Implement streaming WAL segment iterator

Currently, WALSegmentIterator implementations read to the end of
the end of their list of segments and return EOF. This commit adds
the ability to push additional segments to in-process iterators and
notify their callers that new segments are available. This is only
implemented for the file-based iterator but other segment iterators
may get this implementation in the future or have a wrapping
iterator provide a polling-based implementation.

db.go

@@ -45,15 +45,17 @@ const BusyTimeout = 1 * time.Second
 
 // DB represents a managed instance of a SQLite database in the file system.
 type DB struct {
-	mu        sync.RWMutex
+	mu       sync.RWMutex
-	path      string        // part to database
+	path     string        // part to database
-	db        *sql.DB       // target database
+	db       *sql.DB       // target database
-	f         *os.File      // long-running db file descriptor
+	f        *os.File      // long-running db file descriptor
-	rtx       *sql.Tx       // long running read transaction
+	rtx      *sql.Tx       // long running read transaction
-	pos       Pos           // cached position
+	pos      Pos           // cached position
-	pageSize  int           // page size, in bytes
+	pageSize int           // page size, in bytes
-	notifyCh  chan struct{} // notifies DB of changes
+	notifyCh chan struct{} // notifies DB of changes
-	walNotify chan struct{} // closes on WAL change
+
+	// Iterators used to stream new WAL changes to replicas
+	itrs map[*FileWALSegmentIterator]struct{}
 
 	// Cached salt & checksum from current shadow header.
 	hdr              []byte
@@ -111,9 +113,10 @@ type DB struct {
 // NewDB returns a new instance of DB for a given path.
 func NewDB(path string) *DB {
 	db := &DB{
-		path:      path,
+		path:     path,
-		notifyCh:  make(chan struct{}, 1),
+		notifyCh: make(chan struct{}, 1),
-		walNotify: make(chan struct{}),
+
+		itrs: make(map[*FileWALSegmentIterator]struct{}),
 
 		MinCheckpointPageN: DefaultMinCheckpointPageN,
 		MaxCheckpointPageN: DefaultMaxCheckpointPageN,
@@ -245,7 +248,7 @@ func (db *DB) invalidatePos(ctx context.Context) error {
 	}
 
 	// Iterate over all segments to find the last one.
-	itr, err := db.WALSegments(context.Background(), generation)
+	itr, err := db.walSegments(context.Background(), generation, false)
 	if err != nil {
 		return err
 	}
@@ -363,13 +366,6 @@ func (db *DB) NotifyCh() chan<- struct{} {
 	return db.notifyCh
 }
 
-// WALNotify returns a channel that closes when the shadow WAL changes.
-func (db *DB) WALNotify() <-chan struct{} {
-	db.mu.RLock()
-	defer db.mu.RUnlock()
-	return db.walNotify
-}
-
 // PageSize returns the page size of the underlying database.
 // Only valid after database exists & Init() has successfully run.
 func (db *DB) PageSize() int {
@@ -440,6 +436,14 @@ func (db *DB) Close() (err error) {
 		}
 	}
 
+	// Remove all iterators.
+	db.mu.Lock()
+	for itr := range db.itrs {
+		itr.SetErr(ErrDBClosed)
+		delete(db.itrs, itr)
+	}
+	db.mu.Unlock()
+
 	// Release the read lock to allow other applications to handle checkpointing.
 	if db.rtx != nil {
 		if e := db.releaseReadLock(); e != nil && err == nil {
@@ -833,7 +837,6 @@ func (db *DB) sync(ctx context.Context) (err error) {
 			return fmt.Errorf("invalidate: %w", err)
 		}
 	}
-	origPos := db.pos
 
 	// If sync fails, reset position & cache.
 	defer func() {
@@ -934,12 +937,6 @@ func (db *DB) sync(ctx context.Context) (err error) {
 	db.shadowWALIndexGauge.Set(float64(db.pos.Index))
 	db.shadowWALSizeGauge.Set(float64(db.pos.Offset))
 
