Plugin notify

Hello,

I want to write a plugin that will link to an external database and pull info from there, then cache it and display it on blog.

My problem is that I don’t know how to notify the plugin when something new is available on the main site. How do I do that? Is there any way that I can call the plugin from “outside” or do I need to make a call like http://www.blogdomain.com/?somethingnew=1 and then have the plugin check the site for the proper info.

Thank you