Fix release action

Previously, the sync would validate the last page written to ensure
that replication picked up from the last position. However, a large
WAL file followed by a series of shorter checkpointed WAL files means
that the last page could be the same even if multiple checkpoints
have occurred.

To fix this, the WAL header must match the shadow WAL header when
starting litestream since there are no guarantees about checkpoints.

.github/workflows/release.yml

@@ -5,7 +5,7 @@ on:
 
 name: release
 jobs:
-  release:
+  linux:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
@@ -16,10 +16,21 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ github.token }}
 
+      - name: Install nfpm
+        run: |
+          wget https://github.com/goreleaser/nfpm/releases/download/v2.2.3/nfpm_2.2.3_Linux_x86_64.tar.gz
+          tar zxvf nfpm_2.2.3_Linux_x86_64.tar.gz
+
       - name: Build litestream
         run: |
-          go build -ldflags "-X 'main.Version=${{ steps.release.outputs.tag_name }}'" -o litestream ./cmd/litestream
+          mkdir -p dist
+          cp etc/litestream.yml etc/litestream.service dist
+          cat etc/nfpm.yml | LITESTREAM_VERSION=${{ steps.release.outputs.tag_name }} envsubst > dist/nfpm.yml
+          go build -ldflags "-X 'main.Version=${{ steps.release.outputs.tag_name }}'" -o dist/litestream ./cmd/litestream
+          
+          cd dist
           tar -czvf litestream-${{ steps.release.outputs.tag_name }}-linux-amd64.tar.gz litestream
+          ../nfpm pkg --config nfpm.yml --packager deb --target litestream-${{ steps.release.outputs.tag_name }}-linux-amd64.deb 
 
       - name: Upload release binary
         uses: actions/upload-release-asset@v1.0.2
@@ -27,6 +38,16 @@ jobs:
           GITHUB_TOKEN: ${{ github.token }}
         with:
           upload_url: ${{ steps.release.outputs.upload_url }}
-          asset_path: ./litestream-${{ steps.release.outputs.tag_name }}-linux-amd64.tar.gz
+          asset_path: ./dist/litestream-${{ steps.release.outputs.tag_name }}-linux-amd64.tar.gz
           asset_name: litestream-${{ steps.release.outputs.tag_name }}-linux-amd64.tar.gz
           asset_content_type: application/gzip
+
+      - name: Upload debian package
+        uses: actions/upload-release-asset@v1.0.2
+        env:
+          GITHUB_TOKEN: ${{ github.token }}
+        with:
+          upload_url: ${{ steps.release.outputs.upload_url }}
+          asset_path: ./dist/litestream-${{ steps.release.outputs.tag_name }}-linux-amd64.deb
+          asset_name: litestream-${{ steps.release.outputs.tag_name }}-linux-amd64.deb
+          asset_content_type: application/octet-stream

.github/workflows/test.yml

@@ -5,6 +5,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/setup-go@v2
+        with:
+          go-version: '1.15'
 
       - uses: actions/checkout@v2
 

.gitignore

@@ -0,0 +1,2 @@
+.DS_Store
+/dist

Makefile

@@ -0,0 +1,19 @@
+default:
+
+dist:
+	mkdir -p dist
+	cp etc/litestream.yml dist/litestream.yml
+	docker run --rm -v "${PWD}":/usr/src/litestream -w /usr/src/litestream -e GOOS=linux -e GOARCH=amd64 golang:1.15 go build -v -o dist/litestream ./cmd/litestream
+	tar -cz -f dist/litestream-linux-amd64.tar.gz -C dist litestream
+
+deb: dist
+ifndef LITESTREAM_VERSION
+	$(error LITESTREAM_VERSION is undefined)
+endif
+	cat etc/nfpm.yml | envsubst > dist/nfpm.yml
+	nfpm pkg --config dist/nfpm.yml --packager deb --target dist/litestream.deb 
+
+clean:
+	rm -rf dist
+
+.PHONY: deb dist clean

README.md

@@ -1,17 +1,200 @@
-litestream
+Litestream ![test](https://github.com/benbjohnson/litestream/workflows/test/badge.svg)
 ==========
 
-Streaming replication for SQLite.
+Litestream is a standalone streaming replication tool for SQLite. It runs as a
+background process and safely replicates changes incrementally to another file
+or S3. Litestream only communicates with SQLite through the SQLite API so it
+will not corrupt your database.
+
+If you find this project interesting, please consider starring the project on
+GitHub.
 
 
-## Questions
+## Installation
 
-- How to avoid WAL checkpointing on close?
+### Homebrew
+
+TODO
 
 
-## Notes
+### Linux (Debian)
 
