Litestream lightweight read-replicas. (#328)

2026-01-11 21:49:13 +00:00 · 2025-11-09 13:16:08 +00:00
parent 99b097de3b
commit 8f9a6ca4c1
8 changed files with 675 additions and 1 deletions
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -74,7 +74,7 @@ jobs:
    - name: Test GORM
      shell: bash
      run: gormlite/test.sh
-      if: matrix.os != 'windows-latest'
+      if: matrix.os == 'ubuntu-latest'

    - name: Collect coverage
      run: |
--- a/litestream/README.md
+++ b/litestream/README.md
@@ -0,0 +1,11 @@
+# Litestream lightweight read-replicas
+
+This package implements the **EXPERIMENTAL** `"litestream"` SQLite VFS
+that offers Litestream [lightweight read-replicas](https://fly.io/blog/litestream-revamped/#lightweight-read-replicas).
+
+See the [example](vfs_test.go) for how to use.
+
+To improve performance,
+increase `PollInterval` (and `MinLevel`) as much as you can,
+and set [`PRAGMA cache_size=N`](https://www.sqlite.org/pragma.html#pragma_cache_size)
+(or use `_pragma=cache_size(N)`).
--- a/litestream/api.go
+++ b/litestream/api.go
@@ -0,0 +1,62 @@
+// Package litestream implements a Litestream lightweight read-replica VFS.
+package litestream
+
+import (
+	"log/slog"
+	"sync"
+	"time"
+
+	"github.com/benbjohnson/litestream"
+	"github.com/ncruces/go-sqlite3/vfs"
+)
+
+// The default poll interval.
+const DefaultPollInterval = 1 * time.Second
+
+func init() {
+	vfs.Register("litestream", liteVFS{})
+}
+
+var (
+	liteMtx sync.RWMutex
+	// +checklocks:liteMtx
+	liteDBs = map[string]*liteDB{}
+)
+
+// ReplicaOptions represents options for [NewReplica].
+type ReplicaOptions struct {
+	// Where to log error messages. May be nil.
+	Logger *slog.Logger
+	// Minimum compaction level to track.
+	MinLevel int
+	// Replica poll interval. Must be less than the compaction interval
+	// used by the replica at MinLevel+1.
+	PollInterval time.Duration
+}
+
+// NewReplica creates a read-replica from a Litestream client.
+func NewReplica(name string, client litestream.ReplicaClient, options ReplicaOptions) {
+	if options.Logger != nil {
+		options.Logger = options.Logger.With("name", name)
+	} else {
+		options.Logger = slog.New(slog.DiscardHandler)
+	}
+	if options.PollInterval <= 0 {
+		options.PollInterval = DefaultPollInterval
+	}
+	options.MinLevel = max(0, min(options.MinLevel, litestream.SnapshotLevel))
+
+	liteMtx.Lock()
+	defer liteMtx.Unlock()
+	liteDBs[name] = &liteDB{
+		client: client,
+		opts:   &options,
+	}
+}
+
+// RemoveReplica removes a replica by name.
+func RemoveReplica(name string) {
+	liteMtx.Lock()
+	defer liteMtx.Unlock()
+	delete(liteDBs, name)
+}
--- a/litestream/go.mod
+++ b/litestream/go.mod
@@ -0,0 +1,63 @@
+module github.com/ncruces/go-sqlite3/litestream
+
+go 1.24.4
+
+require (
+	github.com/benbjohnson/litestream v0.5.2
+	github.com/ncruces/go-sqlite3 v0.30.1
+	github.com/ncruces/wbt v0.2.0
+	github.com/superfly/ltx v0.5.0
+)
+
+// github.com/ncruces/go-sqlite3
+require (
+	github.com/ncruces/julianday v1.0.0 // indirect
+	github.com/tetratelabs/wazero v1.10.0 // indirect
+	golang.org/x/sys v0.38.0 // indirect
+)
+
+// github.com/superfly/ltx
+require github.com/pierrec/lz4/v4 v4.1.22 // indirect
+
+// github.com/benbjohnson/litestream
+require (
+	filippo.io/age v1.2.1 // indirect
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/kr/text v0.2.0 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
+	github.com/prometheus/client_golang v1.23.2 // indirect
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.67.2 // indirect
+	github.com/prometheus/procfs v0.19.2 // indirect
+	github.com/psanford/sqlite3vfs v0.0.0-20240315230605-24e1d98cf361 // indirect
+	go.yaml.in/yaml/v2 v2.4.3 // indirect
+	golang.org/x/crypto v0.43.0 // indirect
+	google.golang.org/protobuf v1.36.10 // indirect
+	modernc.org/sqlite v1.40.0 // indirect
+)
+
+// github.com/benbjohnson/litestream/s3
+require (
