Merge pull request #27 from benbjohnson/generations-replica-url

Allow replica URLs for generations command

.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

@@ -1,10 +1,12 @@
-on: [push, pull_request]
+on: push
 name: test
 jobs:
   test:
     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 +1,22 @@
+default:
+
+dist-linux:
+	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
+
+dist-macos:
+ifndef LITESTREAM_VERSION
+	$(error LITESTREAM_VERSION is undefined)
+endif
+	mkdir -p dist
+	go build -v -ldflags "-X 'main.Version=${LITESTREAM_VERSION}'"  -o dist/litestream ./cmd/litestream
+	gon etc/gon.hcl
+	mv dist/litestream.zip dist/litestream-${LITESTREAM_VERSION}-darwin-amd64.zip
+	openssl dgst -sha256 dist/litestream-${LITESTREAM_VERSION}-darwin-amd64.zip
+
+clean:
+	rm -rf dist
+
+.PHONY: default dist-linux dist-macos clean

README.md

@@ -1,17 +1,296 @@
-litestream
+Litestream
+![GitHub release (latest by date)](https://img.shields.io/github/v/release/benbjohnson/litestream)
+![Status](https://img.shields.io/badge/status-beta-blue)
+![GitHub](https://img.shields.io/github/license/benbjohnson/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 need support or have ideas for improving Litestream, please visit the
+[GitHub Discussions](https://github.com/benbjohnson/litestream/discussions) to
+chat. 
+
+If you find this project interesting, please consider starring the project on
+GitHub.
 
 
-## Questions
+## Installation
 
-- How to avoid WAL checkpointing on close?
+### Mac OS (Homebrew)
 
+To install from homebrew, first add the Litestream tap and then install:
 
-## Notes
-
-```sql
--- Disable autocheckpointing.
-PRAGMA wal_autocheckpoint = 0
+```sh
+$ brew install benbjohnson/litestream/litestream
 ```
+
+
+### Linux (Debian)
+
+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.
+
+
+### Building from source
+
+Download and install the [Go toolchain](https://golang.org/) and then run:
+
+```sh
+$ go install ./cmd/litestream
+```
+
+The `litestream` binary should be in your `$GOPATH/bin` folder.
+
+
+## Quick Start
+
+Litestream provides a configuration file that can be used for production
+deployments but you can also specify a single database and replica on the
+command line for testing.
+
+First, you'll need to create an S3 bucket that we'll call `"mybkt"` in this
+example. You'll also need to set your AWS credentials:
+
+```sh
+$ export AWS_ACCESS_KEY_ID=AKIAxxxxxxxxxxxxxxxx
+$ export AWS_SECRET_ACCESS_KEY=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/xxxxxxxxx
+```
+
+Next you can run the `litestream replicate` command with the path to the
+database you want to backup and the URL of your replica destination:
+
+```sh
+$ litestream replicate /path/to/db s3://mybkt/db
+```
+
+If you make changes to your local database, those changes will be replicated
+to S3 every 10 seconds. From another terminal window, you can restore your
+database from your S3 replica:
+
+```
+$ litestream restore -v -o /path/to/restored/db s3://mybkt/db
+```
+
+Voila! 🎉
+
+Your database should be restored to the last replicated state that
+was sent to S3. You can adjust your replication frequency and other options by
+using a configuration-based approach specified below.
+
+
+## Configuration
+
+A configuration-based install gives you more replication options. 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. You can also 
+set the `LITESTREAM_CONFIG` environment variable to specify a new path.
+
+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:
+      - url: 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
+```
+
+### Retention period
+
+By default, replicas will retain a snapshot & subsequent WAL changes for 24
+hours. When the snapshot age exceeds the retention threshold, a new snapshot
+is taken and uploaded and the previous snapshot and WAL files are removed.
+
+You can configure this setting per-replica. Times are parsed using [Go's
+duration](https://golang.org/pkg/time/#ParseDuration) so time units of hours
+(`h`), minutes (`m`), and seconds (`s`) are allowed but days, weeks, months, and
+years are not.