-```sql
--- Disable autocheckpointing.
-PRAGMA wal_autocheckpoint = 0
+You can download the `.deb` file from the [Releases page][releases] page and
+then run the following:
+
+```sh
+$ sudo dpkg -i litestream-v0.3.0-linux-amd64.deb
 ```
+
+Once installed, you'll need to enable & start the service:
+
+```sh
+$ sudo systemctl enable litestream
+$ sudo systemctl start litestream
+```
+
+
+### Release binaries
+
+You can also download the release binary for your system from the
+[Releases page][releases] and run it as a standalone application.
+
+
+## Configuration
+
+Once installed locally, you'll need to create a config file. By default, the
+config file lives at `/etc/litestream.yml` but you can pass in a different
+path to any `litestream` command using the `-config PATH` flag.
+
+The configuration specifies one or more `dbs` and a list of one or more replica
+locations for each db. Below are some common configurations:
+
+### Replicate to S3
+
+This will replicate the database at `/path/to/db` to the `"/db"` path inside
+the S3 bucket named `"mybkt"`.
+
+```yaml
+access-key-id:     AKIAxxxxxxxxxxxxxxxx
+secret-access-key: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/xxxxxxxxx
+
+dbs:
+  - path: /path/to/db
+    replicas:
+      - path: s3://mybkt/db
+```
+
+### Replicate to another file path
+
+This will replicate the database at `/path/to/db` to a directory named
+`/path/to/replica`.
+
+```yaml
+dbs:
+  - path: /path/to/db
+    replicas:
+      - path: /path/to/replica
+```
+
+
+### Other configuration options
+
+These are some additional configuration options available on replicas:
+
+- `type`—Specify the type of replica (`"file"` or `"s3"`). Derived from `"path"`.
+- `name`—Specify an optional name for the replica if you are using multiple replicas.
+- `path`—File path or URL to the replica location.
+- `retention`—Length of time to keep replicated WAL files. Defaults to `24h`.
+- `retention-check-interval`—Time between retention enforcement checks. Defaults to `1h`.
+- `validation-interval`—Interval between periodic checks to ensure restored backup matches current database. Disabled by default.
+
+These replica options are only available for S3 replicas:
+
+- `bucket`—S3 bucket name. Derived from `"path"`.
+- `region`—S3 bucket region. Looked up on startup if unspecified.
+- `sync-interval`—Replication sync frequency.
+
+
+## Usage
+
+### Replication
+
+Once your configuration is saved, you'll need to begin replication. If you
+installed the `.deb` file then run:
+
+```sh
+$ sudo systemctl restart litestream
+```
+
+To run litestream on its own, run:
+
+```sh
+# Replicate using the /etc/litestream.yml configuration.
+$ litestream replicate
+
+# Replicate using a different configuration path.
+$ litestream replicate -config /path/to/litestream.yml
+```
+
+The `litestream` command will initialize and then wait indefinitely for changes.
+You should see your destination replica path is now populated with a
+`generations` directory. Inside there should be a 16-character hex generation
+directory and inside there should be snapshots & WAL files. As you make changes
+to your source database, changes will be copied over to your replica incrementally.
+
+
+### Restoring a backup
+
+Litestream can restore a previous snapshot and replay all replicated WAL files.
+By default, it will restore up to the latest WAL file but you can also perform
+point-in-time restores.
+
+A database can only be restored to a path that does not exist so you don't need
+to worry about accidentally overwriting your current database.
+
+```sh
+# Restore database to original path.
+$ litestream restore /path/to/db
+
+# Restore database to a new location.
+$ litestream restore -o /tmp/mynewdb /path/to/db
+
+# Restore database to a specific point-in-time.
+$ litestream restore -timestamp 2020-01-01T00:00:00Z /path/to/db
+```
+
+Point-in-time restores only have the resolution of the timestamp of the WAL file
+itself. By default, litestream will start a new WAL file every minute so
+point-in-time restores are only accurate to the minute.
+
+
+
+## How it works
+
+SQLite provides a WAL (write-ahead log) journaling mode which writes pages to
+a `-wal` file before eventually being copied over to the original database file.
+This copying process is known as checkpointing. The WAL file works as a circular
+buffer so when the WAL reaches a certain size then it restarts from the beginning.
+
+Litestream works by taking over the checkpointing process and controlling when
+it is restarted to ensure that it copies every new page. Checkpointing is only
+allowed when there are no read transactions so Litestream maintains a
+long-running read transaction against each database until it is ready to
+checkpoint.
+
+The SQLite WAL file is copied to a separate location called the shadow WAL which
+ensures that it will not be overwritten by SQLite. This shadow WAL acts as a
+temporary buffer so that replicas can replicate to their destination (e.g.
+another file path or to S3). The shadow WAL files are removed once they have
+been fully replicated. You can find the shadow directory as a hidden directory
+next to your database file. If you database file is named `/var/lib/my.db` then
+the shadow directory will be `/var/lib/.my.db-litestream`.
+
+Litestream groups a snapshot and all subsequent WAL changes into "generations".
+A generation is started on initial replication of a database and a new
+generation will be started if litestream detects that the WAL replication is
+no longer contiguous. This can occur if the `litestream` process is stopped and
+another process is allowed to checkpoint the WAL.
+
+
+
+## Open-source, not open-contribution
+
+[Similar to SQLite](https://www.sqlite.org/copyright.html), litestream is open
+source but closed to contributions. This keeps the code base free of proprietary
+or licensed code but it also helps me continue to maintain and build litestream.
+
+As the author of [BoltDB](https://github.com/boltdb/bolt), I found that
+accepting and maintaining third party patches contributed to my burn out and
+I eventually archived the project. Writing databases & low-level replication
+tools involves nuance and simple one line changes can have profound and
+unexpected changes in correctness and performance. Small contributions
+typically required hours of my time to properly test and validate them.
+
+I am grateful for community involvement, bug reports, & feature requests. I do
+not wish to come off as anything but welcoming, however, I've
+made the decision to keep this project closed to contributions for my own
+mental health and long term viability of the project.
+
+
+[releases]: https://github.com/benbjohnson/litestream/releases

cmd/litestream/databases.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"strings"
+	"text/tabwriter"
+)
+
+// DatabasesCommand is a command for listing managed databases.
+type DatabasesCommand struct{}
+
+// Run executes the command.
+func (c *DatabasesCommand) Run(ctx context.Context, args []string) (err error) {
+	var configPath string
+	fs := flag.NewFlagSet("litestream-databases", flag.ContinueOnError)
+	registerConfigFlag(fs, &configPath)
+	fs.Usage = c.Usage
+	if err := fs.Parse(args); err != nil {
+		return err
+	} else if fs.NArg() != 0 {
+		return fmt.Errorf("too many argument")
+	}
+
+	// Load configuration.
+	if configPath == "" {
+		return errors.New("-config required")
+	}
+	config, err := ReadConfigFile(configPath)
+	if err != nil {
+		return err
+	}
+
+	// List all databases.
+	w := tabwriter.NewWriter(os.Stdout, 0, 8, 1, '\t', 0)
+	fmt.Fprintln(w, "path\treplicas")
+	for _, dbConfig := range config.DBs {
+		db, err := newDBFromConfig(&config, dbConfig)
+		if err != nil {
+			return err
+		}
+
+		var replicaNames []string
+		for _, r := range db.Replicas {
+			replicaNames = append(replicaNames, r.Name())
+		}
+
+		fmt.Fprintf(w, "%s\t%s\n",
+			db.Path(),
+			strings.Join(replicaNames, ","),
+		)
+	}
+	w.Flush()
+
+	return nil
+}
+
+// Usage prints the help screen to STDOUT.
+func (c *DatabasesCommand) Usage() {
+	fmt.Printf(`
+The databases command lists all databases in the configuration file.
+
+Usage:
+
+	litestream databases [arguments]
+
+Arguments:
+
+	-config PATH
+	    Specifies the configuration file.
+	    Defaults to %s
+
+`[1:],
+		DefaultConfigPath(),
+	)
+}

cmd/litestream/generations.go

@@ -12,89 +12,85 @@ import (
 	"time"
 )
 
-type GenerationsCommand struct {
-	ConfigPath string
-	Config     Config
-
-	DBPath string
-}
-
-func NewGenerationsCommand() *GenerationsCommand {
-	return &GenerationsCommand{}
-}
+// GenerationsCommand represents a command to list all generations for a database.
+type GenerationsCommand struct{}
 
+// Run executes the command.
 func (c *GenerationsCommand) Run(ctx context.Context, args []string) (err error) {
+	var configPath string
 	fs := flag.NewFlagSet("litestream-generations", flag.ContinueOnError)
-	registerConfigFlag(fs, &c.ConfigPath)
+	registerConfigFlag(fs, &configPath)
+	replicaName := fs.String("replica", "", "replica name")
 	fs.Usage = c.Usage
 	if err := fs.Parse(args); err != nil {
 		return err
+	} else if fs.NArg() == 0 || fs.Arg(0) == "" {
+		return fmt.Errorf("database path required")
 	} else if fs.NArg() > 1 {
 		return fmt.Errorf("too many arguments")
 	}
 
 	// Load configuration.
-	if c.ConfigPath == "" {
+	if configPath == "" {
 		return errors.New("-config required")
 	}
-	config, err := ReadConfigFile(c.ConfigPath)
+	config, err := ReadConfigFile(configPath)
 	if err != nil {
 		return err
 	}
 
-	// Determine absolute path for database, if specified.
-	if c.DBPath = fs.Arg(0); c.DBPath != "" {
-		if c.DBPath, err = filepath.Abs(c.DBPath); err != nil {
-			return err
-		}
+	// Determine absolute path for database.
+	dbPath, err := filepath.Abs(fs.Arg(0))
+	if err != nil {
+		return err
+	}
+
+	// Instantiate DB from from configuration.
+	dbConfig := config.DBConfig(dbPath)
+	if dbConfig == nil {
+		return fmt.Errorf("database not found in config: %s", dbPath)
+	}
+	db, err := newDBFromConfig(&config, dbConfig)
+	if err != nil {
+		return err
+	}
+
+	// Determine last time database or WAL was updated.
+	updatedAt, err := db.UpdatedAt()
+	if err != nil {
+		return err
 	}
 
 	// List each generation.
 	w := tabwriter.NewWriter(os.Stdout, 0, 8, 1, '\t', 0)
-	fmt.Fprintln(w, "db\tname\tgeneration\tlag\tstart\tend")
-	for _, dbConfig := range config.DBs {
-		// Filter database, if specified in the arguments.
-		if c.DBPath != "" && dbConfig.Path != c.DBPath {
+	fmt.Fprintln(w, "name\tgeneration\tlag\tstart\tend")
+	for _, r := range db.Replicas {
+		if *replicaName != "" && r.Name() != *replicaName {
 			continue
 		}
 
-		// Instantiate DB from from configuration.
-		db, err := newDBFromConfig(dbConfig)
+		generations, err := r.Generations(ctx)
 		if err != nil {
-			return err
+			log.Printf("%s: cannot list generations: %s", r.Name(), err)
+			continue
 		}
 
-		// Determine last time database or WAL was updated.
-		updatedAt, err := db.UpdatedAt()
-		if err != nil {
-			return err
-		}
-
-		// Iterate over each replica in the database.
-		for _, r := range db.Replicas {
-			generations, err := r.Generations(ctx)
+		// Iterate over each generation for the replica.
+		for _, generation := range generations {
+			stats, err := r.GenerationStats(ctx, generation)
 			if err != nil {
-				log.Printf("%s: cannot list generations: %s", r.Name(), err)
+				log.Printf("%s: cannot find generation stats: %s", r.Name(), err)
 				continue
 			}
 
-			// Iterate over each generation for the replica.
-			for _, generation := range generations {
-				stats, err := r.GenerationStats(ctx, generation)
-				if err != nil {
-					log.Printf("%s: cannot find generation stats: %s", r.Name(), err)
-					continue
-				}
-
-				fmt.Fprintf(w, "%s\t%s\t%s\t%s\t%s\t%s\n",
-					db.Path(),
-					r.Name(),
-					generation,
-					truncateDuration(stats.UpdatedAt.Sub(updatedAt)).String(),
-					stats.CreatedAt.Format(time.RFC3339),
-					stats.UpdatedAt.Format(time.RFC3339),
-				)
-			}
+			fmt.Fprintf(w, "%s\t%s\t%s\t%s\t%s\n",
+				r.Name(),
+				generation,
+				truncateDuration(updatedAt.Sub(stats.UpdatedAt)).String(),
+				stats.CreatedAt.Format(time.RFC3339),
+				stats.UpdatedAt.Format(time.RFC3339),
+			)
+			w.Flush()
 		}
 	}
 	w.Flush()
@@ -102,9 +98,10 @@ func (c *GenerationsCommand) Run(ctx context.Context, args []string) (err error)
 	return nil
 }
 
+// Usage prints the help message to STDOUT.
 func (c *GenerationsCommand) Usage() {
 	fmt.Printf(`
-The generations command lists all generations across all replicas along with
+The generations command lists all generations for a database. It also lists
