diff --git a/spool/spool.go b/spool/spool.go
index b8c302c..da81b00 100644
--- a/spool/spool.go
+++ b/spool/spool.go
@@ -13,6 +13,7 @@ import (
 	"context"
 	"io"
 	"io/fs"
+	"path/filepath"
 
 	"github.com/fogfish/opts"
 	"github.com/fogfish/stream"
@@ -172,6 +173,82 @@ func (spool *Spool) ForEachFile(
 	)
 }
 
+// Apply the parition function over each file in the reader filesystem, producing
+// results to writer file system.
+func (spool *Spool) Partition(
+	ctx context.Context,
+	dir string,
+	f func(context.Context, string, io.Reader) (string, error),
+) error {
+	return fs.WalkDir(spool.reader, dir,
+		func(path string, d fs.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
+
+			if d.IsDir() {
+				return nil
+			}
+
+			fd, err := spool.reader.Open(path)
+			if err != nil {
+				return spool.iserr(err)
+			}
+			defer fd.Close()
+
+			shard, err := f(ctx, path, fd)
+			if err != nil {
+				return spool.iserr(err)
+			}
+			if len(shard) == 0 {
+				return nil
+			}
+
+			cp, err := spool.reader.Open(path)
+			if err != nil {
+				return spool.iserr(err)
+			}
+			defer cp.Close()
+
+			err = spool.write(spool.writer, filepath.Join("/", shard, path), cp)
+			if err != nil {
+				return spool.iserr(err)
+			}
+
+			if spool.mutable == mutable {
+				err = spool.reader.Remove(path)
+				if err != nil {
+					return spool.iserr(err)
+				}
+			}
+
+			return nil
+		},
+	)
+}
+
+func (spool *Spool) PartitionFile(
+	ctx context.Context,
+	dir string,
+	f func(context.Context, string, []byte) (string, error),
+) error {
+	return spool.Partition(ctx, dir,
+		func(ctx context.Context, path string, r io.Reader) (string, error) {
+			in, err := io.ReadAll(r)
+			if err != nil {
+				return "", err
+			}
+
+			shard, err := f(ctx, path, in)
+			if err != nil {
+				return "", err
+			}
+
+			return shard, nil
+		},
+	)
+}
+
 func (spool *Spool) write(fs stream.CreateFS[struct{}], path string, r io.Reader) error {
 	fd, err := fs.Create(path, nil)
 	if err != nil {
diff --git a/spool/spool_test.go b/spool/spool_test.go
index ad503af..574b93a 100644
--- a/spool/spool_test.go
+++ b/spool/spool_test.go
@@ -18,7 +18,7 @@ import (
 	"github.com/fogfish/stream/spool"
 )
 
-func TestSpool(t *testing.T) {
+func TestSpoolForEach(t *testing.T) {
 	in, err := lfs.NewTempFS(os.TempDir(), "in")
 	it.Then(t).Must(it.Nil(err))
 
@@ -45,3 +45,31 @@ func TestSpool(t *testing.T) {
 		it.Seq(seq).Equal(dat...),
 	)
 }
+
+func TestSpoolPartition(t *testing.T) {
+	in, err := lfs.NewTempFS(os.TempDir(), "in")
+	it.Then(t).Must(it.Nil(err))
+
+	to, err := lfs.NewTempFS(os.TempDir(), "to")
+	it.Then(t).Must(it.Nil(err))
+
+	qq := spool.New(in, to)
+
+	seq := []string{"/a", "/b", "/c", "/d", "/e", "/f"}
+	for _, txt := range seq {
+		err := qq.WriteFile(txt, []byte(txt))
+		it.Then(t).Must(it.Nil(err))
+	}
+
+	dat := []string{}
+	qq.PartitionFile(context.Background(), "/",
+		func(ctx context.Context, path string, b []byte) (string, error) {
+			dat = append(dat, path)
+			return path, nil
+		},
+	)
+
+	it.Then(t).Should(
+		it.Seq(seq).Equal(dat...),
+	)
+}