If a regular, scheduled extraction is set up, when the extraction is ready does the client-side application have to continually have to check that the file is available, or for efficiency is there a way to set up so that when the extraction has occurred the application is notified to proceed with getting the extracation.
Today, Datascope Select can only "push" results to private network clients via FTP/SFTP - this is really more of a legacy feature than intended to be used directly with the REST API. I have not seen any "push" features defined for the REST API, but I will have to check with others on what might already be in the backlog.
In the meantime, you are always welcome to submit a feature enhancement request asking for this functionality.
There is no way for the server to notify your application that the extraction is ready.