid)
pd.ch <- pd.err
// We emit one pending watermark as soon as we read from rd.committedentries.
// Since the tasks are executed in goroutines we need one guarding watermark which
// is done only when all the pending sync/applied marks have been emitted.
pd.n.Applied.Done(pd.index)
posting.SyncMarks().Done(pd.index)
}
func (p *proposals) Has(pid uint32) bool {
p.RLock()
defer p.RUnlock()
_, has := p.ids[pid]
return has
}
type node struct {
*conn.Node
// Fields which are never changed after init.
applyCh chan raftpb.Entry
ctx context.Context
stop chan struct{} // to send the stop signal to Run
done chan struct{} // to check whether node is running or not
gid uint32
props proposals
canCampaign bool
sch *scheduler
}
func newNode(gid uint32, id uint64, myAddr string) *node {
x.Printf("Node ID: %v with GroupID: %v\n", id, gid)
rc := &protos.RaftContext{
Addr: myAddr,
Group: gid,
Id: id,
}
m := conn.NewNode(rc)
props := proposals{
ids: make(map[uint32]*proposalCtx),
}
n := &node{
Node: m,
ctx: context.Background(),
gid: gid,
// processConfChange etc are not throttled so some extra delta, so that we don't
// block tick when applyCh is full
applyCh: make(chan raftpb.Entry, Config.NumPendingProposals+1000),
props: props,
stop: make(chan struct{}),
done: make(chan struct{}),
sch: new(scheduler),
}
n.sch.init(n)
return n
}
type header struct {
proposalId uint32
msgId uint16
}
func (h *header) Length() int {
return 6 // 4 bytes for proposalId, 2 bytes for msgId.
}
func (h *header) Encode() []byte {
result := make([]byte, h.Length())
binary.LittleEndian.PutUint32(result[0:4], h.proposalId)
binary.LittleEndian.PutUint16(result[4:6], h.msgId)
return result
}
func (h *header) Decode(in []byte) {
h.proposalId = binary.LittleEndian.Uint32(in[0:4])
h.msgId = binary.LittleEndian.Uint16(in[4:6])
}
func (n *node) ProposeAndWait(ctx context.Context, proposal *protos.Proposal) error {
if n.Raft() == nil {
return x.Errorf("RAFT isn't initialized yet")
}
// TODO: Should be based on number of edges (amount of work)
pendingProposals <- struct{}{}
x.PendingProposals.Add(1)
defer func() { <-pendingProposals; x.PendingProposals.Add(-1) }()
if ctx.Err() != nil {
return ctx.Err()
}
// Do a type check here if schema is present
// In very rare cases invalid entries might pass through raft, which would
// be persisted, we do best effort schema check while writing
if proposal.Mutations != nil {
for _, edge := range proposal.Mutations.Edges {
if tablet := groups().Tablet(edge.Attr); tablet != nil && tablet.ReadOnly {
return errPredicateMoving
}
if typ, err := schema.State().TypeOf(edge.Attr); err != nil {
continue
} else if err := ValidateAndConvert(edge, typ); err != nil {
return err
}
}
for _, schema := range proposal.Mutations.Schema {
if tablet := groups().Tablet(schema.Predicate); tablet != nil && tablet.ReadOnly {
return errPredicateMoving
}
if err := checkSchema(schema); err != nil {
return err
}
}
}
che := make(chan error, 1)
pctx := &proposalCtx{
ch: che,
ctx: ctx,
n: n,
}
for {
id := rand.Uint32()
if n.props.Store(id, pctx) {
proposal.Id = id
break
}
}
sz := proposal.Size()
slice := make([]byte, sz)
upto, err := proposal.MarshalTo(slice)
if err != nil {
return err
}
// We don't timeout on a mutation which has already been proposed.
if err = n.Raft().Propose(ctx, slice[:upto]); err != nil {
return x.Wrapf(err, "While proposing")
}
// Wait for the proposal to be committed.
if proposal.Mutations != nil {
if tr, ok := trace.FromContext(ctx); ok {
tr.LazyPrintf("Waiting for the proposal: mutations.")
}
} else if len(proposal.Kv) > 0 {
if tr, ok := trace.FromContext(ctx); ok {
tr.LazyPrintf("Waiting for the proposal: key-values.")