+
+
+```yaml
+db:
+  - path: /path/to/db
+    replicas:
+      - url: s3://mybkt/db
+        retention: 1h          # 1 hour retention
+```
+
+
+### Monitoring replication
+
+You can also enable a Prometheus metrics endpoint to monitor replication by
+specifying a bind address with the `addr` field:
+
+```yml
+addr: ":9090"
+```
+
+This will make metrics available at: http://localhost:9090/metrics
+
+
+### 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 to the replica location.
+- `url`—URL to the replica location.
+- `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 /path/to/restored/db /path/to/db
+
+# Restore from a replica URL.
+$ litestream restore -o /path/to/restored/db s3://mybkt/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

@@ -7,94 +7,101 @@ import (
 	"fmt"
 	"log"
 	"os"
-	"path/filepath"
 	"text/tabwriter"
 	"time"
+
+	"github.com/benbjohnson/litestream"
 )
 
-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 or replica URL required")
 	} else if fs.NArg() > 1 {
 		return fmt.Errorf("too many arguments")
 	}
 
-	// Load configuration.
-	if c.ConfigPath == "" {
-		return errors.New("-config required")
-	}
-	config, err := ReadConfigFile(c.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 {
+	var db *litestream.DB
+	var r litestream.Replica
+	updatedAt := time.Now()
+	if isURL(fs.Arg(0)) {
+		if r, err = NewReplicaFromURL(fs.Arg(0)); err != nil {
 			return err
 		}
+	} else if configPath != "" {
+		// Load configuration.
+		config, err := ReadConfigFile(configPath)
+		if err != nil {
+			return err
+		}
+
+		// Lookup database from configuration file by path.
+		if path, err := expand(fs.Arg(0)); err != nil {
+			return err
+		} else if dbc := config.DBConfig(path); dbc == nil {
+			return fmt.Errorf("database not found in config: %s", path)
+		} else if db, err = newDBFromConfig(&config, dbc); err != nil {
+			return err
+		}
+
+		// Filter by replica, if specified.
+		if *replicaName != "" {
+			if r = db.Replica(*replicaName); r == nil {
+				return fmt.Errorf("replica %q not found for database %q", *replicaName, db.Path())
+			}
+		}
+
+		// Determine last time database or WAL was updated.
+		if updatedAt, err = db.UpdatedAt(); err != nil {
+			return err
+		}
+	} else {
+		return errors.New("config path or replica URL required")
+	}
+
+	var replicas []litestream.Replica
+	if r != nil {
+		replicas = []litestream.Replica{r}
+	} else {
+		replicas = db.Replicas
 	}
 
 	// 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 replicas {
+		generations, err := r.Generations(ctx)
+		if err != nil {
+			log.Printf("%s: cannot list generations: %s", r.Name(), err)
 			continue
 		}
 
-		// Instantiate DB from from configuration.
-		db, err := newDBFromConfig(dbConfig)
-		if err != nil {
-			return err
-		}
-
-		// 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()
@@ -102,32 +109,51 @@ 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
-stats about their lag behind the primary database and the time range they cover.
+The generations command lists all generations for a database or replica. It also
+lists stats about their lag behind the primary database and the time range they
+cover.
 
 Usage:
 
-	litestream generations [arguments] DB
+	litestream generations [arguments] DB_PATH
+	
+	litestream generations [arguments] REPLICA_URL
 
 Arguments:
 
 	-config PATH
-	    Specifies the configuration file. Defaults to %s
+	    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.
@@ -75,16 +87,29 @@ Usage:
 
 The commands are:
 
-	generations  list available generations across all dbs & replicas
+	databases    list databases specified in config file
+	generations  list available generations for a database
 	replicate    runs a server to replicate databases
 	restore      recovers database backup from a replica
-	version      prints the version
+	snapshots    list available snapshots for a database
+	version      prints the binary version
+	wal          list available WAL files for a database
 `[1:])
 }
 
 // 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"`
 }
 
 // DefaultConfig returns a new instance of Config with defaults set.
@@ -92,6 +117,7 @@ func DefaultConfig() Config {
 	return Config{}
 }
 
+// DBConfig returns database configuration by path.
 func (c *Config) DBConfig(path string) *DBConfig {
 	for _, dbConfig := range c.DBs {
 		if dbConfig.Path == path {
@@ -102,18 +128,13 @@ func (c *Config) DBConfig(path string) *DBConfig {
 }
 
 // ReadConfigFile unmarshals config from filename. Expands path if needed.
