28 Apr 12 17:49
Do you have data that is updated in real time and needs to be transferred over? By this I mean if you had the 2 sites live and people were hitting the old server, would there be any database inserts or any files written to the old server that would need to be transferred over or is everything static?
1) If it's just static files and there are no database inserts etc, then just get everything setup on the new server and when you are ready, make the necessary DNS changes to point the site to the new server. It will take some time for some users to get the new DNS records due to cache but they would hit the old server during the transition so there would be no problem or any downtime involved.
2) If there are any database inserts, you can look into taking everything down for a very short period while you transfer the DB over to the new server and then update the old server so it points to the new DB server address. This way involves minimal downtime and means both front-end interfaces are pointing to the new DB so any updates are stored on the new server.
3) If you have dynamically generated files that need to be written to the server and accessible for future requests, you would probably be best to take the site down on the old server and setup a redirect to the new IP address, so any requests that do hit the old server are transferred over albeit with the IP address but this would only be temporary. This would involve downtime while you move the files over.
I hope this helps but if I maybe didn't understand your request or you want any clarification, please let me know.
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