package raft import ( "errors" "fmt" "sync" "time" "github.com/armon/go-metrics" ) const ( maxFailureScale = 12 failureWait = 10 * time.Millisecond ) var ( // ErrLogNotFound indicates a given log entry is not available. ErrLogNotFound = errors.New("log not found") // ErrPipelineReplicationNotSupported can be returned by the transport to // signal that pipeline replication is not supported in general, and that // no error message should be produced. ErrPipelineReplicationNotSupported = errors.New("pipeline replication not supported") ) // followerReplication is in charge of sending snapshots and log entries from // this leader during this particular term to a remote follower. type followerReplication struct { // peer contains the network address and ID of the remote follower. peer Server // commitment tracks the entries acknowledged by followers so that the // leader's commit index can advance. It is updated on successsful // AppendEntries responses. commitment *commitment // stopCh is notified/closed when this leader steps down or the follower is // removed from the cluster. In the follower removed case, it carries a log // index; replication should be attempted with a best effort up through that // index, before exiting. stopCh chan uint64 // triggerCh is notified every time new entries are appended to the log. triggerCh chan struct{} // currentTerm is the term of this leader, to be included in AppendEntries // requests. currentTerm uint64 // nextIndex is the index of the next log entry to send to the follower, // which may fall past the end of the log. nextIndex uint64 // lastContact is updated to the current time whenever any response is // received from the follower (successful or not). This is used to check // whether the leader should step down (Raft.checkLeaderLease()). lastContact time.Time // lastContactLock protects 'lastContact'. lastContactLock sync.RWMutex // failures counts the number of failed RPCs since the last success, which is // used to apply backoff. failures uint64 // notifyCh is notified to send out a heartbeat, which is used to check that // this server is still leader. notifyCh chan struct{} // notify is a list of futures to be resolved upon receipt of an // acknowledgement, then cleared from this list. notify []*verifyFuture // notifyLock protects 'notify'. notifyLock sync.Mutex // stepDown is used to indicate to the leader that we // should step down based on information from a follower. stepDown chan struct{} // allowPipeline is used to determine when to pipeline the AppendEntries RPCs. // It is private to this replication goroutine. allowPipeline bool } // notifyAll is used to notify all the waiting verify futures // if the follower believes we are still the leader. func (s *followerReplication) notifyAll(leader bool) { // Clear the waiting notifies minimizing lock time s.notifyLock.Lock() n := s.notify s.notify = nil s.notifyLock.Unlock() // Submit our votes for _, v := range n { v.vote(leader) } } // LastContact returns the time of last contact. func (s *followerReplication) LastContact() time.Time { s.lastContactLock.RLock() last := s.lastContact s.lastContactLock.RUnlock() return last } // setLastContact sets the last contact to the current time. func (s *followerReplication) setLastContact() { s.lastContactLock.Lock() s.lastContact = time.Now() s.lastContactLock.Unlock() } // replicate is a long running routine that replicates log entries to a single // follower. func (r *Raft) replicate(s *followerReplication) { // Start an async heartbeating routing stopHeartbeat := make(chan struct{}) defer close(stopHeartbeat) r.goFunc(func() { r.heartbeat(s, stopHeartbeat) }) RPC: shouldStop := false for !shouldStop { select { case maxIndex := <-s.stopCh: // Make a best effort to replicate up to this index if maxIndex > 0 { r.replicateTo(s, maxIndex) } return case <-s.triggerCh: lastLogIdx, _ := r.getLastLog() shouldStop = r.replicateTo(s, lastLogIdx) case <-randomTimeout(r.conf.CommitTimeout): // TODO: what is this? lastLogIdx, _ := r.getLastLog() shouldStop = r.replicateTo(s, lastLogIdx) } // If things looks healthy, switch to pipeline mode if !shouldStop && s.allowPipeline { goto PIPELINE } } return PIPELINE: // Disable until re-enabled s.allowPipeline = false // Replicates using a pipeline for high performance. This method // is not able to gracefully recover from errors, and so we fall back // to standard mode on failure. if err := r.pipelineReplicate(s); err != nil { if err != ErrPipelineReplicationNotSupported { r.logger.Printf("[ERR] raft: Failed to start pipeline replication to %s: %s", s.peer, err) } } goto RPC } // replicateTo is a hepler to replicate(), used to replicate the logs up to a // given last index. // If the follower log is behind, we take care to bring them up to date. func (r *Raft) replicateTo(s *followerReplication, lastIndex uint64) (shouldStop bool) { // Create the base request var req AppendEntriesRequest var resp AppendEntriesResponse var start time.Time START: // Prevent an excessive retry rate on errors if s.failures > 0 { select { case <-time.After(backoff(failureWait, s.failures, maxFailureScale)): case <-r.shutdownCh: } } // Setup the request if err := r.setupAppendEntries(s, &req, s.nextIndex, lastIndex); err == ErrLogNotFound { goto SEND_SNAP } else if err != nil { return } // Make the RPC call start = time.Now() if err := r.trans.AppendEntries(s.peer.Address, &req, &resp); err != nil { r.logger.Printf("[ERR] raft: Failed to AppendEntries to %v: %v", s.peer, err) s.failures++ return } appendStats(string(s.peer.ID), start, float32(len(req.Entries))) // Check for a newer term, stop running if resp.Term > req.Term { r.handleStaleTerm(s) return true } // Update the last contact s.setLastContact() // Update s based on success if resp.Success { // Update our replication state updateLastAppended(s, &req) // Clear any failures, allow pipelining s.failures = 0 s.allowPipeline = true } else { s.nextIndex = max(min(s.nextIndex-1, resp.LastLog+1), 1) if resp.NoRetryBackoff { s.failures = 0 } else { s.failures++ } r.logger.Printf("[WARN] raft: AppendEntries to %v rejected, sending older logs (next: %d)", s.peer, s.nextIndex) } CHECK_MORE: // Poll the stop channel here in case we are looping and have been asked // to stop, or have stepped down as leader. Even for the best effort case // where we are asked to replicate to a given index and then shutdown, // it's better to not loop in here to send lots of entries to a straggler // that's leaving the cluster anyways. select { case <-s.stopCh: return true default: } // Check if there are more logs to replicate if s.nextIndex <= lastIndex { goto START } return // SEND_SNAP is used when we fail to get a log, usually because the follower // is too far behind, and we must ship a snapshot down instead SEND_SNAP: if stop, err := r.sendLatestSnapshot(s); stop { return true } else if err != nil { r.logger.Printf("[ERR] raft: Failed to send snapshot to %v: %v", s.peer, err) return } // Check if there is more to replicate goto CHECK_MORE } // sendLatestSnapshot is used to send the latest snapshot we have // down to our follower. func (r *Raft) sendLatestSnapshot(s *followerReplication) (bool, error) { // Get the snapshots snapshots, err := r.snapshots.List() if err != nil { r.logger.Printf("[ERR] raft: Failed to list snapshots: %v", err) return false, err } // Check we have at least a single snapshot if len(snapshots) == 0 { return false, fmt.Errorf("no snapshots found") } // Open the most recent snapshot snapID := snapshots[0].ID meta, snapshot, err := r.snapshots.Open(snapID) if err != nil { r.logger.Printf("[ERR] raft: Failed to open snapshot %v: %v", snapID, err) return false, err } defer snapshot.Close() // Setup the request req := InstallSnapshotRequest{ RPCHeader: r.getRPCHeader(), SnapshotVersion: meta.Version, Term: s.currentTerm, Leader: r.trans.EncodePeer(r.localAddr), LastLogIndex: meta.Index, LastLogTerm: meta.Term, Peers: meta.Peers, Size: meta.Size, Configuration: encodeConfiguration(meta.Configuration), ConfigurationIndex: meta.ConfigurationIndex, } // Make the call start := time.Now() var resp InstallSnapshotResponse if err := r.trans.InstallSnapshot(s.peer.Address, &req, &resp, snapshot); err != nil { r.logger.Printf("[ERR] raft: Failed to install snapshot %v: %v", snapID, err) s.failures++ return false, err } metrics.MeasureSince([]string{"raft", "replication", "installSnapshot", string(s.peer.ID)}, start) // Check for a newer term, stop running if resp.Term > req.Term { r.handleStaleTerm(s) return true, nil } // Update the last contact s.setLastContact() // Check for success if resp.Success { // Update the indexes s.nextIndex = meta.Index + 1 s.commitment.match(s.peer.ID, meta.Index) // Clear any failures s.failures = 0 // Notify we are still leader s.notifyAll(true) } else { s.failures++ r.logger.Printf("[WARN] raft: InstallSnapshot to %v rejected", s.peer) } return false, nil } // heartbeat is used to periodically invoke AppendEntries on a peer // to ensure they don't time out. This is done async of replicate(), // since that routine could potentially be blocked on disk IO. func (r *Raft) heartbeat(s *followerReplication, stopCh chan struct{}) { var failures uint64 req := AppendEntriesRequest{ RPCHeader: r.getRPCHeader(), Term: s.currentTerm, Leader: r.trans.EncodePeer(r.localAddr), } var resp AppendEntriesResponse for { // Wait for the next heartbeat interval or forced notify select { case <-s.notifyCh: case <-randomTimeout(r.conf.HeartbeatTimeout / 10): case <-stopCh: return } start := time.Now() if err := r.trans.AppendEntries(s.peer.Address, &req, &resp); err != nil { r.logger.Printf("[ERR] raft: Failed to heartbeat to %v: %v", s.peer.Address, err) failures++ select { case <-time.After(backoff(failureWait, failures, maxFailureScale)): case <-stopCh: } } else { s.setLastContact() failures = 0 metrics.MeasureSince([]string{"raft", "replication", "heartbeat", string(s.peer.ID)}, start) s.notifyAll(resp.Success) } } } // pipelineReplicate is used when we have synchronized our state with the follower, // and want to switch to a higher performance pipeline mode of replication. // We only pipeline AppendEntries commands, and if we ever hit an error, we fall // back to the standard replication which can handle more complex situations. func (r *Raft) pipelineReplicate(s *followerReplication) error { // Create a new pipeline pipeline, err := r.trans.AppendEntriesPipeline(s.peer.Address) if err != nil { return err } defer pipeline.Close() // Log start and stop of pipeline r.logger.Printf("[INFO] raft: pipelining replication to peer %v", s.peer) defer r.logger.Printf("[INFO] raft: aborting pipeline replication to peer %v", s.peer) // Create a shutdown and finish channel stopCh := make(chan struct{}) finishCh := make(chan struct{}) // Start a dedicated decoder r.goFunc(func() { r.pipelineDecode(s, pipeline, stopCh, finishCh) }) // Start pipeline sends at the last good nextIndex nextIndex := s.nextIndex shouldStop := false SEND: for !shouldStop { select { case <-finishCh: break SEND case maxIndex := <-s.stopCh: // Make a best effort to replicate up to this index if maxIndex > 0 { r.pipelineSend(s, pipeline, &nextIndex, maxIndex) } break SEND case <-s.triggerCh: lastLogIdx, _ := r.getLastLog() shouldStop = r.pipelineSend(s, pipeline, &nextIndex, lastLogIdx) case <-randomTimeout(r.conf.CommitTimeout): lastLogIdx, _ := r.getLastLog() shouldStop = r.pipelineSend(s, pipeline, &nextIndex, lastLogIdx) } } // Stop our decoder, and wait for it to finish close(stopCh) select { case <-finishCh: case <-r.shutdownCh: } return nil } // pipelineSend is used to send data over a pipeline. It is a helper to // pipelineReplicate. func (r *Raft) pipelineSend(s *followerReplication, p AppendPipeline, nextIdx *uint64, lastIndex uint64) (shouldStop bool) { // Create a new append request req := new(AppendEntriesRequest) if err := r.setupAppendEntries(s, req, *nextIdx, lastIndex); err != nil { return true } // Pipeline the append entries if _, err := p.AppendEntries(req, new(AppendEntriesResponse)); err != nil { r.logger.Printf("[ERR] raft: Failed to pipeline AppendEntries to %v: %v", s.peer, err) return true } // Increase the next send log to avoid re-sending old logs if n := len(req.Entries); n > 0 { last := req.Entries[n-1] *nextIdx = last.Index + 1 } return false } // pipelineDecode is used to decode the responses of pipelined requests. func (r *Raft) pipelineDecode(s *followerReplication, p AppendPipeline, stopCh, finishCh chan struct{}) { defer close(finishCh) respCh := p.Consumer() for { select { case ready := <-respCh: req, resp := ready.Request(), ready.Response() appendStats(string(s.peer.ID), ready.Start(), float32(len(req.Entries))) // Check for a newer term, stop running if resp.Term > req.Term { r.handleStaleTerm(s) return } // Update the last contact s.setLastContact() // Abort pipeline if not successful if !resp.Success { return } // Update our replication state updateLastAppended(s, req) case <-stopCh: return } } } // setupAppendEntries is used to setup an append entries request. func (r *Raft) setupAppendEntries(s *followerReplication, req *AppendEntriesRequest, nextIndex, lastIndex uint64) error { req.RPCHeader = r.getRPCHeader() req.Term = s.currentTerm req.Leader = r.trans.EncodePeer(r.localAddr) req.LeaderCommitIndex = r.getCommitIndex() if err := r.setPreviousLog(req, nextIndex); err != nil { return err } if err := r.setNewLogs(req, nextIndex, lastIndex); err != nil { return err } return nil } // setPreviousLog is used to setup the PrevLogEntry and PrevLogTerm for an // AppendEntriesRequest given the next index to replicate. func (r *Raft) setPreviousLog(req *AppendEntriesRequest, nextIndex uint64) error { // Guard for the first index, since there is no 0 log entry // Guard against the previous index being a snapshot as well lastSnapIdx, lastSnapTerm := r.getLastSnapshot() if nextIndex == 1 { req.PrevLogEntry = 0 req.PrevLogTerm = 0 } else if (nextIndex - 1) == lastSnapIdx { req.PrevLogEntry = lastSnapIdx req.PrevLogTerm = lastSnapTerm } else { var l Log if err := r.logs.GetLog(nextIndex-1, &l); err != nil { r.logger.Printf("[ERR] raft: Failed to get log at index %d: %v", nextIndex-1, err) return err } // Set the previous index and term (0 if nextIndex is 1) req.PrevLogEntry = l.Index req.PrevLogTerm = l.Term } return nil } // setNewLogs is used to setup the logs which should be appended for a request. func (r *Raft) setNewLogs(req *AppendEntriesRequest, nextIndex, lastIndex uint64) error { // Append up to MaxAppendEntries or up to the lastIndex req.Entries = make([]*Log, 0, r.conf.MaxAppendEntries) maxIndex := min(nextIndex+uint64(r.conf.MaxAppendEntries)-1, lastIndex) for i := nextIndex; i <= maxIndex; i++ { oldLog := new(Log) if err := r.logs.GetLog(i, oldLog); err != nil { r.logger.Printf("[ERR] raft: Failed to get log at index %d: %v", i, err) return err } req.Entries = append(req.Entries, oldLog) } return nil } // appendStats is used to emit stats about an AppendEntries invocation. func appendStats(peer string, start time.Time, logs float32) { metrics.MeasureSince([]string{"raft", "replication", "appendEntries", "rpc", peer}, start) metrics.IncrCounter([]string{"raft", "replication", "appendEntries", "logs", peer}, logs) } // handleStaleTerm is used when a follower indicates that we have a stale term. func (r *Raft) handleStaleTerm(s *followerReplication) { r.logger.Printf("[ERR] raft: peer %v has newer term, stopping replication", s.peer) s.notifyAll(false) // No longer leader asyncNotifyCh(s.stepDown) } // updateLastAppended is used to update follower replication state after a // successful AppendEntries RPC. // TODO: This isn't used during InstallSnapshot, but the code there is similar. func updateLastAppended(s *followerReplication, req *AppendEntriesRequest) { // Mark any inflight logs as committed if logs := req.Entries; len(logs) > 0 { last := logs[len(logs)-1] s.nextIndex = last.Index + 1 s.commitment.match(s.peer.ID, last.Index) } // Notify still leader s.notifyAll(true) }