+	github.com/aws/aws-sdk-go-v2 v1.37.1 // indirect
+	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.7.0 // indirect
+	github.com/aws/aws-sdk-go-v2/config v1.30.2 // indirect
+	github.com/aws/aws-sdk-go-v2/credentials v1.18.2 // indirect
+	github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.18.1 // indirect
+	github.com/aws/aws-sdk-go-v2/feature/s3/manager v1.18.2 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/configsources v1.4.1 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.7.1 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/ini v1.8.3 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/v4a v1.4.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.13.0 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/checksum v1.8.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.13.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.19.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/s3 v1.85.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/sso v1.26.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/ssooidc v1.31.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/sts v1.35.1 // indirect
+	github.com/aws/smithy-go v1.22.5 // indirect
+)
+
+replace modernc.org/sqlite => github.com/ncruces/go-sqlite3/litestream/modernc v0.0.0-20251109124432-99b097de3b79
--- a/litestream/go.sum
+++ b/litestream/go.sum
@@ -0,0 +1,197 @@
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--- a/litestream/vfs.go
+++ b/litestream/vfs.go
@@ -0,0 +1,290 @@
+package litestream
+
+import (
+	"context"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"sync"
+	"time"
+
+	"github.com/benbjohnson/litestream"
+	"github.com/ncruces/go-sqlite3"
+	"github.com/ncruces/go-sqlite3/util/vfsutil"
+	"github.com/ncruces/go-sqlite3/vfs"
+	"github.com/ncruces/wbt"
+	"github.com/superfly/ltx"
+)
+
+type liteVFS struct{}
+
+func (liteVFS) Open(name string, flags vfs.OpenFlag) (vfs.File, vfs.OpenFlag, error) {
+	// Temp journals, as used by the sorter, use SliceFile.
+	if flags&vfs.OPEN_TEMP_JOURNAL != 0 {
+		return &vfsutil.SliceFile{}, flags | vfs.OPEN_MEMORY, nil
+	}
+	// Refuse to open all other file types.
+	if flags&vfs.OPEN_MAIN_DB == 0 {
+		return nil, flags, sqlite3.CANTOPEN
+	}
+
+	liteMtx.RLock()
+	defer liteMtx.RUnlock()
+	if db, ok := liteDBs[name]; ok {
+		// Build the page index so we can lookup individual pages.
+		if err := db.buildIndex(context.Background()); err != nil {
+			db.opts.Logger.Error("build index", "error", err)
+			return nil, 0, err
+		}
+		return &liteFile{db: db}, flags | vfs.OPEN_READONLY, nil
+	}
+	return nil, flags, sqlite3.CANTOPEN
+}
+
+func (liteVFS) Delete(name string, dirSync bool) error {
+	// notest // used to delete journals
+	return sqlite3.IOERR_DELETE_NOENT
+}
+
+func (liteVFS) Access(name string, flag vfs.AccessFlag) (bool, error) {
+	// notest // used to check for journals
+	return false, nil
+}
+
+func (liteVFS) FullPathname(name string) (string, error) {
+	return name, nil
+}
+
+type liteFile struct {
+	db       *liteDB
+	conn     *sqlite3.Conn
+	pages    *pageIndex
+	txid     ltx.TXID
+	pageSize uint32
+}
+
+func (f *liteFile) Close() error { return nil }
+
+func (f *liteFile) ReadAt(p []byte, off int64) (n int, err error) {
+	ctx := f.context()
+	pages, txid := f.pages, f.txid
+	if pages == nil {
+		pages, txid, err = f.db.pollReplica(ctx)
+	}
+	if err != nil {
+		return 0, err
+	}
+
+	pgno := uint32(1)
+	if off >= 512 {
+		pgno += uint32(off / int64(f.pageSize))
+	}
+
+	elem, ok := pages.Get(pgno)
+	if !ok {
+		return 0, io.EOF
+	}
+
+	_, data, err := litestream.FetchPage(ctx, f.db.client, elem.Level, elem.MinTXID, elem.MaxTXID, elem.Offset, elem.Size)
+	if err != nil {
+		f.db.opts.Logger.Error("fetch page", "error", err)
+		return 0, err
+	}
+
+	// Update the first page to pretend we are in journal mode,
+	// load the page size and track changes to the database.
