diff --git a/vendor/go.etcd.io/raft/v3/log.go b/vendor/go.etcd.io/raft/v3/log.go
new file mode 100644
index 0000000..bd7c2fe
--- /dev/null
+++ b/vendor/go.etcd.io/raft/v3/log.go
@@ -0,0 +1,574 @@
+// 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 (
+	"fmt"
+
+	pb "go.etcd.io/raft/v3/raftpb"
+)
+
+type raftLog struct {
+	// storage contains all stable entries since the last snapshot.
+	storage Storage
+
+	// unstable contains all unstable entries and snapshot.
+	// they will be saved into storage.
+	unstable unstable
+
+	// committed is the highest log position that is known to be in
+	// stable storage on a quorum of nodes.
+	committed uint64
+	// applying is the highest log position that the application has
+	// been instructed to apply to its state machine. Some of these
+	// entries may be in the process of applying and have not yet
+	// reached applied.
+	// Use: The field is incremented when accepting a Ready struct.
+	// Invariant: applied <= applying && applying <= committed
+	applying uint64
+	// applied is the highest log position that the application has
+	// successfully applied to its state machine.
+	// Use: The field is incremented when advancing after the committed
+	// entries in a Ready struct have been applied (either synchronously
+	// or asynchronously).
+	// Invariant: applied <= committed
+	applied uint64
+
+	logger Logger
+
+	// maxApplyingEntsSize limits the outstanding byte size of the messages
+	// returned from calls to nextCommittedEnts that have not been acknowledged
+	// by a call to appliedTo.
+	maxApplyingEntsSize entryEncodingSize
+	// applyingEntsSize is the current outstanding byte size of the messages
+	// returned from calls to nextCommittedEnts that have not been acknowledged
+	// by a call to appliedTo.
+	applyingEntsSize entryEncodingSize
+	// applyingEntsPaused is true when entry application has been paused until
+	// enough progress is acknowledged.
+	applyingEntsPaused bool
+}
+
+// newLog returns log using the given storage and default options. It
+// recovers the log to the state that it just commits and applies the
+// latest snapshot.
+func newLog(storage Storage, logger Logger) *raftLog {
+	return newLogWithSize(storage, logger, noLimit)
+}
+
+// newLogWithSize returns a log using the given storage and max
+// message size.
+func newLogWithSize(storage Storage, logger Logger, maxApplyingEntsSize entryEncodingSize) *raftLog {
+	firstIndex, err := storage.FirstIndex()
+	if err != nil {
+		panic(err) // TODO(bdarnell)
+	}
+	lastIndex, err := storage.LastIndex()
+	if err != nil {
+		panic(err) // TODO(bdarnell)
+	}
+	return &raftLog{
+		storage: storage,
+		unstable: unstable{
+			offset:           lastIndex + 1,
+			offsetInProgress: lastIndex + 1,
+			logger:           logger,
+		},
+		maxApplyingEntsSize: maxApplyingEntsSize,
+
+		// Initialize our committed and applied pointers to the time of the last compaction.
+		committed: firstIndex - 1,
+		applying:  firstIndex - 1,
+		applied:   firstIndex - 1,
+
+		logger: logger,
+	}
+}
+
+func (l *raftLog) String() string {
+	return fmt.Sprintf("committed=%d, applied=%d, applying=%d, unstable.offset=%d, unstable.offsetInProgress=%d, len(unstable.Entries)=%d",
+		l.committed, l.applied, l.applying, l.unstable.offset, l.unstable.offsetInProgress, len(l.unstable.entries))
+}
+
+// maybeAppend returns (0, false) if the entries cannot be appended. Otherwise,
+// it returns (last index of new entries, true).
+func (l *raftLog) maybeAppend(a logSlice, committed uint64) (lastnewi uint64, ok bool) {
+	if !l.matchTerm(a.prev) {
+		return 0, false
+	}
+	// TODO(pav-kv): propagate logSlice down the stack. It will be used all the
+	// way down in unstable, for safety checks, and for useful bookkeeping.
+
+	lastnewi = a.prev.index + uint64(len(a.entries))
+	ci := l.findConflict(a.entries)
+	switch {
+	case ci == 0:
+	case ci <= l.committed:
+		l.logger.Panicf("entry %d conflict with committed entry [committed(%d)]", ci, l.committed)
+	default:
+		offset := a.prev.index + 1
+		if ci-offset > uint64(len(a.entries)) {
+			l.logger.Panicf("index, %d, is out of range [%d]", ci-offset, len(a.entries))
+		}
+		l.append(a.entries[ci-offset:]...)
