diff --git a/streamdata.go b/streamdata.go
new file mode 100644
index 0000000..a34f864
--- /dev/null
+++ b/streamdata.go
@@ -0,0 +1,199 @@
+// Package streamdata provides a simple API for downloading VODs and maintaining
+// basic metadata alongside them.
+package streamdata
+
+import (
+	"encoding/json"
+	"errors"
+	"io"
+	"os"
+	"path"
+	"strconv"
+	"syscall"
+	"time"
+)
+
+// Channel represents a Twitch channel.
+type Channel struct {
+	// Numerical identifier specific to the channel, returned by Twitch.
+	Identifier uint64 `json:"id"`
+	// Unique, mutable channel name.
+	Name string `json:"name"`
+
+	// Directory to place persistent data in.
+	root *os.Root
+}
+
+// Is returns whether target is equivalent to c.
+func (c *Channel) Is(target *Channel) bool {
+	return (c == nil && target == nil) || (c != nil && target != nil &&
+		c.Identifier == target.Identifier &&
+		c.Name == target.Name)
+}
+
+// Close closes the underlying on-disk representation.
+func (c *Channel) Close() error {
+	if c == nil || c.root == nil {
+		return syscall.EINVAL
+	}
+	return c.root.Close()
+}
+
+const (
+	// channelPathMetadata points to the metadata file relative to Channel.root.
+	channelPathMetadata = "channel"
+	// channelPathVOD points to the vod directory relative to Channel.root.
+	channelPathVOD = "vod"
+	// ChannelVODSuffix is the (apparently) hardcoded asset file name suffix.
+	ChannelVODSuffix = ".mp4"
+	// channelPathPending points to the transaction backing file relative to Channel.root.
+	channelPathPending = "pending"
+)
+
+// Create initialises the on-disk representation of a [Channel].
+func (c *Channel) Create(pathname string) error {
+	if c == nil || c.root != nil || c.Identifier == 0 || c.Name == "" {
+		return syscall.EINVAL
+	}
+
+	if err := os.MkdirAll(pathname, 0755); err != nil {
+		return err
+	}
+	if root, err := os.OpenRoot(pathname); err != nil {
+		return err
+	} else {
+		c.root = root
+	}
+
+	if w, err := c.root.OpenFile(
+		channelPathMetadata,
+		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
+		0444,
+	); err != nil {
+		_ = c.root.Close()
+		return err
+	} else if err = json.NewEncoder(w).Encode(c); err != nil {
+		_ = c.root.Close()
+		return err
+	}
+
+	if err := c.root.Mkdir(channelPathVOD, 0755); err != nil {
+		_ = c.root.Close()
+		return err
+	}
+
+	return nil
+}
+
+// ErrDanglingTransaction is returned when attempting to [Open] the on-disk
+// representation of a [Channel] with a dangling transaction backing file.
+var ErrDanglingTransaction = errors.New("dangling transaction backing file")
+
+// Open opens the on-disk representation of [Channel] and returns its address.
+func Open(pathname string) (*Channel, error) {
+	var c Channel
+	if root, err := os.OpenRoot(pathname); err != nil {
+		return nil, err
+	} else {
+		c.root = root
+	}
+
+	if _, err := c.root.Lstat(channelPathPending); err != nil {
+		if !errors.Is(err, os.ErrNotExist) {
+			_ = c.root.Close()
+			return nil, err
+		}
+	} else {
+		_ = c.root.Close()
+		return nil, ErrDanglingTransaction
+	}
+
+	if f, err := c.root.Open(channelPathMetadata); err != nil {
+		_ = c.root.Close()
+		return nil, err
+	} else if err = json.NewDecoder(f).Decode(&c); err != nil {
+		_ = c.root.Close()
+		return nil, err
+	}
+	return &c, nil
+}
+
+// VOD holds additional metadata associated with a vod.
+type VOD struct {
+	// Stream title.
+	Title string `json:"title"`
+	// Day of stream start.
+	Date time.Time `json:"date"`
+	// Free-form category string.
+	Category string `json:"category"`
+}
+
+// ChannelMismatchError describes a mismatching [Ident.Channel] passed to [Channel.Add].
+type ChannelMismatchError struct {
+	Got, Want uint64
+}
+
+func (c *ChannelMismatchError) Error() string {
+	return "attempting to add VOD from channel " +
+		strconv.FormatUint(c.Got, 10) + " to channel " +
+		strconv.FormatUint(c.Want, 10)
+}
+
+// Add adds a [VOD] and its corresponding asset to the on-disk representation.