-func ReadConfigFile(filename string) (Config, error) {
+func ReadConfigFile(filename string) (_ Config, err error) {
 	config := DefaultConfig()
 
 	// Expand filename, if necessary.
-	if prefix := "~" + string(os.PathSeparator); strings.HasPrefix(filename, prefix) {
-		u, err := user.Current()
-		if err != nil {
-			return config, err
-		} else if u.HomeDir == "" {
-			return config, fmt.Errorf("home directory unset")
-		}
-		filename = filepath.Join(u.HomeDir, strings.TrimPrefix(filename, prefix))
+	filename, err = expand(filename)
+	if err != nil {
+		return config, err
 	}
 
 	// Read & deserialize configuration.
@@ -124,32 +145,123 @@ func ReadConfigFile(filename string) (Config, error) {
 	} else if err := yaml.Unmarshal(buf, &config); err != nil {
 		return config, err
 	}
+
+	// Normalize paths.
+	for _, dbConfig := range config.DBs {
+		if dbConfig.Path, err = expand(dbConfig.Path); 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"`
 }
 
+// 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"`
+	URL                    string        `yaml:"url"`
+	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"`
+}
+
+// NewReplicaFromURL returns a new Replica instance configured from a URL.
+// The replica's database is not set.
+func NewReplicaFromURL(s string) (litestream.Replica, error) {
+	scheme, host, path, err := ParseReplicaURL(s)
+	if err != nil {
+		return nil, err
+	}
+
+	switch scheme {
+	case "file":
+		return litestream.NewFileReplica(nil, "", path), nil
+	case "s3":
+		r := s3.NewReplica(nil, "")
+		r.Bucket, r.Path = host, path
+		return r, nil
+	default:
+		return nil, fmt.Errorf("invalid replica url type: %s", s)
+	}
+}
+
+// ParseReplicaURL parses a replica URL.
+func ParseReplicaURL(s string) (scheme, host, urlpath string, err error) {
+	u, err := url.Parse(s)
+	if err != nil {
+		return "", "", "", err
+	}
+
+	switch u.Scheme {
+	case "file":
+		scheme, u.Scheme = u.Scheme, ""
+		return scheme, "", path.Clean(u.String()), nil
+
+	case "":
+		return u.Scheme, u.Host, u.Path, fmt.Errorf("replica url scheme required: %s", s)
+
+	default:
+		return u.Scheme, u.Host, strings.TrimPrefix(path.Clean(u.Path), "/"), nil
+	}
+}
+
+// isURL returns true if s can be parsed and has a scheme.
+func isURL(s string) bool {
+	u, err := url.Parse(s)
+	return err == nil && u.Scheme != ""
+}
+
+// ReplicaType returns the type based on the type field or extracted from the URL.
+func (c *ReplicaConfig) ReplicaType() string {
+	typ, _, _, _ := ParseReplicaURL(c.URL)
+	if typ != "" {
+		return typ
+	} else if c.Type != "" {
+		return c.Type
+	}
+	return "file"
+}
+
+// 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) {
+	path, err := expand(dbc.Path)
+	if err != nil {
+		return nil, err
+	}
+
 	// Initialize database with given path.
-	db := litestream.NewDB(config.Path)
+	db := litestream.NewDB(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 +272,129 @@ 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 {
-	case "", "file":
-		return newFileReplicaFromConfig(db, config)
+func newReplicaFromConfig(db *litestream.DB, c *Config, dbc *DBConfig, rc *ReplicaConfig) (litestream.Replica, error) {
+	// Ensure user did not specify URL in path.
