Add Storage.Term() method and hide the first entry from other methods.

The first entry in the log is a dummy which is used for matchTerm
but may not have an actual payload. This change permits Storage
implementations to treat this term value specially instead of
storing it as a dummy Entry.

Storage.FirstIndex() no longer includes the term-only entry.

This reverses a recent decision to create entry zero as initially
unstable; Storage implementations are now required to make
Term(0) == 0 and the first unstable entry is now index 1.
stableTo(0) is no longer allowed.

etcdserver/server_test.go

@@ -840,9 +840,29 @@ func TestSnapshot(t *testing.T) {
 	n := raft.StartNode(0xBAD0, mustMakePeerSlice(t, 0xBAD0), 10, 1, s)
 	defer n.Stop()
 
-	// Save the initial state to storage so we have something to snapshot.
+	// Progress the node to the point where it has something to snapshot.
+	// TODO(bdarnell): this could be improved with changes in the raft internals.
+	// First, we must apply the initial conf changes so we can have an election.
 	rd := <-n.Ready()
 	s.Append(rd.Entries)
+	for _, e := range rd.CommittedEntries {
+		if e.Type == raftpb.EntryConfChange {
+			var cc raftpb.ConfChange
+			err := cc.Unmarshal(e.Data)
+			if err != nil {
+				t.Fatal(err)
+			}
+			n.ApplyConfChange(cc)
+		}
+	}
+	n.Advance()
+
+	// Now we can have an election and persist the rest of the log.
+	// This causes HardState.Commit to advance. HardState.Commit must
+	// be > 0 to snapshot.
+	n.Campaign(context.Background())
+	rd = <-n.Ready()
+	s.Append(rd.Entries)
 	n.Advance()
 
 	st := &storeRecorder{}
@@ -854,7 +874,7 @@ func TestSnapshot(t *testing.T) {
 		raftStorage: s,
 	}
 
-	srv.snapshot(0, []uint64{1})
+	srv.snapshot(1, []uint64{1})
 	gaction := st.Action()
 	if len(gaction) != 1 {
 		t.Fatalf("len(action) = %d, want 1", len(gaction))

raft/log.go

@@ -53,14 +53,11 @@ func newLog(storage Storage) *raftLog {
 		storage: storage,
 	}
 	lastIndex, err := storage.LastIndex()
-	if err == ErrStorageEmpty {
-		// When starting from scratch populate the list with a dummy entry at term zero.
-		log.unstableEnts = make([]pb.Entry, 1)
-	} else if err == nil {
-		log.unstable = lastIndex + 1
-	} else {
+	if err != nil {
 		panic(err) // TODO(bdarnell)
 	}
+	log.unstable = lastIndex + 1
+
 	return log
 }
 
@@ -190,27 +187,28 @@ func (l *raftLog) lastTerm() uint64 {
 }
 
 func (l *raftLog) term(i uint64) uint64 {
-	if e := l.at(i); e != nil {
-		return e.Term
+	if i < l.unstable {
+		t, err := l.storage.Term(i)
+		if err == ErrSnapshotRequired {
+			return 0
+		} else if err != nil {
+			panic(err) // TODO(bdarnell)
+		}
+		return t
 	}
-	return 0
+	if i >= l.unstable+uint64(len(l.unstableEnts)) {
+		return 0
+	}
+	return l.unstableEnts[i-l.unstable].Term
 }
 
 func (l *raftLog) entries(i uint64) []pb.Entry {
-	// never send out the first entry
-	// first entry is only used for matching
-	// prevLogTerm
-	if i == 0 {
-		panic("cannot return the first entry in log")
-	}
 	return l.slice(i, l.lastIndex()+1)
 }
 
-// allEntries returns all entries in the log, including the initial
-// entry that is only used for prevLogTerm validation. This method
-// should only be used for testing.
+// allEntries returns all entries in the log.
 func (l *raftLog) allEntries() []pb.Entry {
-	return l.slice(l.firstIndex(), l.lastIndex()+1)
+	return l.entries(l.firstIndex())
 }
 