+func (c *Channel) Add(ident *Ident, f func(v *VOD, w io.Writer) error) error {
+	if ident == nil || f == nil {
+		return syscall.EINVAL
+	}
+	if ident.Channel != c.Identifier {
+		return &ChannelMismatchError{ident.Channel, c.Identifier}
+	}
+
+	w, err := c.root.OpenFile(
+		channelPathPending,
+		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
+		0,
+	)
+	if err != nil {
+		return err
+	}
+
+	var v VOD
+	err = f(&v, w)
+	if closeErr := w.Close(); err == nil {
+		err = closeErr
+	}
+	if err != nil {
+		_ = c.root.Remove(channelPathPending)
+		return err
+	}
+
+	if err = c.root.Chmod(channelPathPending, 0444); err != nil {
+		return err
+	}
+	pathname := path.Join(channelPathVOD, ident.String())
+
+	if w, err = c.root.OpenFile(
+		pathname,
+		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
+		0444,
+	); err != nil {
+		return err
+	} else if err = json.NewEncoder(w).Encode(&v); err != nil {
+		_ = w.Close()
+		_ = c.root.Remove(pathname)
+		return err
+	} else if err = w.Close(); err != nil {
+		_ = c.root.Remove(pathname)
+		return err
+	}
+
+	if err = c.root.Rename(
+		channelPathPending,
+		pathname+ChannelVODSuffix,
+	); err != nil {
+		_ = c.root.Remove(pathname)
+		return err
+	}
+
+	return nil
+}
diff --git a/streamdata_test.go b/streamdata_test.go
index 11a1ff5..60ab268 100644
--- a/streamdata_test.go
+++ b/streamdata_test.go
@@ -1,11 +1,312 @@
 package streamdata_test
 
 import (
+	"encoding/json"
+	"errors"
+	"io"
+	"os"
+	"path"
+	"reflect"
+	"syscall"
 	"testing"
+	"time"
 
 	"git.gensokyo.uk/yonah/streamdata"
 )
 
+func TestChannelInvalid(t *testing.T) {
+	t.Parallel()
+
+	if err := (*streamdata.Channel)(nil).Create(""); !reflect.DeepEqual(err, syscall.EINVAL) {
+		t.Fatalf("Create: error = %v", err)
+	}
+	if err := (*streamdata.Channel)(nil).Close(); !reflect.DeepEqual(err, syscall.EINVAL) {
+		t.Fatalf("Close: error = %v", err)
+	}
+}
+
+func TestChannelOpenPerm(t *testing.T) {
+	t.Parallel()
+
+	d := t.TempDir()
+	if err := os.Chmod(d, 0); err != nil {
+		t.Fatal(err)
+	}
+
+	wantErr := &os.PathError{
+		Op:   "open",
+		Path: d,
+		Err:  syscall.EACCES,
+	}
+
+	t.Run("open", func(t *testing.T) {
+		t.Parallel()
+
+		if _, err := streamdata.Open(d); !reflect.DeepEqual(err, wantErr) {
+			t.Fatalf("Open: error = %#v, want %#v", err, wantErr)
+		}
+	})
+
+	t.Run("create", func(t *testing.T) {
+		t.Parallel()
+
+		t.Run("root", func(t *testing.T) {
+			t.Parallel()
+
+			if err := (&streamdata.Channel{
+				Identifier: 0xcafe,
+				Name:       ":3",
+			}).Create(d); !reflect.DeepEqual(err, wantErr) {
+				t.Fatalf("Create: error = %#v, want %#v", err, wantErr)
+			}
+		})
+
+		t.Run("mkdir", func(t *testing.T) {
+			t.Parallel()
+
+			wantErrMkdir := &os.PathError{
+				Op:   "mkdir",
+				Path: path.Join(d, "stream"),
+				Err:  syscall.EACCES,
+			}
+			if err := (&streamdata.Channel{
+				Identifier: 0xcafe,
+				Name:       ":3",
+			}).Create(path.Join(d, "stream")); !reflect.DeepEqual(err, wantErrMkdir) {
+				t.Fatalf("Create: error = %#v, want %#v", err, wantErrMkdir)
+			}
+		})
+
