What is the best way to implement network in this case?

I'm planning to deploy a network mainly for downloading files, it serves internationally so CDN is required to boost user's download speed.

And here is the thing, the network should serve different files via a single url by some LOGIC,
like http://address/file
user A gets fileA from CDN_A
user B gets fileB from CDN_B

I'm thinking about write a CGI program in the center server, implement the LOGIC there, redirect the url http://address/file to http://static.Aaddress/fileA or http://static.Baddress/fileB.

It should work, right? And any better ideas?