 // isUpToDate determines if the given (lastIndex,term) log is more up-to-date
@@ -224,10 +222,7 @@ func (l *raftLog) isUpToDate(lasti, term uint64) bool {
 }
 
 func (l *raftLog) matchTerm(i, term uint64) bool {
-	if e := l.at(i); e != nil {
-		return e.Term == term
-	}
-	return false
+	return l.term(i) == term
 }
 
 func (l *raftLog) maybeCommit(maxIndex, term uint64) bool {
@@ -303,7 +298,9 @@ func (l *raftLog) slice(lo uint64, hi uint64) []pb.Entry {
 	var ents []pb.Entry
 	if lo < l.unstable {
 		storedEnts, err := l.storage.Entries(lo, min(hi, l.unstable))
-		if err != nil {
+		if err == ErrSnapshotRequired {
+			return nil
+		} else if err != nil {
 			panic(err) // TODO(bdarnell)
 		}
 		ents = append(ents, storedEnts...)

raft/log_test.go

@@ -91,7 +91,7 @@ func TestIsUpToDate(t *testing.T) {
 }
 
 func TestAppend(t *testing.T) {
-	previousEnts := []pb.Entry{{}, {Term: 1}, {Term: 2}}
+	previousEnts := []pb.Entry{{Term: 1}, {Term: 2}}
 	tests := []struct {
 		after     uint64
 		ents      []pb.Entry
@@ -283,7 +283,7 @@ func TestCompactionSideEffects(t *testing.T) {
 	unstableIndex := uint64(750)
 	lastTerm := lastIndex
 	storage := NewMemoryStorage()
-	for i = 0; i <= unstableIndex; i++ {
+	for i = 1; i <= unstableIndex; i++ {
 		storage.Append([]pb.Entry{{Term: uint64(i), Index: uint64(i)}})
 	}
 	raftLog := newLog(storage)
@@ -337,22 +337,23 @@ func TestCompactionSideEffects(t *testing.T) {
 }
 
 func TestUnstableEnts(t *testing.T) {
-	previousEnts := []pb.Entry{{}, {Term: 1, Index: 1}, {Term: 2, Index: 2}}
+	previousEnts := []pb.Entry{{Term: 1, Index: 1}, {Term: 2, Index: 2}}
 	tests := []struct {
 		unstable  uint64
 		wents     []pb.Entry
 		wunstable uint64
 	}{
 		{3, nil, 3},
-		{1, previousEnts[1:], 3},
-		{0, append([]pb.Entry{{}}, previousEnts...), 3},
+		{1, previousEnts, 3},
 	}
 
 	for i, tt := range tests {
 		storage := NewMemoryStorage()
-		storage.Append(previousEnts[:tt.unstable])
+		if tt.unstable > 0 {
+			storage.Append(previousEnts[:tt.unstable-1])
+		}
 		raftLog := newLog(storage)
-		raftLog.append(raftLog.lastIndex(), previousEnts[tt.unstable:]...)
+		raftLog.append(raftLog.lastIndex(), previousEnts[tt.unstable-1:]...)
 		ents := raftLog.unstableEntries()
 		if l := len(ents); l > 0 {
 			raftLog.stableTo(ents[l-1].Index)
@@ -371,7 +372,6 @@ func TestStableTo(t *testing.T) {
 		stable    uint64
 		wunstable uint64
 	}{
-		{0, 1},
 		{1, 2},
 		{2, 3},
 	}
@@ -396,9 +396,9 @@ func TestCompaction(t *testing.T) {
 	}{
 		// out of upper bound
 		{1000, 1000, []uint64{1001}, []int{-1}, false},
-		{1000, 1000, []uint64{300, 500, 800, 900}, []int{701, 501, 201, 101}, true},
+		{1000, 1000, []uint64{300, 500, 800, 900}, []int{700, 500, 200, 100}, true},
 		// out of lower bound
-		{1000, 1000, []uint64{300, 299}, []int{701, -1}, false},
+		{1000, 1000, []uint64{300, 299}, []int{700, -1}, false},
 		{0, 1000, []uint64{1}, []int{-1}, false},
 	}
 
@@ -413,7 +413,7 @@ func TestCompaction(t *testing.T) {
 			}()
 
 			storage := NewMemoryStorage()
-			for i := uint64(0); i <= tt.lastIndex; i++ {
+			for i := uint64(1); i <= tt.lastIndex; i++ {
 				storage.Append([]pb.Entry{{}})
 			}
 			raftLog := newLog(storage)
@@ -442,11 +442,11 @@ func TestLogRestore(t *testing.T) {
 	raftLog.restore(pb.Snapshot{Index: index, Term: term})
 