 stats about their lag behind the primary database and the time range they cover.
 
 Usage:
@@ -116,18 +113,32 @@ Arguments:
 	-config PATH
 	    Specifies the configuration file. Defaults to %s
 
+	-replica NAME
+	    Optional, filters by replica.
+
 `[1:],
-		DefaultConfigPath,
+		DefaultConfigPath(),
 	)
 }
 
 func truncateDuration(d time.Duration) time.Duration {
-	if d > time.Hour {
-		return d.Truncate(time.Hour)
-	} else if d > time.Minute {
-		return d.Truncate(time.Minute)
-	} else if d > time.Second {
+	if d < 0 {
+		if d < -10*time.Second {
+			return d.Truncate(time.Second)
+		} else if d < -time.Second {
+			return d.Truncate(time.Second / 10)
+		} else if d < -time.Millisecond {
+			return d.Truncate(time.Millisecond)
+		} else if d < -time.Microsecond {
+			return d.Truncate(time.Microsecond)
+		}
+		return d
+	}
+
+	if d > 10*time.Second {
 		return d.Truncate(time.Second)
+	} else if d > time.Second {
+		return d.Truncate(time.Second / 10)
 	} else if d > time.Millisecond {
 		return d.Truncate(time.Millisecond)
 	} else if d > time.Microsecond {

cmd/litestream/main.go

@@ -6,12 +6,17 @@ import (
 	"fmt"
 	"io/ioutil"
 	"log"
+	"net/url"
 	"os"
 	"os/user"
+	"path"
 	"path/filepath"
 	"strings"
+	"time"
 
 	"github.com/benbjohnson/litestream"
+	"github.com/benbjohnson/litestream/s3"
+	_ "github.com/mattn/go-sqlite3"
 	"gopkg.in/yaml.v2"
 )
 
@@ -20,9 +25,6 @@ var (
 	Version = "(development build)"
 )
 
-// DefaultConfigPath is the default configuration path.
-const DefaultConfigPath = "/etc/litestream.yml"
-
 func main() {
 	log.SetFlags(0)
 
@@ -35,12 +37,15 @@ func main() {
 	}
 }
 
+// Main represents the main program execution.
 type Main struct{}
 
+// NewMain returns a new instance of Main.
 func NewMain() *Main {
 	return &Main{}
 }
 
+// Run executes the program.
 func (m *Main) Run(ctx context.Context, args []string) (err error) {
 	var cmd string
 	if len(args) > 0 {
@@ -48,14 +53,20 @@ func (m *Main) Run(ctx context.Context, args []string) (err error) {
 	}
 
 	switch cmd {
+	case "databases":
+		return (&DatabasesCommand{}).Run(ctx, args)
 	case "generations":
 		return (&GenerationsCommand{}).Run(ctx, args)
 	case "replicate":
 		return (&ReplicateCommand{}).Run(ctx, args)
 	case "restore":
 		return (&RestoreCommand{}).Run(ctx, args)
+	case "snapshots":
+		return (&SnapshotsCommand{}).Run(ctx, args)
 	case "version":
 		return (&VersionCommand{}).Run(ctx, args)
+	case "wal":
+		return (&WALCommand{}).Run(ctx, args)
 	default:
 		if cmd == "" || cmd == "help" || strings.HasPrefix(cmd, "-") {
 			m.Usage()
@@ -65,6 +76,7 @@ func (m *Main) Run(ctx context.Context, args []string) (err error) {
 	}
 }
 
+// Usage prints the help screen to STDOUT.
 func (m *Main) Usage() {
 	fmt.Println(`
 litestream is a tool for replicating SQLite databases.
@@ -75,23 +87,54 @@ Usage:
 
 The commands are:
 
-	generations  list available generations across all dbs & replicas
+	generations  list available generations for a database
 	replicate    runs a server to replicate databases
 	restore      recovers database backup from a replica
+	snapshots    list available snapshots for a database
+	validate     checks replica to ensure a consistent state with primary
 	version      prints the version
+	wal          list available WAL files for a database
 `[1:])
 }
 
+// Default configuration settings.
+const (
+	DefaultAddr = ":9090"
+)
+
 // Config represents a configuration file for the litestream daemon.
 type Config struct {
-	DBs []*DBConfig `yaml:"databases"`
+	// Bind address for serving metrics.
+	Addr string `yaml:"addr"`
+
+	// List of databases to manage.
+	DBs []*DBConfig `yaml:"dbs"`
+
+	// Global S3 settings
+	AccessKeyID     string `yaml:"access-key-id"`
+	SecretAccessKey string `yaml:"secret-access-key"`
+	Region          string `yaml:"region"`
+	Bucket          string `yaml:"bucket"`
+}
+
+// Normalize expands paths and parses URL-specified replicas.
+func (c *Config) Normalize() error {
+	for i := range c.DBs {
+		if err := c.DBs[i].Normalize(); err != nil {
+			return err
+		}
+	}
+	return nil
 }
 
 // DefaultConfig returns a new instance of Config with defaults set.
 func DefaultConfig() Config {
-	return Config{}
+	return Config{
+		Addr: DefaultAddr,
+	}
 }
 
+// DBConfig returns database configuration by path.
 func (c *Config) DBConfig(path string) *DBConfig {
 	for _, dbConfig := range c.DBs {
 		if dbConfig.Path == path {
@@ -124,32 +167,107 @@ func ReadConfigFile(filename string) (Config, error) {
 	} else if err := yaml.Unmarshal(buf, &config); err != nil {
 		return config, err
 	}
+
+	if err := config.Normalize(); err != nil {
+		return config, err
+	}
 	return config, nil
 }
 
+// DBConfig represents the configuration for a single database.
 type DBConfig struct {
 	Path     string           `yaml:"path"`
 	Replicas []*ReplicaConfig `yaml:"replicas"`
 }
 
+// Normalize expands paths and parses URL-specified replicas.
+func (c *DBConfig) Normalize() error {
+	for i := range c.Replicas {
+		if err := c.Replicas[i].Normalize(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// ReplicaConfig represents the configuration for a single replica in a database.
 type ReplicaConfig struct {
-	Type string `yaml:"type"` // "file", "s3"
-	Name string `yaml:"name"` // name of replica, optional.
-	Path string `yaml:"path"` // used for file replicas
+	Type                   string        `yaml:"type"` // "file", "s3"
+	Name                   string        `yaml:"name"` // name of replica, optional.
+	Path                   string        `yaml:"path"`
+	Retention              time.Duration `yaml:"retention"`
+	RetentionCheckInterval time.Duration `yaml:"retention-check-interval"`
+	SyncInterval           time.Duration `yaml:"sync-interval"` // s3 only
+	ValidationInterval     time.Duration `yaml:"validation-interval"`
+
+	// S3 settings
+	AccessKeyID     string `yaml:"access-key-id"`
+	SecretAccessKey string `yaml:"secret-access-key"`
+	Region          string `yaml:"region"`
+	Bucket          string `yaml:"bucket"`
+}
+
+// Normalize expands paths and parses URL-specified replicas.
+func (c *ReplicaConfig) Normalize() error {
+	// Expand path filename, if necessary.
+	if prefix := "~" + string(os.PathSeparator); strings.HasPrefix(c.Path, prefix) {
+		u, err := user.Current()
+		if err != nil {
+			return err
+		} else if u.HomeDir == "" {
+			return fmt.Errorf("cannot expand replica path, no home directory available")
+		}
+		c.Path = filepath.Join(u.HomeDir, strings.TrimPrefix(c.Path, prefix))
+	}
+
+	// Attempt to parse as URL. Ignore if it is not a URL or if there is no scheme.
+	u, err := url.Parse(c.Path)
+	if err != nil || u.Scheme == "" {
+		return nil
+	}
+
+	switch u.Scheme {
+	case "file":
+		u.Scheme = ""
+		c.Type = u.Scheme
+		c.Path = path.Clean(u.String())
+		return nil
+
+	case "s3":
+		c.Type = u.Scheme
+		c.Path = strings.TrimPrefix(path.Clean(u.Path), "/")
+		c.Bucket = u.Host
+		if u := u.User; u != nil {
+			c.AccessKeyID = u.Username()
+			c.SecretAccessKey, _ = u.Password()
+		}
+		return nil
+
+	default:
+		return fmt.Errorf("unrecognized replica type in path scheme: %s", c.Path)
+	}
+}
+
+// DefaultConfigPath returns the default config path.
+func DefaultConfigPath() string {
+	if v := os.Getenv("LITESTREAM_CONFIG"); v != "" {
+		return v
+	}
+	return "/etc/litestream.yml"
 }
 
 func registerConfigFlag(fs *flag.FlagSet, p *string) {
-	fs.StringVar(p, "config", DefaultConfigPath, "config path")
+	fs.StringVar(p, "config", DefaultConfigPath(), "config path")
 }
 
 // newDBFromConfig instantiates a DB based on a configuration.
-func newDBFromConfig(config *DBConfig) (*litestream.DB, error) {
+func newDBFromConfig(c *Config, dbc *DBConfig) (*litestream.DB, error) {
 	// Initialize database with given path.
-	db := litestream.NewDB(config.Path)
+	db := litestream.NewDB(dbc.Path)
 
 	// Instantiate and attach replicas.
-	for _, rconfig := range config.Replicas {
-		r, err := newReplicaFromConfig(db, rconfig)
+	for _, rc := range dbc.Replicas {
+		r, err := newReplicaFromConfig(db, c, dbc, rc)
 		if err != nil {
 			return nil, err
 		}
@@ -160,19 +278,84 @@ func newDBFromConfig(config *DBConfig) (*litestream.DB, error) {
 }
 
 // newReplicaFromConfig instantiates a replica for a DB based on a config.
-func newReplicaFromConfig(db *litestream.DB, config *ReplicaConfig) (litestream.Replica, error) {
-	switch config.Type {
+func newReplicaFromConfig(db *litestream.DB, c *Config, dbc *DBConfig, rc *ReplicaConfig) (litestream.Replica, error) {
+	switch rc.Type {
 	case "", "file":
-		return newFileReplicaFromConfig(db, config)
+		return newFileReplicaFromConfig(db, c, dbc, rc)
+	case "s3":
+		return newS3ReplicaFromConfig(db, c, dbc, rc)
 	default:
-		return nil, fmt.Errorf("unknown replica type in config: %q", config.Type)
+		return nil, fmt.Errorf("unknown replica type in config: %q", rc.Type)
 	}
 }
 
 // newFileReplicaFromConfig returns a new instance of FileReplica build from config.
-func newFileReplicaFromConfig(db *litestream.DB, config *ReplicaConfig) (*litestream.FileReplica, error) {
-	if config.Path == "" {
-		return nil, fmt.Errorf("file replica path require for db %q", db.Path())
+func newFileReplicaFromConfig(db *litestream.DB, c *Config, dbc *DBConfig, rc *ReplicaConfig) (*litestream.FileReplica, error) {
+	if rc.Path == "" {
+		return nil, fmt.Errorf("%s: file replica path required", db.Path())
 	}
-	return litestream.NewFileReplica(db, config.Name, config.Path), nil
+
+	r := litestream.NewFileReplica(db, rc.Name, rc.Path)
+	if v := rc.Retention; v > 0 {
+		r.Retention = v
+	}
+	if v := rc.RetentionCheckInterval; v > 0 {
+		r.RetentionCheckInterval = v
+	}
+	if v := rc.ValidationInterval; v > 0 {
+		r.ValidationInterval = v
+	}
+	return r, nil
+}
+
+// newS3ReplicaFromConfig returns a new instance of S3Replica build from config.
+func newS3ReplicaFromConfig(db *litestream.DB, c *Config, dbc *DBConfig, rc *ReplicaConfig) (*s3.Replica, error) {
+	// Use global or replica-specific S3 settings.
+	accessKeyID := c.AccessKeyID
+	if v := rc.AccessKeyID; v != "" {
+		accessKeyID = v
+	}
+	secretAccessKey := c.SecretAccessKey
+	if v := rc.SecretAccessKey; v != "" {
+		secretAccessKey = v
+	}
+	bucket := c.Bucket
+	if v := rc.Bucket; v != "" {
+		bucket = v
+	}
+	region := c.Region
+	if v := rc.Region; v != "" {
+		region = v
+	}
+
+	// Ensure required settings are set.
+	if accessKeyID == "" {
+		return nil, fmt.Errorf("%s: s3 access key id required", db.Path())
+	} else if secretAccessKey == "" {
+		return nil, fmt.Errorf("%s: s3 secret access key required", db.Path())
+	} else if bucket == "" {
+		return nil, fmt.Errorf("%s: s3 bucket required", db.Path())
+	}
+
+	// Build replica.
+	r := s3.NewReplica(db, rc.Name)
+	r.AccessKeyID = accessKeyID
+	r.SecretAccessKey = secretAccessKey
+	r.Region = region
+	r.Bucket = bucket
+	r.Path = rc.Path
+
+	if v := rc.Retention; v > 0 {
+		r.Retention = v
+	}
+	if v := rc.RetentionCheckInterval; v > 0 {
+		r.RetentionCheckInterval = v
+	}
+	if v := rc.SyncInterval; v > 0 {
+		r.SyncInterval = v
+	}
+	if v := rc.ValidationInterval; v > 0 {
+		r.ValidationInterval = v
+	}
+	return r, nil
 }

cmd/litestream/replicate.go

@@ -5,12 +5,19 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"log"
+	"net"
+	"net/http"
+	_ "net/http/pprof"
 	"os"
 	"os/signal"
 
 	"github.com/benbjohnson/litestream"
+	"github.com/benbjohnson/litestream/s3"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
+// ReplicateCommand represents a command that continuously replicates SQLite databases.
 type ReplicateCommand struct {
 	ConfigPath string
 	Config     Config
@@ -19,13 +26,10 @@ type ReplicateCommand struct {
 	DBs []*litestream.DB
 }
 
-func NewReplicateCommand() *ReplicateCommand {
-	return &ReplicateCommand{}
-}
-
 // Run loads all databases specified in the configuration.
 func (c *ReplicateCommand) Run(ctx context.Context, args []string) (err error) {
 	fs := flag.NewFlagSet("litestream-replicate", flag.ContinueOnError)
+	verbose := fs.Bool("v", false, "verbose logging")
 	registerConfigFlag(fs, &c.ConfigPath)
 	fs.Usage = c.Usage
 	if err := fs.Parse(args); err != nil {
@@ -41,8 +45,13 @@ func (c *ReplicateCommand) Run(ctx context.Context, args []string) (err error) {
 		return err
 	}
 
+	// Enable trace logging.
+	if *verbose {
+		litestream.Tracef = log.Printf
+	}
+
 	// Setup signal handler.
-	ctx, cancel := context.WithCancel(context.Background())
+	ctx, cancel := context.WithCancel(ctx)
 	ch := make(chan os.Signal, 1)
 	signal.Notify(ch, os.Interrupt)
 	go func() { <-ch; cancel() }()
@@ -51,11 +60,11 @@ func (c *ReplicateCommand) Run(ctx context.Context, args []string) (err error) {
 	fmt.Printf("litestream %s\n", Version)
 
