diff --git a/replicator/driver.go b/replicator/driver.go
index fbb3ebe..7f8f03d 100644
--- a/replicator/driver.go
+++ b/replicator/driver.go
@@ -5,14 +5,15 @@ import (
 	"fmt"
 	"net/url"
 	"path"
+	"time"
 )
 
 type (
 	Driver interface {
-		Keys(context.Context) (chan Key, *error)
-		Get(context.Context, Key) (Value, error)
-		Set(context.Context, Key, Value) error
-		Del(context.Context, Key) error
+		KeysSince(context.Context, time.Time) (chan KeyVersion, *error)
+		Get(context.Context, Key) (ValueVersion, error)
+		Set(context.Context, Key, Value, Version) error
+		Del(context.Context, Key, Version) error
 	}
 
 	Key struct {
@@ -20,10 +21,19 @@ type (
 		Key       string
 	}
 
-	Value struct {
-		Value   []byte
+	Value []byte
+
+	Version []byte
+
+	KeyVersion struct {
+		Key     Key
 		Version []byte
 	}
+
+	ValueVersion struct {
+		Value   []byte
+		Version Version
+	}
 )
 
 func NewDriver(ctx context.Context, driver url.URL) (Driver, error) {
@@ -34,6 +44,8 @@ func NewDriver(ctx context.Context, driver url.URL) (Driver, error) {
 			p = path.Join(driver.Host, p)
 		}
 		return NewFileTree(p)
+	case "map":
+		return NewMap(), nil
 	default:
 		return nil, fmt.Errorf("unknown driver spec %s", driver.String())
 	}
diff --git a/replicator/driver_test.go b/replicator/driver_test.go
index b91dada..f5a05b7 100644
--- a/replicator/driver_test.go
+++ b/replicator/driver_test.go
@@ -9,6 +9,7 @@ import (
 
 func TestDriverInterface(t *testing.T) {
 	var _ Driver = FileTree("")
+	var _ Driver = Map{}
 }
 
 func testDriver(t *testing.T, d Driver) {
@@ -16,7 +17,7 @@ func testDriver(t *testing.T, d Driver) {
 	defer can()
 
 	key := Key{Namespace: "x/y", Key: "z"}
-	value := Value{Value: []byte(t.Name()), Version: []byte("1")}
+	value := ValueVersion{Value: []byte(t.Name()), Version: []byte("1")}
 
 	t.Run("get does not exist", func(t *testing.T) {
 		v, err := d.Get(ctx, key)
@@ -29,7 +30,7 @@ func testDriver(t *testing.T, d Driver) {
 	})
 
 	t.Run("404 set get", func(t *testing.T) {
-		if err := d.Set(ctx, key, value); err != nil {
+		if err := d.Set(ctx, key, value.Value, value.Version); err != nil {
 			t.Fatal(err)
 		}
 
@@ -44,13 +45,13 @@ func testDriver(t *testing.T, d Driver) {
 			t.Errorf("version didnt match set-get: want %q, got %q", value.Version, v.Version)
 		}
 
-		if err := d.Del(ctx, key); err != nil {
+		if err := d.Del(ctx, key, nil); err != nil {
 			t.Errorf("failed to clean up: %v", err)
 		}
 	})
 
 	t.Run("keys of nothing", func(t *testing.T) {
-		ch, err := d.Keys(ctx)
+		ch, err := d.KeysSince(ctx, time.Time{})
 		for got := range ch {
 			t.Error("expected nothing but got", got)
 		}
@@ -60,16 +61,18 @@ func testDriver(t *testing.T, d Driver) {
 	})
 
 	t.Run("keys of one key", func(t *testing.T) {
-		if err := d.Set(ctx, key, value); err != nil {
+		if err := d.Set(ctx, key, value.Value, value.Version); err != nil {
 			t.Fatal(err)
 		}
-		defer d.Del(ctx, key)
+		defer d.Del(ctx, key, nil)
 
-		ch, err := d.Keys(ctx)
+		ch, err := d.KeysSince(ctx, time.Time{})
 		n := 0
 		for got := range ch {
 			n += 1
-			if got != key {
+			if got.Key != key {
+				t.Error(got)
+			} else if !bytes.Equal(got.Version, value.Version) {
 				t.Error(got)
 			}
 		}
diff --git a/replicator/filetree.go b/replicator/filetree.go
index 91230b2..f3fb93d 100644
--- a/replicator/filetree.go
+++ b/replicator/filetree.go
@@ -20,8 +20,8 @@ func NewFileTree(root string) (FileTree, error) {
 	return FileTree(root), nil
 }
 
-func (tree FileTree) Keys(ctx context.Context) (chan Key, *error) {
-	result := make(chan Key)
+func (tree FileTree) KeysSince(ctx context.Context, t time.Time) (chan KeyVersion, *error) {
+	result := make(chan KeyVersion)
 	var final error
 	go func() {
 		defer close(result)
@@ -33,8 +33,12 @@ func (tree FileTree) Keys(ctx context.Context) (chan Key, *error) {
 				return ctx.Err()
 			}
 			key := tree.toKey(p)
+			version, err := tree.getVersion(key)
+			if err != nil {
+				return err
+			}
 			select {
-			case result <- key:
+			case result <- KeyVersion{Key: key, Version: version}:
 			case <-ctx.Done():
 			}
 			return ctx.Err()
@@ -45,40 +49,46 @@ func (tree FileTree) Keys(ctx context.Context) (chan Key, *error) {
 	return result, &final
 }
 