-	// Notify replicas of WAL changes.
-	if db.pos != origPos {
-		close(db.walNotify)
-		db.walNotify = make(chan struct{})
-	}
-
 	return nil
 }
 
@@ -1263,7 +1260,8 @@ func (db *DB) writeWALSegment(ctx context.Context, pos Pos, rd io.Reader) error
 	}
 	defer f.Close()
 
-	if _, err := io.Copy(f, rd); err != nil {
+	n, err := io.Copy(f, rd)
+	if err != nil {
 		return err
 	} else if err := f.Sync(); err != nil {
 		return err
@@ -1276,14 +1274,47 @@ func (db *DB) writeWALSegment(ctx context.Context, pos Pos, rd io.Reader) error
 		return err
 	}
 
+	// Generate
+	info := WALSegmentInfo{
+		Generation: pos.Generation,
+		Index:      pos.Index,
+		Offset:     pos.Offset,
+		Size:       n,
+		CreatedAt:  time.Now(),
+	}
+
+	// Notify all managed segment iterators.
+	for itr := range db.itrs {
+		// Notify iterators of generation change.
+		if itr.Generation() != pos.Generation {
+			itr.SetErr(ErrGenerationChanged)
+			delete(db.itrs, itr)
+			continue
+		}
+
+		// Attempt to append segment to end of iterator.
+		// On error, mark it on the iterator and remove from future notifications.
+		if err := itr.Append(info); err != nil {
+			itr.SetErr(fmt.Errorf("cannot append wal segment: %w", err))
+			delete(db.itrs, itr)
+			continue
+		}
+	}
+
 	return nil
 }
 
 // WALSegments returns an iterator over all available WAL files for a generation.
-func (db *DB) WALSegments(ctx context.Context, generation string) (WALSegmentIterator, error) {
+func (db *DB) WALSegments(ctx context.Context, generation string) (*FileWALSegmentIterator, error) {
+	db.mu.Lock()
+	defer db.mu.Unlock()
+	return db.walSegments(ctx, generation, true)
+}
+
+func (db *DB) walSegments(ctx context.Context, generation string, managed bool) (*FileWALSegmentIterator, error) {
 	ents, err := os.ReadDir(db.ShadowWALDir(generation))
 	if os.IsNotExist(err) {
-		return NewWALSegmentInfoSliceIterator(nil), nil
+		return NewFileWALSegmentIterator(db.ShadowWALDir(generation), generation, nil), nil
 	} else if err != nil {
 		return nil, err
 	}
@@ -1300,7 +1331,27 @@ func (db *DB) WALSegments(ctx context.Context, generation string) (WALSegmentIte
 
 	sort.Ints(indexes)
 
-	return newFileWALSegmentIterator(db.ShadowWALDir(generation), generation, indexes), nil
+	itr := NewFileWALSegmentIterator(db.ShadowWALDir(generation), generation, indexes)
+
+	// Managed iterators will have new segments pushed to them.
+	if managed {
+		itr.closeFunc = func() error {
+			return db.CloseWALSegmentIterator(itr)
+		}
+
+		db.itrs[itr] = struct{}{}
+	}
+
+	return itr, nil
+}
+
+// CloseWALSegmentIterator removes itr from the list of managed iterators.
+func (db *DB) CloseWALSegmentIterator(itr *FileWALSegmentIterator) error {
+	db.mu.Lock()
+	defer db.mu.Unlock()
+
+	delete(db.itrs, itr)
+	return nil
 }
 
 // SQLite WAL constants

file_replica_client.go

@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/benbjohnson/litestream/internal"
 )
@@ -285,7 +286,7 @@ func (c *FileReplicaClient) WALSegments(ctx context.Context, generation string)
 
 	sort.Ints(indexes)
 
-	return newFileWALSegmentIterator(dir, generation, indexes), nil
+	return NewFileWALSegmentIterator(dir, generation, indexes), nil
 }
 
 // WriteWALSegment writes LZ4 compressed data from rd into a file on disk.
@@ -360,33 +361,74 @@ func (c *FileReplicaClient) DeleteWALSegments(ctx context.Context, a []Pos) erro
 	return nil
 }
 
