A critical component to any ArcIMS web site is the ArcIMS Application Server. All map service requests pass through the Application Server on their way to an ArcIMS Spatial Server. You can use multiple Application Server machines to help ensure that your site is always up and running, or to help make more efficient use of your hardware resources.
The ArcIMS Servlet Connector can be configured to communicate with more than one Application Server. See the ArcIMS Help for details on how to set this up: Help > Contents > Administering the ArcIMS Servlet Connector > "Linking additional ArcIMS Application Servers to the ArcIMS Servlet Connector".
ArcIMS 3.1 and higher includes enhanced configuration settings that make it easier to set up multiple Application Servers for your ArcIMS system in one of two ways:
1) Failover - In this configuration, the Servlet Connector will only utilize Application Server B if Application Server A fails to respond.
2) Round Robin - In this configuration, the Servlet Connector will pass the first request to Application Server A, the second request to B, the third to A, and so on.
Both configurations, Failover and Round Robin, have their pros and cons. Failover is designed to ensure that requests are always channeled to a working Application Server. Round robin is designed to make efficient use of the system resources to handle high request volume. Choose the configuration that best suits your needs.