 	if len(config.DBs) == 0 {
-		return errors.New("configuration must specify at least one database")
+		fmt.Println("no databases specified in configuration")
 	}
 
 	for _, dbConfig := range config.DBs {
-		db, err := newDBFromConfig(dbConfig)
+		db, err := newDBFromConfig(&config, dbConfig)
 		if err != nil {
 			return err
 		}
@@ -68,7 +77,31 @@ func (c *ReplicateCommand) Run(ctx context.Context, args []string) (err error) {
 	}
 
 	// Notify user that initialization is done.
-	fmt.Printf("Initialized with %d databases.\n", len(c.DBs))
+	for _, db := range c.DBs {
+		fmt.Printf("initialized db: %s\n", db.Path())
+		for _, r := range db.Replicas {
+			switch r := r.(type) {
+			case *litestream.FileReplica:
+				fmt.Printf("replicating to: name=%q type=%q path=%q\n", r.Name(), r.Type(), r.Path())
+			case *s3.Replica:
+				fmt.Printf("replicating to: name=%q type=%q bucket=%q path=%q region=%q\n", r.Name(), r.Type(), r.Bucket, r.Path, r.Region)
+			default:
+				fmt.Printf("replicating to: name=%q type=%q\n", r.Name(), r.Type())
+			}
+		}
+	}
+
+	// Serve metrics over HTTP if enabled.
+	if config.Addr != "" {
+		_, port, _ := net.SplitHostPort(config.Addr)
+		fmt.Printf("serving metrics on http://localhost:%s/metrics\n", port)
+		go func() {
+			http.Handle("/metrics", promhttp.Handler())
+			if err := http.ListenAndServe(config.Addr, nil); err != nil {
+				log.Printf("cannot start metrics server: %s", err)
+			}
+		}()
+	}
 
 	// Wait for signal to stop program.
 	<-ctx.Done()
@@ -96,6 +129,7 @@ func (c *ReplicateCommand) Close() (err error) {
 	return err
 }
 
+// Usage prints the help screen to STDOUT.
 func (c *ReplicateCommand) Usage() {
 	fmt.Printf(`
 The replicate command starts a server to monitor & replicate databases 
@@ -110,5 +144,8 @@ Arguments:
 	-config PATH
 	    Specifies the configuration file. Defaults to %s
 
-`[1:], DefaultConfigPath)
+	-v
+	    Enable verbose logging output.
+
+`[1:], DefaultConfigPath())
 }

cmd/litestream/restore.go

@@ -13,22 +13,19 @@ import (
 	"github.com/benbjohnson/litestream"
 )
 
-type RestoreCommand struct {
-	DBPath string
-}
-
-func NewRestoreCommand() *RestoreCommand {
-	return &RestoreCommand{}
-}
+// RestoreCommand represents a command to restore a database from a backup.
+type RestoreCommand struct{}
 
+// Run executes the command.
 func (c *RestoreCommand) Run(ctx context.Context, args []string) (err error) {
 	var configPath string
-	var opt litestream.RestoreOptions
+	opt := litestream.NewRestoreOptions()
 	fs := flag.NewFlagSet("litestream-restore", flag.ContinueOnError)
 	registerConfigFlag(fs, &configPath)
 	fs.StringVar(&opt.OutputPath, "o", "", "output path")
 	fs.StringVar(&opt.ReplicaName, "replica", "", "replica name")
 	fs.StringVar(&opt.Generation, "generation", "", "generation name")
+	fs.IntVar(&opt.Index, "index", opt.Index, "wal index")
 	fs.BoolVar(&opt.DryRun, "dry-run", false, "dry run")
 	timestampStr := fs.String("timestamp", "", "timestamp")
 	verbose := fs.Bool("v", false, "verbose output")
@@ -67,17 +64,18 @@ func (c *RestoreCommand) Run(ctx context.Context, args []string) (err error) {
 		opt.Logger = log.New(os.Stderr, "", log.LstdFlags)
 	}
 
-	// Determine absolute path for database, if specified.
-	if c.DBPath, err = filepath.Abs(fs.Arg(0)); err != nil {
+	// Determine absolute path for database.
+	dbPath, err := filepath.Abs(fs.Arg(0))
+	if err != nil {
 		return err
 	}
 
 	// Instantiate DB.
-	dbConfig := config.DBConfig(c.DBPath)
+	dbConfig := config.DBConfig(dbPath)
 	if dbConfig == nil {
-		return fmt.Errorf("database not found in config: %s", c.DBPath)
+		return fmt.Errorf("database not found in config: %s", dbPath)
 	}
-	db, err := newDBFromConfig(dbConfig)
+	db, err := newDBFromConfig(&config, dbConfig)
 	if err != nil {
 		return err
 	}
@@ -85,6 +83,7 @@ func (c *RestoreCommand) Run(ctx context.Context, args []string) (err error) {
 	return db.Restore(ctx, opt)
 }
 
+// Usage prints the help screen to STDOUT.
 func (c *RestoreCommand) Usage() {
 	fmt.Printf(`
 The restore command recovers a database from a previous snapshot and WAL.
@@ -107,6 +106,10 @@ Arguments:
 	    Restore from a specific generation.
 	    Defaults to generation with latest data.
 
+	-index NUM
+	    Restore up to a specific WAL index (inclusive).
+	    Defaults to use the highest available index.
+
 	-timestamp TIMESTAMP
 	    Restore to a specific point-in-time.
 	    Defaults to use the latest available backup.
@@ -141,6 +144,6 @@ Examples:
 	$ litestream restore -replica s3 -generation xxxxxxxx /path/to/db
 
 `[1:],
-		DefaultConfigPath,
+		DefaultConfigPath(),
 	)
 }

cmd/litestream/snapshots.go

@@ -0,0 +1,120 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+	"text/tabwriter"
+	"time"
+
+	"github.com/benbjohnson/litestream"
+)
+
+// SnapshotsCommand represents a command to list snapshots for a command.
+type SnapshotsCommand struct{}
+
+// Run executes the command.
+func (c *SnapshotsCommand) Run(ctx context.Context, args []string) (err error) {
+	var configPath string
+	fs := flag.NewFlagSet("litestream-snapshots", flag.ContinueOnError)
+	registerConfigFlag(fs, &configPath)
+	replicaName := fs.String("replica", "", "replica name")
+	fs.Usage = c.Usage
+	if err := fs.Parse(args); err != nil {
+		return err
+	} else if fs.NArg() == 0 || fs.Arg(0) == "" {
+		return fmt.Errorf("database path required")
+	} else if fs.NArg() > 1 {
+		return fmt.Errorf("too many arguments")
+	}
+
+	// Load configuration.
+	if configPath == "" {
+		return errors.New("-config required")
+	}
+	config, err := ReadConfigFile(configPath)
+	if err != nil {
+		return err
+	}
+
+	// Determine absolute path for database.
+	dbPath, err := filepath.Abs(fs.Arg(0))
+	if err != nil {
+		return err
+	}
+
+	// Instantiate DB.
+	dbConfig := config.DBConfig(dbPath)
+	if dbConfig == nil {
+		return fmt.Errorf("database not found in config: %s", dbPath)
+	}
+	db, err := newDBFromConfig(&config, dbConfig)
+	if err != nil {
+		return err
+	}
+
+	// Find snapshots by db or replica.
+	var infos []*litestream.SnapshotInfo
+	if *replicaName != "" {
+		if r := db.Replica(*replicaName); r == nil {
+			return fmt.Errorf("replica %q not found for database %q", *replicaName, dbPath)
+		} else if infos, err = r.Snapshots(ctx); err != nil {
+			return err
+		}
+	} else {
+		if infos, err = db.Snapshots(ctx); err != nil {
+			return err
+		}
+	}
+
+	// List all snapshots.
+	w := tabwriter.NewWriter(os.Stdout, 0, 8, 1, '\t', 0)
+	fmt.Fprintln(w, "replica\tgeneration\tindex\tsize\tcreated")
+	for _, info := range infos {
+		fmt.Fprintf(w, "%s\t%s\t%d\t%d\t%s\n",
+			info.Replica,
+			info.Generation,
+			info.Index,
+			info.Size,
+			info.CreatedAt.Format(time.RFC3339),
+		)
+	}
+	w.Flush()
+
+	return nil
+}
+
+// Usage prints the help screen to STDOUT.
+func (c *SnapshotsCommand) Usage() {
+	fmt.Printf(`
+The snapshots command lists all snapshots available for a database.
+
+Usage:
+
+	litestream snapshots [arguments] DB
+
+Arguments:
+
+	-config PATH
+	    Specifies the configuration file.
+	    Defaults to %s
+
+	-replica NAME
+	    Optional, filter by a specific replica.
+
+
+Examples:
+
+	# List all snapshots for a database.
+	$ litestream snapshots /path/to/db
+
+	# List all snapshots on S3.
+	$ litestream snapshots -replica s3 /path/to/db
+
+`[1:],
+		DefaultConfigPath(),
+	)
+}

cmd/litestream/version.go

@@ -6,8 +6,10 @@ import (
 	"fmt"
 )
 
+// VersionCommand represents a command to print the current version.
 type VersionCommand struct{}
 
+// Run executes the command.
 func (c *VersionCommand) Run(ctx context.Context, args []string) (err error) {
 	fs := flag.NewFlagSet("litestream-version", flag.ContinueOnError)
 	fs.Usage = c.Usage
@@ -20,6 +22,7 @@ func (c *VersionCommand) Run(ctx context.Context, args []string) (err error) {
 	return nil
 }
 
+// Usage prints the help screen to STDOUT.
 func (c *VersionCommand) Usage() {
 	fmt.Println(`
 Prints the version.

cmd/litestream/wal.go

@@ -0,0 +1,129 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+	"text/tabwriter"
+	"time"
+
+	"github.com/benbjohnson/litestream"
+)
+
+// WALCommand represents a command to list WAL files for a database.
+type WALCommand struct{}
+
+// Run executes the command.
+func (c *WALCommand) Run(ctx context.Context, args []string) (err error) {
+	var configPath string
+	fs := flag.NewFlagSet("litestream-wal", flag.ContinueOnError)
+	registerConfigFlag(fs, &configPath)
+	replicaName := fs.String("replica", "", "replica name")
+	generation := fs.String("generation", "", "generation name")
+	fs.Usage = c.Usage
+	if err := fs.Parse(args); err != nil {
+		return err
+	} else if fs.NArg() == 0 || fs.Arg(0) == "" {
+		return fmt.Errorf("database path required")
+	} else if fs.NArg() > 1 {
+		return fmt.Errorf("too many arguments")
+	}
+
+	// Load configuration.
+	if configPath == "" {
+		return errors.New("-config required")
+	}
+	config, err := ReadConfigFile(configPath)
+	if err != nil {
+		return err
+	}
+
+	// Determine absolute path for database.
+	dbPath, err := filepath.Abs(fs.Arg(0))
+	if err != nil {
+		return err
+	}
+
+	// Instantiate DB.
+	dbConfig := config.DBConfig(dbPath)
+	if dbConfig == nil {
+		return fmt.Errorf("database not found in config: %s", dbPath)
+	}
+	db, err := newDBFromConfig(&config, dbConfig)
+	if err != nil {
+		return err
+	}
+
+	// Find snapshots by db or replica.
+	var infos []*litestream.WALInfo
+	if *replicaName != "" {
+		if r := db.Replica(*replicaName); r == nil {
+			return fmt.Errorf("replica %q not found for database %q", *replicaName, dbPath)
+		} else if infos, err = r.WALs(ctx); err != nil {
+			return err
+		}
+	} else {
+		if infos, err = db.WALs(ctx); err != nil {
+			return err
+		}
+	}
+
+	// List all WAL files.
+	w := tabwriter.NewWriter(os.Stdout, 0, 8, 1, '\t', 0)
+	fmt.Fprintln(w, "replica\tgeneration\tindex\toffset\tsize\tcreated")
+	for _, info := range infos {
+		if *generation != "" && info.Generation != *generation {
+			continue
+		}
+
+		fmt.Fprintf(w, "%s\t%s\t%d\t%d\t%d\t%s\n",
+			info.Replica,
+			info.Generation,
+			info.Index,
+			info.Offset,
+			info.Size,
+			info.CreatedAt.Format(time.RFC3339),
+		)
+	}
+	w.Flush()
+
+	return nil
+}
+
+// Usage prints the help screen to STDOUT.
+func (c *WALCommand) Usage() {
+	fmt.Printf(`
+The wal command lists all wal files available for a database.
+
+Usage:
+
+	litestream wal [arguments] DB
+
+Arguments:
+
+	-config PATH
+	    Specifies the configuration file.
+	    Defaults to %s
+
+	-replica NAME
+	    Optional, filter by a specific replica.
+
+	-generation NAME
+	    Optional, filter by a specific generation.
+
+
+Examples:
+
+	# List all WAL files for a database.
+	$ litestream wal /path/to/db
+
+	# List all WAL files on S3 for a specific generation.
+	$ litestream snapshots -replica s3 -generation xxxxxxxx /path/to/db
+
+`[1:],
+		DefaultConfigPath(),
+	)
+}

db_test.go

@@ -0,0 +1,647 @@
+package litestream_test
+
+import (
+	"database/sql"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/benbjohnson/litestream"
+)
+
+func TestDB_Path(t *testing.T) {
+	db := litestream.NewDB("/tmp/db")
+	if got, want := db.Path(), `/tmp/db`; got != want {
+		t.Fatalf("Path()=%v, want %v", got, want)
+	}
+}
+
+func TestDB_WALPath(t *testing.T) {
+	db := litestream.NewDB("/tmp/db")
+	if got, want := db.WALPath(), `/tmp/db-wal`; got != want {
+		t.Fatalf("WALPath()=%v, want %v", got, want)
+	}
+}
+
+func TestDB_MetaPath(t *testing.T) {
+	t.Run("Absolute", func(t *testing.T) {
+		db := litestream.NewDB("/tmp/db")
+		if got, want := db.MetaPath(), `/tmp/.db-litestream`; got != want {
+			t.Fatalf("MetaPath()=%v, want %v", got, want)
+		}
+	})
+	t.Run("Relative", func(t *testing.T) {
+		db := litestream.NewDB("db")
+		if got, want := db.MetaPath(), `.db-litestream`; got != want {
+			t.Fatalf("MetaPath()=%v, want %v", got, want)
+		}
+	})
+}
+
+func TestDB_GenerationNamePath(t *testing.T) {
+	db := litestream.NewDB("/tmp/db")
+	if got, want := db.GenerationNamePath(), `/tmp/.db-litestream/generation`; got != want {
+		t.Fatalf("GenerationNamePath()=%v, want %v", got, want)
+	}
+}
+
+func TestDB_GenerationPath(t *testing.T) {
+	db := litestream.NewDB("/tmp/db")
+	if got, want := db.GenerationPath("xxxx"), `/tmp/.db-litestream/generations/xxxx`; got != want {
+		t.Fatalf("GenerationPath()=%v, want %v", got, want)
+	}
+}
+
+func TestDB_ShadowWALDir(t *testing.T) {
+	db := litestream.NewDB("/tmp/db")
+	if got, want := db.ShadowWALDir("xxxx"), `/tmp/.db-litestream/generations/xxxx/wal`; got != want {
+		t.Fatalf("ShadowWALDir()=%v, want %v", got, want)
+	}
+}
+
+func TestDB_ShadowWALPath(t *testing.T) {
+	db := litestream.NewDB("/tmp/db")