-func (tree FileTree) Get(ctx context.Context, key Key) (Value, error) {
+func (tree FileTree) Get(ctx context.Context, key Key) (ValueVersion, error) {
 	version, err := tree.getVersion(key)
 	if version == nil {
-		return Value{}, err
+		return ValueVersion{}, err
 	}
 
 	f, err := os.Open(tree.realpath(key))
 	if err != nil {
-		return Value{}, err
+		return ValueVersion{}, err
 	}
 	defer f.Close()
 	b, _ := io.ReadAll(f)
 
-	return Value{
+	return ValueVersion{
 		Value:   b,
 		Version: version,
 	}, nil
 }
 
-func (tree FileTree) Set(ctx context.Context, key Key, value Value) error {
-	version, err := tree.fromVersion(value.Version)
+func (tree FileTree) Set(ctx context.Context, key Key, value Value, version Version) error {
+	v, err := tree.fromVersion(version)
 	if err != nil {
 		return err
 	}
 
+	if oldv, err := tree._getVersion(key); err != nil {
+		return err
+	} else if v.Before(oldv) {
+		return nil // conflict
+	}
+
 	f2, err := ioutil.TempFile(os.TempDir(), "*.*")
 	if err != nil {
 		return err
 	}
 	defer os.Remove(f2.Name())
 
-	if n, err := f2.Write(value.Value); err != nil {
+	if n, err := f2.Write(value); err != nil {
 		return err
-	} else if n != len(value.Value) {
+	} else if n != len(value) {
 		return fmt.Errorf("truncated write")
 	}
 
@@ -86,7 +96,7 @@ func (tree FileTree) Set(ctx context.Context, key Key, value Value) error {
 		return err
 	}
 
-	if err := os.Chtimes(f2.Name(), time.Time{}, version); err != nil {
+	if err := os.Chtimes(f2.Name(), time.Time{}, v); err != nil {
 		return err
 	}
 
@@ -94,12 +104,20 @@ func (tree FileTree) Set(ctx context.Context, key Key, value Value) error {
 	return os.Rename(f2.Name(), tree.realpath(key))
 }
 
-func (tree FileTree) Del(ctx context.Context, key Key) error {
-	if _, err := os.Stat(tree.realpath(key)); os.IsNotExist(err) {
-		return nil
-	} else if err != nil {
-		return err
+func (tree FileTree) Del(ctx context.Context, key Key, version Version) error {
+	if version != nil {
+		v, err := tree.fromVersion(version)
+		if err != nil {
+			return err
+		}
+
+		if oldv, err := tree._getVersion(key); err != nil {
+			return err
+		} else if v.Before(oldv) {
+			return nil // conflict
+		}
 	}
+
 	return os.Remove(tree.realpath(key))
 }
 
@@ -115,14 +133,22 @@ func (tree FileTree) realpath(key Key) string {
 }
 
 func (tree FileTree) getVersion(key Key) ([]byte, error) {
-	info, err := os.Stat(tree.realpath(key))
-	if os.IsNotExist(err) {
-		return nil, nil
-	}
-	if err != nil {
+	v, err := tree._getVersion(key)
+	if v == (time.Time{}) {
 		return nil, err
 	}
-	return []byte(strconv.FormatInt(info.ModTime().UnixNano(), 10)), nil
+	return []byte(strconv.FormatInt(v.UnixNano(), 10)), err
+}
+
+func (tree FileTree) _getVersion(key Key) (time.Time, error) {
+	info, err := os.Stat(tree.realpath(key))
+	if os.IsNotExist(err) {
+		return time.Time{}, nil
+	}
+	if err != nil {
+		return time.Time{}, err
+	}
+	return info.ModTime(), nil
 }
 
 func (tree FileTree) fromVersion(v []byte) (time.Time, error) {
diff --git a/replicator/map.go b/replicator/map.go
new file mode 100644
index 0000000..1654301
--- /dev/null
+++ b/replicator/map.go
@@ -0,0 +1,43 @@
+package replicator
+
+import (
+	"context"
+	"io"
+	"time"
+)
+
+type Map map[Key]ValueVersion
+
+func NewMap() Map {
+	return make(Map)
+}
+
+func (m Map) KeysSince(ctx context.Context, t time.Time) (chan KeyVersion, *error) {
+	result := make(chan KeyVersion)
+	var err error
+	go func() {
+		defer close(result)
+		for k := range m {
+			select {
+			case result <- KeyVersion{Key: k, Version: m[k].Version}:
+			case <-ctx.Done():
+				err = ctx.Err()
+				return
+			}
+		}
+	}()
+	return result, &err
+}
+
+func (m Map) Get(_ context.Context, k Key) (ValueVersion, error) {
+	return ValueVersion{}, io.EOF
+}
+
+func (m Map) Set(_ context.Context, key Key, value Value, version Version) error {
+	m[key] = ValueVersion{Value: value, Version: version}
+	return nil
+}
+
+func (m Map) Del(_ context.Context, k Key, v Version) error {
+	return io.EOF
+}
diff --git a/replicator/map_test.go b/replicator/map_test.go
new file mode 100644
index 0000000..e9ba044
--- /dev/null
+++ b/replicator/map_test.go
@@ -0,0 +1,9 @@
+package replicator
+
+import (
+	"testing"
+)
+
+func TestMap(t *testing.T) {
+	testDriver(t, NewMap())
+}