+		t.Run("metadata", func(t *testing.T) {
+			t.Parallel()
+
+			nx := t.TempDir()
+			if err := os.Chmod(nx, 0400); err != nil {
+				t.Fatal(err)
+			}
+
+			wantErrMetadata := &os.PathError{
+				Op:   "openat",
+				Path: "channel",
+				Err:  syscall.EACCES,
+			}
+			if err := (&streamdata.Channel{
+				Identifier: 0xcafe,
+				Name:       ":3",
+			}).Create(nx); !reflect.DeepEqual(err, wantErrMetadata) {
+				t.Fatalf("Create: error = %#v, want %#v", err, wantErrMetadata)
+			}
+		})
+	})
+}
+
+func TestChannelCreate(t *testing.T) {
+	t.Parallel()
+
+	d := t.TempDir()
+	c := streamdata.Channel{
+		Identifier: 0xcafe,
+		Name:       ":3",
+	}
+
+	if err := c.Create(d); err != nil {
+		t.Fatalf("Create: error = %v", err)
+	}
+	if err := c.Close(); err != nil {
+		t.Fatalf("Close: error = %v", err)
+	}
+
+	if got, err := streamdata.Open(d); err != nil {
+		t.Fatalf("Open: error = %v", err)
+	} else if err = got.Close(); err != nil {
+		t.Fatalf("Close: error = %v", err)
+	} else if !got.Is(&c) {
+		t.Errorf("Open: %#v, want %#v", got, c)
+	}
+}
+
+func TestChannelDangling(t *testing.T) {
+	t.Parallel()
+
+	t.Run("dangling", func(t *testing.T) {
+		t.Parallel()
+
+		d := t.TempDir()
+		if err := os.WriteFile(path.Join(d, "pending"), nil, 0); err != nil {
+			t.Fatal(err)
+		}
+		if _, err := streamdata.Open(d); !reflect.DeepEqual(err, streamdata.ErrDanglingTransaction) {
+			t.Errorf("Open: error = %v", err)
+		}
+	})
+
+	t.Run("perm", func(t *testing.T) {
+		t.Parallel()
+
+		d := t.TempDir()
+		if err := os.Chmod(d, 0400); err != nil {
+			t.Fatal(err)
+		}
+
+		wantErr := &os.PathError{
+			Op:   "statat",
+			Path: "pending",
+			Err:  syscall.EACCES,
+		}
+		if _, err := streamdata.Open(d); !reflect.DeepEqual(err, wantErr) {
+			t.Errorf("Open: error = %#v, want %#v", err, wantErr)
+		}
+	})
+}
+
+func TestChannelBadMetadata(t *testing.T) {
+	t.Run("perm", func(t *testing.T) {
+		t.Parallel()
+
+		d := t.TempDir()
+		if err := os.WriteFile(path.Join(d, "channel"), nil, 0); err != nil {
+			t.Fatal(err)
+		}
+
+		wantErr := &os.PathError{
+			Op:   "openat",
+			Path: "channel",
+			Err:  syscall.EACCES,
+		}
+		if _, err := streamdata.Open(d); !reflect.DeepEqual(err, wantErr) {
+			t.Errorf("Open: error = %#v, want %#v", err, wantErr)
+		}
+	})
+
+	t.Run("invalid", func(t *testing.T) {
+		t.Parallel()
+
+		d := t.TempDir()
+		if err := os.WriteFile(path.Join(d, "channel"), nil, 0400); err != nil {
+			t.Fatal(err)
+		}
+
+		wantErr := io.EOF
+		if _, err := streamdata.Open(d); !reflect.DeepEqual(err, wantErr) {
+			t.Errorf("Open: error = %#v, want %#v", err, wantErr)
+		}
+	})
+}
+
+func TestChannelAdd(t *testing.T) {
+	t.Parallel()
+
+	d := t.TempDir()
+	c := streamdata.Channel{
+		Identifier: 0xcafe,
+		Name:       ":3",
+	}
+
+	if err := c.Create(d); err != nil {
+		t.Fatalf("Create: error = %v", err)
+	}
+	t.Cleanup(func() {
+		if err := c.Close(); err != nil {
+			t.Fatalf("Close: error = %v", err)
+		}
+	})
+	var wantErr error
+
+	if err := c.Add(nil, nil); !reflect.DeepEqual(err, syscall.EINVAL) {
