Whenever your app need some maintenance, you can take steps as following to prevent your app from interacting with app users.
Go to Design | Core App Info | Advanced Settings page, from App Settings section, uncheck [App Enabled] checkbox.
Click [Edit Disabled Reason] button, put a custom message to the value of "message", Save it.
Go to your app, after initialize your app, when calling authencition methods, you will get an error message with the value you input return.
{
"status_message": "Processing exception (bundle): App is disabled.",
"reason_code": 40330,
"severity": "ERROR",
"disabledReason": {
"message": "custom this message -- App has been disabled"
},
"status": 403
}Capture this returned message and show it to your app users.