+	}
+	l.commitTo(min(committed, lastnewi))
+	return lastnewi, true
+}
+
+func (l *raftLog) append(ents ...pb.Entry) uint64 {
+	if len(ents) == 0 {
+		return l.lastIndex()
+	}
+	if after := ents[0].Index - 1; after < l.committed {
+		l.logger.Panicf("after(%d) is out of range [committed(%d)]", after, l.committed)
+	}
+	l.unstable.truncateAndAppend(ents)
+	return l.lastIndex()
+}
+
+// findConflict finds the index of the conflict.
+// It returns the first pair of conflicting entries between the existing
+// entries and the given entries, if there are any.
+// If there is no conflicting entries, and the existing entries contains
+// all the given entries, zero will be returned.
+// If there is no conflicting entries, but the given entries contains new
+// entries, the index of the first new entry will be returned.
+// An entry is considered to be conflicting if it has the same index but
+// a different term.
+// The index of the given entries MUST be continuously increasing.
+func (l *raftLog) findConflict(ents []pb.Entry) uint64 {
+	for i := range ents {
+		if id := pbEntryID(&ents[i]); !l.matchTerm(id) {
+			if id.index <= l.lastIndex() {
+				// TODO(pav-kv): can simply print %+v of the id. This will change the
+				// log format though.
+				l.logger.Infof("found conflict at index %d [existing term: %d, conflicting term: %d]",
+					id.index, l.zeroTermOnOutOfBounds(l.term(id.index)), id.term)
+			}
+			return id.index
+		}
+	}
+	return 0
+}
+
+// findConflictByTerm returns a best guess on where this log ends matching
+// another log, given that the only information known about the other log is the
+// (index, term) of its single entry.
+//
+// Specifically, the first returned value is the max guessIndex <= index, such
+// that term(guessIndex) <= term or term(guessIndex) is not known (because this
+// index is compacted or not yet stored).
+//
+// The second returned value is the term(guessIndex), or 0 if it is unknown.
+//
+// This function is used by a follower and leader to resolve log conflicts after
+// an unsuccessful append to a follower, and ultimately restore the steady flow
+// of appends.
+func (l *raftLog) findConflictByTerm(index uint64, term uint64) (uint64, uint64) {
+	for ; index > 0; index-- {
+		// If there is an error (likely ErrCompacted or ErrUnavailable), we don't
+		// know whether it's a match or not, so assume a possible match and return
+		// the index, with 0 term indicating an unknown term.
+		if ourTerm, err := l.term(index); err != nil {
+			return index, 0
+		} else if ourTerm <= term {
+			return index, ourTerm
+		}
+	}
+	return 0, 0
+}
+
+// nextUnstableEnts returns all entries that are available to be written to the
+// local stable log and are not already in-progress.
+func (l *raftLog) nextUnstableEnts() []pb.Entry {
+	return l.unstable.nextEntries()
+}
+
+// hasNextUnstableEnts returns if there are any entries that are available to be
+// written to the local stable log and are not already in-progress.
+func (l *raftLog) hasNextUnstableEnts() bool {
+	return len(l.nextUnstableEnts()) > 0
+}
+
+// hasNextOrInProgressUnstableEnts returns if there are any entries that are
+// available to be written to the local stable log or in the process of being
+// written to the local stable log.
+func (l *raftLog) hasNextOrInProgressUnstableEnts() bool {
+	return len(l.unstable.entries) > 0
+}
+
+// nextCommittedEnts returns all the available entries for execution.
+// Entries can be committed even when the local raft instance has not durably
+// appended them to the local raft log yet. If allowUnstable is true, committed
+// entries from the unstable log may be returned; otherwise, only entries known
+// to reside locally on stable storage will be returned.
+func (l *raftLog) nextCommittedEnts(allowUnstable bool) (ents []pb.Entry) {
+	if l.applyingEntsPaused {
+		// Entry application outstanding size limit reached.
+		return nil
+	}
+	if l.hasNextOrInProgressSnapshot() {
+		// See comment in hasNextCommittedEnts.
+		return nil
+	}
+	lo, hi := l.applying+1, l.maxAppliableIndex(allowUnstable)+1 // [lo, hi)
+	if lo >= hi {
+		// Nothing to apply.