+		t.Errorf("(invalid) Add: error = %#v", err)
+	}
+
+	wantErr = &streamdata.ChannelMismatchError{
+		Got:  0xbabe,
+		Want: 0xcafe,
+	}
+	if err := c.Add(&streamdata.Ident{
+		Channel: 0xbabe,
+	}, func(*streamdata.VOD, io.Writer) error {
+		panic("unreachable")
+	}); !reflect.DeepEqual(err, wantErr) {
+		t.Fatalf("(channel) Add: error = %#v, want %#v", err, wantErr)
+	}
+
+	wantErr = &os.PathError{
+		Op:   "openat",
+		Path: "pending",
+		Err:  syscall.EEXIST,
+	}
+	if err := os.WriteFile(path.Join(d, "pending"), nil, 0); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Add(&streamdata.Ident{
+		Channel: 0xcafe,
+	}, func(*streamdata.VOD, io.Writer) error {
+		panic("unreachable")
+	}); !reflect.DeepEqual(err, wantErr) {
+		t.Fatalf("(create) Add: error = %#v, want %#v", err, wantErr)
+	}
+	if err := os.Remove(path.Join(d, "pending")); err != nil {
+		t.Fatal(err)
+	}
+
+	wantErr = errors.New("unique error")
+	if err := c.Add(&streamdata.Ident{
+		Channel: 0xcafe,
+	}, func(*streamdata.VOD, io.Writer) error {
+		return wantErr
+	}); !reflect.DeepEqual(err, wantErr) {
+		t.Fatalf("(callback) Add: error = %#v, want %#v", err, wantErr)
+	}
+	if _, err := os.Stat(path.Join(d, "pending")); !errors.Is(err, os.ErrNotExist) {
+		t.Fatalf("Stat: error = %v", err)
+	}
+
+	wantData := []byte{0xde, 0xad, 0xbe, 0xef}
+	wantVOD := streamdata.VOD{
+		Title:    "\x00",
+		Date:     time.Unix(0, 0).UTC(),
+		Category: "\t",
+	}
+	if err := c.Add(&streamdata.Ident{
+		Serial:  0xfdfdfdfd,
+		Channel: 0xcafe,
+		Data: [streamdata.IdentFFLen]byte{
+			0xfe, 0xe1, 0xde, 0xad,
+			0xfe, 0xed,
+			0x0b, 0xad,
+			0xf0, 0x0d,
+			0, 0, 0, 0, 0, 0,
+		},
+	}, func(v *streamdata.VOD, w io.Writer) error {
+		*v = wantVOD
+		_, err := w.Write(wantData)
+		return err
+	}); err != nil {
+		t.Fatalf("Add: error = %v", err)
+	}
+
+	if dents, err := os.ReadDir(path.Join(d, "vod")); err != nil {
+		t.Fatal(err)
+	} else if len(dents) != 2 {
+		t.Fatalf("ReadDir: %#v", dents)
+	}
+
+	const wantIdent = "4261281277-51966-fee1dead-feed-0bad-f00d-000000000000"
+	var got streamdata.VOD
+	if r, err := os.Open(path.Join(d, "vod", wantIdent)); err != nil {
+		t.Fatal(err)
+	} else if err = json.NewDecoder(r).Decode(&got); err != nil {
+		_ = r.Close()
+		t.Fatalf("Decode: error = %v", err)
+	} else if got != wantVOD {
+		t.Errorf("Add: %#v, want %#v", got, wantVOD)
+	}
+
+	if gotData, err := os.ReadFile(path.Join(d, "vod", wantIdent+streamdata.ChannelVODSuffix)); err != nil {
+		t.Fatal(err)
+	} else if string(gotData) != string(wantData) {
+		t.Errorf("Add: data = %#v, want %#v", gotData, wantData)
+	}
+}
+
 func TestErrors(t *testing.T) {
 	t.Parallel()
 
@@ -18,6 +319,8 @@ func TestErrors(t *testing.T) {
 			"got 51966 field(s) instead of 7"},
 		{"IdentFFError", &streamdata.IdentFFError{Got: 0xcafe, Want: 0xbabe},
 			"got 51966 bytes for a 47806-byte long free-form field"},
+		{"ChannelMismatchError", &streamdata.ChannelMismatchError{Got: 0xcafe, Want: 0xbabe},
+			"attempting to add VOD from channel 51966 to channel 47806"},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {