diff --git a/docs/developer_guides/chap04_message_stream.md b/docs/developer_guides/chap04_message_stream.md
index 1782ad3182..02e4f0b1d1 100644
--- a/docs/developer_guides/chap04_message_stream.md
+++ b/docs/developer_guides/chap04_message_stream.md
@@ -172,6 +172,7 @@ type RepackFunc func(msgs []TsMsg, hashKeys [][]int32) (map[int32]*MsgPack, erro
 // Interface
 type UnmarshalFunc func(interface{}) (TsMsg, error)
 
+// UnmarshalDispatcher is an interface contains method Unmarshal
 type UnmarshalDispatcher interface {
 	Unmarshal(input interface{}, msgType commonpb.MsgType) (TsMsg, error)
 	AddMsgTemplate(msgType commonpb.MsgType, unmarshalFunc UnmarshalFunc)