+		return nil
+	}
+	maxSize := l.maxApplyingEntsSize - l.applyingEntsSize
+	if maxSize <= 0 {
+		l.logger.Panicf("applying entry size (%d-%d)=%d not positive",
+			l.maxApplyingEntsSize, l.applyingEntsSize, maxSize)
+	}
+	ents, err := l.slice(lo, hi, maxSize)
+	if err != nil {
+		l.logger.Panicf("unexpected error when getting unapplied entries (%v)", err)
+	}
+	return ents
+}
+
+// hasNextCommittedEnts returns if there is any available entries for execution.
+// This is a fast check without heavy raftLog.slice() in nextCommittedEnts().
+func (l *raftLog) hasNextCommittedEnts(allowUnstable bool) bool {
+	if l.applyingEntsPaused {
+		// Entry application outstanding size limit reached.
+		return false
+	}
+	if l.hasNextOrInProgressSnapshot() {
+		// If we have a snapshot to apply, don't also return any committed
+		// entries. Doing so raises questions about what should be applied
+		// first.
+		return false
+	}
+	lo, hi := l.applying+1, l.maxAppliableIndex(allowUnstable)+1 // [lo, hi)
+	return lo < hi
+}
+
+// maxAppliableIndex returns the maximum committed index that can be applied.
+// If allowUnstable is true, committed entries from the unstable log can be
+// applied; otherwise, only entries known to reside locally on stable storage
+// can be applied.
+func (l *raftLog) maxAppliableIndex(allowUnstable bool) uint64 {
+	hi := l.committed
+	if !allowUnstable {
+		hi = min(hi, l.unstable.offset-1)
+	}
+	return hi
+}
+
+// nextUnstableSnapshot returns the snapshot, if present, that is available to
+// be applied to the local storage and is not already in-progress.
+func (l *raftLog) nextUnstableSnapshot() *pb.Snapshot {
+	return l.unstable.nextSnapshot()
+}
+
+// hasNextUnstableSnapshot returns if there is a snapshot that is available to
+// be applied to the local storage and is not already in-progress.
+func (l *raftLog) hasNextUnstableSnapshot() bool {
+	return l.unstable.nextSnapshot() != nil
+}
+
+// hasNextOrInProgressSnapshot returns if there is pending snapshot waiting for
+// applying or in the process of being applied.
+func (l *raftLog) hasNextOrInProgressSnapshot() bool {
+	return l.unstable.snapshot != nil
+}
+
+func (l *raftLog) snapshot() (pb.Snapshot, error) {
+	if l.unstable.snapshot != nil {
+		return *l.unstable.snapshot, nil
+	}
+	return l.storage.Snapshot()
+}
+
+func (l *raftLog) firstIndex() uint64 {
+	if i, ok := l.unstable.maybeFirstIndex(); ok {
+		return i
+	}
+	index, err := l.storage.FirstIndex()
+	if err != nil {
+		panic(err) // TODO(bdarnell)
+	}
+	return index
+}
+
+func (l *raftLog) lastIndex() uint64 {
+	if i, ok := l.unstable.maybeLastIndex(); ok {
+		return i
+	}
+	i, err := l.storage.LastIndex()
+	if err != nil {
+		panic(err) // TODO(bdarnell)
+	}
+	return i
+}
+
+func (l *raftLog) commitTo(tocommit uint64) {
+	// never decrease commit
+	if l.committed < tocommit {
+		if l.lastIndex() < tocommit {
+			l.logger.Panicf("tocommit(%d) is out of range [lastIndex(%d)]. Was the raft log corrupted, truncated, or lost?", tocommit, l.lastIndex())
+		}
+		l.committed = tocommit
+	}
+}
+
+func (l *raftLog) appliedTo(i uint64, size entryEncodingSize) {
+	if l.committed < i || i < l.applied {
+		l.logger.Panicf("applied(%d) is out of range [prevApplied(%d), committed(%d)]", i, l.applied, l.committed)
+	}
+	l.applied = i
+	l.applying = max(l.applying, i)
+	if l.applyingEntsSize > size {
+		l.applyingEntsSize -= size
+	} else {
+		// Defense against underflow.
+		l.applyingEntsSize = 0
+	}
+	l.applyingEntsPaused = l.applyingEntsSize >= l.maxApplyingEntsSize
+}
+
+func (l *raftLog) acceptApplying(i uint64, size entryEncodingSize, allowUnstable bool) {
+	if l.committed < i {
+		l.logger.Panicf("applying(%d) is out of range [prevApplying(%d), committed(%d)]", i, l.applying, l.committed)
+	}
+	l.applying = i
+	l.applyingEntsSize += size
+	// Determine whether to pause entry application until some progress is
+	// acknowledged. We pause in two cases:
+	// 1. the outstanding entry size equals or exceeds the maximum size.
