diff --git a/go-demo-4/account/vault.go b/go-demo-4/account/vault.go
index 8954904..b76de27 100644
--- a/go-demo-4/account/vault.go
+++ b/go-demo-4/account/vault.go
@@ -8,11 +8,20 @@ import (
 	"github.com/fatih/color"
 )
 
-type Db interface {
+type ByteReader interface {
 	Read() ([]byte, error)
+}
+
+type ByteWriter interface {
 	Write([]byte)
 }
 
+// embedded interface
+type Db interface {
+	ByteReader
+	ByteWriter
+}
+
 type Vault struct {
 	Accounts  []Account `json:"accounts"`
 	UpdatedAt time.Time `json:"updatedAt"`