I am challenged with writing a piece of code that first finds whether
a domain name has been registered yet and if yes who is the domain
name registrar.

The problem is that it should work with all domain extensions and not
just the .com .net and .org etc.

As you can see on http://websiteregistrar.us.com/domain_whois_server.php
there are now 250+ domain name extensions and growing!

The best product I have found is http://wp-whois-proxy.sourceforge.net
but it also uses a static text file based mapping between domain name
extensions and their root servers. The text file can be seen on
http://geektools.com/dist/whoislist.gz

problem:

As you can clearly see the text file based approach is not
sustainable. I need to update the text file on all installations
whenever there is a change in the root servers for any extension or a
new extension is introduced.

Expected answer:

My browser understands all new domain extensions as they are
introduced without being told anything. I don't know how does it find
the root DNS servers for all those extensions but this is what I would
like to have i.e. a web service or a central repository which is
continuously updated and is from the issuing authority preferably by
ICANN.org or IANA.org

I should give them the full domain name and they return me the root
server info for the "domain extension". I should ask that root server
and it should return me that whois server for the exact domain name. I
should then proceed to the whois server of the domain name to get
details about that particular domain name.

To resolve a name, your web browser simply asks the domain server
which in turn asks the root server if required. There is no root
server for particular extensions, the DNS root servers are for . (they
also handle com and net indeed). They might bounce to other servers
depending on the extension.

I don't think there is any way to get the whois server from DNS
queries. You can only proceed by guessing with whois.nic.DOMAIN. But
it's not perfect indeed.

That's why every global whois solution proceeds with a list AFAIK.