php - General CURL and Order of operations

This is a very general question but I'm not sure the best way to go about things here.

I have two applications that want to interface. One is a Windows based app that has a database and can send CURL commands. The other is a very simple website with a MySQL database.

The main feature is that these two apps can swap database data between each other. The Windows app is currently using SQLAnywhere but could be converted to MySQL.

Anyway: On the web app there is a js function to dump all data requested into a .txt file, essentially a mysql dump. This function will be called by the Windows app via CURL. It will say "Hey, dump the data for this table in to a txt file, then let me download it."

What I am unsure of is: Once the request to dump the data is complete, the Windows app will want the file right away. How do I say back to it, "Wait until the file is completed, and then you can download it."

I was thinking of making a dummy file and then a .txt file so the Windows app essentially gets stuck in a loop (with a timeout) until the file is renamed to .txt. Is this a good way to approach this?

Thank you.