« Archives in May, 2006

Moving Servers Realtively Painlessly

Let’s face it, moving from one server to another server is nasty and something any of us would try to avoid as often as we can. Unfortunetly, there are instances where we can not avoid this and we must bite the bullet and do it.

Here are some tips that I have gathered up that will help you maintain your sanity as much as possible. Honestly even with all the planning and preperation, there will be issues so just prepare yourself mentally for that. It will happen.

So here are some things you can do to help the transition:

  • DNS Caching: DNS has to be one of the most difficult things to deal with, because you only have a certain level of control over what occurs. Caching of DNS results will often cause many problems. The trick here is to plan ahead and remove the caching from the picture until the move is completed.

    Setting your DNS to a low TTL (I use 5 seconds versus the recommended 86400 seconds/ 24 hours). The TTL tells other DNS servers and clients how long they should hold the information they just received, in their cache. Since we are going to be moving the site to a new IP soon, we don’t want them caching the old IP once the site is on the new IP. The low TTL will help prevent this.

  • Pathing: If you are moving to a server with different paths, you should at least try to setup symbolic links to help any hard coded paths in the site’s content.

  • PHP register_globals: This one has bitten me a couple of times. Most ofthen when you move to a new server the server will have the default settings for PHP which turns off register_globals. This will cause all sorts of problems that you won’t be able to track down easily. Check it early on and make sure that both servers match settings wise. (A a general rule register_globals should be set to off, but there are some legacy PHP apps that need it.)

  • SSH Tunnels: Most of the time you should adjust your TTL for your DNS long before you do the actual move, but sometimes that isn’t an option. In a pinch you can use an SSL tunnel to redirect traffic from your old server to the new server. This will help users that have the old IP cached, still access the new site. It should be noted that SSH tunnels are pretty unstable and shouldn’t be really be depended on, but like I said, they are good in a pinch. Here is an example of one, you run this on the old server as root with the httpd process stopped:

    ssh -g -C -N -f -L 80:newserverip:80 root@newserverip

    This will prompt you for a password (if you don’t have key based access already setup). You will get a warning ‘bind: Address already in use’, but the process is working. This message is just letting you know that the remote endof the tunnel has something already on port 80, which is actually a good thing.

    You can read more about this process in my blog post Redirecting TCP Ports with SSH

  • DNS: Once you have everything moved over to the new server, update the current authoritative DNS servers with the new IP as soon as possible. Once you have done that change the domains name server records also. The name server change takes longer to complete (will normally be done in 8 hours at the root servers).

  • How To Deal With a Highly Dynamic Website: For sites that have alot of content, or for sites that have users contributing data often (mainly forums) you need to draw a line in the sand and take the site offline during the move. This ensures that you have all the content and that no one loses any content that was contributed during the move.

  • Rsync: Also in the same vein as dynamic sites (user images etc) rsync is your best friend. Run rsync, and run it often. Run it even before you move. It keeps the two sites in sync with each other and can be run while the old site is still up and running. The more you run rsync leading up to the actual move, the faster the actual move will occur. Faster move == less downtime for your users.

There are many other issues to consider, but if you take these items into consideration you should not have any major surprises coming your way.