+	if pgno == 1 && len(data) >= 100 &&
+		data[18] >= 1 && data[19] >= 1 &&
+		data[18] <= 3 && data[19] <= 3 {
+		data[18], data[19] = 0x01, 0x01
+		binary.BigEndian.PutUint32(data[24:28], uint32(txid))
+		f.pageSize = uint32(256 * binary.LittleEndian.Uint16(data[16:18]))
+	}
+
+	n = copy(p, data[off%int64(len(data)):])
+	return n, nil
+}
+
+func (f *liteFile) WriteAt(b []byte, off int64) (n int, err error) {
+	// notest // OPEN_READONLY
+	return 0, sqlite3.IOERR_WRITE
+}
+
+func (f *liteFile) Truncate(size int64) error {
+	// notest // OPEN_READONLY
+	return sqlite3.IOERR_TRUNCATE
+}
+
+func (f *liteFile) Sync(flag vfs.SyncFlag) error {
+	// notest // OPEN_READONLY
+	return sqlite3.IOERR_FSYNC
+}
+
+func (f *liteFile) Size() (size int64, err error) {
+	if max := f.pages.Max(); max != nil {
+		size = int64(max.Key()) * int64(f.pageSize)
+	}
+	return
+}
+
+func (f *liteFile) Lock(lock vfs.LockLevel) (err error) {
+	if lock >= vfs.LOCK_RESERVED {
+		return sqlite3.IOERR_LOCK
+	}
+	f.pages, f.txid, err = f.db.pollReplica(f.context())
+	return err
+}
+
+func (f *liteFile) Unlock(lock vfs.LockLevel) error {
+	f.pages, f.txid = nil, 0
+	return nil
+}
+
+func (f *liteFile) CheckReservedLock() (bool, error) {
+	// notest // used to check for hot journals
+	return false, nil
+}
+
+func (f *liteFile) SectorSize() int {
+	// notest // safe default
+	return 0
+}
+
+func (f *liteFile) DeviceCharacteristics() vfs.DeviceCharacteristic {
+	// notest // safe default
+	return 0
+}
+
+func (f *liteFile) SetDB(conn any) {
+	f.conn = conn.(*sqlite3.Conn)
+}
+
+func (f *liteFile) context() context.Context {
+	if f.conn != nil {
+		return f.conn.GetInterrupt()
+	}
+	return context.Background()
+}
+
+type liteDB struct {
+	client   litestream.ReplicaClient
+	opts     *ReplicaOptions
+	pages    *pageIndex // +checklocks:mtx
+	lastPoll time.Time  // +checklocks:mtx
+	txids    levelTXIDs // +checklocks:mtx
+	mtx      sync.Mutex
+}
+
+func (f *liteDB) buildIndex(ctx context.Context) error {
+	f.mtx.Lock()
+	defer f.mtx.Unlock()
+
+	// Skip if we already have an index.
+	if f.pages != nil {
+		return nil
+	}
+
+	// Build the index from scratch from a Litestream restore plan.
+	infos, err := litestream.CalcRestorePlan(ctx, f.client, 0, time.Time{}, f.opts.Logger)
+	if err != nil {
+		if !errors.Is(err, litestream.ErrTxNotAvailable) {
+			return fmt.Errorf("calc restore plan: %w", err)
+		}
+		return nil
+	}
+
+	for _, info := range infos {
+		err := f.updateInfo(ctx, info)
+		if err != nil {
+			return err
+		}
+	}
+
+	f.lastPoll = time.Now()
+	return nil
+}
+
+func (f *liteDB) pollReplica(ctx context.Context) (*pageIndex, ltx.TXID, error) {
+	f.mtx.Lock()
+	defer f.mtx.Unlock()
+
+	// Limit polling interval.
+	if time.Since(f.lastPoll) < f.opts.PollInterval {
+		return f.pages, f.txids[0], nil
+	}
+
+	// Updating from MinLevel to SnapshotLevel is non-racy,
+	// since LTX files are compacted into higher levels
+	// before the lower level LTX files are deleted.
+	for level := f.opts.MinLevel; level <= litestream.SnapshotLevel; level++ {
+		if err := f.updateLevel(ctx, level); err != nil {
+			f.opts.Logger.Error("cannot poll replica", "error", err)
+			return nil, 0, err
+		}
+	}
+
+	f.lastPoll = time.Now()
+	return f.pages, f.txids[0], nil
+}
+
+// +checklocks:f.mtx
+func (f *liteDB) updateLevel(ctx context.Context, level int) error {
+	var nextTXID ltx.TXID
+	// Snapshots must start from scratch,
+	// other levels can start from where they were left.
