Configure IP Failover — Part 2

Last week, in part one of this article, we took a look at configuring basic IP failover features on Mac OS X to provide high availability. This week, we'll take a look at building on the process with custom shell scripts, and the secret to successful AFP setup for failover systems.

Extending IP Failover Features:

When IP failover is triggered, the system first checks if any scripts exist in the /Library/IPFailover directory. This script system can be used for additional failover logging, email notification, or even mounting and unmounting volumes in environments without a SAN. The folder doesn't exist by default, but creating it allows you to add functionality to the failover process.

Shell scripts are placed inside a folder named for the main server IP (192.168.0.250 in our example from part one), and must begin with one of four prefixes: PreAcq, PostAcq, PreRel, and PostRel. The PreAcq and PostAcq scripts are run before and after IP acquisition from the main server, while the PreRel and PostRel scripts handle actions surrounding IP release when the main server comes back online.

IP Failover and File Sharing:

If you're configuring IP failover on Macintosh, there's likely one more thing you'll want to know. While there are articles elsewhere on configuring Postfix, BIND, and Apache for high availability, even Apple doesn't currently have support documents for IP failover of AFP. This can make setting up file sharing failover both difficult and frustrating. Should your main file server fail, you can make new connections to an alternate server, but your existing connection is lost (and unfortunately, with it, your data).

The secret is that AFP requires access to a per-session token, established in order to seamlessly reconnect an existing connection. It's this cache that allows AFP to survive network interruptions without data loss. For single-server setups that file is located on the startup volume, but in a failover environment it needs to be accessible to both servers. Open /Library/Preferences/com.apple.AppleFileServer.plist and find these lines near the bottom:

<key>reconnectKeyLocation</key>
<string>/private/etc/AFP.conf</string>

Change the string above, and you change the where OS X Server stores the reconnect key. This allows the to be kept in the shared storage available to both your servers, and provides the ability to "reconnect" to a new server during IP failover.