Saving away URL query parameters in an analytics application
Your server sets up a cluster, then listens for requests. It parses a parameter from the request URL, and writes it to file.

For performance reasons, that write to file should be asyncrhonous. Currently your client has to wait for your log to write before the client continues.... but, here's the real flaw in your code....

... you do not send a response at all to the client!

You need to actually process the response by sending a status code, and ending the communication:

 response.writeHeader(200); // success
 response.end();


Client sessions will just sit there hanging until you do that.