+	if got, want := db.ShadowWALPath("xxxx", 1000), `/tmp/.db-litestream/generations/xxxx/wal/000003e8.wal`; got != want {
+		t.Fatalf("ShadowWALPath()=%v, want %v", got, want)
+	}
+}
+
+// Ensure we can check the last modified time of the real database and its WAL.
+func TestDB_UpdatedAt(t *testing.T) {
+	t.Run("ErrNotExist", func(t *testing.T) {
+		db := MustOpenDB(t)
+		defer MustCloseDB(t, db)
+		if _, err := db.UpdatedAt(); !os.IsNotExist(err) {
+			t.Fatalf("unexpected error: %#v", err)
+		}
+	})
+
+	t.Run("DB", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		if t0, err := db.UpdatedAt(); err != nil {
+			t.Fatal(err)
+		} else if time.Since(t0) > 10*time.Second {
+			t.Fatalf("unexpected updated at time: %s", t0)
+		}
+	})
+
+	t.Run("WAL", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		t0, err := db.UpdatedAt()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if os.Getenv("CI") != "" {
+			time.Sleep(1 * time.Second)
+		}
+		if _, err := sqldb.Exec(`CREATE TABLE t (id INT);`); err != nil {
+			t.Fatal(err)
+		}
+
+		if t1, err := db.UpdatedAt(); err != nil {
+			t.Fatal(err)
+		} else if !t1.After(t0) {
+			t.Fatalf("expected newer updated at time: %s > %s", t1, t0)
+		}
+	})
+}
+
+// Ensure we can compute a checksum on the real database.
+func TestDB_CRC64(t *testing.T) {
+	t.Run("ErrNotExist", func(t *testing.T) {
+		db := MustOpenDB(t)
+		defer MustCloseDB(t, db)
+		if _, _, err := db.CRC64(); !os.IsNotExist(err) {
+			t.Fatalf("unexpected error: %#v", err)
+		}
+	})
+
+	t.Run("DB", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		chksum0, _, err := db.CRC64()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Issue change that is applied to the WAL. Checksum should not change.
+		if _, err := sqldb.Exec(`CREATE TABLE t (id INT);`); err != nil {
+			t.Fatal(err)
+		} else if chksum1, _, err := db.CRC64(); err != nil {
+			t.Fatal(err)
+		} else if chksum0 == chksum1 {
+			t.Fatal("expected different checksum event after WAL change")
+		}
+
+		// Checkpoint change into database. Checksum should change.
+		if _, err := sqldb.Exec(`PRAGMA wal_checkpoint(TRUNCATE);`); err != nil {
+			t.Fatal(err)
+		}
+
+		if chksum2, _, err := db.CRC64(); err != nil {
+			t.Fatal(err)
+		} else if chksum0 == chksum2 {
+			t.Fatal("expected different checksums after checkpoint")
+		}
+	})
+}
+
+// Ensure we can sync the real WAL to the shadow WAL.
+func TestDB_Sync(t *testing.T) {
+	// Ensure sync is skipped if no database exists.
+	t.Run("NoDB", func(t *testing.T) {
+		db := MustOpenDB(t)
+		defer MustCloseDB(t, db)
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+	})
+
+	// Ensure sync can successfully run on the initial sync.
+	t.Run("Initial", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify page size if now available.
+		if db.PageSize() == 0 {
+			t.Fatal("expected page size after initial sync")
+		}
+
+		// Obtain real WAL size.
+		fi, err := os.Stat(db.WALPath())
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Ensure position now available.
+		if pos, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos.Generation == "" {
+			t.Fatal("expected generation")
+		} else if got, want := pos.Index, 0; got != want {
+			t.Fatalf("pos.Index=%v, want %v", got, want)
+		} else if got, want := pos.Offset, fi.Size(); got != want {
+			t.Fatalf("pos.Offset=%v, want %v", got, want)
+		}
+	})
+
+	// Ensure DB can keep in sync across multiple Sync() invocations.
+	t.Run("MultiSync", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		}
+
+		// Perform initial sync & grab initial position.
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Insert into table.
+		if _, err := sqldb.Exec(`INSERT INTO foo (bar) VALUES ('baz');`); err != nil {
+			t.Fatal(err)
+		}
+
+		// Sync to ensure position moves forward one page.
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		} else if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation != pos1.Generation {
+			t.Fatal("expected the same generation")
+		} else if got, want := pos1.Index, pos0.Index; got != want {
+			t.Fatalf("Index=%v, want %v", got, want)
+		} else if got, want := pos1.Offset, pos0.Offset+4096+litestream.WALFrameHeaderSize; got != want {
+			t.Fatalf("Offset=%v, want %v", got, want)
+		}
+	})
+
+	// Ensure a WAL file is created if one does not already exist.
+	t.Run("NoWAL", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Issue initial sync and truncate WAL.
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Obtain initial position.
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Checkpoint & fully close which should close WAL file.
+		if err := db.Checkpoint(litestream.CheckpointModeTruncate); err != nil {
+			t.Fatal(err)
+		} else if err := db.Close(); err != nil {
+			t.Fatal(err)
+		} else if err := sqldb.Close(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify WAL does not exist.
+		if _, err := os.Stat(db.WALPath()); !os.IsNotExist(err) {
+			t.Fatal(err)
+		}
+
+		// Reopen the managed database.
+		db = MustOpenDBAt(t, db.Path())
+		defer MustCloseDB(t, db)
+
+		// Re-sync and ensure new generation has been created.
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Obtain initial position.
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation == pos1.Generation {
+			t.Fatal("expected new generation after truncation")
+		}
+	})
+
+	// Ensure DB can start new generation if it detects it cannot verify last position.
+	t.Run("OverwritePrevPosition", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		}
+
+		// Issue initial sync and truncate WAL.
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Obtain initial position.
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Fully close which should close WAL file.
+		if err := db.Close(); err != nil {
+			t.Fatal(err)
+		} else if err := sqldb.Close(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify WAL does not exist.
+		if _, err := os.Stat(db.WALPath()); !os.IsNotExist(err) {
+			t.Fatal(err)
+		}
+
+		// Insert into table multiple times to move past old offset
+		sqldb = MustOpenSQLDB(t, db.Path())
+		defer MustCloseSQLDB(t, sqldb)
+		for i := 0; i < 100; i++ {
+			if _, err := sqldb.Exec(`INSERT INTO foo (bar) VALUES ('baz');`); err != nil {
+				t.Fatal(err)
+			}
+		}
+
+		// Reopen the managed database.
+		db = MustOpenDBAt(t, db.Path())
+		defer MustCloseDB(t, db)
+
+		// Re-sync and ensure new generation has been created.
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Obtain initial position.
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation == pos1.Generation {
+			t.Fatal("expected new generation after truncation")
+		}
+	})
+
+	// Ensure DB can handle a mismatched header-only and start new generation.
+	t.Run("WALHeaderMismatch", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL and then sync.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Grab initial position & close.
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		} else if err := db.Close(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Read existing file, update header checksum, and write back only header
+		// to simulate a header with a mismatched checksum.
+		shadowWALPath := db.ShadowWALPath(pos0.Generation, pos0.Index)
+		if buf, err := ioutil.ReadFile(shadowWALPath); err != nil {
+			t.Fatal(err)
+		} else if err := ioutil.WriteFile(shadowWALPath, append(buf[:litestream.WALHeaderSize-8], 0, 0, 0, 0, 0, 0, 0, 0), 0600); err != nil {
+			t.Fatal(err)
+		}
+
+		// Reopen managed database & ensure sync will still work.
+		db = MustOpenDBAt(t, db.Path())
+		defer MustCloseDB(t, db)
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify a new generation was started.
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation == pos1.Generation {
+			t.Fatal("expected new generation")
+		}
+	})
+
+	// Ensure DB can handle partial shadow WAL header write.
+	t.Run("PartialShadowWALHeader", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL and then sync.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Close & truncate shadow WAL to simulate a partial header write.
+		if err := db.Close(); err != nil {
+			t.Fatal(err)
+		} else if err := os.Truncate(db.ShadowWALPath(pos0.Generation, pos0.Index), litestream.WALHeaderSize-1); err != nil {
+			t.Fatal(err)
+		}
+
+		// Reopen managed database & ensure sync will still work.
+		db = MustOpenDBAt(t, db.Path())
+		defer MustCloseDB(t, db)
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify a new generation was started.
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation == pos1.Generation {
+			t.Fatal("expected new generation")
+		}
+	})
+
+	// Ensure DB can handle partial shadow WAL writes.
+	t.Run("PartialShadowWALFrame", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL and then sync.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Obtain current shadow WAL size.
+		fi, err := os.Stat(db.ShadowWALPath(pos0.Generation, pos0.Index))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Close & truncate shadow WAL to simulate a partial frame write.
+		if err := db.Close(); err != nil {
+			t.Fatal(err)
+		} else if err := os.Truncate(db.ShadowWALPath(pos0.Generation, pos0.Index), fi.Size()-1); err != nil {
+			t.Fatal(err)
+		}
+
+		// Reopen managed database & ensure sync will still work.
+		db = MustOpenDBAt(t, db.Path())
+		defer MustCloseDB(t, db)
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify same generation is kept.
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, want := pos1, pos0; got != want {
+			t.Fatalf("Pos()=%s want %s", got, want)
+		}
+
+		// Ensure shadow WAL has recovered.
+		if fi0, err := os.Stat(db.ShadowWALPath(pos0.Generation, pos0.Index)); err != nil {
+			t.Fatal(err)
+		} else if got, want := fi0.Size(), fi.Size(); got != want {
+			t.Fatalf("Size()=%v, want %v", got, want)
+		}
+	})
+
+	// Ensure DB can handle a generation directory with a missing shadow WAL.
+	t.Run("NoShadowWAL", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL and then sync.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		pos0, err := db.Pos()
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Close & delete shadow WAL to simulate dir created but not WAL.
+		if err := db.Close(); err != nil {
+			t.Fatal(err)
+		} else if err := os.Remove(db.ShadowWALPath(pos0.Generation, pos0.Index)); err != nil {
+			t.Fatal(err)
+		}
+
+		// Reopen managed database & ensure sync will still work.
+		db = MustOpenDBAt(t, db.Path())
+		defer MustCloseDB(t, db)
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Verify new generation created but index/offset the same.
+		if pos1, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if pos0.Generation == pos1.Generation {
+			t.Fatal("expected new generation")
+		} else if got, want := pos1.Index, pos0.Index; got != want {
+			t.Fatalf("Index=%v want %v", got, want)
+		} else if got, want := pos1.Offset, pos0.Offset; got != want {
+			t.Fatalf("Offset=%v want %v", got, want)
+		}
+	})
+
+	// Ensure DB checkpoints after minimum number of pages.
+	t.Run("MinCheckpointPageN", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL and then sync.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Write at least minimum number of pages to trigger rollover.
+		for i := 0; i < db.MinCheckpointPageN; i++ {
+			if _, err := sqldb.Exec(`INSERT INTO foo (bar) VALUES ('baz');`); err != nil {
+				t.Fatal(err)
+			}
+		}
+
+		// Sync to shadow WAL.
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Ensure position is now on the second index.
+		if pos, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, want := pos.Index, 1; got != want {
+			t.Fatalf("Index=%v, want %v", got, want)
+		}
+	})
+
+	// Ensure DB checkpoints after interval.
+	t.Run("CheckpointInterval", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+
+		// Execute a query to force a write to the WAL and then sync.
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Reduce checkpoint interval to ensure a rollover is triggered.
+		db.CheckpointInterval = 1 * time.Nanosecond
+
+		// Write to WAL & sync.
+		if _, err := sqldb.Exec(`INSERT INTO foo (bar) VALUES ('baz');`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		}
+
+		// Ensure position is now on the second index.
+		if pos, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, want := pos.Index, 1; got != want {
+			t.Fatalf("Index=%v, want %v", got, want)
+		}
+	})
+}
+
+// MustOpenDBs returns a new instance of a DB & associated SQL DB.
+func MustOpenDBs(tb testing.TB) (*litestream.DB, *sql.DB) {
+	db := MustOpenDB(tb)
+	return db, MustOpenSQLDB(tb, db.Path())
+}
+
+// MustCloseDBs closes db & sqldb and removes the parent directory.
+func MustCloseDBs(tb testing.TB, db *litestream.DB, sqldb *sql.DB) {
+	MustCloseDB(tb, db)