}
} else {
if tr, ok := trace.FromContext(ctx); ok {
tr.LazyPrintf("Waiting for the proposal: %v.", proposal)
}
}
err = <-che
if err != nil {
if tr, ok := trace.FromContext(ctx); ok {
tr.LazyPrintf(err.Error())
}
}
return err
}
func (n *node) handleUpsert(task *task) (bool, error) {
if task.upsert == nil {
return true, nil
}
edge := task.edge
attr := task.upsert.Attr
if attr != edge.Attr {
return true, nil
}
gid := groups().BelongsTo(attr)
res, err := n.processUpsertTask(n.ctx, task, gid)
if err != nil {
return false, err
}
uids := algo.MergeSorted(res.UidMatrix)
if len(uids.Uids) > 0 {
return false, nil
}
return true, nil
}
func (n *node) processMutation(task *task) error {
pid := task.pid
ridx := task.rid
edge := task.edge
if cont, err := n.handleUpsert(task); err != nil {
return err
} else if !cont {
// We found a uid which has this value, so don't continue.
return nil
}
var ctx context.Context
var has bool
if ctx, has = n.props.Ctx(pid); !has {
ctx = n.ctx
}
rv := x.RaftValue{Group: n.gid, Index: ridx}
ctx = context.WithValue(ctx, "raft", rv)
if err := runMutation(ctx, edge); err != nil {
if tr, ok := trace.FromContext(ctx); ok {
tr.LazyPrintf(err.Error())
}
return err
}
return nil
}
func (n *node) processSchemaMutations(pid uint32, index uint64, s *protos.SchemaUpdate) error {
var ctx context.Context
var has bool
if ctx, has = n.props.Ctx(pid); !has {
ctx = n.ctx
}
rv := x.RaftValue{Group: n.gid, Index: index}
ctx = context.WithValue(n.ctx, "raft", rv)
if err := runSchemaMutation(ctx, s); err != nil {
if tr, ok := trace.FromContext(n.ctx); ok {
tr.LazyPrintf(err.Error())
}
return err
}
return nil
}
func (n *node) applyConfChange(e raftpb.Entry) {
var cc raftpb.ConfChange
cc.Unmarshal(e.Data)
if len(cc.Context) > 0 {
var rc protos.RaftContext
x.Check(rc.Unmarshal(cc.Context))
n.Connect(rc.Id, rc.Addr)
}
cs := n.Raft().ApplyConfChange(cc)
n.SetConfState(cs)
// Not present in proposal map
n.Applied.Done(e.Index)
posting.SyncMarks().Done(e.Index)
groups().triggerMembershipSync()
}
func (n *node) processApplyCh() {
for e := range n.applyCh {
if len(e.Data) == 0 {
// This is not in the proposal map
n.Applied.Done(e.Index)
posting.SyncMarks().Done(e.Index)
continue
}
if e.Type == raftpb.EntryConfChange {
n.applyConfChange(e)
continue
}
x.AssertTrue(e.Type == raftpb.EntryNormal)
proposal := &protos.Proposal{}
if err := proposal.Unmarshal(e.Data); err != nil {
log.Fatalf("Unable to unmarshal proposal: %v %q\n", err, e.Data)
}
// One final applied and synced watermark would be emitted when proposal ctx ref count
// becomes zero.
if !n.props.Has(proposal.Id) {
pctx := &proposalCtx{
ch: make(chan error, 1),
ctx: n.ctx,
n: n,
index: e.Index,
cnt: 1,
}
n.props.Store(proposal.Id, pctx)
} else {
n.props.SetIndex(proposal.Id, e.Index)
}
if proposal.Mutations != nil {
n.sch.schedule(proposal, e.Index)
} else if proposal.Membership != nil {
x.Fatalf("Dgraph does not handle membership proposals anymore.")
} else if len(proposal.Kv) > 0 {
go n.processKeyValues(e.Index, proposal.Id, proposal.Kv)
} else if proposal.State != nil {
// This state needn't be snapshotted in this group, on restart we would fetch
// a state which is latest or equal to this.
groups().applyState(proposal.State)
// When proposal is done it emits done watermarks.
n.props.Done(proposal.Id, nil)
} else if len(proposal.CleanPredicate) > 0 {
go n.deletePredicate(e.Index, proposal.Id, proposal.CleanPredicate)
} else {
x.Fatalf("Unknown proposal")
}
}
}
func (n *node) deletePredicate(index uint64, pid uint32, predicate string) {
ctx, _ := n.props.Ctx(pid)
rv := x.RaftValue{Group: n.gid, Index: index}
ctx = context.WithValue(n.ctx, "raft", rv)
err := posting.DeletePredicate(ctx, predicate)
n.props.Done(pid, err)
}
func (n *node) processKeyValues(index uint64, pid uint32, kvs []*protos.KV) error {
ctx, _ := n.props.Ctx(pid)
err := populateKeyValues(ctx, kvs)
n.props.Done(pid, err)
return nil
}
func (n *node) retrieveSnapshot(peerID uint64) {
addr, _ := n.Peer(peerID)
pool, err := conn.Get().Get(addr)
if err != nil {
// err is just going to be errNoConnection
log.Fatalf("Cannot retrieve snapshot from peer %v, no connection. Error: %v\n",
peerID, err)
}
// Get index of last committed.