 	// only has the guard entry
-	if len(raftLog.allEntries()) != 1 {
+	if len(raftLog.allEntries()) != 0 {
 		t.Errorf("len = %d, want 1", len(raftLog.allEntries()))
 	}
-	if raftLog.firstIndex() != index {
-		t.Errorf("firstIndex = %d, want %d", raftLog.firstIndex(), index)
+	if raftLog.firstIndex() != index+1 {
+		t.Errorf("firstIndex = %d, want %d", raftLog.firstIndex(), index+1)
 	}
 	if raftLog.applied != index {
 		t.Errorf("applied = %d, want %d", raftLog.applied, index)
@@ -474,7 +474,7 @@ func TestIsOutOfBounds(t *testing.T) {
 		w     bool
 	}{
 		{offset - 1, true},
-		{offset, false},
+		{offset, true},
 		{offset + num/2, false},
 		{offset + num, false},
 		{offset + num + 1, true},
@@ -504,7 +504,7 @@ func TestAt(t *testing.T) {
 		w     *pb.Entry
 	}{
 		{offset - 1, nil},
-		{offset, &pb.Entry{Term: 0}},
+		{offset, nil},
 		{offset + num/2, &pb.Entry{Term: num / 2}},
 		{offset + num - 1, &pb.Entry{Term: num - 1}},
 		{offset + num, nil},
@@ -518,6 +518,36 @@ func TestAt(t *testing.T) {
 	}
 }
 
+func TestTerm(t *testing.T) {
+	var i uint64
+	offset := uint64(100)
+	num := uint64(100)
+
+	l := newLog(NewMemoryStorage())
+	l.restore(pb.Snapshot{Index: offset})
+	for i = 1; i < num; i++ {
+		l.append(offset+i-1, pb.Entry{Term: i})
+	}
+
+	tests := []struct {
+		index uint64
+		w     uint64
+	}{
+		{offset - 1, 0},
+		{offset, 0},
+		{offset + num/2, num / 2},
+		{offset + num - 1, num - 1},
+		{offset + num, 0},
+	}
+
+	for i, tt := range tests {
+		term := l.term(tt.index)
+		if !reflect.DeepEqual(term, tt.w) {
+			t.Errorf("#%d: at = %d, want %d", i, term, tt.w)
+		}
+	}
+}
+
 func TestSlice(t *testing.T) {
 	var i uint64
 	offset := uint64(100)
@@ -535,7 +565,7 @@ func TestSlice(t *testing.T) {
 		w    []pb.Entry
 	}{
 		{offset - 1, offset + 1, nil},
-		{offset, offset + 1, []pb.Entry{{Term: 0}}},
+		{offset, offset + 1, nil},
 		{offset + num/2, offset + num/2 + 1, []pb.Entry{{Term: num / 2}}},
 		{offset + num - 1, offset + num, []pb.Entry{{Term: num - 1}}},
 		{offset + num, offset + num + 1, nil},

raft/node_test.go

@@ -308,7 +308,6 @@ func TestNodeStart(t *testing.T) {
 			SoftState: &SoftState{Lead: 1, Nodes: []uint64{1}, RaftState: StateLeader},
 			HardState: raftpb.HardState{Term: 1, Commit: 2},
 			Entries: []raftpb.Entry{
-				{},
 				{Type: raftpb.EntryConfChange, Term: 1, Index: 1, Data: ccdata},
 				{Term: 1, Index: 2},
 			},
@@ -352,7 +351,6 @@ func TestNodeStart(t *testing.T) {
 
 func TestNodeRestart(t *testing.T) {
 	entries := []raftpb.Entry{
-		{},
 		{Term: 1, Index: 1},
 		{Term: 1, Index: 2, Data: []byte("foo")},
 	}
@@ -361,7 +359,7 @@ func TestNodeRestart(t *testing.T) {
 	want := Ready{
 		HardState: emptyState,
 		// commit upto index commit index in st
-		CommittedEntries: entries[1 : st.Commit+1],
+		CommittedEntries: entries[:st.Commit],
 	}
 
 	storage := NewMemoryStorage()
@@ -429,7 +427,7 @@ func TestNodeCompact(t *testing.T) {
 	}
 	n.Stop()
 