+	// 2. the outstanding entry size does not equal or exceed the maximum size,
+	//    but we determine that the next entry in the log will push us over the
+	//    limit. We determine this by comparing the last entry returned from
+	//    raftLog.nextCommittedEnts to the maximum entry that the method was
+	//    allowed to return had there been no size limit. If these indexes are
+	//    not equal, then the returned entries slice must have been truncated to
+	//    adhere to the memory limit.
+	l.applyingEntsPaused = l.applyingEntsSize >= l.maxApplyingEntsSize ||
+		i < l.maxAppliableIndex(allowUnstable)
+}
+
+func (l *raftLog) stableTo(id entryID) { l.unstable.stableTo(id) }
+
+func (l *raftLog) stableSnapTo(i uint64) { l.unstable.stableSnapTo(i) }
+
+// acceptUnstable indicates that the application has started persisting the
+// unstable entries in storage, and that the current unstable entries are thus
+// to be marked as being in-progress, to avoid returning them with future calls
+// to Ready().
+func (l *raftLog) acceptUnstable() { l.unstable.acceptInProgress() }
+
+// lastEntryID returns the ID of the last entry in the log.
+func (l *raftLog) lastEntryID() entryID {
+	index := l.lastIndex()
+	t, err := l.term(index)
+	if err != nil {
+		l.logger.Panicf("unexpected error when getting the last term at %d: %v", index, err)
+	}
+	return entryID{term: t, index: index}
+}
+
+func (l *raftLog) term(i uint64) (uint64, error) {
+	// Check the unstable log first, even before computing the valid term range,
+	// which may need to access stable Storage. If we find the entry's term in
+	// the unstable log, we know it was in the valid range.
+	if t, ok := l.unstable.maybeTerm(i); ok {
+		return t, nil
+	}
+
+	// The valid term range is [firstIndex-1, lastIndex]. Even though the entry at
+	// firstIndex-1 is compacted away, its term is available for matching purposes
+	// when doing log appends.
+	if i+1 < l.firstIndex() {
+		return 0, ErrCompacted
+	}
+	if i > l.lastIndex() {
+		return 0, ErrUnavailable
+	}
+
+	t, err := l.storage.Term(i)
+	if err == nil {
+		return t, nil
+	}
+	if err == ErrCompacted || err == ErrUnavailable {
+		return 0, err
+	}
+	panic(err) // TODO(bdarnell)
+}
+
+func (l *raftLog) entries(i uint64, maxSize entryEncodingSize) ([]pb.Entry, error) {
+	if i > l.lastIndex() {
+		return nil, nil
+	}
+	return l.slice(i, l.lastIndex()+1, maxSize)
+}
+
+// allEntries returns all entries in the log.
+func (l *raftLog) allEntries() []pb.Entry {
+	ents, err := l.entries(l.firstIndex(), noLimit)
+	if err == nil {
+		return ents
+	}
+	if err == ErrCompacted { // try again if there was a racing compaction
+		return l.allEntries()
+	}
+	// TODO (xiangli): handle error?
+	panic(err)
+}
+
+// isUpToDate determines if a log with the given last entry is more up-to-date
+// by comparing the index and term of the last entries in the existing logs.
+//
+// If the logs have last entries with different terms, then the log with the
+// later term is more up-to-date. If the logs end with the same term, then
+// whichever log has the larger lastIndex is more up-to-date. If the logs are
+// the same, the given log is up-to-date.
+func (l *raftLog) isUpToDate(their entryID) bool {
+	our := l.lastEntryID()
+	return their.term > our.term || their.term == our.term && their.index >= our.index
+}
+
+func (l *raftLog) matchTerm(id entryID) bool {
+	t, err := l.term(id.index)
+	if err != nil {
+		return false
+	}
+	return t == id.term
+}
+
+func (l *raftLog) maybeCommit(at entryID) bool {
+	// NB: term should never be 0 on a commit because the leader campaigned at
+	// least at term 1. But if it is 0 for some reason, we don't consider this a
+	// term match.