+	if isURL(rc.Path) {
+		return nil, fmt.Errorf("replica path cannot be a url, please use the 'url' field instead: %s", rc.Path)
+	}
+
+	switch rc.ReplicaType() {
+	case "file":
+		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, err error) {
+	path := rc.Path
+	if rc.URL != "" {
+		_, _, path, err = ParseReplicaURL(rc.URL)
+		if err != nil {
+			return nil, err
+		}
 	}
-	return litestream.NewFileReplica(db, config.Name, config.Path), nil
+
+	if path == "" {
+		return nil, fmt.Errorf("%s: file replica path required", db.Path())
+	}
+
+	if path, err = expand(path); err != nil {
+		return nil, err
+	}
+
+	r := litestream.NewFileReplica(db, rc.Name, 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, err error) {
+	bucket := c.Bucket
+	if v := rc.Bucket; v != "" {
+		bucket = v
+	}
+
+	path := rc.Path
+	if rc.URL != "" {
+		_, bucket, path, err = ParseReplicaURL(rc.URL)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	// 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
+	}
+	region := c.Region
+	if v := rc.Region; v != "" {
+		region = v
+	}
+
+	// Ensure required settings are set.
+	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 = 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
+}
+
+// expand returns an absolute path for s.
+func expand(s string) (string, error) {
+	// Just expand to absolute path if there is no home directory prefix.
+	prefix := "~" + string(os.PathSeparator)
+	if s != "~" && !strings.HasPrefix(s, prefix) {
+		return filepath.Abs(s)
+	}
+
+	// Look up home directory.
+	u, err := user.Current()
+	if err != nil {
+		return "", err
+	} else if u.HomeDir == "" {
+		return "", fmt.Errorf("cannot expand path %s, no home directory available", s)
+	}
+
+	// Return path with tilde replaced by the home directory.
+	if s == "~" {
+		return u.HomeDir, nil
+	}
+	return filepath.Join(u.HomeDir, strings.TrimPrefix(s, prefix)), 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
@@ -19,30 +26,42 @@ 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 {
 		return err
 	}
 
-	// Load configuration.
-	if c.ConfigPath == "" {
-		return errors.New("-config required")
+	// Load configuration or use CLI args to build db/replica.
+	var config Config
+	if fs.NArg() == 1 {
+		return fmt.Errorf("must specify at least one replica URL for %s", fs.Arg(0))
+	} else if fs.NArg() > 1 {
+		dbConfig := &DBConfig{Path: fs.Arg(0)}
+		for _, u := range fs.Args()[1:] {
+			dbConfig.Replicas = append(dbConfig.Replicas, &ReplicaConfig{URL: u})
+		}
+		config.DBs = []*DBConfig{dbConfig}
+	} else if c.ConfigPath != "" {
+		config, err = ReadConfigFile(c.ConfigPath)
+		if err != nil {
+			return err
+		}
+	} else {
+		return errors.New("-config flag or database/replica arguments required")
 	}
-	config, err := ReadConfigFile(c.ConfigPath)
-	if err != nil {
-		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 +70,11 @@ 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 +87,31 @@ 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()
@@ -96,19 +139,28 @@ 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 
-specified in your configuration file.
+The replicate command starts a server to monitor & replicate databases. 
+You can specify your database & replicas in a configuration file or you can
+replicate a single database file by specifying its path and its replicas in the
+command line arguments.
 
 Usage:
 
 	litestream replicate [arguments]
 
+	litestream replicate [arguments] DB_PATH REPLICA_URL [REPLICA_URL...]
+
 Arguments:
 
 	-config PATH
-	    Specifies the configuration file. Defaults to %s
+	    Specifies the configuration file.
+	    Defaults to %s
 
-`[1:], DefaultConfigPath)
+	-v
+	    Enable verbose logging output.
+
+`[1:], DefaultConfigPath())
 }

cmd/litestream/restore.go

@@ -7,28 +7,24 @@ import (
 	"fmt"
 	"log"
 	"os"
-	"path/filepath"
 	"time"
 
 	"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")
@@ -36,20 +32,11 @@ func (c *RestoreCommand) Run(ctx context.Context, args []string) (err error) {
 	if err := fs.Parse(args); err != nil {
 		return err
 	} else if fs.NArg() == 0 || fs.Arg(0) == "" {
-		return fmt.Errorf("database path required")
+		return fmt.Errorf("database path or replica URL 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
-	}
-
 	// Parse timestamp, if specified.
 	if *timestampStr != "" {
 		if opt.Timestamp, err = time.Parse(time.RFC3339, *timestampStr); err != nil {
@@ -67,31 +54,84 @@ 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 {
-		return err
+	// Determine replica & generation to restore from.