-type fileWalSegmentIterator struct {
+type FileWALSegmentIterator struct {
+	mu        sync.Mutex
+	notifyCh  chan struct{}
+	closeFunc func() error
+
 	dir        string
 	generation string
 	indexes    []int
 
-	infos []WALSegmentInfo
+	buffered bool
-	err   error
+	infos    []WALSegmentInfo
+	err      error
 }
 
-func newFileWALSegmentIterator(dir, generation string, indexes []int) *fileWalSegmentIterator {
+func NewFileWALSegmentIterator(dir, generation string, indexes []int) *FileWALSegmentIterator {
-	return &fileWalSegmentIterator{
+	return &FileWALSegmentIterator{
 		dir:        dir,
 		generation: generation,
 		indexes:    indexes,
+
+		notifyCh: make(chan struct{}, 1),
 	}
 }
 
-func (itr *fileWalSegmentIterator) Close() (err error) {
+func (itr *FileWALSegmentIterator) Close() (err error) {
-	return itr.err
+	if itr.closeFunc != nil {
+		if e := itr.closeFunc(); e != nil && err == nil {
+			err = e
+		}
+	}
+
+	if e := itr.Err(); e != nil && err == nil {
+		err = e
+	}
+	return err
 }
 
-func (itr *fileWalSegmentIterator) Next() bool {
+func (itr *FileWALSegmentIterator) NotifyCh() <-chan struct{} {
+	return itr.notifyCh
+}
+
+// Generation returns the generation this iterator was initialized with.
+func (itr *FileWALSegmentIterator) Generation() string {
+	return itr.generation
+}
+
+// Indexes returns the pending indexes. Only used for testing.
+func (itr *FileWALSegmentIterator) Indexes() []int {
+	itr.mu.Lock()
+	defer itr.mu.Unlock()
+	return itr.indexes
+}
+
+func (itr *FileWALSegmentIterator) Next() bool {
+	itr.mu.Lock()
+	defer itr.mu.Unlock()
+
 	// Exit if an error has already occurred.
 	if itr.err != nil {
 		return false
 	}
 
+	// Read first info, if buffered.
+	if itr.buffered {
+		itr.buffered = false
+		return true
+	}
+
 	for {
 		// Move to the next segment in cache, if available.
 		if len(itr.infos) > 1 {
@@ -448,11 +490,94 @@ func (itr *fileWalSegmentIterator) Next() bool {
 	}
 }
 
-func (itr *fileWalSegmentIterator) Err() error { return itr.err }
+// SetErr sets the error on the iterator and notifies it of the change.
+func (itr *FileWALSegmentIterator) SetErr(err error) {
+	itr.mu.Lock()
+	defer itr.mu.Unlock()
+	if itr.err == nil {
+		itr.err = err
+	}
+
+	select {
+	case itr.notifyCh <- struct{}{}:
+	default:
+	}
+}
+
+// Err returns the first error that occurs on the iterator.
+func (itr *FileWALSegmentIterator) Err() error {
+	itr.mu.Lock()
+	defer itr.mu.Unlock()
+	return itr.err
+}
+
+func (itr *FileWALSegmentIterator) WALSegment() WALSegmentInfo {
+	itr.mu.Lock()
+	defer itr.mu.Unlock()
 