+	MustCloseSQLDB(tb, sqldb)
+}
+
+// MustOpenDB returns a new instance of a DB.
+func MustOpenDB(tb testing.TB) *litestream.DB {
+	dir := tb.TempDir()
+	return MustOpenDBAt(tb, filepath.Join(dir, "db"))
+}
+
+// MustOpenDBAt returns a new instance of a DB for a given path.
+func MustOpenDBAt(tb testing.TB, path string) *litestream.DB {
+	tb.Helper()
+	db := litestream.NewDB(path)
+	db.MonitorInterval = 0 // disable background goroutine
+	if err := db.Open(); err != nil {
+		tb.Fatal(err)
+	}
+	return db
+}
+
+// MustCloseDB closes db and removes its parent directory.
+func MustCloseDB(tb testing.TB, db *litestream.DB) {
+	tb.Helper()
+	if err := db.Close(); err != nil {
+		tb.Fatal(err)
+	} else if err := os.RemoveAll(filepath.Dir(db.Path())); err != nil {
+		tb.Fatal(err)
+	}
+}
+
+// MustOpenSQLDB returns a database/sql DB.
+func MustOpenSQLDB(tb testing.TB, path string) *sql.DB {
+	tb.Helper()
+	d, err := sql.Open("sqlite3", path)
+	if err != nil {
+		tb.Fatal(err)
+	} else if _, err := d.Exec(`PRAGMA journal_mode = wal;`); err != nil {
+		tb.Fatal(err)
+	}
+	return d
+}
+
+// MustCloseSQLDB closes a database/sql DB.
+func MustCloseSQLDB(tb testing.TB, d *sql.DB) {
+	tb.Helper()
+	if err := d.Close(); err != nil {
+		tb.Fatal(err)
+	}
+}

doc/DESIGN.md

@@ -1,88 +0,0 @@
-DESIGN
-======
-
-Litestream is a sidecar process that replicates the write ahead log (WAL) for
-a SQLite database. To ensure that it can replicate every page, litestream takes
-control over the checkpointing process by issuing a long running read
-transaction against the database to prevent checkpointing. It then releases
-this transaction once it obtains a write lock and issues the checkpoint itself.
-
-The daemon polls the database on an interval to breifly obtain a write
-transaction lock and copy over new WAL pages. Once the WAL has reached a
-threshold size, litestream will issue a checkpoint and a single page write
-to a table called `_litestream` to start the new WAL.
-
-
-## Workflow
-
-When litestream first loads a database, it checks if there is an existing
-sidecar directory which is named `.<DB>-litestream`. If not, it initializes
-the directory and starts a new generation.
-
-A generation is a snapshot of the database followed by a continuous stream of
-WAL files. A new generation is started on initialization & whenever litestream
-cannot verify that it has a continuous record of WAL files. This could happen
-if litestream is stopped and another process checkpoints the WAL. In this case,
-a new generation ID is randomly created and a snapshot is replicated to the
-appropriate destinations.
-
-Generations also prevent two servers from replicating to the same destination
-and corrupting each other's data. In this case, each server would replicate
-to a different generation directory. On recovery, there will be duplicate 
-databases and the end user can choose which generation to recover but each
-database will be uncorrupted.
-
-
-## File Layout
-
-Litestream maintains a shadow WAL which is a historical record of all previous
-WAL files. These files can be deleted after a time or size threshold but should
-be replicated before being deleted.
-
-### Local
-
-Given a database file named `db`, SQLite will create a WAL file called `db-wal`.
-Litestream will then create a hidden directory called `.db-litestream` that
-contains the historical record of all WAL files for the current generation.
-
-```
-db                                      # SQLite database
-db-wal                                  # SQLite WAL
-.db-litestream/
-	generation                          # current generation number
-	generations/
-		xxxxxxxx/
-			wal/                        # WAL files
-				000000000000001.wal     
-				000000000000002.wal     
-				000000000000003.wal     # active WAL
-```
-
-### Remote (S3)
-
-```
-bkt/
-	db/                                       # database path
-		generations/
-			xxxxxxxx/
-				snapshots/                    # snapshots w/ timestamp+offset
-					20000101T000000Z-000000000000023.snapshot
-				wal/                          # compressed WAL files
-					000000000000001-0.wal.gz    
-					000000000000001-<offset>.wal.gz
-					000000000000002-0.wal.gz
-		00000002/
-			snapshot/
-				000000000000000.snapshot
-				scheduled/
-					daily/
-						20000101T000000Z-000000000000023.snapshot
-						20000102T000000Z-000000000000036.snapshot
-					monthly/
-						20000101T000000Z-000000000000023.snapshot
-
-			wal/
-				000000000000001.wal.gz
-```
-
-

etc/litestream.service

@@ -0,0 +1,9 @@
+[Unit]
+Description=Litestream
+
+[Service]
+Restart=always
+ExecStart=/usr/bin/litestream replicate
+
+[Install]
+WantedBy=multi-user.target

etc/litestream.yml

@@ -0,0 +1,10 @@
+# AWS credentials
+# access-key-id:     AKIAxxxxxxxxxxxxxxxx
+# secret-access-key: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/xxxxxxxxx
+
+# dbs:
+#  - path: /path/to/primary/db            # Database to replicate from
+#    replicas:
+#      - path: /path/to/replica           # File-based replication
+#      - path: s3://my.bucket.com/db      # S3-based replication
+

etc/nfpm.yml

@@ -0,0 +1,19 @@
+name: litestream
+arch: amd64
+platform: linux
+version: "${LITESTREAM_VERSION}"
+section: "default"
+priority: "extra"
+maintainer: "Ben Johnson <benbjohnson@yahoo.com>"
+description: Litestream is a tool for real-time replication of SQLite databases.
+homepage: "https://github.com/benbjohnson/litestream"
+license: "GPLv3"
+contents:
+- src: ./litestream
+  dst: /usr/bin/litestream
+- src: ./litestream.yml
+  dst: /etc/litestream.yml
+  type: config
+- src: ./litestream.service
+  dst: /usr/lib/systemd/system/litestream.service
+  type: config

go.mod

@@ -3,7 +3,10 @@ module github.com/benbjohnson/litestream
 go 1.15
 
 require (
+	github.com/aws/aws-sdk-go v1.27.0
+	github.com/davecgh/go-spew v1.1.1
 	github.com/mattn/go-sqlite3 v1.14.5
-	github.com/pelletier/go-toml v1.8.1
+	github.com/pierrec/lz4/v4 v4.1.3
+	github.com/prometheus/client_golang v1.9.0
 	gopkg.in/yaml.v2 v2.4.0
 )

go.sum

@@ -1,38 +1,410 @@
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internal/internal.go

@@ -0,0 +1,32 @@
+package internal
+
+import (
+	"io"
+)
+
+// ReadCloser wraps a reader to also attach a separate closer.
+type ReadCloser struct {
+	r io.Reader
+	c io.Closer
+}
+
+// NewReadCloser returns a new instance of ReadCloser.
+func NewReadCloser(r io.Reader, c io.Closer) *ReadCloser {
+	return &ReadCloser{r, c}
+}
+
+// Read reads bytes into the underlying reader.
+func (r *ReadCloser) Read(p []byte) (n int, err error) {
+	return r.r.Read(p)
+}
+
+// Close closes the reader (if implementing io.ReadCloser) and the Closer.
+func (r *ReadCloser) Close() error {
+	if rc, ok := r.r.(io.Closer); ok {
+		if err := rc.Close(); err != nil {
+			r.c.Close()
+			return err
+		}
+	}
+	return r.c.Close()
+}

internal/metrics.go

@@ -0,0 +1,44 @@
+package internal
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+// Shared replica metrics.
+var (
+	ReplicaSnapshotTotalGaugeVec = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "litestream",
+		Subsystem: "replica",
+		Name:      "snapshot_total",
+		Help:      "The current number of snapshots",
+	}, []string{"db", "name"})
+
+	ReplicaWALBytesCounterVec = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "litestream",
+		Subsystem: "replica",
+		Name:      "wal_bytes",
+		Help:      "The number wal bytes written",
+	}, []string{"db", "name"})
+
+	ReplicaWALIndexGaugeVec = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "litestream",
+		Subsystem: "replica",
+		Name:      "wal_index",
+		Help:      "The current WAL index",
+	}, []string{"db", "name"})
+
+	ReplicaWALOffsetGaugeVec = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "litestream",
+		Subsystem: "replica",
+		Name:      "wal_offset",
+		Help:      "The current WAL offset",
+	}, []string{"db", "name"})
+
+	ReplicaValidationTotalCounterVec = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "litestream",
+		Subsystem: "replica",
+		Name:      "validation_total",
+		Help:      "The number of validations performed",
+	}, []string{"db", "name", "status"})
+)

litestream.go

@@ -1,10 +1,8 @@
 package litestream
 
 import (
-	"compress/gzip"
 	"database/sql"
 	"encoding/binary"
-	"encoding/hex"
 	"errors"
 	"fmt"
 	"io"
@@ -13,10 +11,11 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
-
-	_ "github.com/mattn/go-sqlite3"
+	"syscall"
+	"time"
 )
 
+// Naming constants.
 const (
 	MetaDirSuffix = "-litestream"
 
@@ -27,11 +26,65 @@ const (
 	GenerationNameLen = 16
 )
 
+// SQLite checkpoint modes.
+const (
+	CheckpointModePassive  = "PASSIVE"
+	CheckpointModeFull     = "FULL"
+	CheckpointModeRestart  = "RESTART"
+	CheckpointModeTruncate = "TRUNCATE"
+)
+
 // Litestream errors.
 var (
-	ErrNoSnapshots = errors.New("no snapshots available")
+	ErrNoSnapshots      = errors.New("no snapshots available")
+	ErrChecksumMismatch = errors.New("invalid replica, checksum mismatch")
 )
 
+// SnapshotInfo represents file information about a snapshot.
+type SnapshotInfo struct {
+	Name       string
+	Replica    string
+	Generation string
+	Index      int
+	Size       int64
+	CreatedAt  time.Time
+}
+
+// FilterSnapshotsAfter returns all snapshots that were created on or after t.
+func FilterSnapshotsAfter(a []*SnapshotInfo, t time.Time) []*SnapshotInfo {
+	other := make([]*SnapshotInfo, 0, len(a))
+	for _, snapshot := range a {
+		if !snapshot.CreatedAt.Before(t) {
+			other = append(other, snapshot)
+		}
+	}
+	return other
+}
+
+// FindMinSnapshotByGeneration finds the snapshot with the lowest index in a generation.
+func FindMinSnapshotByGeneration(a []*SnapshotInfo, generation string) *SnapshotInfo {
+	var min *SnapshotInfo
+	for _, snapshot := range a {
+		if snapshot.Generation != generation {
+			continue
+		} else if min == nil || snapshot.Index < min.Index {
+			min = snapshot
+		}
+	}
+	return min
+}
+
+// WALInfo represents file information about a WAL file.
+type WALInfo struct {
+	Name       string
+	Replica    string
+	Generation string
+	Index      int
+	Offset     int64
+	Size       int64
+	CreatedAt  time.Time
+}
+
 // Pos is a position in the WAL for a generation.
 type Pos struct {
 	Generation string // generation name
@@ -44,7 +97,7 @@ func (p Pos) String() string {
 	if p.IsZero() {
 		return "<>"
 	}
-	return fmt.Sprintf("<%s,%d,%d>", p.Generation, p.Index, p.Offset)
+	return fmt.Sprintf("<%s,%08x,%d>", p.Generation, p.Index, p.Offset)
 }
 
 // IsZero returns true if p is the zero value.
@@ -98,10 +151,6 @@ func readWALHeader(filename string) ([]byte, error) {
 	return buf[:n], err
 }
 
-func readCheckpointSeqNo(hdr []byte) uint32 {
-	return binary.BigEndian.Uint32(hdr[12:])
-}
-
 // readFileAt reads a slice from a file.
 func readFileAt(filename string, offset, n int64) ([]byte, error) {
 	f, err := os.Open(filename)
@@ -119,19 +168,6 @@ func readFileAt(filename string, offset, n int64) ([]byte, error) {
 	return buf, nil
 }
 
-func ParseWALFilename(name string) (index int, err error) {
-	v, err := strconv.ParseInt(strings.TrimSuffix(name, WALExt), 16, 64)
-	if err != nil {
-		return 0, fmt.Errorf("invalid wal filename: %q", name)
-	}
-	return int(v), nil
-}
-
-func FormatWALFilename(index int) string {
-	assert(index >= 0, "wal index must be non-negative")
-	return fmt.Sprintf("%016x%s", index, WALExt)
-}
-
 // removeTmpFiles recursively finds and removes .tmp files.
 func removeTmpFiles(root string) error {
 	return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
@@ -166,17 +202,19 @@ func IsSnapshotPath(s string) bool {
 
 // ParseSnapshotPath returns the index for the snapshot.
 // Returns an error if the path is not a valid snapshot path.
-func ParseSnapshotPath(s string) (index int, typ, ext string, err error) {
+func ParseSnapshotPath(s string) (index int, ext string, err error) {
+	s = filepath.Base(s)
+
 	a := snapshotPathRegex.FindStringSubmatch(s)
 	if a == nil {
-		return 0, "", "", fmt.Errorf("invalid snapshot path: %s", s)
+		return 0, "", fmt.Errorf("invalid snapshot path: %s", s)
 	}
 
 	i64, _ := strconv.ParseUint(a[1], 16, 64)
-	return int(i64), a[2], a[3], nil
+	return int(i64), a[2], nil
 }
 
-var snapshotPathRegex = regexp.MustCompile(`^([0-9a-f]{16})(?:-(\w+))?(.snapshot(?:.gz)?)$`)
+var snapshotPathRegex = regexp.MustCompile(`^([0-9a-f]{8})(.snapshot(?:.lz4)?)$`)
 
 // IsWALPath returns true if s is a path to a WAL file.
 func IsWALPath(s string) bool {
@@ -186,6 +224,8 @@ func IsWALPath(s string) bool {
 // ParseWALPath returns the index & offset for the WAL file.
 // Returns an error if the path is not a valid snapshot path.
 func ParseWALPath(s string) (index int, offset int64, ext string, err error) {
