Last week I had to move a live heavily trafficed web application from a data center in south america, to a top class offering at Contegix. They really go above and beyond. Anyway, back to the move. Of course, the first step was to have migrated the application to the new machine, and run the battery of tests to make sure all is well.
The magic is simple. At some point you need to tell DNS that your server is actually at a new location. We all know that it can take roughly up to 24 hours or so for this to propogate through the internet. There are some tricks such as taking the TTL down a notch and making sure that gets through a cycle , but it will still take time for the full propogation. You only want people going to ONE machine though, and as soon as you flip the switch, you want it to be the new machine.
For this to happen, you simply make sure that anyone finding their way to the old machine gets proxied through to the correct place. In our case, our app relies a lot on the URL coming in to do different things respond with different applications. Anyway, to do the proxy thing, we simply go through each virtual host in our httpd.
We had to have foo. This form of proxy means that if someone connects to the old machine, that old machine connects to the new one and sends the data across. If you looked at the logs on your new machine you will see that each request is coming from the old machines ip address so your web stats may be a bit weird for this day. Live mode is going to be the same as the httpd. When you are ready to do the switch say, 3am on a Sunday? Now, you could bring down the old system for this to happen, and then you will get some downtime.
A set of our applications are very heavy read applications typical in web apps , so rather than have downtime you can make the choice of:. Depending on the app, this is valid. Now things are running fine on the new machine it is time to switch your DNS to point to the new boy. Over time you will slowly see more different ip addresses on the new machine as the dns change propogates. You will also slowly see less and less traffic on the old machine. When you are done with traffic there you can take it out of commision.