+	var r litestream.Replica
+	if isURL(fs.Arg(0)) {
+		if r, err = c.loadFromURL(ctx, fs.Arg(0), &opt); err != nil {
+			return err
+		}
+	} else if configPath != "" {
+		if r, err = c.loadFromConfig(ctx, fs.Arg(0), configPath, &opt); err != nil {
+			return err
+		}
+	} else {
+		return errors.New("config path or replica URL required")
 	}
 
-	// Instantiate DB.
-	dbConfig := config.DBConfig(c.DBPath)
-	if dbConfig == nil {
-		return fmt.Errorf("database not found in config: %s", c.DBPath)
-	}
-	db, err := newDBFromConfig(dbConfig)
-	if err != nil {
-		return err
+	// Return an error if no matching targets found.
+	if opt.Generation == "" {
+		return fmt.Errorf("no matching backups found")
 	}
 
-	return db.Restore(ctx, opt)
+	return litestream.RestoreReplica(ctx, r, opt)
 }
 
+// loadFromURL creates a replica & updates the restore options from a replica URL.
+func (c *RestoreCommand) loadFromURL(ctx context.Context, replicaURL string, opt *litestream.RestoreOptions) (litestream.Replica, error) {
+	r, err := NewReplicaFromURL(replicaURL)
+	if err != nil {
+		return nil, err
+	}
+	opt.Generation, _, err = litestream.CalcReplicaRestoreTarget(ctx, r, *opt)
+	return r, err
+}
+
+// loadFromConfig returns a replica & updates the restore options from a DB reference.
+func (c *RestoreCommand) loadFromConfig(ctx context.Context, dbPath, configPath string, opt *litestream.RestoreOptions) (litestream.Replica, error) {
+	// Load configuration.
+	config, err := ReadConfigFile(configPath)
+	if err != nil {
+		return nil, err
+	}
+
+	// Lookup database from configuration file by path.
+	if dbPath, err = expand(dbPath); err != nil {
+		return nil, err
+	}
+	dbConfig := config.DBConfig(dbPath)
+	if dbConfig == nil {
+		return nil, fmt.Errorf("database not found in config: %s", dbPath)
+	}
+	db, err := newDBFromConfig(&config, dbConfig)
+	if err != nil {
+		return nil, err
+	}
+
+	// Restore into original database path if not specified.
+	if opt.OutputPath == "" {
+		opt.OutputPath = dbPath
+	}
+
+	// Determine the appropriate replica & generation to restore from,
+	r, generation, err := db.CalcRestoreTarget(ctx, *opt)
+	if err != nil {
+		return nil, err
+	}
+	opt.Generation = generation
+
+	return r, nil
+}
+
+// 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.
 
 Usage:
 
-	litestream restore [arguments] DB
+	litestream restore [arguments] DB_PATH
+
+	litestream restore [arguments] REPLICA_URL
 
 Arguments:
 
@@ -107,6 +147,10 @@ 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 +185,6 @@ Examples:
 	$ litestream restore -replica s3 -generation xxxxxxxx /path/to/db
 
 `[1:],
-		DefaultConfigPath,
+		DefaultConfigPath(),
 	)
 }

cmd/litestream/snapshots.go

@@ -0,0 +1,128 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"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")
+	}
+
+	var db *litestream.DB
+	var r litestream.Replica
+	if isURL(fs.Arg(0)) {
+		if r, err = NewReplicaFromURL(fs.Arg(0)); err != nil {
+			return err
+		}
+	} else if configPath != "" {
+		// Load configuration.
+		config, err := ReadConfigFile(configPath)
+		if err != nil {
+			return err
+		}
+
+		// Lookup database from configuration file by path.
+		if path, err := expand(fs.Arg(0)); err != nil {
+			return err
+		} else if dbc := config.DBConfig(path); dbc == nil {
+			return fmt.Errorf("database not found in config: %s", path)
+		} else if db, err = newDBFromConfig(&config, dbc); err != nil {
+			return err
+		}
+
+		// Filter by replica, if specified.