+	if level != litestream.SnapshotLevel {
+		nextTXID = f.txids[level] + 1
+	}
+
+	// Start reading from the next LTX file after the current position.
+	itr, err := f.client.LTXFiles(ctx, level, nextTXID, false)
+	if err != nil {
+		return fmt.Errorf("ltx files: %w", err)
+	}
+	defer itr.Close()
+
+	// Build an update across all new LTX files.
+	for itr.Next() {
+		info := itr.Item()
+
+		// Skip LTX files already fully loaded into the index.
+		if info.MaxTXID <= f.txids[level] {
+			continue
+		}
+
+		err := f.updateInfo(ctx, info)
+		if err != nil {
+			return err
+		}
+	}
+	if err := itr.Err(); err != nil {
+		return err
+	}
+	return itr.Close()
+}
+
+// +checklocks:f.mtx
+func (f *liteDB) updateInfo(ctx context.Context, info *ltx.FileInfo) error {
+	idx, err := litestream.FetchPageIndex(ctx, f.client, info)
+	if err != nil {
+		return fmt.Errorf("fetch page index: %w", err)
+	}
+
+	// Replace pages in the index with new pages.
+	for k, v := range idx {
+		// Patch avoids mutating the index for an unmodified page.
+		f.pages = f.pages.Patch(k, func(node *pageIndex) (ltx.PageIndexElem, bool) {
+			return v, node == nil || v != node.Value()
+		})
+	}
+
+	// Track the MaxTXID for each level.
+	maxTXID := &f.txids[info.Level]
+	*maxTXID = max(*maxTXID, info.MaxTXID)
+	return nil
+}
+
+// Type aliases; these are a mouthful.
+type pageIndex = wbt.Tree[uint32, ltx.PageIndexElem]
+type levelTXIDs = [litestream.SnapshotLevel + 1]ltx.TXID
--- a/litestream/vfs_test.go
+++ b/litestream/vfs_test.go
@@ -0,0 +1,47 @@
+package litestream_test
+
+import (
+	"log"
+	"time"
+
+	"github.com/benbjohnson/litestream/s3"
+	"github.com/ncruces/go-sqlite3/driver"
+	_ "github.com/ncruces/go-sqlite3/embed"
+	"github.com/ncruces/go-sqlite3/litestream"
+)
+
+func ExampleNewReplica() {
+	client := s3.NewReplicaClient()
+	client.Bucket = "test-bucket"
+	client.Path = "fruits.db"
+
+	litestream.NewReplica("fruits.db", client, litestream.ReplicaOptions{
+		PollInterval: 5 * time.Second,
+	})
+
+	db, err := driver.Open("file:fruits.db?vfs=litestream")
+	if err != nil {
+		log.Fatalln(err)
+	}
+	defer db.Close()
+
+	for {
+		time.Sleep(time.Second)
+		rows, err := db.Query("SELECT * FROM fruits")
+		if err != nil {
+			log.Fatalln(err)
+		}
+
+		for rows.Next() {
+			var name, color string
+			err := rows.Scan(&name, &color)
+			if err != nil {
+				log.Fatalln(err)
+			}
+			log.Println(name, color)
+		}
+
+		log.Println("===")
+		rows.Close()
+	}
+}
--- a/vfs/README.md
+++ b/vfs/README.md
@@ -117,9 +117,13 @@ The VFS can be customized with a few build tags:

 - [`github.com/ncruces/go-sqlite3/vfs/memdb`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/memdb)
  implements an in-memory VFS.
+- [`github.com/ncruces/go-sqlite3/vfs/mvcc`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/mvcc)
+  implements an in-memory MVCC VFS.
 - [`github.com/ncruces/go-sqlite3/vfs/readervfs`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/readervfs)
  implements a VFS for immutable databases.
 - [`github.com/ncruces/go-sqlite3/vfs/adiantum`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/adiantum)
  wraps a VFS to offer encryption at rest.
 - [`github.com/ncruces/go-sqlite3/vfs/xts`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs/xts)
  wraps a VFS to offer encryption at rest.
+- [`github.com/ncruces/go-sqlite3/litestream`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/litestream)
+  implements Litestream [lightweight read-replicas](https://fly.io/blog/litestream-revamped/#lightweight-read-replicas).