diff --git a/head.go b/head.go
index 6e3dd21c4c..6edbae8948 100644
--- a/head.go
+++ b/head.go
@@ -51,7 +51,7 @@ var (
 type HeadBlock struct {
 	mtx sync.RWMutex
 	dir string
-	wal *WAL
+	wal WAL
 
 	activeWriters uint64
 	closed        bool
@@ -101,7 +101,7 @@ func CreateHeadBlock(dir string, seq int, l log.Logger, mint, maxt int64) (*Head
 
 // OpenHeadBlock opens the head block in dir.
 func OpenHeadBlock(dir string, l log.Logger) (*HeadBlock, error) {
-	wal, err := OpenWAL(dir, log.With(l, "component", "wal"), 5*time.Second)
+	wal, err := OpenSegmentWAL(dir, log.With(l, "component", "wal"), 5*time.Second)
 	if err != nil {
 		return nil, err
 	}
@@ -122,7 +122,6 @@ func OpenHeadBlock(dir string, l log.Logger) (*HeadBlock, error) {
 
 	r := wal.Reader()
 
-Outer:
 	for r.Next() {
 		series, samples := r.At()
 
@@ -132,8 +131,7 @@ Outer:
 		}
 		for _, s := range samples {
 			if int(s.Ref) >= len(h.series) {
-				l.Log("msg", "unknown series reference, abort WAL restore", "got", s.Ref, "max", len(h.series)-1)
-				break Outer
+				return nil, errors.Errorf("unknown series reference %d (max %d); abort WAL restore", s.Ref, len(h.series))
 			}
 			h.series[s.Ref].append(s.T, s.V)
 
diff --git a/wal.go b/wal.go
index 80f3508e8e..251944f0bf 100644
--- a/wal.go
+++ b/wal.go
@@ -49,9 +49,8 @@ const (
 	WALEntrySamples WALEntryType = 3
 )
 
-// WAL is a write ahead log for series data. It can only be written to.
-// Use walReader to read back from a write ahead log.
-type WAL struct {
+// SegmentWAL is a write ahead log for series data.
+type SegmentWAL struct {
 	mtx sync.Mutex
 
 	dirFile *os.File
@@ -69,6 +68,21 @@ type WAL struct {
 	donec chan struct{}
 }
 
+// WAL is a write ahead log that can log new series labels and samples.
+// It must be completely read before new entries are logged.
+type WAL interface {
+	Reader() WALReader
+	Log([]labels.Labels, []RefSample) error
+	Close() error
+}
+
+// WALReader reads entries from a WAL.
+type WALReader interface {
+	At() ([]labels.Labels, []RefSample)
+	Next() bool
+	Err() error
+}
+
 // RefSample is a timestamp/value pair associated with a reference to a series.
 type RefSample struct {
 	Ref uint64
@@ -90,9 +104,9 @@ func init() {
 	castagnoliTable = crc32.MakeTable(crc32.Castagnoli)
 }
 
-// OpenWAL opens or creates a write ahead log in the given directory.
+// OpenSegmentWAL opens or creates a write ahead log in the given directory.
 // The WAL must be read completely before new data is written.
-func OpenWAL(dir string, logger log.Logger, flushInterval time.Duration) (*WAL, error) {
+func OpenSegmentWAL(dir string, logger log.Logger, flushInterval time.Duration) (*SegmentWAL, error) {
 	dir = filepath.Join(dir, walDirName)
 
 	if err := os.MkdirAll(dir, 0777); err != nil {
@@ -106,7 +120,7 @@ func OpenWAL(dir string, logger log.Logger, flushInterval time.Duration) (*WAL,
 		logger = log.NewNopLogger()
 	}
 
-	w := &WAL{
+	w := &SegmentWAL{
 		dirFile:       df,
 		logger:        logger,
 		flushInterval: flushInterval,
@@ -126,12 +140,12 @@ func OpenWAL(dir string, logger log.Logger, flushInterval time.Duration) (*WAL,
 
 // Reader returns a new reader over the the write ahead log data.
 // It must be completely consumed before writing to the WAL.
-func (w *WAL) Reader() WALReader {
+func (w *SegmentWAL) Reader() WALReader {
 	return newWALReader(w, w.logger)
 }
 
 // Log writes a batch of new series labels and samples to the log.
-func (w *WAL) Log(series []labels.Labels, samples []RefSample) error {
+func (w *SegmentWAL) Log(series []labels.Labels, samples []RefSample) error {
 	if err := w.encodeSeries(series); err != nil {
 		return err
 	}
@@ -146,7 +160,7 @@ func (w *WAL) Log(series []labels.Labels, samples []RefSample) error {
 