lastIndex, err := n.Store.LastIndex()
x.Checkf(err, "Error while getting last index")
// Wait for watermarks to sync since populateShard writes directly to db, otherwise
// the values might get overwritten
// Safe to keep this line
n.syncAllMarks(n.ctx, lastIndex)
// Need to clear pl's stored in memory for the case when retrieving snapshot with
// index greater than this node's last index
// Should invalidate/remove pl's to this group only ideally
posting.EvictLRU()
if _, err := populateShard(n.ctx, pstore, pool, n.gid); err != nil {
// TODO: We definitely don't want to just fall flat on our face if we can't
// retrieve a simple snapshot.
log.Fatalf("Cannot retrieve snapshot from peer %v, error: %v\n", peerID, err)
}
// Populate shard stores the streamed data directly into db, so we need to refresh
// schema for current group id
x.Checkf(schema.LoadFromDb(), "Error while initilizating schema")
groups().triggerMembershipSync()
}
func (n *node) Run() {
firstRun := true
var leader bool
// See also our configuration of HeartbeatTick and ElectionTick.
ticker := time.NewTicker(20 * time.Millisecond)
defer ticker.Stop()
rcBytes, err := n.RaftContext.Marshal()
x.Check(err)
// This chan could have capacity zero, because runReadIndexLoop never blocks without selecting
// on readStateCh. It's 2 so that sending rarely blocks (so the Go runtime doesn't have to
// switch threads as much.)
readStateCh := make(chan raft.ReadState, 2)
closer := y.NewCloser(1)
// We only stop runReadIndexLoop after the for loop below has finished interacting with it.
// That way we know sending to readStateCh will not deadlock.
defer closer.SignalAndWait()
go n.RunReadIndexLoop(closer, readStateCh)
for {
select {
case <-ticker.C:
n.Raft().Tick()
case rd := <-n.Raft().Ready():
for _, rs := range rd.ReadStates {
readStateCh <- rs
}
if rd.SoftState != nil {
groups().triggerMembershipSync()
leader = rd.RaftState == raft.StateLeader
}
if leader {
// Leader can send messages in parallel with writing to disk.
for _, msg := range rd.Messages {
// NOTE: We can do some optimizations here to drop messages.
msg.Context = rcBytes
n.Send(msg)
}
}
// First store the entries, then the hardstate and snapshot.
x.Check(n.Wal.Store(n.gid, rd.HardState, rd.Entries))
x.Check(n.Wal.StoreSnapshot(n.gid, rd.Snapshot))
// Now store them in the in-memory store.
n.SaveToStorage(rd.Snapshot, rd.HardState, rd.Entries)
if !raft.IsEmptySnap(rd.Snapshot) {
// We don't send snapshots to other nodes. But, if we get one, that means
// either the leader is trying to bring us up to state; or this is the
// snapshot that I created. Only the former case should be handled.
var rc protos.RaftContext
x.Check(rc.Unmarshal(rd.Snapshot.Data))
x.AssertTrue(rc.Group == n.gid)
if rc.Id != n.Id {
// NOTE: Retrieving snapshot here is OK, after storing it above in WAL, because
// rc.Id != n.Id.
x.Printf("-------> SNAPSHOT [%d] from %d\n", n.gid, rc.Id)
// It's ok to block tick while retrieving snapshot, since it's a follower
n.retrieveSnapshot(rc.Id)
x.Printf("-------> SNAPSHOT [%d]. DONE.\n", n.gid)
} else {
x.Printf("-------> SNAPSHOT [%d] from %d [SELF]. Ignoring.\n", n.gid, rc.Id)
}
}
if len(rd.CommittedEntries) > 0 {
if tr, ok := trace.FromContext(n.ctx); ok {
tr.LazyPrintf("Found %d committed entries", len(rd.CommittedEntries))
}
}
// Now schedule or apply committed entries.
for _, entry := range rd.CommittedEntries {
// Need applied watermarks for schema mutation also for read linearazibility
// Applied watermarks needs to be emitted as soon as possible sequentially.
// If we emit Mark{4, false} and Mark{4, true} before emitting Mark{3, false}
// then doneUntil would be set as 4 as soon as Mark{4,true} is done and before
// Mark{3, false} is emitted. So it's safer to emit watermarks as soon as
// possible sequentially
n.Applied.Begin(entry.Index)
posting.SyncMarks().Begin(entry.Index)
if !leader && entry.Type == raftpb.EntryConfChange {
// Config changes in followers must be applied straight away.
n.applyConfChange(entry)
} else {
// TODO: Stop accepting requests when applyCh is full
// Just queue up to be processed. Don't wait on them.
n.applyCh <- entry
}
}
if !leader {
// Followers should send messages later.
for _, msg := range rd.Messages {
// NOTE: We can do some optimizations here to drop messages.