-	if r.raftLog.firstIndex() != w.Index {
+	if r.raftLog.firstIndex() != w.Index+1 {
 		t.Errorf("log.offset = %d, want %d", r.raftLog.firstIndex(), w.Index)
 	}
 }

raft/raft.go

@@ -133,7 +133,7 @@ func newRaft(id uint64, peers []uint64, election, heartbeat int, storage Storage
 	}
 	r.rand = rand.New(rand.NewSource(int64(id)))
 	for _, p := range peers {
-		r.prs[p] = &progress{}
+		r.prs[p] = &progress{next: 1}
 	}
 	r.becomeFollower(0, None)
 	return r
@@ -187,12 +187,12 @@ func (r *raft) sendAppend(to uint64) {
 	pr := r.prs[to]
 	m := pb.Message{}
 	m.To = to
-	m.Index = pr.next - 1
-	if r.needSnapshot(m.Index) {
+	if r.needSnapshot(pr.next) {
 		m.Type = pb.MsgSnap
 		m.Snapshot = r.raftLog.snapshot
 	} else {
 		m.Type = pb.MsgApp
+		m.Index = pr.next - 1
 		m.LogTerm = r.raftLog.term(pr.next - 1)
 		m.Entries = r.raftLog.entries(pr.next)
 		m.Commit = r.raftLog.committed

raft/raft_paper_test.go

@@ -514,7 +514,7 @@ func TestLeaderCommitPrecedingEntries(t *testing.T) {
 	}
 	for i, tt := range tests {
 		storage := NewMemoryStorage()
-		storage.Append(append([]pb.Entry{{}}, tt...))
+		storage.Append(tt)
 		r := newRaft(1, []uint64{1, 2, 3}, 10, 1, storage)
 		r.loadState(pb.HardState{Term: 2})
 		r.becomeCandidate()
@@ -591,17 +591,17 @@ func TestFollowerCommitEntry(t *testing.T) {
 // append entries.
 // Reference: section 5.3
 func TestFollowerCheckMsgApp(t *testing.T) {
-	ents := []pb.Entry{{}, {Term: 1, Index: 1}, {Term: 2, Index: 2}}
+	ents := []pb.Entry{{Term: 1, Index: 1}, {Term: 2, Index: 2}}
 	tests := []struct {
 		term    uint64
 		index   uint64
 		wreject bool
 	}{
+		{0, 0, false},
 		{ents[0].Term, ents[0].Index, false},
 		{ents[1].Term, ents[1].Index, false},
-		{ents[2].Term, ents[2].Index, false},
-		{ents[1].Term, ents[1].Index + 1, true},
-		{ents[1].Term + 1, ents[1].Index, true},
+		{ents[0].Term, ents[0].Index + 1, true},
+		{ents[0].Term + 1, ents[0].Index, true},
 		{3, 3, true},
 	}
 	for i, tt := range tests {
@@ -638,31 +638,31 @@ func TestFollowerAppendEntries(t *testing.T) {
 		{
 			2, 2,
 			[]pb.Entry{{Term: 3, Index: 3}},
-			[]pb.Entry{{}, {Term: 1, Index: 1}, {Term: 2, Index: 2}, {Term: 3, Index: 3}},
+			[]pb.Entry{{Term: 1, Index: 1}, {Term: 2, Index: 2}, {Term: 3, Index: 3}},
 			[]pb.Entry{{Term: 3, Index: 3}},
 		},
 		{
 			1, 1,
 			[]pb.Entry{{Term: 3, Index: 3}, {Term: 4, Index: 4}},
-			[]pb.Entry{{}, {Term: 1, Index: 1}, {Term: 3, Index: 3}, {Term: 4, Index: 4}},
+			[]pb.Entry{{Term: 1, Index: 1}, {Term: 3, Index: 3}, {Term: 4, Index: 4}},
 			[]pb.Entry{{Term: 3, Index: 3}, {Term: 4, Index: 4}},
 		},
 		{
 			0, 0,
 			[]pb.Entry{{Term: 1, Index: 1}},
-			[]pb.Entry{{}, {Term: 1, Index: 1}, {Term: 2, Index: 2}},
+			[]pb.Entry{{Term: 1, Index: 1}, {Term: 2, Index: 2}},
 			nil,
 		},
 		{
 			0, 0,
 			[]pb.Entry{{Term: 3, Index: 3}},
-			[]pb.Entry{{}, {Term: 3, Index: 3}},
+			[]pb.Entry{{Term: 3, Index: 3}},
 			[]pb.Entry{{Term: 3, Index: 3}},
 		},
 	}
 	for i, tt := range tests {
 		storage := NewMemoryStorage()
-		storage.Append([]pb.Entry{{}, {Term: 1, Index: 1}, {Term: 2, Index: 2}})
+		storage.Append([]pb.Entry{{Term: 1, Index: 1}, {Term: 2, Index: 2}})
 		r := newRaft(1, []uint64{1, 2, 3}, 10, 1, storage)
 		r.becomeFollower(2, 2)
 
