Understudy is a software-based server clustering utility that implements load balancing and failover protection for Linux (Red Hat, Debian and Slackware), Solaris, Cobalt, FreeBSD and Windows NT.

The instruction manual states that all messages between the remote console and the servers are signed for security. The remote console will work with a firewall, and the servers will record a message to the log if somebody tries to “replay” internal UDP messages to try to confuse the servers.

Use in the Wild

To use Understudy in a production environment, you will want to configure any services (such as web, FTP, mail, TELNET) to respond to the virtual IP addresses (as well as the real IP addresses). There are complete instructions on adding the Virtual Hosts to your Apache or Microsoft IIS web servers.

Understudy does not automatically mirror information from one server to another, although Polyserve has stated that is a goal for a future version. You should think about whether servers will need to have up-to-the-second data copies, and plan accordingly. Some database applications might require extra hardware, such as a RAID array connected to multiple servers. I would visit the Linux High Availability web site, at http://linux-ha.org/, for LAN mirroring ideas.

Pricing and Support

Understudy can be downloaded and demoed free for 30 days, during which time technical support via e-mail is also free. It is trial-ware; during the trial period, the dæmon will turn itself off after two hours of use, requiring you to restart the dæmon. A permanent license with a service contract costs a little under $1000. Without the service contract, the price is roughly half as much. Polyserve has various support options, so you should contact them for a complete listing. I have good things to say about their customer service. Owing to network problems related to the Understudy software (but not the fault of the software), I spent a fair amount of time talking with Polyserve's support staff on the phone. They were technically competent and very helpful in pointing me toward a good solution to my problems.

The documentation is downloaded from the site in PDF form. It is complete and useful, although the product manual shows a few signs of poor editing. Unlike the white papers, the manual incorrectly states that you can use it with only two servers, not ten.

For further help, there is also a six-page help facility which describes the program's operation. For some reason, on my computer the help pages kept throwing Java exceptions. However, the information was still accessible, and they were minor distractions. These were the only bugs I found in the program.


Understudy should be a godsend for the beleaguered system administrator, server farm or ISP that needs to have services up 24/7 through reboots, failures and planned outages. One strong point in favor of this software is its ability to work in any network with all kinds of hosts. Even if your backup server is a 33MHz 486, Understudy can keep your network limping along until you can fix the primary server. It seems to be a good solution for those who cannot afford $10,000 to $50,000 for a dedicated failover and load-balancing server, or simply are not willing to pay a $2000 license for each server in the cluster.

The Good; The Bad

Daniel Allen (da@coder.com) discovered UNIX courtesy of a 1200-baud modem, a free local dialup and a guest account at MIT, back when those things existed. He has been an enthusiastic Linux user since 1995. He is President and co-founder of Prescient Code Solutions, a software consulting company located in Ithaca, New York.