-func (itr *fileWalSegmentIterator) WALSegment() WALSegmentInfo {
 	if len(itr.infos) == 0 {
 		return WALSegmentInfo{}
 	}
 	return itr.infos[0]
 }
+
+// Append add an additional WAL segment to the end of the iterator. This
+// function expects that info will always be later than all previous infos
+// that the iterator has or has seen.
+func (itr *FileWALSegmentIterator) Append(info WALSegmentInfo) error {
+	itr.mu.Lock()
+	defer itr.mu.Unlock()
+
+	if itr.err != nil {
+		return itr.err
+	} else if itr.generation != info.Generation {
+		return fmt.Errorf("generation mismatch")
+	}
+
+	// If the info has an index that is still waiting to be read from disk into
+	// the cache then simply append it to the end of the indices.
+	//
+	// If we have no pending indices, then append to the end of the infos. If
+	// we don't have either then just append to the infos and avoid validation.
+	if len(itr.indexes) > 0 {
+		maxIndex := itr.indexes[len(itr.indexes)-1]
+
+		if info.Index < maxIndex {
+			return fmt.Errorf("appended index %q below max index %q", FormatIndex(info.Index), FormatIndex(maxIndex))
+		} else if info.Index > maxIndex+1 {
+			return fmt.Errorf("appended index %q skips index %q", FormatIndex(info.Index), FormatIndex(maxIndex+1))
+		} else if info.Index == maxIndex+1 {
+			itr.indexes = append(itr.indexes, info.Index)
+		}
+		// NOTE: no-op if segment index matches the current last index
+
+	} else if len(itr.infos) > 0 {
+		lastInfo := itr.infos[len(itr.infos)-1]
+		if info.Index < lastInfo.Index {
+			return fmt.Errorf("appended index %q below current index %q", FormatIndex(info.Index), FormatIndex(lastInfo.Index))
+		} else if info.Index > lastInfo.Index+1 {
+			return fmt.Errorf("appended index %q skips next index %q", FormatIndex(info.Index), FormatIndex(lastInfo.Index+1))
+		} else if info.Index == lastInfo.Index+1 {
+			itr.indexes = append(itr.indexes, info.Index)
+		} else {
+			// If the index matches the current infos, verify its offset and append.
+			if info.Offset < lastInfo.Offset {
+				return fmt.Errorf("appended offset %s/%s before last offset %s/%s", FormatIndex(info.Index), FormatOffset(info.Offset), FormatIndex(lastInfo.Index), FormatOffset(lastInfo.Offset))
+			} else if info.Offset == lastInfo.Offset {
+				return fmt.Errorf("duplicate offset %s/%s appended", FormatIndex(info.Index), FormatOffset(info.Offset))
+			}
+			itr.infos = append(itr.infos, info)
+		}
+	} else {
+		itr.buffered = true
+		itr.infos = append(itr.infos, info)
+	}
+
+	// Signal that a new segment is available.
+	select {
+	case itr.notifyCh <- struct{}{}:
+	default:
+	}
+
+	return nil
+}

file_replica_client_test.go

@@ -1,6 +1,7 @@
 package litestream_test
 
 import (
+	"reflect"
 	"testing"
 
