diff --git a/common/ipsetsink/sinkcluster/common.go b/common/ipsetsink/sinkcluster/common.go
index 501c753..4360f70 100644
--- a/common/ipsetsink/sinkcluster/common.go
+++ b/common/ipsetsink/sinkcluster/common.go
@@ -1,5 +1,20 @@
 package sinkcluster
 
+/* ClusterWriter, and (ClusterCountResult).Count output a streamed IP set journal file to remember distinct IP address
+
+   its format is as follows:
+
+   This file should be in newline-delimited JSON format(https://jsonlines.org/).
+   For each line, the format of json data should be in the format of:
+   {"recordingStart":"2022-05-30T14:38:44.678610091Z","recordingEnd":"2022-05-30T14:39:48.157630926Z","recorded":""}
+
+	recordingStart:datetime is the time this chunk of recording start.
+
+	recordingEnd:datetime is the time this chunk of recording end.
+
+	recorded is the checkpoint data generated by hyperloglog.
+*/
+
 import "time"
 
 type SinkEntry struct {