 // initSegments finds all existing segment files and opens them in the
 // appropriate file modes.
-func (w *WAL) initSegments() error {
+func (w *SegmentWAL) initSegments() error {
 	fns, err := sequenceFiles(w.dirFile.Name(), "")
 	if err != nil {
 		return err
@@ -187,7 +201,7 @@ func (w *WAL) initSegments() error {
 
 // cut finishes the currently active segments and opens the next one.
 // The encoder is reset to point to the new segment.
-func (w *WAL) cut() error {
+func (w *SegmentWAL) cut() error {
 	// Sync current tail to disk and close.
 	if tf := w.tail(); tf != nil {
 		if err := w.sync(); err != nil {
@@ -236,7 +250,7 @@ func (w *WAL) cut() error {
 	return nil
 }
 
-func (w *WAL) tail() *os.File {
+func (w *SegmentWAL) tail() *os.File {
 	if len(w.files) == 0 {
 		return nil
 	}
@@ -244,14 +258,14 @@ func (w *WAL) tail() *os.File {
 }
 
 // Sync flushes the changes to disk.
-func (w *WAL) Sync() error {
+func (w *SegmentWAL) Sync() error {
 	w.mtx.Lock()
 	defer w.mtx.Unlock()
 
 	return w.sync()
 }
 
-func (w *WAL) sync() error {
+func (w *SegmentWAL) sync() error {
 	if w.cur == nil {
 		return nil
 	}
@@ -261,7 +275,7 @@ func (w *WAL) sync() error {
 	return fileutil.Fdatasync(w.tail())
 }
 
-func (w *WAL) run(interval time.Duration) {
+func (w *SegmentWAL) run(interval time.Duration) {
 	var tick <-chan time.Time
 
 	if interval > 0 {
@@ -284,7 +298,7 @@ func (w *WAL) run(interval time.Duration) {
 }
 
 // Close syncs all data and closes the underlying resources.
-func (w *WAL) Close() error {
+func (w *SegmentWAL) Close() error {
 	close(w.stopc)
 	<-w.donec
 
@@ -312,7 +326,7 @@ const (
 	walPageBytes = 16 * minSectorSize
 )
 
-func (w *WAL) entry(et WALEntryType, flag byte, buf []byte) error {
+func (w *SegmentWAL) entry(et WALEntryType, flag byte, buf []byte) error {
 	w.mtx.Lock()
 	defer w.mtx.Unlock()
 
@@ -376,7 +390,7 @@ func putWALBuffer(b []byte) {
 	walBuffers.Put(b)
 }
 
-func (w *WAL) encodeSeries(series []labels.Labels) error {
+func (w *SegmentWAL) encodeSeries(series []labels.Labels) error {
 	if len(series) == 0 {
 		return nil
 	}
@@ -402,7 +416,7 @@ func (w *WAL) encodeSeries(series []labels.Labels) error {
 	return w.entry(WALEntrySeries, walSeriesSimple, buf)
 }
 
-func (w *WAL) encodeSamples(samples []RefSample) error {
+func (w *SegmentWAL) encodeSamples(samples []RefSample) error {
 	if len(samples) == 0 {
 		return nil
 	}
@@ -439,7 +453,7 @@ func (w *WAL) encodeSamples(samples []RefSample) error {
 type walReader struct {
 	logger log.Logger
 
-	wal   *WAL
+	wal   *SegmentWAL
 	cur   int
 	buf   []byte
 	crc32 hash.Hash32
@@ -449,7 +463,7 @@ type walReader struct {
 	samples []RefSample
 }
 
-func newWALReader(w *WAL, l log.Logger) *walReader {
+func newWALReader(w *SegmentWAL, l log.Logger) *walReader {
 	if l == nil {
 		l = log.NewNopLogger()
 	}
diff --git a/wal_test.go b/wal_test.go
index 99822ec01f..cbd6c62477 100644
--- a/wal_test.go
+++ b/wal_test.go
@@ -27,7 +27,7 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestWAL_initSegments(t *testing.T) {
+func TestSegmentWAL_initSegments(t *testing.T) {
 	tmpdir, err := ioutil.TempDir("", "test_wal_open")
 	require.NoError(t, err)
 	defer os.RemoveAll(tmpdir)
@@ -35,7 +35,7 @@ func TestWAL_initSegments(t *testing.T) {
 	df, err := fileutil.OpenDir(tmpdir)
 	require.NoError(t, err)
 
-	w := &WAL{dirFile: df}
+	w := &SegmentWAL{dirFile: df}
 
 	// Create segment files with an appropriate header.
 	for i := 1; i <= 5; i++ {
@@ -80,7 +80,7 @@ func TestWAL_initSegments(t *testing.T) {
 	_, err = f.WriteAt([]byte{0}, 4)
 	require.NoError(t, err)
 