 	"github.com/benbjohnson/litestream"
@@ -133,3 +134,133 @@ func TestReplicaClient_WALSegmentPath(t *testing.T) {
 		}
 	})
 }
+
+func TestFileWALSegmentIterator_Append(t *testing.T) {
+	t.Run("Empty", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err != nil {
+			t.Fatal(err)
+		}
+
+		select {
+		case <-itr.NotifyCh():
+		default:
+			t.Fatal("expected notification")
+		}
+
+		if !itr.Next() {
+			t.Fatal("expected next")
+		} else if got, want := itr.WALSegment(), (litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); got != want {
+			t.Fatalf("info=%#v, want %#v", got, want)
+		}
+	})
+
+	t.Run("MultiOffset", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 1}); err != nil {
+			t.Fatal(err)
+		}
+
+		select {
+		case <-itr.NotifyCh():
+		default:
+			t.Fatal("expected notification")
+		}
+
+		if !itr.Next() {
+			t.Fatal("expected next")
+		} else if got, want := itr.WALSegment(), (litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); got != want {
+			t.Fatalf("info=%#v, want %#v", got, want)
+		}
+
+		if !itr.Next() {
+			t.Fatal("expected next")
+		} else if got, want := itr.WALSegment(), (litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 1}); got != want {
+			t.Fatalf("info=%#v, want %#v", got, want)
+		}
+	})
+
+	t.Run("MultiIndex", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 1, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 1, Offset: 1}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 2, Offset: 0}); err != nil {
+			t.Fatal(err)
+		}
+
+		if got, want := itr.Indexes(), []int{1, 2}; !reflect.DeepEqual(got, want) {
+			t.Fatalf("indexes=%v, want %v", got, want)
+		}
+	})
+
+	t.Run("ErrGenerationMismatch", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0000000000000000", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err == nil || err.Error() != `generation mismatch` {
+			t.Fatalf("unexpected error: %s", err)
+		}
+	})
+
+	t.Run("ErrBelowMaxIndex", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 1, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err == nil || err.Error() != `appended index "0000000000000000" below max index "0000000000000001"` {
+			t.Fatalf("unexpected error: %s", err)
+		}
+	})
+
+	t.Run("ErrAboveMaxIndex", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 1, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 3, Offset: 0}); err == nil || err.Error() != `appended index "0000000000000003" skips index "0000000000000002"` {
+			t.Fatalf("unexpected error: %s", err)
+		}
+	})
+
+	t.Run("ErrBelowCurrentIndex", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 1, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err == nil || err.Error() != `appended index "0000000000000000" below current index "0000000000000001"` {
+			t.Fatalf("unexpected error: %s", err)
+		}
+	})
+
+	t.Run("ErrSkipsNextIndex", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 0}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 2, Offset: 0}); err == nil || err.Error() != `appended index "0000000000000002" skips next index "0000000000000001"` {
+			t.Fatalf("unexpected error: %s", err)
+		}
+	})
+
+	t.Run("ErrBelowOffset", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 5}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 4}); err == nil || err.Error() != `appended offset 0000000000000000/0000000000000004 before last offset 0000000000000000/0000000000000005` {
+			t.Fatalf("unexpected error: %s", err)
+		}
+	})
+
+	t.Run("ErrDuplicateOffset", func(t *testing.T) {
+		itr := litestream.NewFileWALSegmentIterator(t.TempDir(), "0123456789abcdef", nil)
+		if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 5}); err != nil {
+			t.Fatal(err)
+		} else if err := itr.Append(litestream.WALSegmentInfo{Generation: "0123456789abcdef", Index: 0, Offset: 5}); err == nil || err.Error() != `duplicate offset 0000000000000000/0000000000000005 appended` {
+			t.Fatalf("unexpected error: %s", err)
+		}
+	})
+}

litestream.go

@@ -39,10 +39,12 @@ const (
 
 // Litestream errors.
 var (
-	ErrNoGeneration     = errors.New("no generation available")
+	ErrDBClosed          = errors.New("database closed")
-	ErrNoSnapshots      = errors.New("no snapshots available")
+	ErrNoGeneration      = errors.New("no generation available")
-	ErrNoWALSegments    = errors.New("no wal segments available")
+	ErrGenerationChanged = errors.New("generation changed")
-	ErrChecksumMismatch = errors.New("invalid replica, checksum mismatch")
+	ErrNoSnapshots       = errors.New("no snapshots available")
+	ErrNoWALSegments     = errors.New("no wal segments available")
+	ErrChecksumMismatch  = errors.New("invalid replica, checksum mismatch")
 )
 
 var (

replica.go

@@ -33,6 +33,7 @@ type Replica struct {
 
 	mu  sync.RWMutex
 	pos Pos // current replicated position
+	itr *FileWALSegmentIterator
 
 	muf sync.Mutex
 	f   *os.File // long-running file descriptor to avoid non-OFD lock issues
@@ -126,6 +127,11 @@ func (r *Replica) Start(ctx context.Context) {
 func (r *Replica) Stop() {
 	r.cancel()
 	r.wg.Wait()
+
+	if r.itr != nil {
+		r.itr.Close()
+		r.itr = nil
+	}
 }
 
 // Close will close the DB file descriptor which could release locks on
@@ -155,10 +161,24 @@ func (r *Replica) Sync(ctx context.Context) (err error) {
 	// Find current position of database.
 	dpos := r.db.Pos()
 	if dpos.IsZero() {
-		return fmt.Errorf("no generation, waiting for data")
+		return ErrNoGeneration
 	}
 	generation := dpos.Generation
 