+	s = filepath.Base(s)
+
 	a := walPathRegex.FindStringSubmatch(s)
 	if a == nil {
 		return 0, 0, "", fmt.Errorf("invalid wal path: %s", s)
@@ -196,65 +236,85 @@ func ParseWALPath(s string) (index int, offset int64, ext string, err error) {
 	return int(i64), int64(off64), a[3], nil
 }
 
-var walPathRegex = regexp.MustCompile(`^([0-9a-f]{16})(?:_([0-9a-f]{16}))?(.wal(?:.gz)?)$`)
+// FormatWALPath formats a WAL filename with a given index.
+func FormatWALPath(index int) string {
+	assert(index >= 0, "wal index must be non-negative")
+	return fmt.Sprintf("%08x%s", index, WALExt)
+}
+
+// FormatWALPathWithOffset formats a WAL filename with a given index & offset.
+func FormatWALPathWithOffset(index int, offset int64) string {
+	assert(index >= 0, "wal index must be non-negative")
+	assert(offset >= 0, "wal offset must be non-negative")
+	return fmt.Sprintf("%08x_%08x%s", index, offset, WALExt)
+}
+
+var walPathRegex = regexp.MustCompile(`^([0-9a-f]{8})(?:_([0-9a-f]{8}))?(.wal(?:.lz4)?)$`)
 
 // isHexChar returns true if ch is a lowercase hex character.
 func isHexChar(ch rune) bool {
 	return (ch >= '0' && ch <= '9') || (ch >= 'a' && ch <= 'f')
 }
 
-// gzipReadCloser wraps gzip.Reader to also close the underlying reader on close.
-type gzipReadCloser struct {
-	r      *gzip.Reader
-	closer io.ReadCloser
+// createFile creates the file and attempts to set the UID/GID.
+func createFile(filename string, uid, gid int) (*os.File, error) {
+	f, err := os.Create(filename)
+	if err != nil {
+		return nil, err
+	}
+	_ = f.Chown(uid, gid)
+	return f, nil
 }
 
-func (r *gzipReadCloser) Read(p []byte) (n int, err error) {
-	return r.r.Read(p)
-}
+// mkdirAll is a copy of os.MkdirAll() except that it attempts to set the
+// uid/gid for each created directory.
+func mkdirAll(path string, perm os.FileMode, uid, gid int) error {
+	// Fast path: if we can tell whether path is a directory or file, stop with success or error.
+	dir, err := os.Stat(path)
+	if err == nil {
+		if dir.IsDir() {
+			return nil
+		}
+		return &os.PathError{Op: "mkdir", Path: path, Err: syscall.ENOTDIR}
+	}
 
-func (r *gzipReadCloser) Close() error {
-	if err := r.r.Close(); err != nil {
-		r.closer.Close()
+	// Slow path: make sure parent exists and then call Mkdir for path.
+	i := len(path)
+	for i > 0 && os.IsPathSeparator(path[i-1]) { // Skip trailing path separator.
+		i--
+	}
+
+	j := i
+	for j > 0 && !os.IsPathSeparator(path[j-1]) { // Scan backward over element.
+		j--
+	}
+
+	if j > 1 {
+		// Create parent.
+		err = mkdirAll(fixRootDirectory(path[:j-1]), perm, uid, gid)
+		if err != nil {
+			return err
+		}
+	}
+
+	// Parent now exists; invoke Mkdir and use its result.
+	err = os.Mkdir(path, perm)
+	if err != nil {
+		// Handle arguments like "foo/." by
+		// double-checking that directory doesn't exist.
+		dir, err1 := os.Lstat(path)
+		if err1 == nil && dir.IsDir() {
+			_ = os.Chown(path, uid, gid)
+			return nil
+		}
 		return err
 	}
-	return r.closer.Close()
+	_ = os.Chown(path, uid, gid)
+	return nil
 }
 
-// HexDump returns hexdump output but with duplicate lines removed.
-func HexDump(b []byte) string {
-	const prefixN = len("00000000")
-
-	var output []string
-	var prev string
-	var ellipsis bool
-
-	lines := strings.Split(strings.TrimSpace(hex.Dump(b)), "\n")
-	for i, line := range lines {
-		// Add line to output if it is not repeating or the last line.
-		if i == 0 || i == len(lines)-1 || trimPrefixN(line, prefixN) != trimPrefixN(prev, prefixN) {
-			output = append(output, line)
-			prev, ellipsis = line, false
-			continue
-		}
-
-		// Add an ellipsis for the first duplicate line.
-		if !ellipsis {
-			output = append(output, "...")
-			ellipsis = true
-			continue
-		}
-	}
-
-	return strings.Join(output, "\n")
-}
-
-func trimPrefixN(s string, n int) string {
-	if len(s) < n {
-		return ""
-	}
-	return s[n:]
-}
+// Tracef is used for low-level tracing.
+var Tracef = func(format string, a ...interface{}) {}
 
 func assert(condition bool, message string) {
 	if !condition {

litestream_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/benbjohnson/litestream"
+	_ "github.com/mattn/go-sqlite3"
 )
 
 func TestChecksum(t *testing.T) {

litestream_unix.go

@@ -0,0 +1,18 @@
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package litestream
+
+import (
+	"os"
+	"syscall"
+)
+
+// fileinfo returns syscall fields from a FileInfo object.
+func fileinfo(fi os.FileInfo) (uid, gid int) {
+	stat := fi.Sys().(*syscall.Stat_t)
+	return int(stat.Uid), int(stat.Gid)
+}
+
+func fixRootDirectory(p string) string {
+	return p
+}

litestream_windows.go

@@ -0,0 +1,23 @@
+// +build windows
+
+package litestream
+
+import (
+	"os"
+	"syscall"
+)
+
+// fileinfo returns syscall fields from a FileInfo object.
+func fileinfo(fi os.FileInfo) (uid, gid int) {
+	return -1, -1
+}
+
+// fixRootDirectory is copied from the standard library for use with mkdirAll()
+func fixRootDirectory(p string) string {
+	if len(p) == len(`\\?\c:`) {
+		if IsPathSeparator(p[0]) && IsPathSeparator(p[1]) && p[2] == '?' && IsPathSeparator(p[3]) && p[5] == ':' {
+			return p + `\`
+		}
+	}
+	return p
+}

replica_test.go

@@ -0,0 +1,90 @@
+package litestream_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/benbjohnson/litestream"
+)
+
+func TestFileReplica_Sync(t *testing.T) {
+	// Ensure replica can successfully sync after DB has sync'd.
+	t.Run("InitialSync", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+		r := NewTestFileReplica(t, db)
+
+		// Sync database & then sync replica.
+		if err := db.Sync(); err != nil {
+			t.Fatal(err)
+		} else if err := r.Sync(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+
+		// Ensure posistions match.
+		if pos, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, want := r.LastPos(), pos; got != want {
+			t.Fatalf("LastPos()=%v, want %v", got, want)
+		}
+	})
+
+	// Ensure replica can successfully sync multiple times.
+	t.Run("MultiSync", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+		r := NewTestFileReplica(t, db)
+
+		if _, err := sqldb.Exec(`CREATE TABLE foo (bar TEXT);`); err != nil {
+			t.Fatal(err)
+		}
+
+		// Write to the database multiple times and sync after each write.
+		for i, n := 0, db.MinCheckpointPageN*2; i < n; i++ {
+			if _, err := sqldb.Exec(`INSERT INTO foo (bar) VALUES ('baz')`); err != nil {
+				t.Fatal(err)
+			}
+
+			// Sync periodically.
+			if i%100 == 0 || i == n-1 {
+				if err := db.Sync(); err != nil {
+					t.Fatal(err)
+				} else if err := r.Sync(context.Background()); err != nil {
+					t.Fatal(err)
+				}
+			}
+		}
+
+		// Ensure posistions match.
+		if pos, err := db.Pos(); err != nil {
+			t.Fatal(err)
+		} else if got, want := pos.Index, 2; got != want {
+			t.Fatalf("Index=%v, want %v", got, want)
+		} else if calcPos, err := r.CalcPos(context.Background(), pos.Generation); err != nil {
+			t.Fatal(err)
+		} else if got, want := calcPos, pos; got != want {
+			t.Fatalf("CalcPos()=%v, want %v", got, want)
+		} else if got, want := r.LastPos(), pos; got != want {
+			t.Fatalf("LastPos()=%v, want %v", got, want)
+		}
+	})
+
+	// Ensure replica returns an error if there is no generation available from the DB.
+	t.Run("ErrNoGeneration", func(t *testing.T) {
+		db, sqldb := MustOpenDBs(t)
+		defer MustCloseDBs(t, db, sqldb)
+		r := NewTestFileReplica(t, db)
+
+		if err := r.Sync(context.Background()); err == nil || err.Error() != `no generation, waiting for data` {
+			t.Fatal(err)
+		}
+	})
+}
+
+// NewTestFileReplica returns a new replica using a temp directory & with monitoring disabled.
+func NewTestFileReplica(tb testing.TB, db *litestream.DB) *litestream.FileReplica {
+	r := litestream.NewFileReplica(db, "", tb.TempDir())
+	r.MonitorEnabled = false
+	db.Replicas = []litestream.Replica{r}
+	return r
+}

replicator.go

@@ -1,640 +0,0 @@
-package litestream
-
-import (
-	"compress/gzip"
-	"context"
-	"fmt"
-	"io"
-	"io/ioutil"
-	"log"
-	"os"
-	"path/filepath"
-	"sort"
-	"strings"
-	"sync"
-	"time"
-)
-
-// Replica represents a remote destination to replicate the database & WAL.
-type Replica interface {
-	// The name of the replica. Defaults to type if no name specified.
-	Name() string
-
-	// String identifier for the type of replica ("file", "s3", etc).
-	Type() string
-
-	// Starts replicating in a background goroutine.
-	Start(ctx context.Context)
-
-	// Stops all replication processing. Blocks until processing stopped.
-	Stop()
-
-	// Returns a list of generation names for the replica.
-	Generations(ctx context.Context) ([]string, error)
-
-	// Returns basic information about a generation including the number of
-	// snapshot & WAL files as well as the time range covered.
-	GenerationStats(ctx context.Context, generation string) (GenerationStats, error)
-
-	// Returns the highest index for a snapshot within a generation that occurs
-	// before timestamp. If timestamp is zero, returns the latest snapshot.
-	SnapshotIndexAt(ctx context.Context, generation string, timestamp time.Time) (int, error)
-
-	// Returns the highest index for a WAL file that occurs before timestamp.
-	// If timestamp is zero, returns the highest WAL index.
-	WALIndexAt(ctx context.Context, generation string, timestamp time.Time) (int, error)
-
-	// Returns a reader for snapshot data at the given generation/index.
-	SnapshotReader(ctx context.Context, generation string, index int) (io.ReadCloser, error)
-
-	// Returns a reader for WAL data at the given position.
-	WALReader(ctx context.Context, generation string, index int) (io.ReadCloser, error)
-}
-
-var _ Replica = (*FileReplica)(nil)
-
-// FileReplica is a replica that replicates a DB to a local file path.
-type FileReplica struct {
-	db   *DB    // source database
-	name string // replica name, optional
-	dst  string // destination path
-
-	// mu sync.RWMutex
-	wg sync.WaitGroup
-
-	ctx    context.Context
-	cancel func()
-}
-
-// NewFileReplica returns a new instance of FileReplica.
-func NewFileReplica(db *DB, name, dst string) *FileReplica {
-	return &FileReplica{
-		db:     db,
-		name:   name,
-		dst:    dst,
-		cancel: func() {},
-	}
-}
-
-// Name returns the name of the replica. Returns the type if no name set.
-func (r *FileReplica) Name() string {
-	if r.name != "" {
-		return r.name
-	}
-	return r.Type()
-}
-
-// Type returns the type of replica.
-func (r *FileReplica) Type() string {
-	return "file"
-}
-
-// SnapshotDir returns the path to a generation's snapshot directory.
-func (r *FileReplica) SnapshotDir(generation string) string {
-	return filepath.Join(r.dst, "generations", generation, "snapshots")
-}
-
-// SnapshotPath returns the path to a snapshot file.
-func (r *FileReplica) SnapshotPath(generation string, index int) string {
-	return filepath.Join(r.SnapshotDir(generation), fmt.Sprintf("%016x.snapshot.gz", index))
-}
-
-// WALDir returns the path to a generation's WAL directory
-func (r *FileReplica) WALDir(generation string) string {
-	return filepath.Join(r.dst, "generations", generation, "wal")
-}
-
-// WALPath returns the path to a WAL file.
-func (r *FileReplica) WALPath(generation string, index int) string {
-	return filepath.Join(r.WALDir(generation), fmt.Sprintf("%016x.wal", index))
-}
-
-// Generations returns a list of available generation names.
-func (r *FileReplica) Generations(ctx context.Context) ([]string, error) {
-	fis, err := ioutil.ReadDir(filepath.Join(r.dst, "generations"))
-	if os.IsNotExist(err) {
-		return nil, nil
-	} else if err != nil {
-		return nil, err
-	}
-
-	var generations []string
-	for _, fi := range fis {
-		if !IsGenerationName(fi.Name()) {
-			continue
-		} else if !fi.IsDir() {
-			continue
-		}
-		generations = append(generations, fi.Name())
-	}
-	return generations, nil
-}
-
-// GenerationStats returns stats for a generation.
-func (r *FileReplica) GenerationStats(ctx context.Context, generation string) (stats GenerationStats, err error) {
-	// Determine stats for all snapshots.
-	n, min, max, err := r.snapshotStats(generation)
-	if err != nil {
-		return stats, err
-	}
-	stats.SnapshotN = n
-	stats.CreatedAt, stats.UpdatedAt = min, max
-
-	// Update stats if we have WAL files.
-	n, min, max, err = r.walStats(generation)
-	if err != nil {
-		return stats, err
-	} else if n == 0 {
-		return stats, nil
-	}
-
-	stats.WALN = n
-	if stats.CreatedAt.IsZero() || min.Before(stats.CreatedAt) {
-		stats.CreatedAt = min
-	}
-	if stats.UpdatedAt.IsZero() || max.After(stats.UpdatedAt) {
-		stats.UpdatedAt = max
-	}
-	return stats, nil
-}
-
-func (r *FileReplica) snapshotStats(generation string) (n int, min, max time.Time, err error) {
-	fis, err := ioutil.ReadDir(r.SnapshotDir(generation))
-	if os.IsNotExist(err) {
-		return n, min, max, nil
-	} else if err != nil {
-		return n, min, max, err