+	if at.term != 0 && at.index > l.committed && l.matchTerm(at) {
+		l.commitTo(at.index)
+		return true
+	}
+	return false
+}
+
+func (l *raftLog) restore(s pb.Snapshot) {
+	l.logger.Infof("log [%s] starts to restore snapshot [index: %d, term: %d]", l, s.Metadata.Index, s.Metadata.Term)
+	l.committed = s.Metadata.Index
+	l.unstable.restore(s)
+}
+
+// scan visits all log entries in the [lo, hi) range, returning them via the
+// given callback. The callback can be invoked multiple times, with consecutive
+// sub-ranges of the requested range. Returns up to pageSize bytes worth of
+// entries at a time. May return more if a single entry size exceeds the limit.
+//
+// The entries in [lo, hi) must exist, otherwise scan() eventually returns an
+// error (possibly after passing some entries through the callback).
+//
+// If the callback returns an error, scan terminates and returns this error
+// immediately. This can be used to stop the scan early ("break" the loop).
+func (l *raftLog) scan(lo, hi uint64, pageSize entryEncodingSize, v func([]pb.Entry) error) error {
+	for lo < hi {
+		ents, err := l.slice(lo, hi, pageSize)
+		if err != nil {
+			return err
+		} else if len(ents) == 0 {
+			return fmt.Errorf("got 0 entries in [%d, %d)", lo, hi)
+		}
+		if err := v(ents); err != nil {
+			return err
+		}
+		lo += uint64(len(ents))
+	}
+	return nil
+}
+
+// slice returns a slice of log entries from lo through hi-1, inclusive.
+func (l *raftLog) slice(lo, hi uint64, maxSize entryEncodingSize) ([]pb.Entry, error) {
+	if err := l.mustCheckOutOfBounds(lo, hi); err != nil {
+		return nil, err
+	}
+	if lo == hi {
+		return nil, nil
+	}
+	if lo >= l.unstable.offset {
+		ents := limitSize(l.unstable.slice(lo, hi), maxSize)
+		// NB: use the full slice expression to protect the unstable slice from
+		// appends to the returned ents slice.
+		return ents[:len(ents):len(ents)], nil
+	}
+
+	cut := min(hi, l.unstable.offset)
+	ents, err := l.storage.Entries(lo, cut, uint64(maxSize))
+	if err == ErrCompacted {
+		return nil, err
+	} else if err == ErrUnavailable {
+		l.logger.Panicf("entries[%d:%d) is unavailable from storage", lo, cut)
+	} else if err != nil {
+		panic(err) // TODO(pavelkalinnikov): handle errors uniformly
+	}
+	if hi <= l.unstable.offset {
+		return ents, nil
+	}
+
+	// Fast path to check if ents has reached the size limitation. Either the
+	// returned slice is shorter than requested (which means the next entry would
+	// bring it over the limit), or a single entry reaches the limit.
+	if uint64(len(ents)) < cut-lo {
+		return ents, nil
+	}
+	// Slow path computes the actual total size, so that unstable entries are cut
+	// optimally before being copied to ents slice.
+	size := entsSize(ents)
+	if size >= maxSize {
+		return ents, nil
+	}
+
+	unstable := limitSize(l.unstable.slice(l.unstable.offset, hi), maxSize-size)
+	// Total size of unstable may exceed maxSize-size only if len(unstable) == 1.
+	// If this happens, ignore this extra entry.
+	if len(unstable) == 1 && size+entsSize(unstable) > maxSize {
+		return ents, nil
+	}
+	// Otherwise, total size of unstable does not exceed maxSize-size, so total
+	// size of ents+unstable does not exceed maxSize. Simply concatenate them.
+	return extend(ents, unstable), nil
+}
+
+// l.firstIndex <= lo <= hi <= l.firstIndex + len(l.entries)
+func (l *raftLog) mustCheckOutOfBounds(lo, hi uint64) error {
+	if lo > hi {
+		l.logger.Panicf("invalid slice %d > %d", lo, hi)
+	}
+	fi := l.firstIndex()
+	if lo < fi {
+		return ErrCompacted
+	}
+
+	length := l.lastIndex() + 1 - fi
+	if hi > fi+length {
+		l.logger.Panicf("slice[%d,%d) out of bound [%d,%d]", lo, hi, fi, l.lastIndex())
+	}
+	return nil
+}
+
+func (l *raftLog) zeroTermOnOutOfBounds(t uint64, err error) uint64 {
+	if err == nil {
+		return t
+	}
+	if err == ErrCompacted || err == ErrUnavailable {
+		return 0
+	}
+	l.logger.Panicf("unexpected error (%v)", err)
+	return 0
+}