@@ -813,17 +813,17 @@ func TestVoter(t *testing.T) {
 		wreject bool
 	}{
 		// same logterm
-		{[]pb.Entry{{}, {Term: 1, Index: 1}}, 1, 1, false},
-		{[]pb.Entry{{}, {Term: 1, Index: 1}}, 1, 2, false},
-		{[]pb.Entry{{}, {Term: 1, Index: 1}, {Term: 1, Index: 2}}, 1, 1, true},
+		{[]pb.Entry{{Term: 1, Index: 1}}, 1, 1, false},
+		{[]pb.Entry{{Term: 1, Index: 1}}, 1, 2, false},
+		{[]pb.Entry{{Term: 1, Index: 1}, {Term: 1, Index: 2}}, 1, 1, true},
 		// candidate higher logterm
-		{[]pb.Entry{{}, {Term: 1, Index: 1}}, 2, 1, false},
-		{[]pb.Entry{{}, {Term: 1, Index: 1}}, 2, 2, false},
-		{[]pb.Entry{{}, {Term: 1, Index: 1}, {Term: 1, Index: 2}}, 2, 1, false},
+		{[]pb.Entry{{Term: 1, Index: 1}}, 2, 1, false},
+		{[]pb.Entry{{Term: 1, Index: 1}}, 2, 2, false},
+		{[]pb.Entry{{Term: 1, Index: 1}, {Term: 1, Index: 2}}, 2, 1, false},
 		// voter higher logterm
-		{[]pb.Entry{{}, {Term: 2, Index: 1}}, 1, 1, true},
-		{[]pb.Entry{{}, {Term: 2, Index: 1}}, 1, 2, true},
-		{[]pb.Entry{{}, {Term: 2, Index: 1}, {Term: 1, Index: 2}}, 1, 1, true},
+		{[]pb.Entry{{Term: 2, Index: 1}}, 1, 1, true},
+		{[]pb.Entry{{Term: 2, Index: 1}}, 1, 2, true},
+		{[]pb.Entry{{Term: 2, Index: 1}, {Term: 1, Index: 2}}, 1, 1, true},
 	}
 	for i, tt := range tests {
 		storage := NewMemoryStorage()
@@ -850,7 +850,7 @@ func TestVoter(t *testing.T) {
 // current term are committed by counting replicas.
 // Reference: section 5.4.2
 func TestLeaderOnlyCommitsLogFromCurrentTerm(t *testing.T) {
-	ents := []pb.Entry{{}, {Term: 1, Index: 1}, {Term: 2, Index: 2}}
+	ents := []pb.Entry{{Term: 1, Index: 1}, {Term: 2, Index: 2}}
 	tests := []struct {
 		index   uint64
 		wcommit uint64

raft/raft_test.go

@@ -547,8 +547,8 @@ func TestCompact(t *testing.T) {
 			}
 			sm.compact(tt.compacti, tt.nodes, tt.snapd)
 			sort.Sort(uint64Slice(sm.raftLog.snapshot.Nodes))
-			if sm.raftLog.firstIndex() != tt.compacti {
-				t.Errorf("%d: log.firstIndex = %d, want %d", i, sm.raftLog.firstIndex(), tt.compacti)
+			if sm.raftLog.firstIndex() != tt.compacti+1 {
+				t.Errorf("%d: log.firstIndex = %d, want %d", i, sm.raftLog.firstIndex(), tt.compacti+1)
 			}
 			if !reflect.DeepEqual(sm.raftLog.snapshot.Nodes, tt.nodes) {
 				t.Errorf("%d: snap.nodes = %v, want %v", i, sm.raftLog.snapshot.Nodes, tt.nodes)
@@ -836,9 +836,9 @@ func TestAllServerStepdown(t *testing.T) {
 		wterm  uint64
 		windex uint64
 	}{
-		{StateFollower, StateFollower, 3, 1},
-		{StateCandidate, StateFollower, 3, 1},
-		{StateLeader, StateFollower, 3, 2},
+		{StateFollower, StateFollower, 3, 0},
+		{StateCandidate, StateFollower, 3, 0},
+		{StateLeader, StateFollower, 3, 1},
 	}
 
