Possible issues in shifting sites from one server to another?
It is always preferred to have the web servers in multiple locations. However, I need to understand about various issues that arise while transferring the websites from one server to another with huge DBs and application pools.
Databases are trouble when moving servers, or at least they can be. I encountered this when moving my large and very active forum from host to host.
The big deal is losing new entries to your database. It can be avoided by moving your database to the new host during a slow period, then changing your old files to connect to the database on the new server. Then change your DNS and you won't lose posts. Wait a week (very conservative) and you're good to shut down the old one.
Moving databases and website files is very easy . The only problem that can occur is the downtime when you change the DNS , but that will be for 2 to 3 hours Max as your websites are already registered .
Windows System Administrator.
Moving from server to server is not normally a problem except the dns change which as prashant said, a few hours of bouncing around. That can be avoided to some degree but in most cases it won't be avoided totally. I have done transfers several different ways and also similar to what Jeff Bee said, but there is still a small chance a few posts could be missed.
If your doing a server to server transfer things will go smoother, with a large database if you do the transfer manually it isn't a problem at all except the issues already listed. With importing the db with a staggered db importer such as bigdump there isn't a db size limit so, no time out issues. The issue here is simply the download and upload of the db and then of course making sure you export the db in the correct manner, which depending on the importer your using may vary a little.
The file transfers shouldn't be an issue regardless of how you have to transfer them. server to server ftp would be faster though.
Moving websites is not a big issue itself, however, you might encounter some trouble due to the actual DNS propagation after changing the nameservers. Since a lot of ISPs use cached records for speed, a large part of your visitors will still hit the old server. If both of the websites and databases are active, your new visitors will modify the new database, while some others will modify the old one. Since there is no easy way of merging the two databases, this will cause a world of pain. This can be avoided in a couple of ways. First, like Jeff Bee said, move your database beforehand, make sure it is up to date, and configure your old server to use it and then change the DNS. This means that even if the users will hit the old server, the database entries will be sent to the new database so nothing would be lost. Another simpler way would be to simply disable the original host after the nameserver switch and enable a message that will inform users that if they are seeing that page, it means that they are using cached dns records, and provide some basic instructions on how to clear their DNS cache in order to be able to access the new server.