diff --git a/observer/const.go b/observer/const.go
new file mode 100644
index 0000000..e0b5faa
--- /dev/null
+++ b/observer/const.go
@@ -0,0 +1,15 @@
+package observer
+
+import "strconv"
+
+type KEY int
+func (k KEY) String() string {
+	return strconv.Itoa(int(k))
+}
+
+const (
+	ADD_CRAWLER = KEY(iota)
+	REMOVE_CRAWLER
+)
+
+