+	// Close out iterator if the generation has changed.
+	if r.itr != nil && r.itr.Generation() != generation {
+		_ = r.itr.Close()
+		r.itr = nil
+	}
+
+	// Ensure we obtain a WAL iterator before we snapshot so we don't miss any segments.
+	resetItr := r.itr == nil
+	if resetItr {
+		if r.itr, err = r.db.WALSegments(ctx, generation); err != nil {
+			return fmt.Errorf("wal segments: %w", err)
+		}
+	}
+
 	// Create snapshot if no snapshots exist for generation.
 	snapshotN, err := r.snapshotN(generation)
 	if err != nil {
@@ -174,7 +194,7 @@ func (r *Replica) Sync(ctx context.Context) (err error) {
 	replicaSnapshotTotalGaugeVec.WithLabelValues(r.db.Path(), r.Name()).Set(float64(snapshotN))
 
 	// Determine position, if necessary.
-	if r.Pos().Generation != generation {
+	if resetItr {
 		pos, err := r.calcPos(ctx, generation)
 		if err != nil {
 			return fmt.Errorf("cannot determine replica position: %s", err)
@@ -196,16 +216,11 @@ func (r *Replica) Sync(ctx context.Context) (err error) {
 func (r *Replica) syncWAL(ctx context.Context) (err error) {
 	pos := r.Pos()
 
-	itr, err := r.db.WALSegments(ctx, pos.Generation)
-	if err != nil {
-		return err
-	}
-	defer itr.Close()
-
 	// Group segments by index.
 	var segments [][]WALSegmentInfo
-	for itr.Next() {
+	for r.itr.Next() {
-		info := itr.WALSegment()
+		info := r.itr.WALSegment()
+
 		if cmp, err := ComparePos(pos, info.Pos()); err != nil {
 			return fmt.Errorf("compare pos: %w", err)
 		} else if cmp == 1 {
@@ -624,38 +639,39 @@ func (r *Replica) deleteWALSegmentsBeforeIndex(ctx context.Context, generation s
 
 // monitor runs in a separate goroutine and continuously replicates the DB.
 func (r *Replica) monitor(ctx context.Context) {
-	ticker := time.NewTicker(r.SyncInterval)
+	timer := time.NewTimer(r.SyncInterval)
-	defer ticker.Stop()
+	defer timer.Stop()
 
-	// Continuously check for new data to replicate.
+	for {
-	ch := make(chan struct{})
+		if err := r.Sync(ctx); ctx.Err() != nil {
-	close(ch)
+			return
-	var notify <-chan struct{} = ch
+		} else if err != nil && err != ErrNoGeneration {
+			r.Logger.Printf("monitor error: %s", err)
+		}
 
-	for initial := true; ; initial = false {
+		// Wait for a change to the WAL iterator.
-		// Enforce a minimum time between synchronization.
+		if r.itr != nil {
-		if !initial {
 			select {
 			case <-ctx.Done():
 				return
-			case <-ticker.C:
+			case <-r.itr.NotifyCh():
 			}
 		}
 
-		// Wait for changes to the database.
+		// Wait for the sync interval to collect additional changes.
+		timer.Reset(r.SyncInterval)
 		select {
 		case <-ctx.Done():
 			return
-		case <-notify:
+		case <-timer.C:
 		}
 
-		// Fetch new notify channel before replicating data.
+		// Flush any additional notifications from the WAL iterator.
-		notify = r.db.WALNotify()
+		if r.itr != nil {
-
+			select {
-		// Synchronize the shadow wal into the replication directory.
+			case <-r.itr.NotifyCh():
-		if err := r.Sync(ctx); err != nil {
+			default:
-			r.Logger.Printf("monitor error: %s", err)
+			}
-			continue
 		}
 	}
 }

replica_client_test.go

@@ -489,8 +489,6 @@ func TestFindMaxIndexByGeneration(t *testing.T) {
 	})
 }
 
-func TestRestoreSnapshot(t *testing.T) { t.Skip("TODO") }
-
 func TestRestore(t *testing.T) {
 	t.Run("OK", func(t *testing.T) {
 		testDir := filepath.Join("testdata", "restore", "ok")