msg.Context = rcBytes
n.Send(msg)
}
}
n.Raft().Advance()
if firstRun && n.canCampaign {
go n.Raft().Campaign(n.ctx)
firstRun = false
}
case <-n.stop:
if peerId, has := groups().MyPeer(); has && n.AmLeader() {
n.Raft().TransferLeadership(n.ctx, Config.RaftId, peerId)
go func() {
select {
case <-n.ctx.Done(): // time out
if tr, ok := trace.FromContext(n.ctx); ok {
tr.LazyPrintf("context timed out while transfering leadership")
}
case <-time.After(1 * time.Second):
if tr, ok := trace.FromContext(n.ctx); ok {
tr.LazyPrintf("Timed out transfering leadership")
}
}
n.Raft().Stop()
close(n.done)
}()
} else {
n.Raft().Stop()
close(n.done)
}
case <-n.done:
return
}
}
}
func (n *node) Stop() {
select {
case n.stop <- struct{}{}:
case <-n.done:
// already stopped.
return
}
<-n.done // wait for Run to respond.
}
func (n *node) snapshotPeriodically() {
if n.gid == 0 {
// Group zero is dedicated for membership information, whose state we don't persist.
// So, taking snapshots would end up deleting the RAFT entries that we need to
// regenerate the state on a crash. Therefore, don't take snapshots.
return
}
ticker := time.NewTicker(time.Minute)
defer ticker.Stop()
for {
select {
case <-ticker.C:
n.snapshot(Config.MaxPendingCount)
case <-n.done:
return
}
}
}
func (n *node) snapshot(skip uint64) {
if n.gid == 0 {
// Group zero is dedicated for membership information, whose state we don't persist.
// So, taking snapshots would end up deleting the RAFT entries that we need to
// regenerate the state on a crash. Therefore, don't take snapshots.
return
}
water := posting.SyncMarks()
le := water.DoneUntil()
existing, err := n.Store.Snapshot()
x.Checkf(err, "Unable to get existing snapshot")
si := existing.Metadata.Index
if le <= si+skip {
return
}
snapshotIdx := le - skip
if tr, ok := trace.FromContext(n.ctx); ok {
tr.LazyPrintf("Taking snapshot for group: %d at watermark: %d\n", n.gid, snapshotIdx)
}
rc, err := n.RaftContext.Marshal()
x.Check(err)
s, err := n.Store.CreateSnapshot(snapshotIdx, n.ConfState(), rc)
x.Checkf(err, "While creating snapshot")
x.Checkf(n.Store.Compact(snapshotIdx), "While compacting snapshot")
x.Check(n.Wal.StoreSnapshot(n.gid, s))
}
func (n *node) joinPeers() {
// Get leader information for MY group.
pl := groups().Leader(n.gid)
if pl == nil {
log.Fatalf("Unable to reach leader or any other server in group %d", n.gid)
}
// Bring the instance up to speed first.
// Raft would decide whether snapshot needs to fetched or not
// so populateShard is not needed
// _, err := populateShard(n.ctx, pool, n.gid)
// x.Checkf(err, "Error while populating shard")
gconn := pl.Get()
c := protos.NewRaftClient(gconn)
x.Printf("Calling JoinCluster")
_, err := c.JoinCluster(n.ctx, n.RaftContext)
// TODO: This should keep on indefinitely trying to join the cluster, instead of crashing.
x.Checkf(err, "Error while joining cluster")
x.Printf("Done with JoinCluster call\n")
}
// InitAndStartNode gets called after having at least one membership sync with the cluster.
func (n *node) InitAndStartNode(wal *raftwal.Wal) {
idx, restart, err := n.InitFromWal(wal)
x.Check(err)
n.Applied.SetDoneUntil(idx)
posting.SyncMarks().SetDoneUntil(idx)
if restart {
x.Printf("Restarting node for group: %d\n", n.gid)
found := groups().HasMeInState()
_, hasPeer := groups().MyPeer()
if !found && hasPeer {
n.joinPeers()
}
n.SetRaft(raft.RestartNode(n.Cfg))
} else {
x.Printf("New Node for group: %d\n", n.gid)
if _, hasPeer := groups().MyPeer(); hasPeer {
n.joinPeers()
n.SetRaft(raft.StartNode(n.Cfg, nil))
} else {
peers := []raft.Peer{{ID: n.Id}}
n.SetRaft(raft.StartNode(n.Cfg, peers))
// Trigger election, so this node can become the leader of this single-node cluster.
n.canCampaign = true
}
}
go n.processApplyCh()
go n.Run()
go n.snapshotPeriodically()
go n.BatchAndSendMessages()
}
func (n *node) AmLeader() bool {
if n.Raft() == nil {
return false
}
r := n.Raft()
return r.Status().Lead == r.Status().ID
}