add usage and nil check

README.md

@@ -0,0 +1,5 @@
+usage:
+
+codis-ha --codis-config=localhost:18087 --productName=test
+
+

main.go

@@ -12,7 +12,7 @@ type fnHttpCall func(objPtr interface{}, api string, method string, arg interfac
 type aliveCheckerFactory func(addr string, defaultTimeout time.Duration) AliveChecker
 
 var (
-	apiServer   = flag.String("apiserver", "localhost:18087", "api server address")
+	apiServer   = flag.String("codis-config", "localhost:18087", "api server address")
 	productName = flag.String("productName", "test", "product name, can be found in codis-proxy's config")
 
 	callHttp fnHttpCall          = httpCall

rest.go

@@ -37,5 +37,9 @@ func httpCall(objPtr interface{}, url string, method string, arg interface{}) er
 		return errors.Errorf("error: %d, message: %s", resp.StatusCode, string(msg))
 	}
 
-	return json.NewDecoder(resp.Body).Decode(objPtr)
+	if objPtr != nil {
+		return json.NewDecoder(resp.Body).Decode(objPtr)
+	}
+
+	return nil
 }