Every extension of WP_Async_Task must have a non-empty string value stored in the protected $action property. If it has an empty value when WP_Async_Task::__construct() gets called, the constructor will throw an Exception.

The asynchronous postback will run on the action that is set in the $action property (e.g. save_post).

When the asynchronous request is running, if there isn't a logged in user, the library will automatically prepend 'nopriv_' to the $action property.