app controls other network instance

I have an application that is designed to be run on a local machine but access network resources (databases, etc). I want to be able to notify other network instances of this app when certain events happen.

e.g. maybe one user wants to update one of the network resource files and notify all other users that the file has been updated. Say User A updates the file and the form flashes in Users B + C's applications.

How can I detect if another instance of the app is running on a network (simple workgroup)?

How can an external exe send commands to an app?

Russ