-	w = &WAL{dirFile: df}
+	w = &SegmentWAL{dirFile: df}
 	require.Error(t, w.initSegments(), "init corrupted segments")
 
 	for _, f := range w.files {
@@ -88,13 +88,13 @@ func TestWAL_initSegments(t *testing.T) {
 	}
 }
 
-func TestWAL_cut(t *testing.T) {
+func TestSegmentWAL_cut(t *testing.T) {
 	tmpdir, err := ioutil.TempDir("", "test_wal_cut")
 	require.NoError(t, err)
 	defer os.RemoveAll(tmpdir)
 
 	// This calls cut() implicitly the first time without a previous tail.
-	w, err := OpenWAL(tmpdir, nil, 0)
+	w, err := OpenSegmentWAL(tmpdir, nil, 0)
 	require.NoError(t, err)
 
 	require.NoError(t, w.entry(WALEntrySeries, 1, []byte("Hello World!!")))
@@ -131,7 +131,7 @@ func TestWAL_cut(t *testing.T) {
 }
 
 // Symmetrical test of reading and writing to the WAL via its main interface.
-func TestWAL_Log_Restore(t *testing.T) {
+func TestSegmentWAL_Log_Restore(t *testing.T) {
 	const (
 		numMetrics = 5000
 		iterations = 5
@@ -155,7 +155,7 @@ func TestWAL_Log_Restore(t *testing.T) {
 	// Open WAL a bunch of times, validate all previous data can be read,
 	// write more data to it, close it.
 	for k := 0; k < numMetrics; k += numMetrics / iterations {
-		w, err := OpenWAL(dir, nil, 0)
+		w, err := OpenSegmentWAL(dir, nil, 0)
 		require.NoError(t, err)
 
 		// Set smaller segment size so we can actually write several files.
@@ -222,11 +222,11 @@ func TestWAL_Log_Restore(t *testing.T) {
 func TestWALRestoreCorrupted(t *testing.T) {
 	cases := []struct {
 		name string
-		f    func(*testing.T, *WAL)
+		f    func(*testing.T, *SegmentWAL)
 	}{
 		{
 			name: "truncate_checksum",
-			f: func(t *testing.T, w *WAL) {
+			f: func(t *testing.T, w *SegmentWAL) {
 				f, err := os.OpenFile(w.files[0].Name(), os.O_WRONLY, 0666)
 				require.NoError(t, err)
 				defer f.Close()
@@ -239,7 +239,7 @@ func TestWALRestoreCorrupted(t *testing.T) {
 		},
 		{
 			name: "truncate_body",
-			f: func(t *testing.T, w *WAL) {
+			f: func(t *testing.T, w *SegmentWAL) {
 				f, err := os.OpenFile(w.files[0].Name(), os.O_WRONLY, 0666)
 				require.NoError(t, err)
 				defer f.Close()
@@ -252,7 +252,7 @@ func TestWALRestoreCorrupted(t *testing.T) {
 		},
 		{
 			name: "body_content",
-			f: func(t *testing.T, w *WAL) {
+			f: func(t *testing.T, w *SegmentWAL) {
 				f, err := os.OpenFile(w.files[0].Name(), os.O_WRONLY, 0666)
 				require.NoError(t, err)
 				defer f.Close()
@@ -267,7 +267,7 @@ func TestWALRestoreCorrupted(t *testing.T) {
 		},
 		{
 			name: "checksum",
-			f: func(t *testing.T, w *WAL) {
+			f: func(t *testing.T, w *SegmentWAL) {
 				f, err := os.OpenFile(w.files[0].Name(), os.O_WRONLY, 0666)
 				require.NoError(t, err)
 				defer f.Close()
@@ -289,7 +289,7 @@ func TestWALRestoreCorrupted(t *testing.T) {
 			require.NoError(t, err)
 			defer os.RemoveAll(dir)
 
-			w, err := OpenWAL(dir, nil, 0)
+			w, err := OpenSegmentWAL(dir, nil, 0)
 			require.NoError(t, err)
 
 			require.NoError(t, w.Log(nil, []RefSample{{T: 1, V: 2}}))
@@ -310,7 +310,7 @@ func TestWALRestoreCorrupted(t *testing.T) {
 
 			logger := log.NewLogfmtLogger(os.Stderr)
 
-			w2, err := OpenWAL(dir, logger, 0)
+			w2, err := OpenSegmentWAL(dir, logger, 0)
 			require.NoError(t, err)
 
 			r := w2.Reader()
@@ -333,7 +333,7 @@ func TestWALRestoreCorrupted(t *testing.T) {
 
 			// We should see the first valid entry and the new one, everything after
 			// is truncated.
-			w3, err := OpenWAL(dir, logger, 0)
+			w3, err := OpenSegmentWAL(dir, logger, 0)
 			require.NoError(t, err)
 
 			r = w3.Reader()