 	tmsgTypes := [...]pb.MessageType{pb.MsgVote, pb.MsgApp}
@@ -865,8 +865,11 @@ func TestAllServerStepdown(t *testing.T) {
 			if sm.Term != tt.wterm {
 				t.Errorf("#%d.%d term = %v , want %v", i, j, sm.Term, tt.wterm)
 			}
+			if uint64(sm.raftLog.lastIndex()) != tt.windex {
+				t.Errorf("#%d.%d index = %v , want %v", i, j, sm.raftLog.lastIndex(), tt.windex)
+			}
 			if uint64(len(sm.raftLog.allEntries())) != tt.windex {
-				t.Errorf("#%d.%d index = %v , want %v", i, j, len(sm.raftLog.allEntries()), tt.windex)
+				t.Errorf("#%d.%d len(ents) = %v , want %v", i, j, len(sm.raftLog.allEntries()), tt.windex)
 			}
 			wlead := uint64(2)
 			if msgType == pb.MsgVote {

raft/storage.go

@@ -23,9 +23,9 @@ import (
 	pb "github.com/coreos/etcd/raft/raftpb"
 )
 
-// ErrStorageEmpty is returned by Storage.GetLastIndex when there is
-// no data.
-var ErrStorageEmpty = errors.New("storage is empty")
+// ErrSnapshotRequired is returned by Storage.Entries when a requested
+// index is unavailable because it predates the last snapshot.
+var ErrSnapshotRequired = errors.New("snapshot required; requested index is too old")
 
 // Storage is an interface that may be implemented by the application
 // to retrieve log entries from storage.
@@ -36,11 +36,15 @@ var ErrStorageEmpty = errors.New("storage is empty")
 type Storage interface {
 	// Entries returns a slice of log entries in the range [lo,hi).
 	Entries(lo, hi uint64) ([]pb.Entry, error)
-	// GetLastIndex returns the index of the last entry in the log.
-	// If the log is empty it returns ErrStorageEmpty.
+	// Term returns the term of entry i, which must be in the range
+	// [FirstIndex()-1, LastIndex()]. The term of the entry before
+	// FirstIndex is retained for matching purposes even though the
+	// rest of that entry may not be available.
+	Term(i uint64) (uint64, error)
+	// LastIndex returns the index of the last entry in the log.
 	LastIndex() (uint64, error)
-	// GetFirstIndex returns the index of the first log entry that is
-	// available via GetEntries (older entries have been incorporated
+	// FirstIndex returns the index of the first log entry that is
+	// available via Entries (older entries have been incorporated
 	// into the latest Snapshot).
 	FirstIndex() (uint64, error)
 	// Compact discards all log entries prior to i.
@@ -65,23 +69,36 @@ type MemoryStorage struct {
 
 // NewMemoryStorage creates an empty MemoryStorage.
 func NewMemoryStorage() *MemoryStorage {
-	return &MemoryStorage{}
+	return &MemoryStorage{
+		// When starting from scratch populate the list with a dummy entry at term zero.
+		ents: make([]pb.Entry, 1),
+	}
 }
 
 // Entries implements the Storage interface.
 func (ms *MemoryStorage) Entries(lo, hi uint64) ([]pb.Entry, error) {
 	ms.Lock()
 	defer ms.Unlock()
+	if lo <= ms.offset {
+		return nil, ErrSnapshotRequired
+	}
 	return ms.ents[lo-ms.offset : hi-ms.offset], nil
 }
 
+// Term implements the Storage interface.
+func (ms *MemoryStorage) Term(i uint64) (uint64, error) {
+	ms.Lock()
+	defer ms.Unlock()
+	if i < ms.offset || i > ms.offset+uint64(len(ms.ents)) {
+		return 0, ErrSnapshotRequired
+	}
+	return ms.ents[i-ms.offset].Term, nil
+}
+
 // LastIndex implements the Storage interface.
 func (ms *MemoryStorage) LastIndex() (uint64, error) {
 	ms.Lock()
 	defer ms.Unlock()
-	if len(ms.ents) == 0 {
-		return 0, ErrStorageEmpty
-	}
 	return ms.offset + uint64(len(ms.ents)) - 1, nil
 }
 
@@ -89,7 +106,7 @@ func (ms *MemoryStorage) LastIndex() (uint64, error) {
 func (ms *MemoryStorage) FirstIndex() (uint64, error) {
 	ms.Lock()
 	defer ms.Unlock()
-	return ms.offset, nil
+	return ms.offset + 1, nil
 }
 
 // Compact implements the Storage interface.