+		if *replicaName != "" {
+			if r = db.Replica(*replicaName); r == nil {
+				return fmt.Errorf("replica %q not found for database %q", *replicaName, db.Path())
+			}
+		}
+	} else {
+		return errors.New("config path or replica URL required")
+	}
+
+	// Find snapshots by db or replica.
+	var infos []*litestream.SnapshotInfo
+	if r != nil {
+		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 or replica.
+
+Usage:
+
+	litestream snapshots [arguments] DB_PATH
+
+	litestream snapshots [arguments] REPLICA_URL
+
+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
+
+	# List all snapshots by replica URL.
+	$ litestream snapshots s3://mybkt/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 +1,137 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"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")
+	}
+
+	var db *litestream.DB
+	var r litestream.Replica
+	if isURL(fs.Arg(0)) {
+		if r, err = NewReplicaFromURL(fs.Arg(0)); err != nil {
+			return err
+		}
+	} else if configPath != "" {
+		// Load configuration.
+		config, err := ReadConfigFile(configPath)
+		if err != nil {
+			return err
+		}
+
+		// Lookup database from configuration file by path.
+		if path, err := expand(fs.Arg(0)); err != nil {
+			return err
+		} else if dbc := config.DBConfig(path); dbc == nil {
+			return fmt.Errorf("database not found in config: %s", path)
+		} else if db, err = newDBFromConfig(&config, dbc); err != nil {
+			return err
+		}
+
+		// Filter by replica, if specified.
+		if *replicaName != "" {
+			if r = db.Replica(*replicaName); r == nil {
+				return fmt.Errorf("replica %q not found for database %q", *replicaName, db.Path())
+			}
+		}
+	} else {
+		return errors.New("config path or replica URL required")
+	}
+
+	// Find WAL files by db or replica.
+	var infos []*litestream.WALInfo
+	if r != nil {
+		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_PATH
+
+	litestream wal [arguments] REPLICA_URL
+
+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 wal -replica s3 -generation xxxxxxxx /path/to/db
+
+	# List all WAL files for replica URL.
+	$ litestream wal s3://mybkt/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/gon.hcl

@@ -0,0 +1,15 @@
+source = ["./dist/litestream"]
+bundle_id = "com.middlemost.litestream"
+
+apple_id {
+  username = "benbjohnson@yahoo.com"
+  password = "@env:AC_PASSWORD"
+}
+
+sign {
+  application_identity = "Developer ID Application: Middlemost Systems, LLC"
+}
+
+zip {
+  output_path = "dist/litestream.zip"
+}

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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+gopkg.in/warnings.v0 v0.1.2/go.mod h1:jksf8JmL6Qr/oQM2OXTHunEvvTAsrWBLb6OOjuVWRNI=
+gopkg.in/yaml.v2 v2.0.0-20170812160011-eb3733d160e7/go.mod h1:JAlM8MvJe8wmxCU4Bli9HhUf9+ttbYbLASfIpnQbh74=
+gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v2 v2.2.4/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v2 v2.2.5/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v2 v2.3.0/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.4.0 h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=
 gopkg.in/yaml.v2 v2.4.0/go.mod h1:RDklbk79AGWmwhnvt/jBztapEOGDOx6ZbXqjP6csGnQ=
+honnef.co/go/tools v0.0.0-20180728063816-88497007e858/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
+honnef.co/go/tools v0.0.0-20190102054323-c2f93a96b099/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
+honnef.co/go/tools v0.0.0-20190523083050-ea95bdfd59fc/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
+honnef.co/go/tools v0.0.1-2019.2.3/go.mod h1:a3bituU0lyd329TUQxRnasdCoJDkEUEAqEt0JzvZhAg=
+sigs.k8s.io/yaml v1.1.0/go.mod h1:UJmg0vDUVViEyp3mgSv9WPwZCDxu4rQW1olrI1uml+o=
+sourcegraph.com/sourcegraph/appdash v0.0.0-20190731080439-ebfcffb1b5c0/go.mod h1:hI742Nqp5OhwiqlzhgfbWU4mW4yO10fP+LoT9WOswdU=

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, perm os.FileMode, uid, gid int) (*os.File, error) {
+	f, err := os.OpenFile(filename, os.O_RDWR|os.O_CREATE|os.O_TRUNC, perm)
+	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)
-}