-	}
-
-	for _, fi := range fis {
-		if !IsSnapshotPath(fi.Name()) {
-			continue
-		}
-		modTime := fi.ModTime().UTC()
-
-		n++
-		if min.IsZero() || modTime.Before(min) {
-			min = modTime
-		}
-		if max.IsZero() || modTime.After(max) {
-			max = modTime
-		}
-	}
-	return n, min, max, nil
-}
-
-func (r *FileReplica) walStats(generation string) (n int, min, max time.Time, err error) {
-	fis, err := ioutil.ReadDir(r.WALDir(generation))
-	if os.IsNotExist(err) {
-		return n, min, max, nil
-	} else if err != nil {
-		return n, min, max, err
-	}
-
-	for _, fi := range fis {
-		if !IsWALPath(fi.Name()) {
-			continue
-		}
-		modTime := fi.ModTime().UTC()
-
-		n++
-		if min.IsZero() || modTime.Before(min) {
-			min = modTime
-		}
-		if max.IsZero() || modTime.After(max) {
-			max = modTime
-		}
-	}
-	return n, min, max, nil
-}
-
-type GenerationStats struct {
-	SnapshotN int
-	WALN      int
-	CreatedAt time.Time
-	UpdatedAt time.Time
-}
-
-// Start starts replication for a given generation.
-func (r *FileReplica) Start(ctx context.Context) {
-	// Stop previous replication.
-	r.Stop()
-
-	// Wrap context with cancelation.
-	ctx, r.cancel = context.WithCancel(ctx)
-
-	// Start goroutine to replicate data.
-	r.wg.Add(1)
-	go func() { defer r.wg.Done(); r.monitor(ctx) }()
-}
-
-// Stop cancels any outstanding replication and blocks until finished.
-func (r *FileReplica) Stop() {
-	r.cancel()
-	r.wg.Wait()
-}
-
-// monitor runs in a separate goroutine and continuously replicates the DB.
-func (r *FileReplica) monitor(ctx context.Context) {
-	// Clear old temporary files that my have been left from a crash.
-	if err := removeTmpFiles(r.dst); err != nil {
-		log.Printf("%s(%s): cannot remove tmp files: %s", r.db.Path(), r.Name(), err)
-	}
-
-	// Continuously check for new data to replicate.
-	ch := make(chan struct{})
-	close(ch)
-	var notify <-chan struct{} = ch
-
-	var pos Pos
-	var err error
-	for {
-		select {
-		case <-ctx.Done():
-			return
-		case <-notify:
-		}
-
-		// Fetch new notify channel before replicating data.
-		notify = r.db.Notify()
-
-		// Determine position, if necessary.
-		if pos.IsZero() {
-			if pos, err = r.pos(); err != nil {
-				log.Printf("%s(%s): cannot determine position: %s", r.db.Path(), r.Name(), err)
-				continue
-			} else if pos.IsZero() {
-				log.Printf("%s(%s): no generation, waiting for data", r.db.Path(), r.Name())
-				continue
-			}
-		}
-
-		// If we have no replicated WALs, start from last index in shadow WAL.
-		if pos.Index == 0 && pos.Offset == 0 {
-			if pos.Index, err = r.db.CurrentShadowWALIndex(pos.Generation); err != nil {
-				log.Printf("%s(%s): cannot determine latest shadow wal index: %s", r.db.Path(), r.Name(), err)
-				continue
-			}
-		}
-
-		// Synchronize the shadow wal into the replication directory.
-		if pos, err = r.sync(ctx, pos); err != nil {
-			log.Printf("%s(%s): sync error: %s", r.db.Path(), r.Name(), err)
-			continue
-		}
-
-		// Gzip any old WAL files.
-		if pos.Generation != "" {
-			if err := r.compress(ctx, pos.Generation); err != nil {
-				log.Printf("%s(%s): compress error: %s", r.db.Path(), r.Name(), err)
-				continue
-			}
-		}
-	}
-}
-
-// pos returns the position for the replica for the current generation.
-// Returns a zero value if there is no active generation.
-func (r *FileReplica) pos() (pos Pos, err error) {
-	// Find the current generation from the DB. Return zero pos if no generation.
-	generation, err := r.db.CurrentGeneration()
-	if err != nil {
-		return pos, err
-	} else if generation == "" {
-		return pos, nil // empty position
-	}
-	pos.Generation = generation
-
-	// Find the max WAL file.
-	dir := r.WALDir(generation)
-	fis, err := ioutil.ReadDir(dir)
-	if os.IsNotExist(err) {
-		return pos, nil // no replicated wal, start at beginning of generation
-	} else if err != nil {
-		return pos, err
-	}
-
-	index := -1
-	for _, fi := range fis {
-		name := fi.Name()
-		name = strings.TrimSuffix(name, ".gz")
-
-		if !strings.HasSuffix(name, WALExt) {
-			continue
-		}
-
-		if v, err := ParseWALFilename(filepath.Base(name)); err != nil {
-			continue // invalid wal filename
-		} else if index == -1 || v > index {
-			index = v
-		}
-	}
-	if index == -1 {
-		return pos, nil // wal directory exists but no wal files, return beginning pos
-	}
-	pos.Index = index
-
-	// Determine current offset.
-	fi, err := os.Stat(filepath.Join(dir, FormatWALFilename(pos.Index)))
-	if err != nil {
-		return pos, err
-	}
-	pos.Offset = fi.Size()
-
-	return pos, nil
-}
-
-// snapshot copies the entire database to the replica path.
-func (r *FileReplica) snapshot(ctx context.Context, generation string, index int) error {
-	// Acquire a read lock on the database during snapshot to prevent checkpoints.
-	tx, err := r.db.db.Begin()
-	if err != nil {
-		return err
-	} else if _, err := tx.ExecContext(ctx, `SELECT COUNT(1) FROM _litestream_seq;`); err != nil {
-		tx.Rollback()
-		return err
-	}
-	defer tx.Rollback()
-
-	// Ignore if we already have a snapshot for the given WAL index.
-	snapshotPath := r.SnapshotPath(generation, index)
-	if _, err := os.Stat(snapshotPath); err == nil {
-		return nil
-	}
-
-	if err := os.MkdirAll(filepath.Dir(snapshotPath), 0700); err != nil {
-		return err
-	}
-
-	return compressFile(r.db.Path(), snapshotPath)
-}
-
-// snapshotN returns the number of snapshots for a generation.
-func (r *FileReplica) snapshotN(generation string) (int, error) {
-	fis, err := ioutil.ReadDir(r.SnapshotDir(generation))
-	if os.IsNotExist(err) {
-		return 0, nil
-	} else if err != nil {
-		return 0, err
-	}
-
-	var n int
-	for _, fi := range fis {
-		if _, _, _, err := ParseSnapshotPath(fi.Name()); err == nil {
-			n++
-		}
-	}
-	return n, nil
-}
-
-func (r *FileReplica) sync(ctx context.Context, pos Pos) (_ Pos, err error) {
-	// Read all WAL files since the last position.
-	for {
-		if pos, err = r.syncNext(ctx, pos); err == io.EOF {
-			return pos, nil
-		} else if err != nil {
-			return pos, err
-		}
-	}
-}
-
-func (r *FileReplica) syncNext(ctx context.Context, pos Pos) (_ Pos, err error) {
-	rd, err := r.db.ShadowWALReader(pos)
-	if err == io.EOF {
-		return pos, err
-	} else if err != nil {
-		return pos, fmt.Errorf("wal reader: %w", err)
-	}
-	defer rd.Close()
-
-	// Create snapshot if no snapshots exist.
-	if n, err := r.snapshotN(rd.Pos().Generation); err != nil {
-		return pos, err
-	} else if n == 0 {
-		if err := r.snapshot(ctx, rd.Pos().Generation, rd.Pos().Index); err != nil {
-			return pos, err
-		}
-	}
-
-	// Ensure parent directory exists for WAL file.
-	filename := r.WALPath(rd.Pos().Generation, rd.Pos().Index)
-	if err := os.MkdirAll(filepath.Dir(filename), 0700); err != nil {
-		return pos, err
-	}
-
-	// Create a temporary file to write into so we don't have partial writes.
-	w, err := os.OpenFile(filename, os.O_RDWR|os.O_CREATE, 0600)
-	if err != nil {
-		return pos, err
-	}
-	defer w.Close()
-
-	// Seek, copy & sync WAL contents.
-	if _, err := w.Seek(rd.Pos().Offset, io.SeekStart); err != nil {
-		return pos, err
-	} else if _, err := io.Copy(w, rd); err != nil {
-		return pos, err
-	} else if err := w.Sync(); err != nil {
-		return pos, err
-	} else if err := w.Close(); err != nil {
-		return pos, err
-	}
-
-	// Return ending position of the reader.
-	return rd.Pos(), nil
-}
-
-// compress gzips all WAL files before the current one.
-func (r *FileReplica) compress(ctx context.Context, generation string) error {
-	dir := r.WALDir(generation)
-	filenames, err := filepath.Glob(filepath.Join(dir, "*.wal"))
-	if err != nil {
-		return err
-	} else if len(filenames) <= 1 {
-		return nil // no uncompressed wal files or only one active file
-	}
-
-	// Ensure filenames are sorted & remove the last (active) WAL.
-	sort.Strings(filenames)
-	filenames = filenames[:len(filenames)-1]
-
-	// Compress each file from oldest to newest.
-	for _, filename := range filenames {
-		select {
-		case <-ctx.Done():
-			return err
-		default:
-		}
-
-		dst := filename + ".gz"
-		if err := compressFile(filename, dst); err != nil {
-			return err
-		} else if err := os.Remove(filename); err != nil {
-			return err
-		}
-	}
-
-	return nil
-}
-
-// SnapsotIndexAt returns the highest index for a snapshot within a generation
-// that occurs before timestamp. If timestamp is zero, returns the latest snapshot.
-func (r *FileReplica) SnapshotIndexAt(ctx context.Context, generation string, timestamp time.Time) (int, error) {
-	fis, err := ioutil.ReadDir(r.SnapshotDir(generation))
-	if os.IsNotExist(err) {
-		return 0, ErrNoSnapshots
-	} else if err != nil {
-		return 0, err
-	}
-
-	index := -1
-	var max time.Time
-	for _, fi := range fis {
-		// Read index from snapshot filename.
-		idx, _, _, err := ParseSnapshotPath(fi.Name())
-		if err != nil {
-			continue // not a snapshot, skip
-		} else if !timestamp.IsZero() && fi.ModTime().After(timestamp) {
-			continue // after timestamp, skip
-		}
-
-		// Use snapshot if it newer.
-		if max.IsZero() || fi.ModTime().After(max) {
-			index, max = idx, fi.ModTime()
-		}
-	}
-
-	if index == -1 {
-		return 0, ErrNoSnapshots
-	}
-	return index, nil
-}
-
-// Returns the highest index for a WAL file that occurs before timestamp.
-// If timestamp is zero, returns the highest WAL index.
-func (r *FileReplica) WALIndexAt(ctx context.Context, generation string, timestamp time.Time) (int, error) {
-	fis, err := ioutil.ReadDir(r.WALDir(generation))
-	if os.IsNotExist(err) {
-		return 0, nil
-	} else if err != nil {
-		return 0, err
-	}
-
-	index := -1
-	for _, fi := range fis {
-		// Read index from snapshot filename.
-		idx, _, _, err := ParseWALPath(fi.Name())
-		if err != nil {
-			continue // not a snapshot, skip
-		} else if !timestamp.IsZero() && fi.ModTime().After(timestamp) {
-			continue // after timestamp, skip
-		} else if idx < index {
-			continue // earlier index, skip
-		}
-
-		index = idx
-	}
-
-	if index == -1 {
-		return 0, nil
-	}
-	return index, nil
-}
-
-// SnapshotReader returns a reader for snapshot data at the given generation/index.
-// Returns os.ErrNotExist if no matching index is found.
-func (r *FileReplica) SnapshotReader(ctx context.Context, generation string, index int) (io.ReadCloser, error) {
-	dir := r.SnapshotDir(generation)
-	fis, err := ioutil.ReadDir(dir)
-	if err != nil {
-		return nil, err
-	}
-
-	for _, fi := range fis {
-		// Parse index from snapshot filename. Skip if no match.
-		idx, _, ext, err := ParseSnapshotPath(fi.Name())
-		if err != nil || index != idx {
-			continue
-		}
-
-		// Open & return the file handle if uncompressed.
-		f, err := os.Open(filepath.Join(dir, fi.Name()))
-		if err != nil {
-			return nil, err
-		} else if ext == ".snapshot" {
-			return f, nil // not compressed, return as-is.
-		}
-		assert(ext == ".snapshot.gz", "invalid snapshot extension")
-
-		// If compressed, wrap in a gzip reader and return with wrapper to
-		// ensure that the underlying file is closed.
-		r, err := gzip.NewReader(f)
-		if err != nil {
-			f.Close()
-			return nil, err
-		}
-		return &gzipReadCloser{r: r, closer: f}, nil
-	}
-	return nil, os.ErrNotExist
-}
-
-// WALReader returns a reader for WAL data at the given index.
-// Returns os.ErrNotExist if no matching index is found.
-func (r *FileReplica) WALReader(ctx context.Context, generation string, index int) (io.ReadCloser, error) {
-	filename := r.WALPath(generation, index)
-
-	// Attempt to read uncompressed file first.
-	f, err := os.Open(filename)
-	if err == nil {
-		return f, nil // file exist, return
-	} else if err != nil && !os.IsNotExist(err) {
-		return nil, err
-	}
-
-	// Otherwise read the compressed file. Return error if file doesn't exist.
-	f, err = os.Open(filename + ".gz")
-	if err != nil {
-		return nil, err
-	}
-
-	// If compressed, wrap in a gzip reader and return with wrapper to
-	// ensure that the underlying file is closed.
-	rd, err := gzip.NewReader(f)
-	if err != nil {
-		f.Close()
-		return nil, err
-	}
-	return &gzipReadCloser{r: rd, closer: f}, nil
-}
-
-// compressFile compresses a file and replaces it with a new file with a .gz extension.
-func compressFile(src, dst string) error {
-	r, err := os.Open(src)
-	if err != nil {
-		return err
-	}
-	defer r.Close()
-
-	w, err := os.Create(dst + ".tmp")
-	if err != nil {
-		return err
-	}
-	defer w.Close()
-
-	gz := gzip.NewWriter(w)
-	defer gz.Close()
-
-	// Copy & compress file contents to temporary file.
-	if _, err := io.Copy(gz, r); err != nil {
-		return err
-	} else if err := gz.Close(); err != nil {
-		return err
-	} else if err := w.Sync(); err != nil {
-		return err
-	} else if err := w.Close(); err != nil {
-		return err
-	}
-
-	// Move compressed file to final location.
-	return os.Rename(dst+".tmp", dst)
-}